Optimal periapse height for orbital capture

[steveroof]

Hi everyone,

I like to do "Apollo style" missions where I bring a big ship docked with a lander. If I get to a planet and do my capture burn high up, only the lightweight lander needs to go up and down that gravity gradient while the main ship orbits far away. However this method does not take advantage of the Oberth effect for the capture burn. Also that puts alot of burden on the lander to carry enough fuel to get back to the main ship. On the other hand, If I do the capture really low down, I get the Oberth effect, but I also need to expend fuel hauling the entire heavy main ship back up the slope to escape when it's time to go home. For atmospheres, I am guessing aerobraking is the most efficient, even thought you will need to raise that periapse to escape again?

Tips and advice?

Thanks!

[tavert]

Keep in mind that you don't absolutely need to capture all the way into a circular orbit. One thing you can try is capturing the lander and mothership into a elliptical orbit with low periapsis (to take advantage of the Oberth effect as much as possible) and apoapsis out at the edge of the sphere of influence. Then the lander can kill the rest of the excess velocity on its own to get into a low circular orbit and land. This does mean the lander will need more total delta-V in order to land from a highly elliptical orbit then take off and catch back up to the high velocity of the waiting mothership. If you can get back into an elliptical orbit with the lander but then you run out of fuel before fully matching velocities, you could switch back to the mothership to do the rest of the rendezvous and docking. The timing of rendezvous and escape is trickier with this scheme, especially if the major axis of your capture orbit is not aligned in the direction you want to make your escape later on...

[Pugspaceprogram]

Need orbit advice?

Why not Zoidberg?

[steveroof]

Thanks Tavert - I have tried the elliptical way and it seems to work alright. It seems like a good compromise.

[AmpsterMan]

Aerocapture is the most efficient way to enter a sphere of influence, no question about that. However, whether the ship should be in a high or low orbit is a very good question. I would say that a lower orbit is better because you will be using more efficient engines for those burns (IE, LV-N or Poodles) meaning you will have more delta-v to play with. Furthermore, when ascending back from the planet/moon you would be able to rendezvous with the "mothership" and transfer fuel. This would, again allow for a greater delta-v budget margin that can help with less than perfect maneuvers. Of course, from there you could just decouple the lander module and leave it in orbit or crash it to reduce much weight and you will have much more delta-v than you originally would have. Also, as you stated, this will allow you to take much better advantage of the oberth effect to return home.

Edit: Just noticed original posting date. If you are still looking for help, I hope my paragraph can help you for further missions.