Docking with ISS: Questions on Orbital Mechanics

[LydiaJ]

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)?

 

[CWatters]

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.

https://en.wikipedia.org/wiki/Space_rendezvous

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

.. the Gemini 4 attempts at rendezvous were unsuccessful largely because NASA engineers had yet to learn the orbital mechanics involved in the process. Simply pointing the active vehicle's nose at the target and thrusting was unsuccessful. If the target is ahead in the orbit and the tracking vehicle increases speed, its altitude also increases, actually moving it away from the target. The higher altitude then increases orbital period due to Kepler's third law, putting the tracker not only above, but also behind the target. The proper technique requires changing the tracking vehicle's orbit to allow the rendezvous target to either catch up or be caught up with, and then at the correct moment changing to the same orbit as the target with no relative motion between the vehicles (for example, putting the tracker into a lower orbit, which has a shorter orbital period allowing it to catch up, then executing a Hohmann transfer back to the original orbital height).[6]

 

[CWatters]

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

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)

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"...
https://en.wikipedia.org/wiki/Space_rendezvous#Methods_of_approach

To keep the chaser on the V-vector, other thrusters are fired in the radial direction

 

[Bandersnatch]

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)?

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.

 

[LydiaJ]

you use your engines to 'hover' in an orbit that you don't have the correct velocity to stay in unaided (in free fall).

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.

 

[LydiaJ]

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

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.

 

[gleem]

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.

Maybe the engineers missed that class in their general physics course.