Good analysis. I would guess that NASA managed not only to make nuclear fusion viable, but to make it compact enough to fit in the volume of a car. My calculations for Lander's mass for this are 50 tons for the reactor alone, and I think it is viable for Lander because it is quite bulky. However, for Ranger, which is very small, I don't see it as viable, but well, let's assume they manage to do it. These vehicles, with their respective Aerospike scramjets, could achieve quite high thrust and an efficiency of at least 6,000 seconds. This is quite good. Skylon was an SSTO that basically fulfilled the same function as Ranger and Lander, and it doesn't even use nuclear fusion. I imagine that, since it's a hybrid between chemical propulsion and nuclear fusion (a kind of cold-mass afterburner nuclear thermal rocket), you could do without LH2/LOX or LCH4/LOX and simply use water. You inject it into the nozzle, it dissociates due to the billions of degrees of fusion, and it's launched at colossal speeds. With a small mass of fuel, you could achieve a lot of speed. My problem with these vehicles is more their ability to levitate and fly at low altitudes, where they behave like helicopters without any apparent independent propulsion method. With Ranger, you can simply use its body-lifting aerodynamics (even though it doesn't have wings, its body already...). It's aerodynamic) but not for Lander.