[Enthalpy]

Hi dear friends!

Isar Aerospace, a young company near Munich, wants to develop a launcher called Spectrum to put 1000kg in low-Earth-orbit.
  Company - Launcher

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 for safety.

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

[Enthalpy]

My thoughts about electric pumps to feed rocket engines are there:
  scienceforums
While pressure-fed stages, of steel or of graphite fibers, mixed with messages about re-used low stages, are there:
  scienceforums
Two pressure-fed stages to put 1t in  low-Earth orbit in that message:
  scienceforums
compared with stages pumped by a gas generator cycle.

[Enthalpy]

Spiropentane is an other reasonable fuel choice. It gains 1s over cyclopropane and methane, 4s over propane. Boiling at 1bar +39°C, it's easier to handle, like gasoline versus propane. At room temperature, the density is 755kg/m³.

I compared visually the sizes of a launcher using varied fuels:
  chemicalforums
Diazetidylcyclopropane and others shrink a launcher further and are more reluctant to flame, but they are less known hence must be tried first. Pressure feeding favors amines even more.

Spiropentane isn't massively available, it must be made or bought on demand. The starting material, pentaerythritol C(CH₂OH)₄, is a commodity costing around 1200usd/ton, or it can be produced from formaldehyde and ethanal
  alibaba - alibaba - alibaba - orgsyn

Pentaerythritol becomes pentaerythrityl tetrabromide C(CH₂Br)₄ which zinc converts to spiropentane
  orgsyn - orgsyn - wikipedia

The used Br and Zn are easily reused. I didn't check if P is useful and easy to reuse.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Time to repeat myself, because I wrote too much for anybody to see through.

The widely available, rather safe and cheap rocket fuels include

• Pmdeta
• Hydrogenated cis-pinene (decent fp+48°C), a component of turpentine.
• Hydrogenated myrcene (less good fp+34°C), a product of turpentine.
• Mix of C₁₀-C₁₂ isoparaffins. I got 1L for little money as a solvent. Supposedly a by-product of gasoline additive in refineries.

All these are about 2s better than rocket "kerosene" Rg-1 and available anywhere. Pmdeta is more efficient.

The main advantages of Pmdeta, farnesane, phytane are the higher flash point, wider liquid range and the lesser viscosity at cold.