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.
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.
Extremely low engine pressures also make the cooling task a lot easier -- the plug nozzle could almost certainly live with a pretty lightweight heat barrier / high temp metal fabrication for a six minute burn to orbit.
Composite tanks with a 200 psi burst pressure can be fabricated with SSTO mas fractions fairly easily.
John Carmack
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