2
$\begingroup$

This is an odd one that's been in the back of my head for a while. Consider the LOR architecture of the Apollo missions. To return to earth from the lunar surface, it is most mass-efficient to have multiple stages: one which enters lunar orbit from the surface (AM) and one which transfers the payload (crew) from the previous stage to earth reentry (CSM). To do this, they rendezvous in lunar orbit. This requires the ascent module to enter lunar orbit.

Has it ever been proposed for some ascent stage to rendezvous with an orbital transfer stage while in a suborbital trajectory? Perhaps even mentioned as contingency?

Clearly the disadvantages are stunning:

  • The orbital stage must slow down to enter a suborbital trajectory matching that of the ascent stage, dock, and then accelerate again to exit the suborbital trajectory. These maneuvers will be major ones.
  • Possibly very short rendezvous window with only one chance for success.
  • Unlikely for there to be any time for a checkout period between burns and docking.

But the advantages are present, if niche. Suborbital rendezvous could substantially reduce the dV requirement on the ascent stage. Its application would be most valuable when ascending through a very thick atmosphere, as it reduces the up-mass requirement for the fuel needed to obtain orbital velocity after reaching an altitude where drag losses are minimized. As an example, I could imagine it being used for some Venus sample return mission, where you could use a rockoon to reach a suborbital trajectory with an apocythe above the atmosphere, where it would be snatched up in its suborbital trajectory & accelerated to orbital velocity.

But that is just my speculation. Has anybody actually taken a good look at this idea before?

$\endgroup$
6
  • 3
    $\begingroup$ "Suborbital rendezvous could substantially reduce the dV requirement on the ascent stage. " For every m/s of dV on the ascent stage that you save, you need to spend 2 m/s of dV on the orbiter. $\endgroup$ – Russell Borogove Aug 8 '20 at 19:13
  • $\begingroup$ @RussellBorogove Of course, and usually that's not worth the mass penalties. I suspect that when your drag losses are as steep as they would be on Venus, it ends up saving mass. Regarding that linked question, it's really really similar. I don't like how the first answer is basically "it's not useful" rather than "there's no research". I guess that's all you can ask for if it is a useless technique, but I'm not convinced it is... $\endgroup$ – Anton Hengst Aug 8 '20 at 19:16
  • $\begingroup$ "it would be snatched up in its suborbital trajectory & accelerated to orbital velocity" How could this be done for objects whose velocity differs by multiple km/s? kaboom! $\endgroup$ – Organic Marble Aug 8 '20 at 19:25
  • $\begingroup$ > The orbital stage must slow down to enter a suborbital trajectory matching that of the ascent stage $\endgroup$ – Anton Hengst Aug 8 '20 at 19:28
  • 1
    $\begingroup$ Ah, the "snatched up" part confused me. It's just a regular, although mighty inefficient, rendezvous. Your orbital thingy would have to carry a whopping lot of prop, which has to be placed in orbit itself. Although as a former instructor, I can think of some great sim cases where the orbital thingy's engines fail to ignite after the rendevous. "We're going down!" $\endgroup$ – Organic Marble Aug 8 '20 at 19:33