Couple of sketches

[Pfhorrest]

In the in-game art, the ramscoop (assuming that's what it is) is on the side of the ship facing the camera, so whatever the propulsion system is (which would be opposite the ramscoop) would just have been out of sight.

I wonder what kind of propulsion the Marathon would use. I think ramjets are supposed to be basically Orion-type drives, no? Sucking up hydrogen with the ramscoop and fusing it for propulsion out the back?

I'm not sure what an Orion thruster would look like.

[General Tacticus]

presumably the main components of such an engine would consist of:

-the scoop
-the apparatus to sort matter that is taken in
-storage tanks for the hydrogen
-some way to discard not useful matter
-the magnetic bottle that fusion takes place in (or perhaps lots of smaller reactors, whatever is more efficient)
-an aperture for releasing the high energy plasma to create thrust
-a hell of a lot of radiation shielding

It would make most sense to array the whole thing along a fairly straight line, and use the moon's strata to block radiation emitted by the reaction. If i were to make a practical design, I'd put the majority of the stuff in middle of the moon and have the output aperture be in a big shielding-sided hole in the ground. It would also probably be best not to have anyone living near it.

[Pfhorrest]

Maybe the reason there's lava inside the Marathon is that there is or was an ongoing nuclear fusion reaction in there for a few hundred years, and a lot of waste heat has built up in the rock around that?

[Nathan92]

I always thought the presence of Lava could signify the presence of Forges, for creating whatever is needed out of various metals (sheets, panels, etc) to continually refine the Marathon's construction.

[ravenshining]

One of the most prominent "lava" (or whatever it is) spaces is explicitly labelled as a "Fusion Vent," so presumeably it may have something to do with the propulsion process.