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Old October 13 2012, 08:08 PM   #230
Location: Kentucky
Re: Envisioning the world of 2100

Venture Star was an engineering program to find out if SSTO was quite within reach with liquid hydrogen, because the liftoff weight of an SSTO is extremely sensitive to ISP and engine thrust to weight ratio. It wasn't.

There are some odd propellant combinations that could make an SSTO feasible, but they're not either ready, usable (toxicity), or cheap. Li3AlH6, for example, has slightly better ISP than LH2 and the same density as water (1.0), but the lithium would mean the fuel would cost $100 to $150 a gallon.

There are perhaps other options to make an SSTO feasible, like greatly increasing the Earth's rotation rate to impart a much higher initial horizontal launch velocity, but that would also benefit cheaper multi-stage rockets, making the SSTO face stiffer competition while the 3 or 4 hour day/night cycle would stretch out development schedules.

It is interesting to compare SLS to Saturn II, which would've used essentially the same technology and configuration (INT-19 included engines that are the direct forerunner of the RS-25's on the SLS), but had a 33-foot tank diameter instead of 27.5 feet, giving it 44% more fuel per foot of stack height than the SLS. For big lift, the 1966 design, a downgrade of an earlier 1960's design, wins!
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