In article m,
wrote:
(In fact, it might be worth chilling the LOX below its boiling point too;
Rockwell's X-33 design did that.)

I'm not sure if I understand what you are saying here. Are you
proposing to make a rocket where structural strengh is provided by
pressure in the tanks and where this pressure comes from LOX chilled
below its boiling point?

Not quite. The pressure comes from the tank pressurization, just like it
does with any other balloon-tank scheme. The one blemish is that you
can't easily pressurize subcooled liquids with their own vapor -- you have
to pressurize with something else, unless maybe you can put an insulating
barrier between gas and liquid. Pressurizing with something else is often
done anyway, for other reasons, but here it's pretty much mandatory,
especially for the propane, which is rather drastically subcooled.

(For the oxygen the subcooling is not so dramatic, given LOX's narrow
temperature range. It's also worth noting that most existing LOX-using
rockets effectively run their LOX *slightly* subcooled -- it's boiling at
countdown pressure, but then they go to a higher tank pressure for flight,
which raises the boiling point. It generally doesn't have time to warm up
to the new boiling point before it's burned.)

The orthodox approach is to pressurize with helium, which is light and
inert but does suffer from being relatively expensive and in limited
supply. Some experimenting with alternatives would be in order.

You might be able to pressurize the propane with methane, which
conveniently is also liquid at about LOX temperatures. (You could almost
certainly pressurize propane with hydrogen, but that's a pain to store.)
Some of the methane will dissolve, but that's an issue for pressurizing
with propellant vapor too, and it can be controlled well enough.

Another option, for both propellants, is neon -- still not exactly cheap,
but the supply is essentially unlimited (unlike helium, it's made from
air, as a byproduct of LOX and LN2 production) and it's inert and has a
very low boiling point.

If you were careful to minimize turbulence in the gas, you could probably
pressurize mildly-subcooled LOX with GOX, if you threw in a bit of neon
first: you'd get a layer of cold neon gas on top of the LOX, with warmer
GOX above it. An experiment or two would be needed -- neon *is* somewhat
soluble in LOX, but then, both Delta II and Soyuz pressurize LOX with GN2,
which is very soluble in LOX... Some GOX would get through the neon
layer, so you'd have a layer of warm LOX at the top of the liquid, but
that again is a common issue -- most propellant-vapor pressurization
systems heat the pressurant beyond boiling to reduce its density.

Whether a neon buffer layer would suffice to let you pressurize propane
with propane is less obvious. Maybe.
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