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?