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Orbital rendezvous question.


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I have a contract to rescue Cerzy who is an orbit around the Mun. I am currently in a lower altitude orbit than Cerzy but I am still losing ground to her? My feeble understanding is a lower orbit should have more velocity...I should be gaining on her? Does the mass of the crafts enter into the velocity equation? I still have a very large 2nd stage attached while Cerzy is in her small capsule...does this make a difference to our orbital velocities?

Vic the Newbal

Edited by strider3
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6 minutes ago, Victor3 said:

I have a contract to rescue Cerzy who is an orbit around the Mun. I am currently in a lower altitude orbit than Cerzy but I am still losing ground to her? My feeble understanding is a lower orbit should have more velocity...I should be gaining on her? Does the mass of the crafts enter into the velocity equation? I still have a very large 2nd stage attached while Cerzy is in her small capsule...does this make a difference to our orbital velocities?

Vic the Newbal

Size is irrelevant in this case.  If you're in a lower orbit, you should be gaining on her.

What is your Pe / Ap, and what is Cerzy's Pe / Ap?

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You should definitely be catching up by a few degrees prograde every orbit, if your orbit is lower -- I hope you noticed the word prograde. If she started a little behind you in the orbits, then you have to catch up by going all the way around the Mun the long way. So in that worst case, for the first half of the time she will be moving away in a linear sense -- and then for the second half she will be moving toward you as you catch up a few degrees at a time.

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Are you looking at catch-up/fall-behind in the big picture, or are you only seeing the speed right now? A lower orbit has faster average speed, but eccentricities make a big difference - a higher, eccentric orbit will have higher speed at Pe than a circular orbit that rendezvous with it.

A picture of your orbits would be of great help.

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You are in a lower orbit, but you are also ahead of the orbit (approx. 135 degrees ahead to me = 225 degrees behind). Each orbit you will catch up by a few degrees - maybe 225 degrees behind becomes 220 degrees behind, but that will make you 140 degrees ahead the other way around. So until you get to under 180 degrees behind you are still getting further away by the shortest distance each orbit. 

My suggestions, either:

  1. Keep going around in this orbit, once you get to 180 degrees behind the UI will show you catching up each orbit. (this might take a while) OR
  2. Raise your apo so that you are orbiting slower and let Cerzy catch you up (this will take more dv but means that you are going the quicker way around).
Edited by ineon
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Thank you. Initial orbit insertions on these rendezvous' is not something I'm good at...both attempts at an orbital rendezvous have resulted in being WAY off from the target. The numbers I was seeing in KER, velocity in particular, were throwing me off.

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A couple of pointers, then.

1) This is a fairly high orbit for the Mun, and therefore takes a long time to make one orbit -- so lowering your orbit quite a bit will be a wise option.

2) The interception markers and calculations work best if your orbit crosses the orbit of your target -- your orbit is currently completely inside, and that makes things harder.

So, to catch Cerzy reasonably quickly: raise your Pe to slightly above Cerzy's to make your orbit cross hers -- maybe 140km to 145km.

Then lower your Ap to 12km to 15km. This will make you catch up very fast -- maybe in one orbit. Watch the intercept markers as you are lowering the Ap, and if the "target position" touches the intercept marker, stop burning.

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37 minutes ago, Gojira1000 said:

I hate to wait so I usually bracket an orbit out a good 40-50km and just do a dramatic intercept burn. They sync up more quickly with the difference being greater.

This is the fastest route if your orbit is within a few km of the target.. You can do a burn prograde until you get an encounter of less than 2km on the way back around, only one pass. It helps to use the manuever node to do it at the most efficient time, but it'll work everytime from a mostly circular orbit

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Has anyone completed this contract successfully (rescue Cerzy from her Munar orbit)? If so, please tell me she has a docking port on her capsule and I will be able to dock and transfer her to my capsule. If not, then I am screwed, so to speak. I thought that if she did not have a docking port, I might be able to get her to EVA over to my ship....but it appears I might not have even that option? I cannot "switch to target", with KER, as it says "we don't own that craft"? I'm confused as to how I'm supposed to do anything with her if I can't gain control of her?

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17 minutes ago, Victor3 said:

Has anyone completed this contract successfully (rescue Cerzy from her Munar orbit)? If so, please tell me she has a docking port on her capsule and I will be able to dock and transfer her to my capsule. If not, then I am screwed, so to speak. I thought that if she did not have a docking port, I might be able to get her to EVA over to my ship....but it appears I might not have even that option? I cannot "switch to target", with KER, as it says "we don't own that craft"? I'm confused as to how I'm supposed to do anything with her if I can't gain control of her?

Once you get within about 2.5km (or 2.2km) you'll be able to switch to the stranded vessel using the [ ] keys and EVA the Kerbal over to your ship. Rescues are always in bare capsules or other crew compartments so there won't be a docking port on it.

Edited by Reactordrone
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5 minutes ago, Reactordrone said:

Once you get within about 2.5km (or 2.2km) you'll be able to switch to the stranded vessel using the [ ] keys and EVA the Kerbal over to your ship. Rescues are always in bare capsules or other crew compartments so there won't be a docking port on it.

Excellent...that was the bit of info I have been lacking, my friend. Thank you!

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