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I don't understand why when first launched Space X's Dragon capsule had to orbit the Earth many times in order to match up with the ISS? Was this purely to match it's speed, or to get closer (as in altitude) to the ISS?

In the stages when it gets to about 200m, it seemed like it was able to go directly up to the ISS, how come it couldn't do that the entire way.

(Additionally, in Scifi movies you see smaller shuttles able to go directly to space stations in orbit, is that type if travel not possible?)

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I think it is partly for safety reasons, by slowly matching orbits you can make small corrections far from the station, rather than firing engines in close proximity. –  user2963 May 25 '12 at 15:09
Presumably you mean 200 km? –  Emilio Pisanty Jan 11 '13 at 14:18
No I meant 200m from the station. –  Jonathan. Jan 11 '13 at 15:13
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1 Answer

up vote 38 down vote accepted

In space you don't just "go somewhere". You have to match orbits, while not wasting too much fuel.

If you're in a low circular orbit, and you want to get to a high circular orbit, it takes two tangential burns, one to elongate your orbit into an ellipse, and another at the high point of the ellipse to make it circular again. This is called a Hohman transfer. You may have to do this multiple times, depending on how much thrust you have.

If your orbit is in a different plane from the orbit of the space station, you have to wait until you reach the plane of the other orbit, then do a lateral burn. You may have to do this several times to change your orbit's angle sufficiently, each time having to wait another half-orbit.
EDIT: to give some perspective on this, if your orbit crosses the plane of the other orbit at an angle of 10 degrees, that means you are crossing that plane at about one mile per second. (Orbit velocity times sin(10 degrees).) If your rocket motor generates 1G of thrust, you need to run it around 2.5 minutes to get aligned with that plane. (5280/32/60)

REVISED: If you're in the same orbit as your destination, but some distance behind it (say), the way you catch up is by getting into a lower orbit by a Hohman transfer, with greater angular velocity, and then another such transfer to get back to the original orbit. This is called orbit phasing. If you just accelerate toward the object, that would put you in an orbit that rises above the target, and then eventually falls further behind because it is a higher orbit.

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+1 Very informative. If I understand you right, one decides for radial burn when lowering orbit and tangential burn when rising the orbit. Why is that so? –  Pygmalion May 25 '12 at 15:51
@J.C.Leitão: I seem to remember that astronauts would practice this by driving in circles with jeeps. –  Mike Dunlavey May 25 '12 at 19:49
@MikeDunlavey: nowadays you can practice this at home by downloading one of the many realistic space simulators, many of them are free. –  Lie Ryan May 25 '12 at 21:18
@LieRyan: Didn't know that. Sounds like fun. I'm just a geezer who happened to be at the Instrumentation Lab during Apollo, and later for the Shuttle. –  Mike Dunlavey May 25 '12 at 22:34
I can recommend two space flight sims from personal experience. First is Orbiter (orbit.medphys.ucl.ac.uk), which is free, very detailed and accurate, but also with a fairly steep learning curve. But once things "click" it is an impressive experience. Less serious is Kerbal Space Program (kerbalspaceprogram.com), which is a commercial game currently under development. It is basically lego with rockets, and very cartoony/cute, but the orbital physics is fairly realistic. Both programs have very active modding and add-on development communities. –  Michael Brown Jan 11 '13 at 13:24
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