Others may not be aware of the propellants and not understand that the highest propellant mass fraction stages are also associated with the lowest specific impulse propellants, and vice-versa - so I provided the information. It wasn't intended as a dig. Obviously adding a payload will decrease the propellant mass fraction some more. Actually, I thought the SII-C (which also burned LH2/LOX) had a better propellant mass fraction than the SIII since it was the last design to come together and therefore became the focal point for savings needed to offset weight growth in the rest of the stack.
A SSTO airbreathing vehicle was proposed in ~1984 and it absorbed hundreds of millions of dollars before the government finally figured out that the initial theoretical analyses done by Tony DuPont were way too simplistic and optimistic. In my humble opinion a purely rocket-based system has a better chance of working, but I think it still wouldn't be able to economically compete with expendable multi-stage systems if it had to use the same chemical propellants.
For horizontal take-off & aircraft-like operations, gross take-off weights above about 1,000,000 pounds will significantly increase costs and severely restrict the number of places such a vehicle could be operated. Landing gear weight gets really big. The thrust structure and engine weight in general increase pretty much with the gross vehicle takeoff weight for a given acceleration capability, whether launched vertically or horizontally. Horizontal launch means you need wings, which can't be just any shape, are draggy, and are volumetrically inefficient. The shuttle would be able to haul alot more payload if it didn't have those big wings, which IIRC were originally driven by an Air Force cross-range requirement. They also reduce landing speed, though - which is pretty high already. Weight savings on the propellant tankage certainly helps, but the tank itself isn't necessarily the biggest hitter.
Electric and hybrid cars can and do, but that really isn't the same kind of animal. I can't store the braking energy on a rocket. If I had power to burn, though, I could dispense with the thermal protection system - not that it's very heavy, but failures can obviously be dangerous. Restricting the locations and types of possible catastrophic failures improves overall system reliability and lowers inspection/maintenance costs. We don't have to pore over the skin of an aircraft after each flight, we just inspect engine hot sections once in a while.
Bruce Dunn. Nice guy. I wonder what his neighbors think of his experiments.
I've already located a supply of cantaffordmium, if you are interested. :)
Hypocritical? I'm just publicly examining the extremes to show where that puts us on the map of the possible. The Deep Space 1 xenon ion engine demonstrated 3100 seconds of specific impulse and NASA talks of values up to 13,000 seconds with newer electric engines (very low thrust, though). 2650 seconds isn't a laughably high number, though it is certainly a big challenge for a launch vehicle. At least the answer doesn't violate known physics. If we just need 10,000 m/s delta-v to get to orbit with a little maneuver reserve the required Isp falls to about 1500 seconds with a 50% initial vehicle fuel mass fraction. NERVA demonstrated about 900 seconds, Timberwind would have been better (who knows how much) - maybe a LOX-augmented Timberwind engine could do it. Now *there's* an EX project for you - at least you wouldn't be working with materials that are on the ATF Explosives List :) (or is hydrogen on their List? For that matter, are nuclear explosives like plutonium or tritium on their List? If not, why not? They're set off by detonators, right? Enquiring minds want to know!). It would probably still be thrust/weight challenged, though.