The numbers
Computed with NASA CEA for the same chamber pressure (10 bar) and nozzle area ratio (40), each propellant at the mixture ratio that gives the highest vacuum performance. Bulk density is the average density of fuel and oxidiser together at that ratio.
| LOX / hydrogen | LOX / methane | LOX / kerosene | |
|---|---|---|---|
| Vacuum Isp (ideal) | 454 s | 365 s | 353 s |
| Best mixture ratio O/F | 4.5 | 3.3 | 2.6 |
| Chamber temperature | 3,010 K | 3,290 K | 3,400 K |
| Bulk density | ≈ 0.31 t/m³ | ≈ 0.82 t/m³ | ≈ 1.02 t/m³ |
| Fuel boiling point | 20 K | 112 K | storable |
The trade-offs
Hydrogen wins on efficiency, but its tanks are more than three times larger for the same propellant mass, it must be kept at 20 K, and it leaks through almost anything. Kerosene is dense and easy to store, but it cokes: it leaves carbon deposits in cooling channels and on reused engines. Methane is a compromise: nearly kerosene’s density class, clean combustion, cryogenic but at a temperature close to liquid oxygen, and it can be produced from carbon dioxide and hydrogen, for example on Mars.