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Docking with the ISS

  1. Feb 8, 2016 #1
    I was watching the following video, and I have a question about something they said that seemed kind of vague (I know I could have asked in the video comments section, but this forum tends to supply much better answers, so I hope it's OK that I am asking about a YouTube video here):

    If you don't want to watch the video I'll here's the part I don't get: They say that when a capsule docks with the ISS they fly the capsule into the same orbit as the ISS, but in front of it (that is, moving at the same velocity, and in the same direction the ISS is orbiting). Once the capsule is lined up with the ISS in the same orbit, they slow down to let the ISS catch up with them, and then they steer the capsule to where it needs to go right at the end.

    My question is, wouldn't slowing down the capsule cause it to drop into a lower orbit (in which case the ISS would just fly past them)?
  2. jcsd
  3. Feb 8, 2016 #2


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    I couldn't get that vid to play due to the poor internet bandwidth that I have thanks to BT/Openreach.

    The short answer is yes.


    The first attempt failed because this wasn't understood...

  4. Feb 8, 2016 #3


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    Still can't get it to play but...

    Actually if you slow down and drop into a lower orbit you go faster (orbit the earth faster), so if you are already ahead of the ISS the gap would actually increase.

    See "methods of approach"...

    Last edited: Feb 8, 2016
  5. Feb 8, 2016 #4


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    The final closing in manoeuvres, once you're in the same orbit and reasonably close to the target, are no longer Hohmann transfer manoeuvres. You can use your engines to stay put in the same orbit and move around - for example, if you want to get closer to the ISS that's trailing behind you, you thrust towards it a bit, and make sure to correct for any subsequent tendency to drop in orbit. I.e., you use your engines to 'hover' in an orbit that you don't have the correct velocity to stay in unaided (in free fall).

    If you're interested in orbital mechanics, and have a few hours to waste, head to: http://orbit.medphys.ucl.ac.uk/ it's a freeware, highly accurate simulation of spaceship flight. The tutorial mission (refer to the manual) takes you to the ISS where you do exactly those kinds of things as described in the video.
  6. Feb 8, 2016 #5
    Oh, I see, so they aren't exactly just floating there waiting for the ISS to catch up. They are actually keeping themselves lined up by using their own power.

    Thanks for a help.
  7. Feb 8, 2016 #6
    Thanks for the answer.

    It's kind of crazy that no one thought to study this issue more closely before they went and actually tried to do it in real life. I guess that's easy to say in hindsight though.
  8. Feb 9, 2016 #7
    Maybe the engineers missed that class in their general physics course.
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