So, I'm a rocket scientist. I like playing around with these things.
Consider the following doodle in two frameworks. First, as a Space Launch System alternative, similarly leveraging Shuttle framework (on a 10m base, not an 8.4m one) but rejecting capsules in favor of a further-enlarged HL-42.
Alternately, think of it as something like Shuttle-C, where we're returning the engines and avionics but rejecting a payload-return capability. This gives us something like 150k to LEO, with only the tank and fairing as expendable. We can decouple the crew and lighter payload return capability and stick it on the top of the stack, where it's safe from foam damage. I think the engine pod is safe too, but if we lose one every now and again it's no big deal.
The engine recovery pod for Shuttle-C is something that was extensively studied, and I stole it and traced it directly from here: https://www.aiaa.org/uploadedFiles/Abou ... alAIAA.pdf
HL-60 could carry 20klb of crew and passengers to LEO with almost 800m/s of hypergolic dV, or something more like 35-40klb with Shuttle-like margins. There is a 10x25' payload bay (cf. 15x60 on Shuttle) that would, for ISS missions, be a pressurized passthrough to a tail docking adapter. For missions like Hubble repair or similar, one might imagine a pair of HL-60s docking on-orbit near the target, with one carrying a passenger cabin of about 1400ft3 (Shuttle is 2600) and the other with tools, replacement parts, a manipulation arm, and maybe a bit more crew space. There is a permanent space for two crew, an airlock, and additional lifesystem stuff, to the tune of about 600ft3, although one certainly presumes it could fly unmanned. The notional access tunnel is another 800-900ft3, although I don't think it'd be useful for much.
Pusher solids for abort through the entire launch profile. About 350m2 of solar panel, comparable to ATV. Tail skirts for relocation would fit through the nose door of a 747F, so we might imagine NASA would buy one or two of those and use them for transport. The docking mechanism is a notional 50"-diameter expanded version of the international berthing and docking mechanism, which would allow for transfer of large objects to the ISS (not all current automated cargo vehicles can do this). Note that the pressurized cargo volume would be very similar to that of ATV if used in this role, and substantially greater than any other supply systems. Payload by mass is about double.
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Notional stack is three regenerative RS-68A with a pair of RSRM. Later blocks might do crazy things like a Delta IV Heavy parallel stack, which I suspect (I haven't run the numbers) could well exceed any SLS option yet proposed.
Anyway, I enjoyed playing with it!
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