Ancestor of the orbital bus? | Orbital bus | Forum

Here's a page describing one of NASA's many lunar Gemini concepts which never went ahead.  It basically involved sticking some landing legs and lots of extra engines behind a Gemini capsule and actually landing the capsule itself on the moon.  This picture in particular:

shows what is basically our orbital bus concept, sitting at the bottom of the stack.  If you remove the middle two parts (lunar ascent and descet stages) you basically have our arrangement – a conical crew compartment with a bunch of spherical tanks and an engine behind it.

This orbital bus looks considerably larger than our concept renders (it looks about twice the diameter of the Gemini – our bus is about the same diameter as the CLLARE CM, which is probably a little smaller than a Gemini), but then this arrangement has the bus also carrying the mass of the lunar ascent and descent stages, whereas ours is carrying just the capsule (which will hopefully be lighter than a Gemini due to modern construction materials).

Note that all the engines in the stack shown here were supposed to use dinotrogen tetroxide and unsymmetrical dimethylhydrazine – a hypergolic fuel mix where both the oxidiser and fuel are extremely nasty chemicals that we wouldn't want to think of working with.