Speaking of propellant, one thing I always dreamed of in the Orbiter realm is a true nuclear-powered turbine equiped vessel. Let's take the XR2 operations in an atmosphere as a baseline here:
You take off horizontally or using the hower engines using main fuel, then get up to a speed and altitude you can switch to the scramjet, with it's specific fuel... once out of the atmosphere you switch back to main engines. All while using always the same amount of fuel in a basic rocket design.
However, if you use an indirect nuclear turbine engine cycle, you can use the atmosphere as a working fluid. In the case of the XR2 with such a setup, you'd open intake doors, compress them just like in a jet engine, apply heat through a heat exchanger instead of burning fuel in a combustor and let the heated atmospheric gas work it's way out of a turbine, which powers the compressor and fans up front...
Then, as you switch to the scramjet, you either heat air indirectly through another heat exchanger (cleaner) or you make it cool the reactor elements directly (dirtier). Only when there is no useful atmosphere left to work with do you need to tap the main/scram fuel tanks to provide mass to propel the ship forward.
Therefore, one could cruise around for months while in the atmosphere, be it Earth, Venus, Titan or maybe even Mars without using main fuel. A very quick and practical way to implement this in a XR2 indirect cycle nuclear variant for the turbine part would be:
- if atmosphere density is > turbine operation threshold
- and if true airspeed is < Mach 3
- and if scramjet door is open
-> set the TSFC to some very low value
As soon as you exceed one of these values, you switch back to a normal fuel consumption. As a bonus "realist" feature, operations in the atmosphere using the nuclear-powered turbine could result in a stable coolant temperature without having the radiators deployed, since the waste heat could be dumped in the turbine instead...
Too bad I don't have much time nowadays to code anything because I'd love to implement such a design, but since you're in the process of making a Mk II of one of my favorite ships in the Orbiter realm, I figured I could drop the suggestion to others...
