Jim Kingdon wrote:

My idea is to build a nozzle that has a shape optimized for
vacuum but to add inside something that fills in some space
to make it look like a nozzle optimized for a pressure of 100 kPa.
The filler material would be made of something that melts



Hmm, interesting. A google search for "ablative nozzle altitude
compensation" mostly got aerospike hits (which of course are a
different thing).

My first reaction is to ask whether the ablative material would have
too much mass. But I don't know whether anyone has looked at this
before.



Another approach would be to jettison the inside liner at altitude; sort
of the way that some rocket/ramjet missiles jettison there booster
rocket nozzle from the solid fueled rocket motor case and convert the
case into a nozzle for the ramjet.
I would think that there could be problems getting the liner to ablate
evenly- especially since any uneven ablation could start a turbulence
effect in the nozzle that would cause heating at that point, and
steadily increase the severity of the uneven ablation.

Pat