Hi dear friends!
Isar Aerospace, a young company near Munich, wants to develop a launcher called Spectrum to put 1000kg in low-Earth-orbit
All is copied from Falcon-9 developed by SpaceX >20 years ago. This guarantees to stay 20 years behind them. Worse: turbopumps and a gas generator are too expensive and difficult for the smaller Spectrum, where a few % saved mass don't pay for complexity. Batteries and electric pumps
excel at this size, they're cheaper to develop and build, easier to start, more reliable. Or rather, pressure-fed liquids
make a much cheaper small launcher, and can even evolve into a reused launcher.
Now they tell their fuel will be propane
. Propane is extremely flammable for being a gas. Stored at room temperature, it demands heavy tanks. Cold, it has no advantage over better fuels.
Compare with cyclopropane
, mass-available for fridges. Cyclopropane is stable (someone botched Wiki, the Web copied, I let correct the lies). Cyclopropane is just as dangerous as propane is. Burnt with oxygen and expanded from 80bar to 1bar, propane accelerates to 3049m/s, cyclopropane to 3080m/s, a 3s improvement for free
. Propane weighs 582kg/m3
at -42°C 1atm, cyclopropane 699kg/m3
at -33°C 1atm. Their argument of "energy density" is wrong - little relevant at a launcher anyway.
The very safe liquid Pmdeta
is trivially obtained from the mass-available Deta. mp=-52°C, fp=+77°C, ig=+155°C, bp=+201°C @1atm, ρ=828kg/m3
, η=1.5mPa×s, P=30Pa @+20°C: all perfect! 3027m/s lose only 2s to propane, obvious choice
Or if cooling the engine with oxygen (has been done), then burn ethylene. 3098m/s gain 5s over propane at identical risk
Marc Schaefer, aka Enthalpy