Combustion chambers

On our way to the torchship, among our engineering constraints is temperature. As we energise the exhaust to high speed - increasing our I_sp - something has to heat it. The hottest stuff gets ejected but some errant fallout will go sideways. Absent various unobtania like magnetic shielding (against ions) and the perfect mirror (against radiation), that's a solid chamber wall. Which will get hot.

Tom Mueller notes rhenium radiation cooled combustion chambers. Looking around, I find "Design, analysis, and fabrication of oxide-coated iridium/rhenium combustion chambers"... from 1993. The numbers check out: Mueller says hours at over 2500K, the abstract says 15 hours at 2200°C and - yes - radiation-cooled. The private companies offering these chambers say the same: 2200°C.

As go les refractoires, rhenium melts before tungsten would, and (say the commenters) it's ten times as expensive; but it seems easier to work with chemically. Those old lightbulbs burned in vacuum. For a rocket what's the point of a chassis that doesn't melt if it reacts with the plasma long before it even starts glowing.

Similar constraints will apply to NERVA engines like Ultra Safe's. Maybe they use ceramics. Anyway, one more reason for them to stick with HALEU.

Then the question moves to, can we construct such chambers on the cheap. We might want several of these on the same rocket... like the Raptors. NASA were working on that in 2016.

If we had abundant rhenium down here (or in an orbiting factory for that matter) that would help too. It's the usual story: once in space, everything is cheaper, but we're not in space, so to get into space is expensive.