If plug nozzles actually work to allow automatic altitude compensation, it seems like it should be possible to do an SSTO with blowdown pressurization and light tanks.
My reasoning is that the bulk of the delta-V that an SSTO needs is generated outside the atmosphere, and with no back pressure, a very large expansion ratio nozzle can extract essentially the same Isp regardless of chamber pressure. If you had a huge plug nozzle over the entire base of a vehicle, you could start out with a 150 psi tank pressure and get a crappy 200 or so Isp at sea level (98% peroxide / kerosene), but it would rise to 300 when out of the atmosphere. The time spent at the lower Isp is not actually all that great -- under 20% of the time in the sensible atmosphere, with the full Isp at the end where you really need it.
I've run simulations of this sort of thing; roughly speaking. You're looking at a dry mass fraction of less than 4%, probably closer to 3%; that's about 1/2 that of a Roton... Challenging!
If you have an engine capable of deep throttling, say, with a moveable pintle injector, starting with some ullage and letting the pressure decay to 20% of its starting pressure probably isn't a bad thing at all, since you are going to throw away 95% of your mass by that point. Making a pressurization subsystem just go away is a big deal optimization.
It's not like composites have just been invented; they've been around for decades. Even wound tanks were used on Apollo I believe.
However, when nanotube production ramps up more; currently there's fibers 9 cm long, and fibers with strength of 73 GPa (cf Kevlar about ~4Gpa), then you might be looking at doing this. Still, they are likely to remain expensive for a good long while.
John Carmack
-- -Ian
Motto: "You're Not Authorized to Know Our Motto." "War is never right, unless of course there is a second UN resolution" - Private Eye "War, huh? What is it good for?" - Frankie goes to Hollywood "Getting reelected" - G. Bush
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