The San Francisco Chronicle has an article in the Sunday paper about the NASA’s next generation launch vehicles. It’s like what you’ve read here before, but they address single-stage-to-orbit when talking about the “space plane” concept.
Let me tell you why SSTO fails miserably. With multi-stage rockets, especially liquid-fueled rockets, your structure is also your tanking. You make very thin-walled rockets, stuff barely able to stand up on its own without being under pressure. In fact, if you launched these things unpressurized, these things would just rip completely apart. Why do you do this? Structure is “dumb mass”—it just doesn’t do anything.
When you have a multi-stage rocket, you shed dumb mass as you go through stages. When you’re done with a stage, you jettison it, and the next stage doesn’t have to carry that mass on through the next phase.
When you do SSTO, you’re stuck with the structure for the entire flight. You need structure, of course—millions of pounds of thrust has to be resisted. Newton’s Third Law doesn’t just apply to the overall system, but also to internal components. [In fact, N3L is the basis of finite element analysis of structures.] If you’re carrying structure for the whole flight, that’s less usable mass.
We ran the numbers once in undergrad propulsion … the best an SSTO vehicle can do is to expect to carry 3% mass fraction of usable mass. But that’s everything … crew, the food, water, and air they breathe … everything.
Now, that doesn’t seem bad, does it? Well, when you run the numbers for what that 3% needs to be to do a good mission, that vehicle ends up being prohibitively expensive.
And mind you, 3% is a very liberal figure. The real answer is probably lower.
SSTO was a great concept, but it was a pipe dream. Sticking with multi-stage and going back to capsules and modules is the way to go, and that’s where NASA is headed.