Every rocket relies on the same principle: burning propellant produces hot gas that is expelled at high speed and pushes the vehicle forward. How that propellant is stored and burned differs widely, however. Liquid and solid rocket engines each have strengths that explain why many launchers combine both.
Solid rocket motors
In a solid motor, fuel and oxidiser are mixed into a rubbery propellant that is cast into the motor casing. Once ignited, it burns until the propellant is used up. Solid motors are relatively simple, can be stored for long periods and deliver very high thrust, which makes them ideal as boosters for lift off. Their drawbacks are lower efficiency and the fact that they generally cannot be throttled freely, shut down and restarted.
Liquid rocket engines
Liquid engines store fuel and oxidiser in separate tanks and pump them into a combustion chamber. They are more complex, but they can be throttled, shut down and in many cases restarted, and they are usually more efficient. This makes them the standard choice for core stages and upper stages.
Common propellant combinations
- Liquid hydrogen and liquid oxygen: very efficient, but hydrogen is extremely cold and bulky. It powers the RS-25 engines of NASA's Space Launch System and the Vulcain and Vinci engines of Ariane 6.
- Kerosene and liquid oxygen: dense and easier to handle, used for example on Falcon 9 and Electron.
- Methane and liquid oxygen: a newer choice that burns cleaner than kerosene and suits reusable engines. It is used by several new launchers, including SpaceX Starship and Blue Origin's New Glenn.
- Storable propellants: hypergolic combinations that ignite on contact and can be stored at room temperature, common in spacecraft thrusters.
Combining both
Many rockets pair a liquid core with solid boosters. Europe's Ariane 6 uses P120C solid boosters alongside its liquid core stage, and the same P120C motor serves as the first stage of Vega-C. NASA's Space Launch System combines four RS-25 engines with two large solid boosters.
Why it matters for customers
The propulsion concept affects the launch environment, for example vibration and acoustic loads, and the flexibility of the mission. Restartable liquid upper stages enable more complex orbit insertions, which is especially relevant for rideshare missions with several drop off orbits.
Related reading
- Rocket Staging Explained: Why Rockets Drop Parts on the Way to Orbit
- Reusable Rockets: How Booster Landings Changed the Launch Market
- Upper Stages and Kick Stages: How Rockets Deliver Satellites to Their Final Orbit
Sources
Cover image: RS-25 engine test at NASA's Stennis Space Center. Credit: NASA (SSC-2015-00064), public domain.