A jet engine breathes the atmosphere. A rocket engine cannot — it must carry its own oxidiser, which is why a Saturn V lifted 2,540 tonnes of propellant for every 140 tonnes of payload to low Earth orbit. The trade-off shapes every design choice from nozzle geometry to engine cycle.
Browse space vehiclesA rocket engine accelerates a working fluid — combustion gas — out of a nozzle. The momentum carried out the back equals the momentum imparted to the vehicle going forward. Unlike a turbofan, the rocket carries both the fuel and the oxidiser on board, so it works equally well in the vacuum of space or at sea level. Thrust F equals mass flow rate ṁ times exhaust velocity ve, plus a smaller pressure term that depends on whether the nozzle is matched to the ambient atmosphere.
Two numbers dominate every rocket discussion. Thrust is the force at the engine, measured in kilonewtons or pounds-force. Specific impulse (Isp), in seconds, is how long one kilogram of propellant could produce one kilogram of thrust. Higher Isp means more delta-v from the same propellant mass.
Liquid engines burn a fuel and an oxidiser stored in separate tanks, mixed only inside the combustion chamber. Four propellant families dominate.
Liquid oxygen and refined kerosene. Dense, storable at near-ambient temperature on the fuel side, and the workhorse of first stages. The Rocketdyne F-1 on the first stage of the Saturn V delivered 1,522,000 lbf (6,770 kN) of thrust at sea level — five of them lifted 2,950 tonnes off pad 39A. The SpaceX Merlin 1D on the Falcon 9 booster produces 190,000 lbf at sea level with an Isp of 282 s, and nine of them cluster on each first stage. RP-1 leaves carbon residue inside the engine — manageable for expendable use but a hurdle for rapid reuse.
The highest-Isp chemical combination in routine use — around 450 s in vacuum. Hydrogen is light, so the molar exhaust velocity is high. The Aerojet Rocketdyne RS-25 flown on the Space Shuttle and now on the SLS produces 418,000 lbf at sea level and 512,000 lbf in vacuum, with a vacuum Isp of 452 s — still the benchmark for staged-combustion hydrolox engines. The RL10, first fired in 1959, lives on the Centaur upper stage with 24,750 lbf and an Isp near 465 s; later derivatives push that higher. The penalty is bulk: liquid hydrogen has a density of 71 kg/m³, so LH2 tanks are huge.
Monomethylhydrazine and nitrogen tetroxide ignite on contact — no ignition system needed. Storable at room temperature for years. Used on the Apollo Lunar Module descent and ascent engines, the Space Shuttle OMS pods, the Dragon SuperDraco abort thrusters, and most Russian and Chinese upper stages. Isp sits at 320–340 s. The downside is toxicity — every ground crew handling MMH wears a SCAPE suit.
The newest mainstream choice. Density between RP-1 and LH2, clean burn (no coking), and producible on Mars from CO₂ — the driver for SpaceX's Raptor on Starship and the Starship upper stage. Raptor 3 targets 280 bar chamber pressure (the highest of any flying engine) and 280 tf (617,000 lbf) of thrust. Blue Origin's BE-4 on New Glenn and ULA's Vulcan first stage also burns methalox, producing 550,000 lbf each.
Bipropellant engines need to push liquid propellant into a chamber that may be at 100–300 bar. Two strategies exist.
Full-flow staged combustion is the most efficient cycle but mechanically the hardest. It took until 2019 for a full-flow engine (Raptor) to fly.
A rocket nozzle converts high-pressure, low-velocity combustion gas into low-pressure, high-velocity exhaust. The expansion ratio — exit area divided by throat area — determines the optimum altitude. Sea-level engines run expansion ratios of 14–25:1, vacuum upper-stage engines run 80–280:1, with bell-shape nozzles that would be impossibly long if extended to a perfect match.
The RS-25 has an expansion ratio of 69:1; the RL10C-1 on Centaur runs 285:1. The aerospike nozzle — a flat-faced engine that uses the atmosphere itself as the outer boundary — promises altitude-compensation across the entire ascent but has never flown operationally despite the XRS-2200 ground tests in the late 1990s for the X-33.
A solid motor is a steel or composite tube packed with a rubbery mixture of ammonium perchlorate oxidiser, aluminium powder fuel, and a polymer binder (APCP). Once lit, it cannot be throttled or shut down — the grain burns until exhausted. The trade is brutally simple: low Isp (260–270 s) in exchange for storage for decades, instant readiness, and no plumbing.
The Space Shuttle SRBs each produced 3,300,000 lbf at sea level — the most thrust ever from a single chamber. The Minuteman III ICBM uses three solid stages; the Trident II D5 SLBM uses three solid stages packaged inside a submarine launch tube. The Peacekeeper stacked three solid stages plus a liquid post-boost vehicle for MIRV deployment. Solids dominate strategic missiles because a silo or submarine needs a weapon that lights on command after 20 years of cold storage.
Hybrids combine a solid fuel grain with a liquid (or gaseous) oxidiser injected from a separate tank. The combination throttles, can be shut down, and is safer to handle than a pre-mixed solid. SpaceShipOne in 2004 and SpaceShipTwo both fly hybrid motors burning HTPB rubber with nitrous oxide. Isp is modest (~250 s) and regression rate of the solid grain limits thrust scaling, which is why no orbital launcher uses a pure hybrid first stage.
The mass ratio a rocket needs to reach orbit — roughly 9:1 propellant-to-everything-else for chemical engines — is impossible to achieve in a single stage. Staging discards spent tanks and engines, raising the effective mass ratio. The Saturn V used three stages plus the Apollo spacecraft's own engine to reach the Moon. Falcon 9 uses two stages; the first returns and lands. Starship splits the same way: Super Heavy first stage with 33 Raptors, Starship upper with 6 Raptors.
The Merlin 1D throttles between 40% and 100% — necessary because the first stage gets dramatically lighter as it climbs, and during landing the engine cannot hover (minimum thrust still exceeds vehicle weight), forcing the suicide-burn profile Falcon 9 uses. Each Merlin gimbals up to about 6 degrees on hydraulic actuators for pitch and yaw steering; roll comes from differential gimbaling across the cluster of nine. The RS-25 was designed for ten flights between major overhauls — exceeded on most Shuttle missions, and the same airframes (now flying on SLS) collectively logged over 100 flights.
Through the 2020s every clean-sheet booster engine from a US or European developer chose methalox: Raptor, BE-4, Prometheus (ArianeGroup), Rutherford's successor Archimedes from Rocket Lab. The reasons stack: density between LH2 and RP-1 means tanks stay reasonable; clean combustion enables rapid reuse; ISRU production on Mars is plausible; chamber chemistry is well-understood. The Prometheus / Themis demonstrator targets a per-engine cost an order of magnitude below the Vulcain 2 it could eventually replace.
Engine specifications quoted are from publicly available manufacturer, NASA, and contractor sources and reflect production or flight variants as of 2026.