How Rockets Work
Controlled explosions, the tyranny of the rocket equation, and the engines that beat it
Jet engines breathe air. Rockets bring everything with them — fuel, oxidizer, and no excuses. Here is how a machine that is 90% propellant accelerates to Mach 25.
Newton’s Third Law, Weaponized
A rocket engine is the simplest idea in physics executed at the edge of what materials can survive: throw mass out the back as fast as possible, and the vehicle accelerates forward. No air required — which is why rockets work in space and jet engines don’t. The faster the exhaust, the more speed you get from every kilogram of propellant.
The problem is that speed is expensive. The Tsiolkovsky rocket equation says the velocity a rocket can gain depends on its exhaust velocity and the ratio of full-to-empty mass. Want to go twice as fast? You need exponentially more propellant. This is why a Falcon 9 stands 70 metres tall to deliver a payload the size of a school bus, and why engineers obsess over every gram of dry mass.
Everything on this page — engine cycles, propellant choices, staging, reusability — is a different way of fighting that same equation.
The Four Pillars of Rocket Propulsion
Liquid Rocket Engines
Precision PowerLiquid engines pump fuel and oxidizer from tanks into a combustion chamber, where they burn at pressures up to 300 bar and temperatures beyond 3,300°C. They can be throttled, shut down, and restarted — which is why every crewed vehicle and every landing booster uses them.
The Cycle Wars
The hard part is driving the propellant pumps, which need tens of megawatts. A gas-generator cycle (Merlin, F-1, Vulcain) burns a little propellant in a separate mini-combustor to spin the turbine, dumping the exhaust overboard — simple but slightly wasteful. A staged-combustion cycle (RS-25, RD-180, BE-4) routes that turbine exhaust back into the main chamber, wasting nothing but demanding exotic metallurgy. SpaceX’s Raptor goes furthest with full-flow staged combustion: both propellants pre-burn, both turbines feed the chamber, and efficiency reaches the practical limit of chemistry.
Solid Rocket Boosters
Brute ForceA solid motor is a tube packed with rubbery propellant that burns from the inside out. No pumps, no plumbing, no countdown drama — solids can sit fueled for years and deliver enormous thrust the instant they ignite. The Space Shuttle’s twin boosters produced about 12,500 kN each; SLS’s five-segment versions are the most powerful solid motors ever flown.
The trade-off is control: once lit, a solid cannot be throttled or shut down. It burns until it is done. That is acceptable for the first two minutes of flight when raw thrust matters most, which is why Ariane 6, SLS and many others strap solids to a liquid core.
Staging
The Orbit Cheat CodeHalfway to orbit, a rocket’s first stage is mostly empty tank — dead weight. Staging drops it, letting a smaller, fresh stage accelerate a much lighter vehicle. The mathematics are unforgiving: with chemical propellants, a single-stage rocket reaching orbit would be almost all tank and no payload. Two or three stages turn an impossible problem into an expensive one.
Watch any Falcon 9 launch: main engine cutoff at ~2.5 minutes, separation, second stage ignition — and the discarded booster flips around to fly home. Saturn V did it twice, shedding 2,300 tonnes of spent hardware on the way to trans-lunar injection.
Mass shed during a Saturn V lunar launch
Propellants
Chemistry Sets the CeilingKerosene/LOX (Falcon 9, Saturn V’s first stage, Soyuz) is dense and manageable — the reliable workhorse. Hydrogen/LOX (RS-25, Ariane’s core, SLS) gives the best efficiency chemistry allows, but liquid hydrogen boils at −253°C and needs enormous tanks. Methane/LOX (Raptor, BE-4) is the new favourite: nearly hydrogen-clean, nearly kerosene-dense, cheap, and — crucially for Mars ambitions — manufacturable from local resources. Hypergolics (spacecraft thrusters, Soyuz’s manoeuvring engines) ignite on contact, perfect for reliability, nasty to handle. Solids trade efficiency for instant, storable thrust.
The Great Engines Compared
Six engines that define their eras
| Engine | Cycle | Propellant | Thrust (SL) | Isp (vac) | Flies On |
|---|---|---|---|---|---|
| F-1 | Gas generator | RP-1 / LOX | 6,770 kN | 304 s | Saturn V (5×) |
| RS-25 | Staged combustion | LH2 / LOX | 1,860 kN | 452 s | Space Shuttle, SLS |
| Merlin 1D | Gas generator | RP-1 / LOX | 845 kN | 311 s | Falcon 9 (9×) |
| Raptor | Full-flow staged | CH4 / LOX | ~2,300 kN | ~350 s | Starship (33+6) |
| BE-4 | Ox-rich staged | CH4 / LOX | ~2,400 kN | ~340 s | New Glenn (7×), Vulcan |
| Rutherford | Electric pump | RP-1 / LOX | 25 kN | 343 s | Electron (9×) |
Sea-level thrust per engine, rounded to commonly cited figures. Isp = specific impulse in vacuum. The F-1 remains the most powerful single-chamber liquid engine ever flown.
Reusability — Landing the Unlandable
For sixty years, every orbital rocket was thrown away after one flight — like scrapping an airliner after each trip. Falcon 9 changed that in December 2015 with the first orbital-class booster landing. The returning stage flips around, performs a boostback burn, steers with titanium grid fins through hypersonic reentry, and relights a single engine for a suicide burn — decelerating from over 4,000 km/h to zero exactly at touchdown, because the engine cannot throttle low enough to hover.
Individual boosters have now flown more than twenty times each. Starship pushes the idea to its conclusion: catch the returning booster with the launch tower’s arms, refuel, and fly again — aiming to make reaching orbit look less like artillery and more like aviation.
Frequently Asked Questions
How do rockets work in space if there is no air to push against?
Rockets never push against air — they push against their own exhaust. By Newton’s third law, throwing mass backward at several kilometres per second pushes the rocket forward. Air actually hurts a rocket engine: thrust and efficiency both improve in vacuum, which is why upper-stage nozzles are so much larger.
What fuel do rockets use?
The main pairings are kerosene with liquid oxygen (Falcon 9, Soyuz), liquid hydrogen with liquid oxygen (SLS core, Ariane), and increasingly methane with liquid oxygen (Starship, New Glenn). Solid boosters use aluminium-based propellant, and spacecraft thrusters often use hypergolic propellants that ignite on contact for maximum reliability.
What is specific impulse in simple terms?
Specific impulse (Isp) is a rocket engine’s fuel mileage: how many seconds one kilogram of propellant can produce one kilogram-force of thrust. A hydrogen engine at 450 seconds squeezes about 50% more velocity from each tonne of propellant than a kerosene engine at 300 seconds.
Why does a rocket roll onto its side after launch?
Because orbit is about horizontal speed, not height. Shortly after liftoff a rocket begins its gravity turn, gradually pitching over so that by engine cutoff it is flying almost parallel to the ground at nearly 28,000 km/h. Going straight up would just mean falling straight back down.
Could a jet engine be used to reach space?
No — jet engines need atmospheric oxygen, which runs out long before orbital altitude, and no air-breathing aircraft has come close to orbital speed. The fastest jet-powered aircraft, the SR-71 Blackbird, reached about Mach 3.3; orbit requires Mach 25. Hybrid designs like scramjets remain experimental, as the X-43A showed at Mach 9.6.