The combustion chamber is the enclosed volume in a rocket engine where fuel and oxidizer mix and react, producing high-temperature, high-pressure combustion gases. These gases then expand through a nozzle to produce thrust.
Chamber conditions
Combustion chamber pressure (p_c) and temperature (T_c) define the starting conditions for nozzle expansion:
| Engine | Propellant | p_c (MPa) | T_c (K) | Cycle |
|---|---|---|---|---|
| Merlin 1D (Falcon 9) | RP-1/LOX | 9.7 | ~3,400 | Gas generator |
| RS-25 (Shuttle/SLS) | LH₂/LOX | 20.6 | ~3,500 | Staged combustion |
| Raptor 2 (Starship) | CH₄/LOX | 30+ | ~3,600 | Full-flow staged combustion |
| RD-180 (Atlas V) | RP-1/LOX | 26.7 | ~3,500 | Staged combustion |
| Solid motor (typical) | APCP | 3–10 | ~3,200 | Pressure-fed (grain) |
Higher chamber pressure generally improves engine performance (specific impulse) because it allows greater expansion through the nozzle. But higher pressure demands heavier chamber walls, more powerful turbopumps, and better cooling — engineering trade-offs that have driven decades of engine development.
Cooling the chamber
Combustion temperatures (3,200–3,600 K) exceed the melting point of all structural metals (nickel superalloys melt at ~1,350°C). The chamber and nozzle throat must be actively cooled:
- Regenerative cooling — one propellant (usually fuel) flows through channels in the chamber wall before entering the combustion zone. The propellant absorbs heat, keeping the wall temperature below structural limits. The heated propellant enters combustion slightly preheated, marginally improving efficiency.
- Film cooling — a layer of cooler fuel is injected along the chamber wall, creating a protective boundary layer. Less efficient than regenerative cooling (the unburned fuel reduces I_sp) but simpler.
- Ablative cooling — the chamber wall is made of material that chars and erodes, absorbing heat as it ablates. Used in solid motors and some low-cost liquid engines. The chamber degrades with each firing.
- Radiation cooling — the chamber or nozzle extension glows at high temperature, radiating heat to the surroundings. Only practical for low-heat-flux regions like large nozzle extensions.
Related terms
- Specific Impulse — chamber conditions determine the maximum achievable I_sp
- Gimbal — the mechanism for steering the thrust produced by combustion