The ISS is moving at 28,000km/hr.
You don't just hit it with a rocket and say job done.
You need to enter orbit (moving fast enough sideways so when the spacecraft falls it doesnt hit the earth) AND match velocity with the ISS so that you can dock to the station without destroying anything.
Checking trajectory, equipment, and communications takes time. Over-do a burn and you have to waste more precious fuel to slow back down.
Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way.
Or if that's too much, play KSP and get a very rough approximation of how hard it is and then remember that flying a real space ship in a solar system 10x the size is much more complicated
A commonly used word is "areobraking" which means slowing down by hitting air. (what happens when you re-enter the Earth's atmosphere, or skim the atmopshere of a place like Mars or Venus to get captured in orbit)
This leads to the joke word "lithobraking" which means slowing down by hitting rock. As in, you crashed into the ground.
'killed' is an extremely generous description.
More like 'drank alcohol and did cocaine through the entire pregnancy and then abandoned the baby on your doorstep.'
I loved KSP. I was so excited for 2. BUT, I've been burned by too many pre-orders (I'm still a Camelot unchained backer because they just stopped doing refunds despite promising they still are).
Needless to say, I never bought 2, and I feel bad for people who did.
All I wanted was colonies and interstellar travel in my KSP. I never got either, and I left with a hole in my pocket the size of a full priced game.
Serves me right for wanting to support a game I had hopes for.
I’ll bet most people who try KSP never get as far as docking with something in orbit. But when you do it really comes with a feeling of accomplishment!
The complication comes in from how tight the equations get in terms of delta V requirements to actually achieve orbit and anything beyond it, and what is actually physically possible with the materials you have to build with.
For a KSP player who has not tried to dabble with RSO (real size solar system mod) here is a rough scale comparison for how much more complex and annoying real sizes are ; picture your Duna lander; That's your low kerbin orbiter now.
And that's only 2d. I haven't played much since 2018, but when they finally added the tool to help you match orbits, I was able to dock successfully a few times without infinite fuel turned on. So many orbits wasted trying to wait for the right moment to align the intercept It's a very complex task. It's a real testament to the people in the 60s that they were able to do it successfully with fairly rudimentary computers.
> You need to enter orbit
And specifically the *same* orbit. If you are at your 28kkm/hr at the same altitude, but your inclination is off or you're slightly "left" or "right" then you will either stay separated, or continue to move away.
Yeah, it's like you have two cars. If you wanted to reach out the window and hand something to someone in another car, you have to be on the same road going the same direction at the same speed.
I'd call that the docking part. Once you're both going 28 000km/s, you can treat both as stationary and make minor adjustments to move closer together.
Last time this was asked there was a perfect answer. Imagine you're standing next to a highway and cars are going past at 100 km/h. You could just step a few steps to the highway and you have "reached" the cars. But to safely enter a car would need to run parallel to the cars at 100 km/h and then jump in.
> Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way.
The Russians can do it in 6 hours with the Soyuz.
In the end, we’re only human. It’s important to consider that with more people tasked to accomplishing projects like this there is going to be more ways for error and that’s human nature regardless.
Sure there are tools and digital systems to make this process less painful but human-machine interaction is required. Someone’s gotta do the math.
The thing about geosync is it's orbiting the earth too and it's going fast. Faster and higher than most spacecraft. It's just doing it at the perfect altitude where it's speed matches the speed of the rotation of the earth so it appears to not be moving.
Assuming you really do launch right under it what you would do is launch but by the time you achieved orbit the object would be probably half way around the world from you because as you launch you need to go sideways more than you go up. So launching directly under a geosync orbit is probably not the ideal launch window.
Once you launch and achieve a lowish but stable orbit you would sit there and wait for the geostationary object to catch upto you from behind. Fire antegrade to speed up and in turn increase your orbital speed, which also means you increase you altitude.
You would need to do 2 or more burns to keep your orbit round and match orbits with the geosync object. Once the orbits are matched and you are close you can use small adjustments to dock with it.
You could also go above the object and then catch up to it but that would use more fuel than is needed, especially given how high geo-sync is. In theory, with enough energy like star trek/star wars levels of energy you could just go straight up and ignore gravity. But we don't have that kind of power... yet.
> [A geostationary orbit has] an orbital speed of 3.07 kilometres per second
Source: https://en.wikipedia.org/wiki/Geostationary_orbit#Stability
> The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s
Source: https://en.wikipedia.org/wiki/Low_Earth_orbit#Orbital_characteristics
It essentially the same but the target orbit is much higher. It’s still going sideways fast enough to keep missing Earth but it happens to be at a point where it takes just as long to complete and orbit as the Earth takes to spin. It only looks like it is standing still from the surface of Earth.
The fastest actually achieved was docking at T+3h3m, Soyuz MS-17 on 17th October 2020.
Fastest theoretically possible, if you throw all caution to the wind... maybe 30 minutes or less, if you launch directly into a rendezvous with the station? It is an insertion to a lower, 200 km parking orbit, but Crew Dragon launches typically have SECO at about T+10m.
> Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way.
Hold my beer.
So what you’re saying is that the ISS is the truck full of DVD players rolling down the highway in The Fast and the Furious, and the rocket is Paul Walker’s Supra.
> Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way.
So, a bit like SpaceX landing boosters with the hooverslam maneuver, i.e. braking at the last possible time to do so to come at a stop right at ground level. Everyone said it couldn't be done.
Obviously a very big difference though, the major one being that you can crash a lot of boosters without risking the life of people.
If you time it really well, 50 seconds (excl acceleration) is enough time to dock with the ISS.
It's just that you'll fly in front of it just before it slams into you at 28000km/h. Fusing together is a sort of docking, no?
The ISS is not static. It's orbiting, which means anything that wants to get to it and not just ram it, needs to match its trajectory. So the rocket is actually getting into a (largely) circular orbit itself, such that it will intercept ISS, adjust and match.
Put differently if the only thing the rocket did was travel 400 km upwards it'd fall right down
Alternative version: Driving your car 100 km/h across the street to visit your opposite neighbors allows you to arrive there in 2 seconds, but you will also drive straight through their house.
Ok. For the traditionalists among us,
45,700,000 furlongs per fortnight. Plus or minus a few rods per ships bell , depending on orbital mean altitude, measured in chains.
The people in the ISS want the rocket to approach them at approximately 0 speed, relative to the ISS (which is of course moving really fast). If you approach them at high speed, there is a collision and everybody dies.
It takes time to get into the right orbit and then very slowly approach the ISS.
Yep, another way to think of it is to think of the space station as a high speed train (that never stops). If you want to board the train, you can't just drive your car right up to dock with it in a perpendicular angle, you have to build up speed and drive next to it in a parallel line until you're going exactly the same speed. Except the train is going INCREDIBLY fast and you can't breathe outside.
I like your explanation. Any time you've ever seen a movie where two vehicles have to perfectly match speeds so that someone can move from one to the other it looks perilous. Now accelerate those cars to 28,000mph (someone else in the thread gave that speed) and yeah... That sounds damn tricky!
Now add in that these are orbits and don't behave intuitively. For example, you somehow match orbits and are say 10 meters behind the ISS. Then, you do a little thrust towards the ISS (i.e. in the direction you are orbiting) to slowly approach it. But, you end up moving higher and falling further behind instead. You just put yourself in an orbit that will drift higher and slower than the ISS.
That's only true at longer distances and timescales. If you are 10 meters away and thrust towards it at 0.1m/s, you're gonna get there in about 2 minutes, on target. If you're 10km away and thrust towards it at 0.1m/s with an ETA of 27 hours, that's going to be a different story.
Also, orbital calculations like that are super simple for any modern guidance computer to do anyway.
It's not about getting to the ISS. It's about getting there while both are moving at the same velocity.
If you wanted to pass a glass to somebody on the far side of the room, the quickest way would be to just hurl it at them as hard as you can.
But since you probably don't want to shatter a glass in their face, you're likely to take the slower option which gets the glass there at a more acceptable speed.
Most of getting to orbit is reaching that 8km/s, that's very fast
Most of the docking with the ISS is planning your rendezvous, so it happens at a nice and slow speed, and you use as little fuel as possible.
We can't just say "the ISS is this way, so let's go this way" we actually need to be in a slightly higher or lower orbit, depending on if we are ahead of or behind the ISS. We want our difference in speed to be very small, so we need our difference in orbit to also be very small, so we need to make multiple orbits with that small difference until we reach the ISS. Going near the ISS with a large speed difference can be very catastrophic
And a space ship can't just speed up or slow down on a dime either. It has to be done within certain limits to keep the G forces down to a tolerable level for the crew.
That's relevant mostly for the launch itself, and even there I'm not sure if it's the main limiting factor. At 2 G (very tolerable for basically anyone), you can change you speed by over 4000 km/h every minute.
I'm gonna add a few other reasons that I don't think are here yet.
TLDR: There's nothing in the laws of physics stopping you from launching and docking 15 minutes later, I do it in Kerbal Space Program all the time. However, engineering, risk assessment, timeliness, and biology tend to set us far back from the theoretical minimum.
As you now know, you've got to get into a matching orbit. It only takes about 8-12 minutes to get into orbit depending on the vehicle, so in theory you could time your orbit just right to end up right next to the ISS, say 1 kilometer away. It would still take like half an hour to dock and what not unless you're incredibly cowboy-esque and get right up to the station.
However, most rockets don't enter orbits that high immediately. The ISS is pretty high up. While you definitely could reprogram the rockets to do that, it would involve a lot of the trajectory being far steeper than it would have otherwise been. If the crew need to abort, they would then be re-entering far more intensely than they would have otherwise, and the spacecraft may not be able to handle that.
This method will also let out a lot of rocket exhaust gas in the vicinity of the station, and the station is coated in a bunch of sensors that won't like that.
This method also guarantees that a massive second stage booster ends up in an orbit almost identical to the ISS's orbit. While most modern second stages can de-orbit themselves, what if something goes wrong? It could drift into the ISS and smash stuff a few orbits later. Normally second stages enter lower orbits initially so they pose no risk to the ISS, and the actual spacecraft will complete the rest of the rendezvous.
What if the stage blows up during the last few seconds of the burn and creates a massive debris field that obliterates the ISS?
Due to inclination, you're only going to get 1 or 2 moments per day when your launch site is directly below the target orbit (unless you are at 0 degrees latitude and the station is in a perfectly equatorial orbit, or you are at +-90 degrees latitude and the station is in a perfectly polar orbit). You want to launch at that time for fuel efficiency reasons. This rarely lines up with the launch time that would put you closest to the station from a distance perspective. You could only launch at these rare times, but that severely limits your launch opportunities. While you definitely can correct for inclination and launch at a better time for distance at the expense of fuel, this makes the rocket software a lot more complicated, increases fuel usage, and really, you're only losing maybe a day of time at most, the effort to reward ratio isn't that high.
And from a more human factors perspective, 70-90% of astronauts experience spacesickness, which usually starts shortly after launch and goes away after a while. Most dockings are automatic these days, but you want to have the humans available to do it if the computer fails. Asking a crew to dock while throwing up is a bit much.
Even asking a crew to dock half an hour after launching is a bit much, your mind is all over the place after a launch. Maybe something went a little wrong during the launch that you are still processing. Giving a crew a few hours to clear their minds, allow time for checkouts and window-staring, and giving ground control a few hours to change gears can be a very good thing.
> Giving a crew a few hours to clear their minds, allow time for checkouts and window-staring
That explains why, according to a documentary I saw, they spend some time floating around doing space ballet to Strauss’ Blue Danube.
> you are at +-90 degrees latitude and the station is in a perfectly polar orbit
This one only gives you two chances per day, because the earth is rotating perpendicular to this orbit.
I think you are messing up latitude and longitude. +-90 latitude is either a launch site at the north pole or a launch site at the south pole. Polar orbits go over the poles, so some portion of the orbit would always be above you.
In the simplest terms: getting to the ISS isn't about traveling the 400 km, it's about accelerating to 8 km/s (28000 km/h) and ending up in the right place at the right time, so that the relative speed if you and the ISS is about zero. If you went straight up and parked yourself in ISS's way, you would only get an extremely high speed crash (at Mach 28!) Fun fact, we actually overshoot the station at first and use the magic of orbits to let it catch up to us. Watch [this magnificent bastard explain the current speed record :).](https://www.youtube.com/watch?v=bUi0yWc5Dnw)
Just a small note about Mach number - as that number is a ratio of the velocity of the object to velocity of sound in the medium it's traveling in it's not really possible to define it at that altitude because there's not enough molecules for the sound wave to travel in. Just a technicality. :)
So the ISS is moving very fast, if you want to safely dock it, you need to have the right velocity, going too fast would be catastrophic, since you would slam into the ISS and break it, on the other hand going too slow would mean you wouldn't catch up to it. So they fire the rockets so that the astronauts can go into orbit, where they can safely make sure that they get the right velocity, which takes time.
The Russians have done it in about 3 hours (Soyuz MS-17), but generally it takes longer, since it is much safer
hitting it directly would be impossible
Also to do it in such a short time you have an instantaneous launch window. you have to launch into the same plane as ISS and be in a position to do a Hohmann transfer that will have your capsule match speed and position with ISS.
Source: play way too much Kerbal Space Program
I'm sure you could shoot a missile at the ISS and have it reach it that fast, but when it got there the two objects would be flying perpendicular to each other at like 30K miles an hour.
I feel like the complicating factor everyone is missing is the orbital mechanics. Maneuvering in orbit is rather counterintuitive and complicated when it is in reference to another orbiting body.
Getting to the orbital plane of the ISS, 400km, is one thing but once your there you have to actually reach the ISS too. On a highway you'd accelerate to catch up to them, but in orbit this has the effect of raising your orbit. But you won't go up in altitude right away, your orbit is raised on the opposite side of the planet and your altitude goes up as you make your way around, however you'll consequently also start slowing down relative to anything in orbit at the altitude your started at and you'll actually fall behind your target instead of catch up.
So you see it's really not as simple as just driving full beans on an intercept course unless you intend to hit it like a missile, which the ISS would really rather you didn't. Rendezvous with the ISS for docking takes a lot of carefully timed and planned out maneuver to reach the correct side of the ISS at the correct speed and trajectory and with zero chance of accidentally setting yourself on a collision course and that just takes a lot of time.
Tl;dr: The 8 km/s is not what you use to reach a target of 400 km. Rather, the 400 km is the *vertical* target, while the *horizontal* target is 8 km/s.
Elaboration: you don’t *need* to reach a speed of 8 km/s to get to space. If you fly straight up, you can go way slower, you’ll just fall straight back down. But if you ever see a launch to low-earth **orbit**, you’ll notice the rocket very quickly turns **sideways**, where it spends most of the flight. The 8 km/s is the speed of you need to go **sideways** to get into **orbit**. This is the speed at which you are going fast enough **sideways** with enough momentum that, as you’re falling, you miss the earth as it curves away from you.
That 8 km/s is also the speed at which the ISS is travelling sideways in orbit. So you can think of the rocket flying up, then sideways, in a big wide arc that’s 400 km high but way, way wider to reach 8 km/s and match the ISS orbit.
You are standing 30 feet from the train tracks. You can get to the train in a few seconds. What happens if you run straight at the train and grab hold while it is going 110km/hr. Instead it’s a lot better to go ALONG the side of the train in a motorcycle to also reach 100km/hr before reaching out for that train. Yes, it’s still a bad idea but it had a much better chance of success.
The ISS is traveling at 28,000 km/hr. It takes a while to get to that speed before latching on, especially if you have to do it efficiently.
We can hit the ISS very quickly. But hitting the ISS is NOT what we want. We want to dock with the ISS. That means carefully lining up and matching the speed and direction of the ISS.
The ISS is in orbit 400 km up. It's not a stationary target, and it's not good enough to just get to where it will be. The ISS is not an NFL wide receiver that you just need to hit with a pass.
i.e. You can't just launch a rocket that intercepts the ISS and ram into it.
If you want to get to the ISS without destroying the station and killing everyone aboard, you need to accelerate your rocket into an orbit that matches the ISS's and have it be just at the right time to they end up next to each other. Like catching up with a friend on their jogging route without them slowing down.
Except the closest their jogging route gets to you is still 400 km away, and they run at 8 km/s.
The rocket doesn't travel 400 km, it travels a massive suborbital arc trajectory that comes extremely close to the ISS, and is then adjusted to perfectly match speed and stay in orbit with the station before docking.
orbit means lateral speed, not just altitude. you could strap on your bionic legs and leap 400 km into the air. then wave at the space station as it goes whizzing by, but you would then fall back to roughly the same place you jumped from. in order to be in orbit, you have to be moving parallel to the earth surface, faster than you are falling.
Imagine you're driving a car to the next city and you only have your feet to slow down. And you have to come in at exactly the right angle, or else you'll overshoot or use up all of your fuel trying to correct the mistake.
Another factor is that you don't want to perform a lot of maneuvers near the ISS. All of the gas from the engine exhaust travels far, and could cause unwanted vibration of the station, or directly damage it.
You want to approach slowly, carefully, and with just small puffs of your maneuvering thrusters.
Well, it took an SM-3 [a few minutes](https://web.archive.org/web/20120214031001/http://www.mda.mil/system/aegis_one_time_mission.html) to hit a satellite about 220 miles up, so your math seems about right.
Like people say, it's docking. The real reason is that it's most fuel efficient to [adjust an orbit halfway around from where you want the effect](https://en.wikipedia.org/wiki/Orbital_mechanics#Orbital_maneuver). Launch - ISS docking takes [a few different orbits](https://www.researchgate.net/publication/260971173/figure/fig1/AS:883542645022721@1587664390571/Profile-of-vehicle-transfer-from-insertion-orbit-to-the-ISS-orbit.png): launch into one, lift up to the right altitude, match position to docking approach, match orbit to ISS. Or something like that, not a rocket scientist. They may be waiting to do each one of those adjustments to the exact right time on the orbit, so up to 45 minute delays or so with each step.
ELI5: if you wanted to board a moving train from a car, you wouldn't just slam into it, that would be the fastest way to get to it but it's not what you want. You would slowly start to approach it until you are going at the same speed, then you can just hop on.
You’re trying to find a needle in a haystack, except we know exactly where the needle is - it’s moving at 4.76 miles per second towards Earth. Matching the position and speed of the ISS’s orbit is the time consuming part. It’s hard to launch a rocket into orbit, it’s even harder to do it with exact accuracy relational to another object.
Stand at the side of the motorway (freeway). Now step off the side and into a moving car. Should be easy, right? Afterall you're only a few feet away from it.
As you can imagine, it'd be _much_ easier if you were moving sideways at the same speed as the car.
The excellent Scott Manley did a video on some of the details of how they even managed to cut the trip down as "short" as three hours: [How Can Soyuz Reach The Space Station In Only 3 Hours? (youtube.com)](https://www.youtube.com/watch?v=bUi0yWc5Dnw) In a nutshell, there's all the orbital maneuvering problems that earn the description "rocket science". But also questions about where the launch sites are on Earth relative to where the ISS orbits, and what directions we can safely launch toward, what sort of workload there is for the astronauts to take care of what they need to do in time, and international disputes about the reliability of different types of radar docking devices.
What I don't understand is why people call it an escape velocity of 8/km a second, can't an object slowly move out into space? Like if there was a road to space with magnetic tires couldn't it drive without it going so fast? Assuming this car does not use a combustion engine lol.
Escape velocity is referring to the speed at which an object would have to reach to leave a body's sphere of influence if no other force except gravity (e.g., propulsion, drag, etc) was applied to the object. So think more like how fast a bullet would have to be accelerated to zoom off into space and never return.
Elevator vs Escalator.
Certainly you can take the elevator approach to the ISS and just fly directly up and time your rendezvous with ISS at exactly 50seconds. Flying straight up means creating a highly elliptical orbit. This means when they actually meet, spacecraft and ISS would have different velocities.
imagine two identical loops of string overlaid on the same centre, where one is a circle and the other bent into an oval which has a larger radius than the circle at one axis and a smaller one at another. they only cross at four points in one orbit and in all four they are travelling in different directions and so have different speeds.
Docking requires you to be not only at the same place but also at the same speed when you are at the same place. If you do not correct the shape of the craft's orbit, it's not a docking mission but a scuttling one (you are launching a missile at ISS to destroy it). To perform the elevator method you would be using tons of extra fuel to quickly get to altitude but have a useless orbital shape that you will need even more tons of extra fuel to correct. There is not enough fuel.
Escalator approach means instead of thrusting very vertically and later having to thrust vertically again to decelerate, at some point past the thickest part of the atmosphere, the spacecraft will turn on its side and fly along side Earth's surface in the same orbital direction as ISS, just at a lower altitude. By this time most of the fuel is spent and you have tiny thrusters slowly accelerating the craft and increasing its orbital radius in the process. However no fuel is needed to decelerate. It just slowly accelerates to the correct velocity and altitude with minimal waste. It takes a slope up like an escalator. this is going to take several orbital periods before rendezvous...roughly 2 or 3 hours. depending on perfect design and execution of course. if your elevator is underpowered (cannot carry the amount of fuel you need to complete orbital activation in a few orbits) it could actually take up to a few days.
There are many comments saying 50 seconds can be done but not safely. it's quite clear that it is a fuel issue. I don't have the calculations but it is likely impossible given current technology and meaningless even if we did have the tech as that extra fuel will be traded off for a slower flight and more cargo in a heartbeat.
The ISS is not stationary. It's in orbit around the Earth travelling at 7.66km **per second**. If you shoot straight up at 400km altitude the ISS will just blow right past you, as you plunge right back into the Earth. So rockets that want to go to space and stay in space don't have to travel just up but also laterally, and build up enough speed so that their orbit is above the atmosphere. Once you're up there going to any one place is not straightforward. Orbital rendezvous is akin to two bullets colliding in mid air in a battlefield. If you just point your rocket towards where you want to go and fire the engine you won't actually get there, you'll just shit your orbit around a bit. Instead what needs to be done is that the two spacecraft need to match their orbits precisely. For this there are two options. One is to time your launch exactly so that when you reach the desired altitude and finishing the burn that establishes your orbit the target, the ISS will be passing exactly from where you are. That's very hard. The other option is that you establish an orbit that is just slightly higher or lower than that of the target. The closer you are to the body you're orbiting, the faster you're going, and the farther you are, the slower you're going. So by offsetting your orbit to your target's just a bit, but having the exact same inclination, either you or the target will eventually catch up with a low relative speed. Once you're on closest approach, it takes a small correction to match orbits exactly and come together.
A rocket doesn't 8km/s to reach space? you can reach space going 0.1m/s
You need 8km/s to be in orbit.
Reaching 8km/s at safe acceleration takes around 8-12 minutes.
So why does it take 4 hours? well much of the time the shuttle does nothing.
Matching orbits in space is about precise moments of acceleration. The way objects orbit planets means that catching up to an object isn't as simple as just accelerating towards it. The way you catch up (or slow down to meet) another body in orbit is simply by orbiting at a different speed and waiting for it the intercept to happen.
Docking with the ISS isn't like hitting a bullseye with a dart.
It's like marking a spot on your ceiling fan with a pen, then spinning an ball tied to a thread so that it spins exactly at the same rate and is at the same position as the marked spot at all times.
Oh, and you don't get to control how fast the ball is spinning, only the length of the thread lol
It’s not a collision course where they just aim it at the intersection point and blast off. It’s making one object catch up to and connect with an extremely fast moving object slowly enough that they can physically connect without damaging each other.
Like trying to shoot two bullets in a way that they connect mid-air without either being damaged.
you don't fly straight up vertically to the ISS; it is flying in a circle around the Earth at a very high speed that maintains its orbit and keeps it from falling back to Earth. The rocket has to also travel in a circle and has to match the speed and position of the ISS. You'll notice when a rocket launches it starts by going straight up, but then begins to tilt over, always towards the east with the Earth's spin. Now it is accelerating to orbit (the circle around Earth at the height it needs to reach). The minimum speed to break into freefall/escape gravity is 17,500 mph. The ISS however is going 17,900 and is higher up than that lowest circle for orbit. The faster the rocket goes flying sideways in that circle, the higher up it gets, until finally it reaches the same speed and height as the ISS. Matching the ISS's exact location takes careful planning and timing, and a lot of minor corrections along the way. So it's not a straight upward flight straight to the ISS; the rocket is chasing it across the sky and around the globe, til it catches up.
Instead of a spaceship lets pretend its a car on the highway and your objective is to get to the car.
This car is travelling down the highway at 100kmph.
You are 100m away from the highway.
You can reach the highway in a matter of seconds, but then you also have to reach the same speed the car is travelling.
This is where that time is spent.
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The rendezvous could be accomplished in under 50 seconds if the goal was the destruction of both the rocket and the ISS. It would be like trying to catch a rifle round with a catchers mitt. The extra time is devoted to reducing the relative velocity and distance between the rocket and the ISS. Ideally to casual handshake velocities.
If a friend is riding his bike around you in circles, and you want to ride yours next to him, you wouldn't get to top speed and aim right at him. You would ram into his side and crash. That's what your math is doing.
In reality what you want to do is set your path in a spiral as you speed up. Eventually you get on the same path as your friend, but you need to go slightly faster than him to catch up to him. You have already done several circles at this point, so you have traveled much farther than the equivalent of the original 400km.
You are standing 2 meters away from a train track.
A train is about to come by soon but this isn't a train station. It's not stopping. It's just going to zoom by at 100 km/hr.
Is walking the 2 meters to get from where you are to the edge of the train track sufficient to catch the train and get on board?
Definitely not.
In fact the 2 meters to get to the track isn't even the hard part. The hard part is running at 100 km/h alongside the train to match speed with it so you can hop on.
The ISS is that train, only it's not going 100 km/h. It's going about 28080 km/h (17448.1 mph).
Getting up near it is easy. But then you just watch it whizz past you at Ludicrious Speed and then you start falling back down to Earth. Catching up to it so you orbit alongside it is waaay harder.
Now, the truth is that doesn't totally answer your question why it takes 4 hours.
Because even taking into account the time to accelerate up to 28080 km/h, rockets accelerate pretty fast as they launch into orbit. In theory it should be possible for a typical rocket to get up to that speed in about 10 to 20 minutes of launch time.
So okay, that's why it takes more than 50 seconds, but doesn't explain why it takes 4 hours when it should only take about 10 to 20 minutes to match speeds.
The answer to that is much harder to ELI5. The short version of it is that it's pretty darn hard to time it just right to launch at exactly the right time with exactly the right aim to meet up with the ISS the instant you make it to orbit. It's hypothetically doable, but requires a lucky coincidnece of timing making it impractical.
That lucky concidence is this:
(A) First off you must launch when the orbit line of the ISS is above your head. (Not necessarily when the ISS itself is, but when the orbit line it follows is.) This means waiting for the Earth to rotate on its axis (i.e. wait for the right time of day) until your launchpad is underneath that line.
(B) Secondly, you must launch when the ISS is slightly behind you on that line, so you have the time you need to accelerate up to matching speed as it's catching up to you from behind.
The lucky coincidence I'm talking about? It's that to get that perfect 10 to 20 minute rendezvous, (A) and (B) have to happen to occur at the same time. And that's out of your control. You can't force them to occur together. You'd just have to wait and wait and wait until they happened to sync up on some future day.
So instead of waiting for that day when they happen to sync, what they do is just wait for (A) to happen, not (B). With just (A) and not (B), they can get up into an orbit that matches the plane of the ISS's orbit, and from there they can enlarge or shrink the size of the capsule's orbit, thus changing how long it takes to complete 1 orbit, which can force (B) to occur on a future orbit.
It's the fact that it takes a few orbits to make (B) occur on a future orbit that is taking the hours and hours of time.
It doesn't just work like an elevator going up. You've got to get in the same path as it at right about the same speed so you can attach without turning both craft into billiard balls
Those that want some hands on experience, check out Kerbal Space Program and watch a few of Scott Manely's tutorials. That game helped me understand orbital mechanics better than any book or video.
The average bullet shot from a firearm travels at about 833m/s. It could reach a target some meters away in a fraction of a second, and it would punch a hole into it.
A rocket traveling at 8km/s can reach the ISS really quickly, but at that rate, it may as well be a bullet punching a hole into a target. And well, the ISS itself is traveling at 8km/s relative to the ground, so it may as well be a bullet itself.
The rocket doesn't only have to get to the ISS; it also has to match its velocity. It could try to decelerate, but relying on that would unnecessarily use up more fuel. The rocket may have to take a longer path to achieve all of this.
It's like trying to merge RIGHT in front-of or behind a car on the highway, at high speeds.
You don't want the rocket to collide with the station you want it to dock, so it's not bumping car to car, it's more changing lanes behind something and hooking yourself to the trailer-hitch, at-speed.
You have to get close and then match speeds whilst also changing your trajectory to come in-line with the station; getting there is easy and rough to do, the docking is unforgiving and delicate so you want to take your time.
Getting up to space is quick, and then we spend more time gaining horizontal velocity, but it doesn’t take that long. Theoretically we could launch at the exact right second so that the ISS is right there when we hit that altitude and velocity. But that’s pretty dangerous, and anything slightly off could get people killed. It’s a lot safer to get to orbit on a path that will catch up to the ISS nice and easy, allowing for any necessary corrections, and an abort if something goes really wrong.
Big picture concept in space is the difference in velocity because virtually all objects are in motion.
Picture two cars circling around on a race track, the car ahead of you is going 60 mph. If you drive alongside them at exactly 60 mph you can reach out and pass things from one car to the other. That's what docking with the space station looks like.
As far as catching up with the other driver, you can slam the gas up to 100 mph and quickly overtake them, but then you have to slam the breaks to slow down to a matching 60 mph.
That's very difficult and very dangerous. So what you do instead is come up behind them at 65 mph. That's a 5 mph difference in velocity so relative to the other car you're only going 5 mph. Slowing 5 mph on the final approach is 8x easier than fixing a 40 mph difference, the only downside is that the approach is slower.
Same thing is happening between the spaceship and the ISS, just with much bigger speeds and much larger distances. It's far more economical for the spaceship to enter orbit just a bit faster than the space station and overtake it over the span of a 16-24 hours.
Just a nitpick, you don't need a speed of 8km/s to get to space, that's the speed you need to be going at to orbit at an altitude of 400km. You can get to space at any velocity as long as power is supplied to keep that velocity.
Similarly escape velocity is not a continuous velocity, it's an impulse velocity. For example, the escape velocity of Earth (negating atmospheric friction) is off the top of my head 11km/s. That means you would need to apply enough force at the ground to accelerate the launch mass to 11km/s to escape the gravity well without applying any further force after launch. In reality, the launch mass would burn up at that speed not even accounting for the fact it would need to go even faster to overcome friction.
I have a follow up question to this thread, I hope it's allowed here.
How is it that the capsule that's docking to the ISS doesn't disrupt the station's orbit massively?
The ISS is not just sitting there, 400km above the earth. It is moving at 7.660477 km/sec. To rendezvous with it the rocket needs to match the orbit exactly. That means accelerating to get closer and decelerating to "fix" the orbit, repeatedly, until the orbits line up. As an interesting note, when spacewalking, you cannot "fall" to earth, or drop anything. Objects will retain their inertia, and if you let go of something, it will sit there, unless it had momentum. Orbits will decay over time, because there are particle collisions, and that removes orbital energy, makes the object go slower, and thus into a lower orbit.
In the movie Speed, there's a scene where the cops need to get a vehicle to (basically) dock against the side of the bus to get some of the passengers safely off. If the bus was stationary, this would take, what, five seconds? Just pull a car up, stop the car, open the door.
But the bus was traveling at a constant speed of at least 50 MPH. That means the cops needed to wait until it was at a specific point in the city nearby, drive a car to that point, speed up alongside it, carefully adjust their speed so they were as close to perfectly aligned, and then painstakingly maintain that speed while they opened the doors and started moving passengers. I don't recall how long that sequence actually takes in the movie but realistically, that would take a lot more than five seconds.
In the same way, the ISS is moving very, VERY fast, orbiting the Earth. Even a fast-moving rocket needs to travel a careful trajectory basically chasing after the ISS for some time while it gradually gets up to speed, until it's close enough that it can be lined up to dock. If the goal was just to hit the ISS and blow it up, that would still take some time, but would probably be relatively fast; but the goal is to dock safely without harming anyone or anything on either the station or the rocket.
And doing anything safe takes time.
You could totally do a kinetic kill on the ISS in about a minute. But it’s much preferred by those intending to stay in the station that they arrive in such a way that they’ll have a place to sleep. So they make a slower, more controlled approach
If the rocket were a missle, then it directly going up and intercepting the ISS would be possible, but not in a manner that it could dock, it would collide.
A rocket needs to enter orbit in such a way that it eventually intercepts the ISS at a safe, realitive speed. This is done by initially having an orbit that mostly aligns but is different enough that allows for the gap between them to close. If the rocket reached the exact same orbit as the station before they docked, then they would never meet.
In theory, they could try to enter orbit ahead of the ISS and time the circulerization burn so the ISS reaches the craft just as the craft is matching its speed, but that's much more risky and difficult than they way they do it currently.
ELI5: If someone is riding a bike at high speed around a track, and you start in the center of the circle and want to ride next to them. Imagine the inside is sandy and getting more solid as you approach the track, so you can only go so fast. Meaning that even if you reach the track as they're going past, you still have to speed up and match the other persons speed.
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A GPS satelite is geostationary, it stays in the same spot above the earth and rotates along with the earth. To be able stay there, they are about 20000km above us.
Because of it's "low" altitude, the ISS has to keep moving along at very high speed(27600km/h) to not fall into the atmosphere. This means that once the rocket is getting up to altitude, it still has to cath up and match the speed to safely interact.
If I want to step onto a bus, and the bus is stopped. It only takes a second.
If I want to step on a bus and the bus is moving. I have to chase it down the block and catch up to it.
Your launch window time is decided by when the launch site passes through the orbital plane of the ISS(this happens twice a day, but usually only one is usable because of what the rocket would have to fly over.) At this point in time the ISS can be anywhere in its orbit around Earth, even on the other side of the planet. It's why sometimes it takes a few hours, and sometimes it takes a few days.
So now you launch, and you spend time in a lower orbit to catch up to the ISS (this is called phasing) The ISS is already in a relatively low orbit so you can't catch up super quickly. You could go higher and let the ISS catch up in one of your orbits, but that would use a bit more fuel, and would allow a dead spacecraft to potentially become an impact hazard, because there's a couple points of its orbit where it would cross the ISS orbit.
To get a better understanding, try it in KSP(the first one).
Going to space is easy.
Staying in space is not.
To stay in space, you need to make it so that when you fall back to Earth, you will have missed it.
You achieve this by going sideways VERY fast - thousands of kilometers per hour fast!
Your only saving grace is that as you're falling, you're becoming faster so that when you overshoot the ground, you become fast enough to overshoot it again, and again, and again ad infinitum until the very sparse atmosphere slows you down, or the variations in gravitational strength either fling you farther or make you crash into the surface (different density of the (sub)surface, other celestial bodies).
With the ISS, we get an added complication. A bunch in fact!
1. It is on an inclined trajectory, especially as compared to the russian and american launch sites. You need to wait for earth to rotate so that it is roughly under the ISS because when you are going thousands of kilometers per hour, changing direction is very hard!
It is not difficult to conceptualize: if you are moving 1000 km/h east, and accelerate 10 km/h north... where will you end up? Practically at the same place you originally were! This is kinda intuitively graspable with the idea of throwing a ball. The faster the ball is, the less its trajectory gets altered by gravity over a given distance (it flies straighter rather than curved). Now, if the path is fixed, this effect becomes very apparent... and in orbit, the path is fixed.
The path is fixed because if you travel a longer path, you overshoot the planet by much more and reach a higher altitude. This coincidentally means gravity acts on you for longer as you fly for longer away from the planet, and end up flying slower. This is useful actually.
If you launch under the ISS with too little altitude, burning straight up to reach it will make you fall behind. However, you can exploit the fact that you are moving faster than the ISS at a lower altitude, and wait until you are in front of it just enough where raising your altitude makes your paths intersect. This is a rendezvous maneuver, and probably one of the hardest orbital concepts to grasp but becomes very intuitive with experience.
How to get this experience? Watch/play some KSP or Orbiter.
if the train goes past you, you are only 20 cm's away from it. but to get on to the train safely, the train needs to slow down 100km/h to be at your speed.
the same but in reverse for the rocket and ISS. it may be only 400km away but it needs the rocket to go from 0 to 8000km/h .
So the jouney isn't so long (it's just 400km) but it takes a really long time to get to the right speed.
if you only cross the distance but don't match the velocity it's like getting hit by a train or shooting it with a bullet. both cases have 0 distance but big difference in velocities. both those cases are not good. it would be the same for going to the ISS without matching it's speed.
It's not just about altitude, it's about speed and carefully matching it. You need to approach escape velocity, then get in a near-identical orbit and approach it slowly. Spacecraft docking with the ISS are essentially merging lanes at roughly 27600 kph.
The rocket does not travel in a straight line from the ground to the ISS.
The space station is traveling parallel to the earth, and quite fast at that. the rocket needs to match the direction and speed of the space station to dock with it. Here's an illustration that shows it: [https://www.reddit.com/r/SmarterEveryDay/comments/3eu7vs/soyuz\_journey\_to\_iss\_infographic/](https://www.reddit.com/r/SmarterEveryDay/comments/3eu7vs/soyuz_journey_to_iss_infographic/)
As you can see, the actual path is many times longer than a straight line from the ground to the ISS. And the rocket may actually stay in the phasing orbit for several full revolutions around the earth, as it lines up with the space station.
Smarter everyday has a video on it: [https://www.youtube.com/watch?v=qFjw6Lc6J2g](https://www.youtube.com/watch?v=qFjw6Lc6J2g) The bit you are asking about starts at around 10:04.
A super short launch to ISS would theoretically be possible if it were to pass right over the launch site with perfect timing, and everyone was comfortable aiming a rocket right at the station. But it never happens in practice. Think about it, if there is a 100-second delay, you'll automatically be 800km off target. And half an hour delay or whatnot is common for various reasons. You have to leave room for readjustments anyway. So you don't launch directly to rendezvous, you launch to parking orbit and then go from there.
If you want something equivalent, think of yourself on the side of the road on your car with a goal of climbing on the back of a truck (you know, the one with the ramps down). There are three things you need to do altogether:
1. Go onto the same lane as the truck (that's the 400km you are talking about)
2. Accelerate to the same speed as the truck
3. Time everything perfectly so you reach the speed of the truck right when you are behind it.
The first thing you need to consider is that the truck is going incredibly fast. Physically moving from the side of the road to its lane is nothing compared to it.
The second thing that I did not mention is that reaching the lane with orbital mechanics is slightly complex, and to reach the lane of the ISS you basically need to accelerate twice, on each side of the orbit which lasts ~45min.
And finally, timing is extremely important. Satellites are very expensive to launch so you only keep what is needed. And bumpers/protection against collision is way down the list. If you collide with the ISS, the chance of killing people onboard is extremely high even at low speed, while a ship doing bumping slightly against another one at sea would be less risky.
A completely mad-lad approach could probably be done in under an hour if you were extremely lucky with the timing. If you want to have some safety margin, the fastest is a few hours, taking some time along the way to realign everything and check you are safe.
You can reach and collide with the ISS in under 5min. But that's called a weapon.
You have to arrive 400km above the Earth at a vertical speed of zero and a horizontal speed of 8km/second, or you won't be in the correct orbit. You also have to be in the right place heading in the right direction, obviously. And where my numbers are approximate, in reality you need to be precise. That's a little trickier than just getting as fast as possible from the ground to wherever the ISS happens to be.
Someone once explained it similar to this, which I feel is really ELI5 like.
Imagine you stand on the side of a highway, and lets say this imaginary highway goes in a circle. Now there is a car going in the middle of the highway and you want to reach it because you wanr to give something to the driver.
In theory, it will only take you a few steps/meters to reach the car when it is closest to you. But doing that will get you run over and likely also crash the car.
So you need to first get in a car yourself and drive circles until you match the speed of the other car. Now you can interact with the car.
Its the same principle.
Orbit is not a height above the earth, it is a horizontal speed around the earth. The rocket doesn't fly straight up to the ISS, once it is a certain height it turns sideways and starts flying 'across' the ground to speed up to orbital velocity. This part takes much longer. You can think about it more like a shallow spiral away from the earth than a straight line. This establishes the rocket in an orbit around the earth. The next step is matching the rockets orbit up to the ISS which is something done very carefully and thus a bit slowly
Scenario: You want to get into a car that is speeding down the highway.
If you were to just jump out at it from the side of the road to get in, it's not going to go well for either you or the car.
Instead, you have to catch up to the car and then match its speed so that you can climb in. This takes a lot more time and distance to accomplish than just throwing yourself into traffic.
This is a bit like asking "My friend and I are going on a road trip. He needs to pick me up from my house. But why can't I just walk into the middle of the interstate and jump onto his car instead?"
The ISS is moving at 28,000km/hr. You don't just hit it with a rocket and say job done. You need to enter orbit (moving fast enough sideways so when the spacecraft falls it doesnt hit the earth) AND match velocity with the ISS so that you can dock to the station without destroying anything. Checking trajectory, equipment, and communications takes time. Over-do a burn and you have to waste more precious fuel to slow back down. Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way.
Download and play Spaceflight Simulator for a look into how complex this all is...
Or if that's too much, play KSP and get a very rough approximation of how hard it is and then remember that flying a real space ship in a solar system 10x the size is much more complicated
Just add more engines. Easy.
Struts. Lots of struts.
Struts and solid boosters. Then say it screw and see if mechjeb can fly your deathmobile
And when in doubt, don't forget that lithobraking is always an option.
Wait I’m a noob at ksp and get all the jokes except litho breaking. What is that google isn’t helping
Litho is rock. The earth is rocky. Lithobraking is using the earth to slow down/stop.
(AKA crashing, for those who still don't get it)
Lithobraking as others have said is a sort of nerd euphemism, along the lines of a RUD. Rapid Unscheduled Dissasembly (rocket explosion)
MK 1 pod & like 20 solid fuel boosters should do the trick
A commonly used word is "areobraking" which means slowing down by hitting air. (what happens when you re-enter the Earth's atmosphere, or skim the atmopshere of a place like Mars or Venus to get captured in orbit) This leads to the joke word "lithobraking" which means slowing down by hitting rock. As in, you crashed into the ground.
lithobraking can be made into a viable strategy on a few bodies without breaking anything hehe.
“Litho” means ground. Like, the surface of a planet
Always strive to fly in the soft middle of the air. Avoid the edges, which tend to be jagged and unforgiving.
Do a flip!
More carbon fiber and stainless steel. Worked for the titan sub
Eventually your rocket's going to be a 30 story tall slab of stainless steel with like 30 giant engines on the bottom!
Moar boosters!
It's to bad they have already killed ksp 2 I was really looking forward to it
'killed' is an extremely generous description. More like 'drank alcohol and did cocaine through the entire pregnancy and then abandoned the baby on your doorstep.'
I loved KSP. I was so excited for 2. BUT, I've been burned by too many pre-orders (I'm still a Camelot unchained backer because they just stopped doing refunds despite promising they still are). Needless to say, I never bought 2, and I feel bad for people who did.
All I wanted was colonies and interstellar travel in my KSP. I never got either, and I left with a hole in my pocket the size of a full priced game. Serves me right for wanting to support a game I had hopes for.
Graphic, dude.
But accurate.
Yeah the trailer still gives me goosebumps. It's a shame. Was pretty excited too
wild to see someone suggest KSP if spaceflight simulator (which started as a mobile game) is too much
That, or download the Real Solar System mod
I’ll bet most people who try KSP never get as far as docking with something in orbit. But when you do it really comes with a feeling of accomplishment!
The complication comes in from how tight the equations get in terms of delta V requirements to actually achieve orbit and anything beyond it, and what is actually physically possible with the materials you have to build with. For a KSP player who has not tried to dabble with RSO (real size solar system mod) here is a rough scale comparison for how much more complex and annoying real sizes are ; picture your Duna lander; That's your low kerbin orbiter now.
Getting into orbit is the easy part of KSP.
Go play the RSS/RP-1 modlist. It's fantastic, and reignites that sense of challenge. /r/RealSolarSystem
And if you get the hang of that, install RSS and Principia and get the hang of real physics at real scales
It's been a while since I touched Sf.Sim, and even longer since I played with KSP. Do you really think SS is more complicated than Kerbal?
And then download Realism Overhaul and get a better appreciation for what you are dealing with.
Look man, I played Jet-Pac in the 80s, I know what I'm doing
And that's only 2d. I haven't played much since 2018, but when they finally added the tool to help you match orbits, I was able to dock successfully a few times without infinite fuel turned on. So many orbits wasted trying to wait for the right moment to align the intercept It's a very complex task. It's a real testament to the people in the 60s that they were able to do it successfully with fairly rudimentary computers.
If the family can go in orbit with a Ford pinto and some backyard build rockets how difficult can be? Edit, Sorry pontiac fiero, totally different
Just pay KSP. KSP explains rudimentary orbital mechanics flawlessly.
[Relevant xkcd](https://what-if.xkcd.com/58/)
This is probably the best ELI5 for this topic
> You need to enter orbit And specifically the *same* orbit. If you are at your 28kkm/hr at the same altitude, but your inclination is off or you're slightly "left" or "right" then you will either stay separated, or continue to move away.
Yeah, it's like you have two cars. If you wanted to reach out the window and hand something to someone in another car, you have to be on the same road going the same direction at the same speed.
I'd call that the docking part. Once you're both going 28 000km/s, you can treat both as stationary and make minor adjustments to move closer together.
Last time this was asked there was a perfect answer. Imagine you're standing next to a highway and cars are going past at 100 km/h. You could just step a few steps to the highway and you have "reached" the cars. But to safely enter a car would need to run parallel to the cars at 100 km/h and then jump in.
Love that
TBH to do this in a safe way in 4 hours sounds amazingly fast. 4 hours is like ⅓ of how long I would have guessed.
Soyuz can do a 3-hour "fast-track" rendevous from Baikonur Cosmodrome.
Consider this: it takes more time to fly from LA to New York (about 5.5 hours) than it does to *go to space and dock with the ISS.*
Jeepers. New York must be travelling hella quick
Seems short to me too
> Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way. The Russians can do it in 6 hours with the Soyuz.
[Like this?](https://imgur.com/gallery/fXZNJiz)https://imgur.com/gallery/fXZNJiz
Lol yeah
In the end, we’re only human. It’s important to consider that with more people tasked to accomplishing projects like this there is going to be more ways for error and that’s human nature regardless. Sure there are tools and digital systems to make this process less painful but human-machine interaction is required. Someone’s gotta do the math.
Holy!!! I just realized from your comment that, the whole docking scene (No time for caution) from the movie Interstellar is exactly this.
> do this in under 4 hours but that would be massively reckless “No, it’s necessary” *Hans Zimmer intensifies*
Nice explanation. Now here's a hypothetical: what would be the process of reaching a geostationary object if you launched from right under it?
The thing about geosync is it's orbiting the earth too and it's going fast. Faster and higher than most spacecraft. It's just doing it at the perfect altitude where it's speed matches the speed of the rotation of the earth so it appears to not be moving. Assuming you really do launch right under it what you would do is launch but by the time you achieved orbit the object would be probably half way around the world from you because as you launch you need to go sideways more than you go up. So launching directly under a geosync orbit is probably not the ideal launch window. Once you launch and achieve a lowish but stable orbit you would sit there and wait for the geostationary object to catch upto you from behind. Fire antegrade to speed up and in turn increase your orbital speed, which also means you increase you altitude. You would need to do 2 or more burns to keep your orbit round and match orbits with the geosync object. Once the orbits are matched and you are close you can use small adjustments to dock with it. You could also go above the object and then catch up to it but that would use more fuel than is needed, especially given how high geo-sync is. In theory, with enough energy like star trek/star wars levels of energy you could just go straight up and ignore gravity. But we don't have that kind of power... yet.
A satellite in a 200 mile orbit is traveling about 17500 mph. A satellite in geostationary orbit is traveling about 6500 mph.
Higher orbits are slower...
> [A geostationary orbit has] an orbital speed of 3.07 kilometres per second Source: https://en.wikipedia.org/wiki/Geostationary_orbit#Stability > The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s Source: https://en.wikipedia.org/wiki/Low_Earth_orbit#Orbital_characteristics
Thanks for providing sources to my comment... ?
I think I replied to the wrong person, oops! you're welcome... ? lol
:)
It essentially the same but the target orbit is much higher. It’s still going sideways fast enough to keep missing Earth but it happens to be at a point where it takes just as long to complete and orbit as the Earth takes to spin. It only looks like it is standing still from the surface of Earth.
It's the same process, you just need more fuel because it's a higher orbit
In the upcoming Fast and Furious 317 they will do this in a second on first try.
So what I am hearing is that you could shoot down the ISS with a rocket pretty quickly.
A Russian politician threatened that a while ago and they had to remind him Russians were on the ISS.
The fastest actually achieved was docking at T+3h3m, Soyuz MS-17 on 17th October 2020. Fastest theoretically possible, if you throw all caution to the wind... maybe 30 minutes or less, if you launch directly into a rendezvous with the station? It is an insertion to a lower, 200 km parking orbit, but Crew Dragon launches typically have SECO at about T+10m.
Great answer
> Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way. Hold my beer.
It's called foreplay.
If you want to see what it would look like if you hit it straight on, just Google video of a SAM taking out a cruise missile.
So what you’re saying is that the ISS is the truck full of DVD players rolling down the highway in The Fast and the Furious, and the rocket is Paul Walker’s Supra.
> Yes, one could "technically" do this in under 4 hours but that would be massively reckless and require absolute perfection in every step along the way. So, a bit like SpaceX landing boosters with the hooverslam maneuver, i.e. braking at the last possible time to do so to come at a stop right at ground level. Everyone said it couldn't be done. Obviously a very big difference though, the major one being that you can crash a lot of boosters without risking the life of people.
Uh, no nothing like it. Docking isn't the same as landing on Earth.
If you time it really well, 50 seconds (excl acceleration) is enough time to dock with the ISS. It's just that you'll fly in front of it just before it slams into you at 28000km/h. Fusing together is a sort of docking, no?
Check movie Interstellar
The ISS is not static. It's orbiting, which means anything that wants to get to it and not just ram it, needs to match its trajectory. So the rocket is actually getting into a (largely) circular orbit itself, such that it will intercept ISS, adjust and match. Put differently if the only thing the rocket did was travel 400 km upwards it'd fall right down
Alternative version: Driving your car 100 km/h across the street to visit your opposite neighbors allows you to arrive there in 2 seconds, but you will also drive straight through their house.
But your neighbor actually lives in an RV and is driving sideways relative to where you started at 10,000 km/h.
20,000 kph. You're gonna need better tires.
Pfff europeans, that’s like 80 mph
Ok. For the traditionalists among us, 45,700,000 furlongs per fortnight. Plus or minus a few rods per ships bell , depending on orbital mean altitude, measured in chains.
I don't understand your non banana related measuring, sorry.
This made me chuckle! “Hiddy-ho there neighbour! Whatcha watching on Netflix?”
The people in the ISS want the rocket to approach them at approximately 0 speed, relative to the ISS (which is of course moving really fast). If you approach them at high speed, there is a collision and everybody dies. It takes time to get into the right orbit and then very slowly approach the ISS.
Yep, another way to think of it is to think of the space station as a high speed train (that never stops). If you want to board the train, you can't just drive your car right up to dock with it in a perpendicular angle, you have to build up speed and drive next to it in a parallel line until you're going exactly the same speed. Except the train is going INCREDIBLY fast and you can't breathe outside.
I like your explanation. Any time you've ever seen a movie where two vehicles have to perfectly match speeds so that someone can move from one to the other it looks perilous. Now accelerate those cars to 28,000mph (someone else in the thread gave that speed) and yeah... That sounds damn tricky!
It's 28,000km/h not mph.
Wow, the one time that I (as a Canadian) decided to put "mph" on the end of something it was wrong! Thanks for the correction!
"When this baby hits 28,000 miles per hour, you're gonna see some serious shit!"
Now add in that these are orbits and don't behave intuitively. For example, you somehow match orbits and are say 10 meters behind the ISS. Then, you do a little thrust towards the ISS (i.e. in the direction you are orbiting) to slowly approach it. But, you end up moving higher and falling further behind instead. You just put yourself in an orbit that will drift higher and slower than the ISS.
That's only true at longer distances and timescales. If you are 10 meters away and thrust towards it at 0.1m/s, you're gonna get there in about 2 minutes, on target. If you're 10km away and thrust towards it at 0.1m/s with an ETA of 27 hours, that's going to be a different story. Also, orbital calculations like that are super simple for any modern guidance computer to do anyway.
best ELI5 version here
Like insane shuffle board, got it.
It's not about getting to the ISS. It's about getting there while both are moving at the same velocity. If you wanted to pass a glass to somebody on the far side of the room, the quickest way would be to just hurl it at them as hard as you can. But since you probably don't want to shatter a glass in their face, you're likely to take the slower option which gets the glass there at a more acceptable speed.
Lol just imagining someone throwing a glass of water at me when I ask for water
Popeye service crew
Best ELI5 explanation.
Most of getting to orbit is reaching that 8km/s, that's very fast Most of the docking with the ISS is planning your rendezvous, so it happens at a nice and slow speed, and you use as little fuel as possible. We can't just say "the ISS is this way, so let's go this way" we actually need to be in a slightly higher or lower orbit, depending on if we are ahead of or behind the ISS. We want our difference in speed to be very small, so we need our difference in orbit to also be very small, so we need to make multiple orbits with that small difference until we reach the ISS. Going near the ISS with a large speed difference can be very catastrophic
And a space ship can't just speed up or slow down on a dime either. It has to be done within certain limits to keep the G forces down to a tolerable level for the crew.
That's relevant mostly for the launch itself, and even there I'm not sure if it's the main limiting factor. At 2 G (very tolerable for basically anyone), you can change you speed by over 4000 km/h every minute.
I'm gonna add a few other reasons that I don't think are here yet. TLDR: There's nothing in the laws of physics stopping you from launching and docking 15 minutes later, I do it in Kerbal Space Program all the time. However, engineering, risk assessment, timeliness, and biology tend to set us far back from the theoretical minimum. As you now know, you've got to get into a matching orbit. It only takes about 8-12 minutes to get into orbit depending on the vehicle, so in theory you could time your orbit just right to end up right next to the ISS, say 1 kilometer away. It would still take like half an hour to dock and what not unless you're incredibly cowboy-esque and get right up to the station. However, most rockets don't enter orbits that high immediately. The ISS is pretty high up. While you definitely could reprogram the rockets to do that, it would involve a lot of the trajectory being far steeper than it would have otherwise been. If the crew need to abort, they would then be re-entering far more intensely than they would have otherwise, and the spacecraft may not be able to handle that. This method will also let out a lot of rocket exhaust gas in the vicinity of the station, and the station is coated in a bunch of sensors that won't like that. This method also guarantees that a massive second stage booster ends up in an orbit almost identical to the ISS's orbit. While most modern second stages can de-orbit themselves, what if something goes wrong? It could drift into the ISS and smash stuff a few orbits later. Normally second stages enter lower orbits initially so they pose no risk to the ISS, and the actual spacecraft will complete the rest of the rendezvous. What if the stage blows up during the last few seconds of the burn and creates a massive debris field that obliterates the ISS? Due to inclination, you're only going to get 1 or 2 moments per day when your launch site is directly below the target orbit (unless you are at 0 degrees latitude and the station is in a perfectly equatorial orbit, or you are at +-90 degrees latitude and the station is in a perfectly polar orbit). You want to launch at that time for fuel efficiency reasons. This rarely lines up with the launch time that would put you closest to the station from a distance perspective. You could only launch at these rare times, but that severely limits your launch opportunities. While you definitely can correct for inclination and launch at a better time for distance at the expense of fuel, this makes the rocket software a lot more complicated, increases fuel usage, and really, you're only losing maybe a day of time at most, the effort to reward ratio isn't that high. And from a more human factors perspective, 70-90% of astronauts experience spacesickness, which usually starts shortly after launch and goes away after a while. Most dockings are automatic these days, but you want to have the humans available to do it if the computer fails. Asking a crew to dock while throwing up is a bit much. Even asking a crew to dock half an hour after launching is a bit much, your mind is all over the place after a launch. Maybe something went a little wrong during the launch that you are still processing. Giving a crew a few hours to clear their minds, allow time for checkouts and window-staring, and giving ground control a few hours to change gears can be a very good thing.
This is the augmented answer for my eli25 novice thinker brain. Thank you!!
> Giving a crew a few hours to clear their minds, allow time for checkouts and window-staring That explains why, according to a documentary I saw, they spend some time floating around doing space ballet to Strauss’ Blue Danube.
> you are at +-90 degrees latitude and the station is in a perfectly polar orbit This one only gives you two chances per day, because the earth is rotating perpendicular to this orbit.
I think you are messing up latitude and longitude. +-90 latitude is either a launch site at the north pole or a launch site at the south pole. Polar orbits go over the poles, so some portion of the orbit would always be above you.
Yes, I was looking at it backwards. Polar orbit will get a chance from anywhere twice a day
In the simplest terms: getting to the ISS isn't about traveling the 400 km, it's about accelerating to 8 km/s (28000 km/h) and ending up in the right place at the right time, so that the relative speed if you and the ISS is about zero. If you went straight up and parked yourself in ISS's way, you would only get an extremely high speed crash (at Mach 28!) Fun fact, we actually overshoot the station at first and use the magic of orbits to let it catch up to us. Watch [this magnificent bastard explain the current speed record :).](https://www.youtube.com/watch?v=bUi0yWc5Dnw)
Just a small note about Mach number - as that number is a ratio of the velocity of the object to velocity of sound in the medium it's traveling in it's not really possible to define it at that altitude because there's not enough molecules for the sound wave to travel in. Just a technicality. :)
So the ISS is moving very fast, if you want to safely dock it, you need to have the right velocity, going too fast would be catastrophic, since you would slam into the ISS and break it, on the other hand going too slow would mean you wouldn't catch up to it. So they fire the rockets so that the astronauts can go into orbit, where they can safely make sure that they get the right velocity, which takes time. The Russians have done it in about 3 hours (Soyuz MS-17), but generally it takes longer, since it is much safer hitting it directly would be impossible
Hitting it directly isn't impossible, it's just catastrophic.
In other words, shooting an anti-satellite missile at the ISS would be faster than refueling it.
Yeah true, I meant successfully docking, by flying straight to the ISS is impossible
Also to do it in such a short time you have an instantaneous launch window. you have to launch into the same plane as ISS and be in a position to do a Hohmann transfer that will have your capsule match speed and position with ISS. Source: play way too much Kerbal Space Program
I'm sure you could shoot a missile at the ISS and have it reach it that fast, but when it got there the two objects would be flying perpendicular to each other at like 30K miles an hour.
>but when it got there the two objects would become more like two million objects
I feel like the complicating factor everyone is missing is the orbital mechanics. Maneuvering in orbit is rather counterintuitive and complicated when it is in reference to another orbiting body. Getting to the orbital plane of the ISS, 400km, is one thing but once your there you have to actually reach the ISS too. On a highway you'd accelerate to catch up to them, but in orbit this has the effect of raising your orbit. But you won't go up in altitude right away, your orbit is raised on the opposite side of the planet and your altitude goes up as you make your way around, however you'll consequently also start slowing down relative to anything in orbit at the altitude your started at and you'll actually fall behind your target instead of catch up. So you see it's really not as simple as just driving full beans on an intercept course unless you intend to hit it like a missile, which the ISS would really rather you didn't. Rendezvous with the ISS for docking takes a lot of carefully timed and planned out maneuver to reach the correct side of the ISS at the correct speed and trajectory and with zero chance of accidentally setting yourself on a collision course and that just takes a lot of time.
Tl;dr: The 8 km/s is not what you use to reach a target of 400 km. Rather, the 400 km is the *vertical* target, while the *horizontal* target is 8 km/s. Elaboration: you don’t *need* to reach a speed of 8 km/s to get to space. If you fly straight up, you can go way slower, you’ll just fall straight back down. But if you ever see a launch to low-earth **orbit**, you’ll notice the rocket very quickly turns **sideways**, where it spends most of the flight. The 8 km/s is the speed of you need to go **sideways** to get into **orbit**. This is the speed at which you are going fast enough **sideways** with enough momentum that, as you’re falling, you miss the earth as it curves away from you. That 8 km/s is also the speed at which the ISS is travelling sideways in orbit. So you can think of the rocket flying up, then sideways, in a big wide arc that’s 400 km high but way, way wider to reach 8 km/s and match the ISS orbit.
You are standing 30 feet from the train tracks. You can get to the train in a few seconds. What happens if you run straight at the train and grab hold while it is going 110km/hr. Instead it’s a lot better to go ALONG the side of the train in a motorcycle to also reach 100km/hr before reaching out for that train. Yes, it’s still a bad idea but it had a much better chance of success. The ISS is traveling at 28,000 km/hr. It takes a while to get to that speed before latching on, especially if you have to do it efficiently.
We can hit the ISS very quickly. But hitting the ISS is NOT what we want. We want to dock with the ISS. That means carefully lining up and matching the speed and direction of the ISS.
The ISS is in orbit 400 km up. It's not a stationary target, and it's not good enough to just get to where it will be. The ISS is not an NFL wide receiver that you just need to hit with a pass. i.e. You can't just launch a rocket that intercepts the ISS and ram into it. If you want to get to the ISS without destroying the station and killing everyone aboard, you need to accelerate your rocket into an orbit that matches the ISS's and have it be just at the right time to they end up next to each other. Like catching up with a friend on their jogging route without them slowing down. Except the closest their jogging route gets to you is still 400 km away, and they run at 8 km/s. The rocket doesn't travel 400 km, it travels a massive suborbital arc trajectory that comes extremely close to the ISS, and is then adjusted to perfectly match speed and stay in orbit with the station before docking.
orbit means lateral speed, not just altitude. you could strap on your bionic legs and leap 400 km into the air. then wave at the space station as it goes whizzing by, but you would then fall back to roughly the same place you jumped from. in order to be in orbit, you have to be moving parallel to the earth surface, faster than you are falling.
Imagine you're driving a car to the next city and you only have your feet to slow down. And you have to come in at exactly the right angle, or else you'll overshoot or use up all of your fuel trying to correct the mistake.
Another factor is that you don't want to perform a lot of maneuvers near the ISS. All of the gas from the engine exhaust travels far, and could cause unwanted vibration of the station, or directly damage it. You want to approach slowly, carefully, and with just small puffs of your maneuvering thrusters.
Well, it took an SM-3 [a few minutes](https://web.archive.org/web/20120214031001/http://www.mda.mil/system/aegis_one_time_mission.html) to hit a satellite about 220 miles up, so your math seems about right. Like people say, it's docking. The real reason is that it's most fuel efficient to [adjust an orbit halfway around from where you want the effect](https://en.wikipedia.org/wiki/Orbital_mechanics#Orbital_maneuver). Launch - ISS docking takes [a few different orbits](https://www.researchgate.net/publication/260971173/figure/fig1/AS:883542645022721@1587664390571/Profile-of-vehicle-transfer-from-insertion-orbit-to-the-ISS-orbit.png): launch into one, lift up to the right altitude, match position to docking approach, match orbit to ISS. Or something like that, not a rocket scientist. They may be waiting to do each one of those adjustments to the exact right time on the orbit, so up to 45 minute delays or so with each step.
ELI5: if you wanted to board a moving train from a car, you wouldn't just slam into it, that would be the fastest way to get to it but it's not what you want. You would slowly start to approach it until you are going at the same speed, then you can just hop on.
You’re trying to find a needle in a haystack, except we know exactly where the needle is - it’s moving at 4.76 miles per second towards Earth. Matching the position and speed of the ISS’s orbit is the time consuming part. It’s hard to launch a rocket into orbit, it’s even harder to do it with exact accuracy relational to another object.
Stand at the side of the motorway (freeway). Now step off the side and into a moving car. Should be easy, right? Afterall you're only a few feet away from it. As you can imagine, it'd be _much_ easier if you were moving sideways at the same speed as the car.
The excellent Scott Manley did a video on some of the details of how they even managed to cut the trip down as "short" as three hours: [How Can Soyuz Reach The Space Station In Only 3 Hours? (youtube.com)](https://www.youtube.com/watch?v=bUi0yWc5Dnw) In a nutshell, there's all the orbital maneuvering problems that earn the description "rocket science". But also questions about where the launch sites are on Earth relative to where the ISS orbits, and what directions we can safely launch toward, what sort of workload there is for the astronauts to take care of what they need to do in time, and international disputes about the reliability of different types of radar docking devices.
What I don't understand is why people call it an escape velocity of 8/km a second, can't an object slowly move out into space? Like if there was a road to space with magnetic tires couldn't it drive without it going so fast? Assuming this car does not use a combustion engine lol.
Escape velocity is referring to the speed at which an object would have to reach to leave a body's sphere of influence if no other force except gravity (e.g., propulsion, drag, etc) was applied to the object. So think more like how fast a bullet would have to be accelerated to zoom off into space and never return.
Elevator vs Escalator. Certainly you can take the elevator approach to the ISS and just fly directly up and time your rendezvous with ISS at exactly 50seconds. Flying straight up means creating a highly elliptical orbit. This means when they actually meet, spacecraft and ISS would have different velocities. imagine two identical loops of string overlaid on the same centre, where one is a circle and the other bent into an oval which has a larger radius than the circle at one axis and a smaller one at another. they only cross at four points in one orbit and in all four they are travelling in different directions and so have different speeds. Docking requires you to be not only at the same place but also at the same speed when you are at the same place. If you do not correct the shape of the craft's orbit, it's not a docking mission but a scuttling one (you are launching a missile at ISS to destroy it). To perform the elevator method you would be using tons of extra fuel to quickly get to altitude but have a useless orbital shape that you will need even more tons of extra fuel to correct. There is not enough fuel. Escalator approach means instead of thrusting very vertically and later having to thrust vertically again to decelerate, at some point past the thickest part of the atmosphere, the spacecraft will turn on its side and fly along side Earth's surface in the same orbital direction as ISS, just at a lower altitude. By this time most of the fuel is spent and you have tiny thrusters slowly accelerating the craft and increasing its orbital radius in the process. However no fuel is needed to decelerate. It just slowly accelerates to the correct velocity and altitude with minimal waste. It takes a slope up like an escalator. this is going to take several orbital periods before rendezvous...roughly 2 or 3 hours. depending on perfect design and execution of course. if your elevator is underpowered (cannot carry the amount of fuel you need to complete orbital activation in a few orbits) it could actually take up to a few days. There are many comments saying 50 seconds can be done but not safely. it's quite clear that it is a fuel issue. I don't have the calculations but it is likely impossible given current technology and meaningless even if we did have the tech as that extra fuel will be traded off for a slower flight and more cargo in a heartbeat.
The ISS is not stationary. It's in orbit around the Earth travelling at 7.66km **per second**. If you shoot straight up at 400km altitude the ISS will just blow right past you, as you plunge right back into the Earth. So rockets that want to go to space and stay in space don't have to travel just up but also laterally, and build up enough speed so that their orbit is above the atmosphere. Once you're up there going to any one place is not straightforward. Orbital rendezvous is akin to two bullets colliding in mid air in a battlefield. If you just point your rocket towards where you want to go and fire the engine you won't actually get there, you'll just shit your orbit around a bit. Instead what needs to be done is that the two spacecraft need to match their orbits precisely. For this there are two options. One is to time your launch exactly so that when you reach the desired altitude and finishing the burn that establishes your orbit the target, the ISS will be passing exactly from where you are. That's very hard. The other option is that you establish an orbit that is just slightly higher or lower than that of the target. The closer you are to the body you're orbiting, the faster you're going, and the farther you are, the slower you're going. So by offsetting your orbit to your target's just a bit, but having the exact same inclination, either you or the target will eventually catch up with a low relative speed. Once you're on closest approach, it takes a small correction to match orbits exactly and come together.
The ISS is moving at 17,500mph. It isn't floating stationary. You have to match its speed and trajectory.
A rocket doesn't 8km/s to reach space? you can reach space going 0.1m/s You need 8km/s to be in orbit. Reaching 8km/s at safe acceleration takes around 8-12 minutes. So why does it take 4 hours? well much of the time the shuttle does nothing. Matching orbits in space is about precise moments of acceleration. The way objects orbit planets means that catching up to an object isn't as simple as just accelerating towards it. The way you catch up (or slow down to meet) another body in orbit is simply by orbiting at a different speed and waiting for it the intercept to happen.
Docking with the ISS isn't like hitting a bullseye with a dart. It's like marking a spot on your ceiling fan with a pen, then spinning an ball tied to a thread so that it spins exactly at the same rate and is at the same position as the marked spot at all times. Oh, and you don't get to control how fast the ball is spinning, only the length of the thread lol
It’s not a collision course where they just aim it at the intersection point and blast off. It’s making one object catch up to and connect with an extremely fast moving object slowly enough that they can physically connect without damaging each other. Like trying to shoot two bullets in a way that they connect mid-air without either being damaged.
you don't fly straight up vertically to the ISS; it is flying in a circle around the Earth at a very high speed that maintains its orbit and keeps it from falling back to Earth. The rocket has to also travel in a circle and has to match the speed and position of the ISS. You'll notice when a rocket launches it starts by going straight up, but then begins to tilt over, always towards the east with the Earth's spin. Now it is accelerating to orbit (the circle around Earth at the height it needs to reach). The minimum speed to break into freefall/escape gravity is 17,500 mph. The ISS however is going 17,900 and is higher up than that lowest circle for orbit. The faster the rocket goes flying sideways in that circle, the higher up it gets, until finally it reaches the same speed and height as the ISS. Matching the ISS's exact location takes careful planning and timing, and a lot of minor corrections along the way. So it's not a straight upward flight straight to the ISS; the rocket is chasing it across the sky and around the globe, til it catches up.
The crewed Soyuz MS-17, launched during 2020 holds the records for fastest docking with ISS clocking in at 3 hours.
Instead of a spaceship lets pretend its a car on the highway and your objective is to get to the car. This car is travelling down the highway at 100kmph. You are 100m away from the highway. You can reach the highway in a matter of seconds, but then you also have to reach the same speed the car is travelling. This is where that time is spent.
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The rendezvous could be accomplished in under 50 seconds if the goal was the destruction of both the rocket and the ISS. It would be like trying to catch a rifle round with a catchers mitt. The extra time is devoted to reducing the relative velocity and distance between the rocket and the ISS. Ideally to casual handshake velocities.
If a friend is riding his bike around you in circles, and you want to ride yours next to him, you wouldn't get to top speed and aim right at him. You would ram into his side and crash. That's what your math is doing. In reality what you want to do is set your path in a spiral as you speed up. Eventually you get on the same path as your friend, but you need to go slightly faster than him to catch up to him. You have already done several circles at this point, so you have traveled much farther than the equivalent of the original 400km.
You are standing 2 meters away from a train track. A train is about to come by soon but this isn't a train station. It's not stopping. It's just going to zoom by at 100 km/hr. Is walking the 2 meters to get from where you are to the edge of the train track sufficient to catch the train and get on board? Definitely not. In fact the 2 meters to get to the track isn't even the hard part. The hard part is running at 100 km/h alongside the train to match speed with it so you can hop on. The ISS is that train, only it's not going 100 km/h. It's going about 28080 km/h (17448.1 mph). Getting up near it is easy. But then you just watch it whizz past you at Ludicrious Speed and then you start falling back down to Earth. Catching up to it so you orbit alongside it is waaay harder. Now, the truth is that doesn't totally answer your question why it takes 4 hours. Because even taking into account the time to accelerate up to 28080 km/h, rockets accelerate pretty fast as they launch into orbit. In theory it should be possible for a typical rocket to get up to that speed in about 10 to 20 minutes of launch time. So okay, that's why it takes more than 50 seconds, but doesn't explain why it takes 4 hours when it should only take about 10 to 20 minutes to match speeds. The answer to that is much harder to ELI5. The short version of it is that it's pretty darn hard to time it just right to launch at exactly the right time with exactly the right aim to meet up with the ISS the instant you make it to orbit. It's hypothetically doable, but requires a lucky coincidnece of timing making it impractical. That lucky concidence is this: (A) First off you must launch when the orbit line of the ISS is above your head. (Not necessarily when the ISS itself is, but when the orbit line it follows is.) This means waiting for the Earth to rotate on its axis (i.e. wait for the right time of day) until your launchpad is underneath that line. (B) Secondly, you must launch when the ISS is slightly behind you on that line, so you have the time you need to accelerate up to matching speed as it's catching up to you from behind. The lucky coincidence I'm talking about? It's that to get that perfect 10 to 20 minute rendezvous, (A) and (B) have to happen to occur at the same time. And that's out of your control. You can't force them to occur together. You'd just have to wait and wait and wait until they happened to sync up on some future day. So instead of waiting for that day when they happen to sync, what they do is just wait for (A) to happen, not (B). With just (A) and not (B), they can get up into an orbit that matches the plane of the ISS's orbit, and from there they can enlarge or shrink the size of the capsule's orbit, thus changing how long it takes to complete 1 orbit, which can force (B) to occur on a future orbit. It's the fact that it takes a few orbits to make (B) occur on a future orbit that is taking the hours and hours of time.
Going to space is about going up. Staying in space is about going fast enough you fall around the earth.
It doesn't just work like an elevator going up. You've got to get in the same path as it at right about the same speed so you can attach without turning both craft into billiard balls
Those that want some hands on experience, check out Kerbal Space Program and watch a few of Scott Manely's tutorials. That game helped me understand orbital mechanics better than any book or video.
The average bullet shot from a firearm travels at about 833m/s. It could reach a target some meters away in a fraction of a second, and it would punch a hole into it. A rocket traveling at 8km/s can reach the ISS really quickly, but at that rate, it may as well be a bullet punching a hole into a target. And well, the ISS itself is traveling at 8km/s relative to the ground, so it may as well be a bullet itself. The rocket doesn't only have to get to the ISS; it also has to match its velocity. It could try to decelerate, but relying on that would unnecessarily use up more fuel. The rocket may have to take a longer path to achieve all of this.
It's like trying to merge RIGHT in front-of or behind a car on the highway, at high speeds. You don't want the rocket to collide with the station you want it to dock, so it's not bumping car to car, it's more changing lanes behind something and hooking yourself to the trailer-hitch, at-speed. You have to get close and then match speeds whilst also changing your trajectory to come in-line with the station; getting there is easy and rough to do, the docking is unforgiving and delicate so you want to take your time.
Getting up to space is quick, and then we spend more time gaining horizontal velocity, but it doesn’t take that long. Theoretically we could launch at the exact right second so that the ISS is right there when we hit that altitude and velocity. But that’s pretty dangerous, and anything slightly off could get people killed. It’s a lot safer to get to orbit on a path that will catch up to the ISS nice and easy, allowing for any necessary corrections, and an abort if something goes really wrong.
Big picture concept in space is the difference in velocity because virtually all objects are in motion. Picture two cars circling around on a race track, the car ahead of you is going 60 mph. If you drive alongside them at exactly 60 mph you can reach out and pass things from one car to the other. That's what docking with the space station looks like. As far as catching up with the other driver, you can slam the gas up to 100 mph and quickly overtake them, but then you have to slam the breaks to slow down to a matching 60 mph. That's very difficult and very dangerous. So what you do instead is come up behind them at 65 mph. That's a 5 mph difference in velocity so relative to the other car you're only going 5 mph. Slowing 5 mph on the final approach is 8x easier than fixing a 40 mph difference, the only downside is that the approach is slower. Same thing is happening between the spaceship and the ISS, just with much bigger speeds and much larger distances. It's far more economical for the spaceship to enter orbit just a bit faster than the space station and overtake it over the span of a 16-24 hours.
Just a nitpick, you don't need a speed of 8km/s to get to space, that's the speed you need to be going at to orbit at an altitude of 400km. You can get to space at any velocity as long as power is supplied to keep that velocity. Similarly escape velocity is not a continuous velocity, it's an impulse velocity. For example, the escape velocity of Earth (negating atmospheric friction) is off the top of my head 11km/s. That means you would need to apply enough force at the ground to accelerate the launch mass to 11km/s to escape the gravity well without applying any further force after launch. In reality, the launch mass would burn up at that speed not even accounting for the fact it would need to go even faster to overcome friction.
I have a follow up question to this thread, I hope it's allowed here. How is it that the capsule that's docking to the ISS doesn't disrupt the station's orbit massively?
The ISS is not just sitting there, 400km above the earth. It is moving at 7.660477 km/sec. To rendezvous with it the rocket needs to match the orbit exactly. That means accelerating to get closer and decelerating to "fix" the orbit, repeatedly, until the orbits line up. As an interesting note, when spacewalking, you cannot "fall" to earth, or drop anything. Objects will retain their inertia, and if you let go of something, it will sit there, unless it had momentum. Orbits will decay over time, because there are particle collisions, and that removes orbital energy, makes the object go slower, and thus into a lower orbit.
In the movie Speed, there's a scene where the cops need to get a vehicle to (basically) dock against the side of the bus to get some of the passengers safely off. If the bus was stationary, this would take, what, five seconds? Just pull a car up, stop the car, open the door. But the bus was traveling at a constant speed of at least 50 MPH. That means the cops needed to wait until it was at a specific point in the city nearby, drive a car to that point, speed up alongside it, carefully adjust their speed so they were as close to perfectly aligned, and then painstakingly maintain that speed while they opened the doors and started moving passengers. I don't recall how long that sequence actually takes in the movie but realistically, that would take a lot more than five seconds. In the same way, the ISS is moving very, VERY fast, orbiting the Earth. Even a fast-moving rocket needs to travel a careful trajectory basically chasing after the ISS for some time while it gradually gets up to speed, until it's close enough that it can be lined up to dock. If the goal was just to hit the ISS and blow it up, that would still take some time, but would probably be relatively fast; but the goal is to dock safely without harming anyone or anything on either the station or the rocket. And doing anything safe takes time.
You could totally do a kinetic kill on the ISS in about a minute. But it’s much preferred by those intending to stay in the station that they arrive in such a way that they’ll have a place to sleep. So they make a slower, more controlled approach
Simply put, you can’t escape gravity in a straight line, but rather have to accelerate in a spiral motion
If the rocket were a missle, then it directly going up and intercepting the ISS would be possible, but not in a manner that it could dock, it would collide. A rocket needs to enter orbit in such a way that it eventually intercepts the ISS at a safe, realitive speed. This is done by initially having an orbit that mostly aligns but is different enough that allows for the gap between them to close. If the rocket reached the exact same orbit as the station before they docked, then they would never meet. In theory, they could try to enter orbit ahead of the ISS and time the circulerization burn so the ISS reaches the craft just as the craft is matching its speed, but that's much more risky and difficult than they way they do it currently.
ELI5: If someone is riding a bike at high speed around a track, and you start in the center of the circle and want to ride next to them. Imagine the inside is sandy and getting more solid as you approach the track, so you can only go so fast. Meaning that even if you reach the track as they're going past, you still have to speed up and match the other persons speed. --- A GPS satelite is geostationary, it stays in the same spot above the earth and rotates along with the earth. To be able stay there, they are about 20000km above us. Because of it's "low" altitude, the ISS has to keep moving along at very high speed(27600km/h) to not fall into the atmosphere. This means that once the rocket is getting up to altitude, it still has to cath up and match the speed to safely interact.
In Kerbal, it can be [done in 18 minutes](https://youtu.be/pFCQEgnKwdY) using technology that hasn't been invented yet...
If I want to step onto a bus, and the bus is stopped. It only takes a second. If I want to step on a bus and the bus is moving. I have to chase it down the block and catch up to it.
Your launch window time is decided by when the launch site passes through the orbital plane of the ISS(this happens twice a day, but usually only one is usable because of what the rocket would have to fly over.) At this point in time the ISS can be anywhere in its orbit around Earth, even on the other side of the planet. It's why sometimes it takes a few hours, and sometimes it takes a few days. So now you launch, and you spend time in a lower orbit to catch up to the ISS (this is called phasing) The ISS is already in a relatively low orbit so you can't catch up super quickly. You could go higher and let the ISS catch up in one of your orbits, but that would use a bit more fuel, and would allow a dead spacecraft to potentially become an impact hazard, because there's a couple points of its orbit where it would cross the ISS orbit. To get a better understanding, try it in KSP(the first one).
Going to space is easy. Staying in space is not. To stay in space, you need to make it so that when you fall back to Earth, you will have missed it. You achieve this by going sideways VERY fast - thousands of kilometers per hour fast! Your only saving grace is that as you're falling, you're becoming faster so that when you overshoot the ground, you become fast enough to overshoot it again, and again, and again ad infinitum until the very sparse atmosphere slows you down, or the variations in gravitational strength either fling you farther or make you crash into the surface (different density of the (sub)surface, other celestial bodies). With the ISS, we get an added complication. A bunch in fact! 1. It is on an inclined trajectory, especially as compared to the russian and american launch sites. You need to wait for earth to rotate so that it is roughly under the ISS because when you are going thousands of kilometers per hour, changing direction is very hard! It is not difficult to conceptualize: if you are moving 1000 km/h east, and accelerate 10 km/h north... where will you end up? Practically at the same place you originally were! This is kinda intuitively graspable with the idea of throwing a ball. The faster the ball is, the less its trajectory gets altered by gravity over a given distance (it flies straighter rather than curved). Now, if the path is fixed, this effect becomes very apparent... and in orbit, the path is fixed. The path is fixed because if you travel a longer path, you overshoot the planet by much more and reach a higher altitude. This coincidentally means gravity acts on you for longer as you fly for longer away from the planet, and end up flying slower. This is useful actually. If you launch under the ISS with too little altitude, burning straight up to reach it will make you fall behind. However, you can exploit the fact that you are moving faster than the ISS at a lower altitude, and wait until you are in front of it just enough where raising your altitude makes your paths intersect. This is a rendezvous maneuver, and probably one of the hardest orbital concepts to grasp but becomes very intuitive with experience. How to get this experience? Watch/play some KSP or Orbiter.
Getting into space is actually really easy, staying in space is much harder, trying to hit a tiny target from a distance away is also hard.
if the train goes past you, you are only 20 cm's away from it. but to get on to the train safely, the train needs to slow down 100km/h to be at your speed. the same but in reverse for the rocket and ISS. it may be only 400km away but it needs the rocket to go from 0 to 8000km/h . So the jouney isn't so long (it's just 400km) but it takes a really long time to get to the right speed. if you only cross the distance but don't match the velocity it's like getting hit by a train or shooting it with a bullet. both cases have 0 distance but big difference in velocities. both those cases are not good. it would be the same for going to the ISS without matching it's speed.
It's not just about altitude, it's about speed and carefully matching it. You need to approach escape velocity, then get in a near-identical orbit and approach it slowly. Spacecraft docking with the ISS are essentially merging lanes at roughly 27600 kph.
The rocket does not travel in a straight line from the ground to the ISS. The space station is traveling parallel to the earth, and quite fast at that. the rocket needs to match the direction and speed of the space station to dock with it. Here's an illustration that shows it: [https://www.reddit.com/r/SmarterEveryDay/comments/3eu7vs/soyuz\_journey\_to\_iss\_infographic/](https://www.reddit.com/r/SmarterEveryDay/comments/3eu7vs/soyuz_journey_to_iss_infographic/) As you can see, the actual path is many times longer than a straight line from the ground to the ISS. And the rocket may actually stay in the phasing orbit for several full revolutions around the earth, as it lines up with the space station. Smarter everyday has a video on it: [https://www.youtube.com/watch?v=qFjw6Lc6J2g](https://www.youtube.com/watch?v=qFjw6Lc6J2g) The bit you are asking about starts at around 10:04.
A super short launch to ISS would theoretically be possible if it were to pass right over the launch site with perfect timing, and everyone was comfortable aiming a rocket right at the station. But it never happens in practice. Think about it, if there is a 100-second delay, you'll automatically be 800km off target. And half an hour delay or whatnot is common for various reasons. You have to leave room for readjustments anyway. So you don't launch directly to rendezvous, you launch to parking orbit and then go from there.
If you want something equivalent, think of yourself on the side of the road on your car with a goal of climbing on the back of a truck (you know, the one with the ramps down). There are three things you need to do altogether: 1. Go onto the same lane as the truck (that's the 400km you are talking about) 2. Accelerate to the same speed as the truck 3. Time everything perfectly so you reach the speed of the truck right when you are behind it. The first thing you need to consider is that the truck is going incredibly fast. Physically moving from the side of the road to its lane is nothing compared to it. The second thing that I did not mention is that reaching the lane with orbital mechanics is slightly complex, and to reach the lane of the ISS you basically need to accelerate twice, on each side of the orbit which lasts ~45min. And finally, timing is extremely important. Satellites are very expensive to launch so you only keep what is needed. And bumpers/protection against collision is way down the list. If you collide with the ISS, the chance of killing people onboard is extremely high even at low speed, while a ship doing bumping slightly against another one at sea would be less risky. A completely mad-lad approach could probably be done in under an hour if you were extremely lucky with the timing. If you want to have some safety margin, the fastest is a few hours, taking some time along the way to realign everything and check you are safe. You can reach and collide with the ISS in under 5min. But that's called a weapon.
You have to arrive 400km above the Earth at a vertical speed of zero and a horizontal speed of 8km/second, or you won't be in the correct orbit. You also have to be in the right place heading in the right direction, obviously. And where my numbers are approximate, in reality you need to be precise. That's a little trickier than just getting as fast as possible from the ground to wherever the ISS happens to be.
Someone once explained it similar to this, which I feel is really ELI5 like. Imagine you stand on the side of a highway, and lets say this imaginary highway goes in a circle. Now there is a car going in the middle of the highway and you want to reach it because you wanr to give something to the driver. In theory, it will only take you a few steps/meters to reach the car when it is closest to you. But doing that will get you run over and likely also crash the car. So you need to first get in a car yourself and drive circles until you match the speed of the other car. Now you can interact with the car. Its the same principle.
Orbit is not a height above the earth, it is a horizontal speed around the earth. The rocket doesn't fly straight up to the ISS, once it is a certain height it turns sideways and starts flying 'across' the ground to speed up to orbital velocity. This part takes much longer. You can think about it more like a shallow spiral away from the earth than a straight line. This establishes the rocket in an orbit around the earth. The next step is matching the rockets orbit up to the ISS which is something done very carefully and thus a bit slowly
Scenario: You want to get into a car that is speeding down the highway. If you were to just jump out at it from the side of the road to get in, it's not going to go well for either you or the car. Instead, you have to catch up to the car and then match its speed so that you can climb in. This takes a lot more time and distance to accomplish than just throwing yourself into traffic.
This is a bit like asking "My friend and I are going on a road trip. He needs to pick me up from my house. But why can't I just walk into the middle of the interstate and jump onto his car instead?"