For VTO SSTO rocket spaceplanes, only the NERVA (or better) option looks to be technically feasible. Under the same price/launch weight assumptions, the unit price for delivered payload looks at-best more-or-less comparable to the one-shot LOX-RP1 VTO TSTO baseline, probably more expensive.
Figure 2 – Basic Calculations and Related Conditions
Figure 4 – Basic Results for One-Shot One-Stage Rocket Launchers
For those not so familiar with rocket work, these plots can be a little confusing or misleading. First: these are for single-stage operations only. You cannot use these directly for staged vehicles. Nor can you do anything useful with these plots toward airbreathing-assist, it's just too coarse for that, although concepts can be illustrated.
For airbreathing-assist, you have to "account" for highly-variable airbreather Isp effects, how much of the thrust is airbreather, and what fraction of the whole trajectory is actually assisted by the airbreather. You also have to worry about having enough thrust to take off, and that these charts embody only vertical takeoff on a fast ascent trajectory.
Second, the slanted curves are just physics as embodied by the classic rocket equation. There's only 3 categories of vehicle mass considered here: inerts, propellant, and dead-head payload. The curves show the interplay among the three, with two explicitly shown, calculated to a fixed velocity-change requirement. None of those curves would ever change, given the same velocity requirement.
The horizontal lines represent the performance levels of typical rocket propulsion technologies. In essence, this is the influence of that portion of the mass budget that is propellant. I showed 3 chemical and one old nuclear system as a guide.
Technologies can improve, shifting these horizontal lines slightly, but chemistry has been "stalled" for decades, pretty much where it is depicted. The nuclear technology offers the most hope of improvement, but has not been seriously worked-on in 4 decades. What I show is what was cancelled right before it could be flight-tested, the variant that was most mature back then.
The vertical lines represent the effects of materials and construction techniques upon the inert weight. This has seen the most change in recent decades. The modern 5-10% inert range is now pretty typical of commercial launcher stages. Rolled textured aluminum alloy panels are what make this possible, in concert with higher-tech versions of the engines that have lower engine weight for the same thrust. Long ago, that was closer to 20% with things more like frame-and-stringer type construction.
I have to caution readers and users of these graphs that these 5-10% inert weight percentages are typical of one-shot (throwaway) stages, not anything that might be reusable. One-shot designs contend with ascent loads and ascent heating only. Descent loads and descent heating are not only worse, they are totally different in character. You have to deliberately design for them from the outset in a reusable design. You also have to have a service lifetime in mind for a reusable design, something totally different than "just-surviving-the-mission" with a one-shot design.
The early history of aircraft design is the most recent example of a technology arena where we have learned a very fundamental lesson the hard way (with many lives lost): the robustness of a long service life is simply heavier, because more materials are required to withstand the forces. There is no escaping that fact-of-life, and that is why I spotted recent modern aircraft values on the figure 6. These are basically dry weight divided by max gross weight. The difference is really both payload and fuel together. (Airplanes are different from rockets, after all.)
The 50% I show as "typical" of a long-life transport or bomber aircraft might not be representative of a reusable winged space launcher, but the 40% of the all-metal X-15 rocket airplane is a good startpoint for guessing what might be suitable for a reusable winged craft. Those are fundamentally different from "not-winged" vertical launch stages, reusable or not.
Composites typically have at least twice the strength to weight of aluminum, but are even more vulnerable to overheating. You cannot replace all the metal with all-composites, except in minimum-velocity suborbital flight, and even that is on a heat-sink transient.
I hope these comments help provide additional guidance for those wishing to use my results. I really do appreciate the comments, Google +1's, and other feedback. Thanks, and have some fun playing with this stuff. I certainly did.