Each propellant burns in its own combustion chamber, spraying exhaust particles colored to match its real flame chemistry. The Tsiolkovsky rocket equation converts the propellant's specific impulse (Isp) and the chosen mass ratio into the delta-v a stage could deliver — the actual velocity change achievable, independent of the rocket's size.
Δv = Isp · g₀ · ln(m0 / mf)
g₀ = 9.80665 m/s²
- Solid propellant — a rubbery oxidizer/fuel grain cast directly into the case. Simple and storable indefinitely, but once lit it cannot be throttled or shut off. Used in boosters and military rockets.
- Hypergolic — fuel and oxidizer (e.g. hydrazine + N₂O₄) ignite on contact, needing no igniter. Extremely reliable restart for maneuvering thrusters, but the fuels are highly toxic to handle.
- LOX/LH2 cryogenic — liquid oxygen and liquid hydrogen give the highest Isp of any chemical propellant, but liquid hydrogen boils at −253°C and needs bulky, heavily insulated tanks.
- LOX/methane — a newer cryogenic combination; lower Isp than LH2 but methane is far denser and easier to store, and it can in principle be produced from Mars ISRU.
- Mass ratio — how many times heavier the fueled stage is than the empty stage; raising it (more propellant, less dry mass) increases Δv logarithmically for any propellant.
Real-world relevance: this exact Isp-vs-storability tradeoff is why real rockets mix propellant types by stage — solid boosters for cheap initial thrust, hypergolics for restartable maneuvering, and cryogenics for the efficiency-critical upper stages.