Rockets

What a rocket is

Rockets lift spacecraft off Earth and up to low Earth orbit or farther, so they are also known as launch vehicles.^[6] Strip it down and it is a chamber of gas under pressure with a small hole at one end: gas escaping through the hole drives the chamber the other way.^[1] In a space rocket that gas comes from burning propellants, which can be solid, liquid or a mixture of the two.^[1]

How thrust works

The idea is Newton’s third law: if one object pushes on a second, the second pushes back just as hard in the reverse direction.^[4] The engine blasts hot exhaust out of its rear (the action), and the rocket moves off the other way (the reaction).^{[5, 7]}

A shaped nozzle speeds the gas up: it is squeezed through a narrow throat, then gains still more speed as the nozzle flares and the gas spreads out.^[12] To steer, the nozzle can be gimbaled, meaning tilted to point the push in a new direction.^{[8, 3]} The rocket rises only once its thrust is bigger than its weight.^{[7, 3]}

A jet engine takes oxygen from the air, but a rocket brings its own oxidizer (the ingredient that lets fuel burn), so it can work beyond the atmosphere.^{[9, 2]}

Propellant is by far the heaviest part of a rocket, and it is burned off as the engines run, so the vehicle keeps getting lighter.^[1] Its acceleration therefore rises, which is why a launch begins slowly and picks up pace on the way up.^[1]

Solid and liquid propellants

A propellant is a fuel paired with an oxidizer.^[8] A solid rocket is little more than a case packed with propellant.^[8] Solid motors cost less to build, but once lit they cannot be stopped.^{[12, 9]}

Liquid engines get more thrust out of each unit of propellant weight, and they can be switched off and, in some types, relit in flight.^{[12, 9]} The cost is complexity: a liquid engine is complex and subject to failure, and carries more structure than a comparable solid rocket.^[8] Oxygen must be cooled to −183 °C to become liquid, and chilling propellants shrinks them so smaller, lighter tanks will do.^[8]

Mass, staging and efficiency

NASA’s Basics of Space Flight guide sums up the problem: getting fuel off the ground takes fuel, and lifting that extra fuel takes more again.^[10] The classic answer is staging: pile smaller rockets on top of a big one, with the payload at the very top.^[8] Each stage fires until its tanks are empty, then separates, and the next takes over.^[14] Shedding dead weight in flight makes the upper stages more effective.^[8]

JPL’s thruster notes explain this through conservation of momentum: the mass of propellant thrown out times its speed equals the spacecraft’s mass times the speed it gains.^[11] Delta-v is the largest change in speed a vehicle can reach by expelling part of its own mass, ignoring outside forces.^[9] The rocket equation, built on Newton’s second law, links exhaust speed to that change in the vehicle’s speed.^[8] JPL adds that the propellant needed grows exponentially as the required delta-v rises.^[11]

Specific impulse (Isp) measures engine efficiency; JPL defines it as thrust divided by the rate at which propellant is used up.^[11] The higher the Isp, the less propellant a given gain in momentum costs.^[9] Exhaust speeds are a few kilometres per second: the top figures are near 4.5 km/s for liquid engines and near 2.5 km/s for solids.^[9] JPL’s thruster notes give typical values of up to 4 km/s for chemical rockets, limited by the energy held in the propellant’s chemical bonds.^[11]

Getting to orbit

Orbit is a mix of height and sideways speed.^[3] A launcher first climbs out of the thick lower air as fast as it can, then builds speed, its lighter upper stage tipping over toward horizontal.^[3] Low Earth orbit takes more than 28,000 km/h, and lower orbits need more speed than higher ones.^{[1, 7]} Leaving Earth for deep space takes escape velocity, over 40,250 km/h.^[1]

Four famous launchers

Saturn V flew the Apollo missions of the 1960s and 1970s and also put the Skylab space station into orbit.^[14] It stood 111 m tall, weighed 2.8 million kg fully fuelled, and could put about 118,000 kg into Earth orbit.^[14] Its first flight, Apollo 4, tested the whole stack at once rather than stage by stage.^[15]

The Space Shuttle first flew on 12 April 1981, when Columbia carried John Young and Bob Crippen; it was the first reusable vehicle for trips to low Earth orbit.^[16] Its solid boosters ran beside three main engines for the first two minutes, and the main engines ran for about 8.5 minutes, after which the big fuel tank, the only part never reused, was dropped.^[16]

SLS, the Space Launch System, uses four RS-25 engines burning liquid hydrogen and oxygen, and its maximum thrust is 15 percent above Saturn V’s.^[8] It flew an uncrewed test with the Orion spacecraft in November 2022.^[17]

Ariane is Europe’s family. ESA records that Ariane 1 first launched on 24 December 1979 from Kourou, built mainly to send commercial satellites to geostationary orbit, and that Ariane 4 made 113 successful launches between 1988 and 2003.^[20] ESA gives Ariane 6 two versions, Ariane 62 with two boosters and Ariane 64 with four, lifting roughly 10.3 and 21.6 tonnes to low Earth orbit.^[21]

Escaping a launch in trouble

Mercury and Gemini capsules carried launch escape systems, Mercury’s built around two solid motors.^{[19, 13]} These protect only the flight crew after the hatch is shut, and the forces that yank a capsule clear can injure the people inside.^[18] Pad workers need their own way out: in the Apollo era a cab carried up to nine people down a steel cable, and the Shuttle pad used slidewire baskets for four.^[18] Each Artemis pad basket, about the size of a small SUV, holds up to five people.^[19]

A short history

By 1232, rockets tied to arrows were fired at Mongol invaders at the battle of Kai-keng.^[8] In 1926 Robert Goddard flew the first liquid-propellant rocket, burning liquid oxygen and gasoline; it climbed 41 feet in 2.5 seconds.^[9] The V-2 became the first human-made object to reach outer space.^[12] Sergei Korolev’s R-7 launched the first artificial satellite, Sputnik 1, on 4 October 1957, and Yuri Gagarin reached space on 12 April 1961.^{[12, 8]}

Common misconceptions

“Rockets push against the air.” They push against their own exhaust, which is why they work in a vacuum, and work better there.^{[1, 2]}

“Getting to orbit just means going high enough.” Orbit needs a great sideways speed as well as height.^{[3, 7]}

What this page does not cover

Rocket-equation arithmetic or a delta-v budget for orbit, launch costs, commercial reusable boosters and booster landings, ion, Hall-effect or nuclear engines, abort modes and escape-motor figures, Artemis flights after November 2022, and the size and mass of the Shuttle stack, SLS or Ariane 6.

Going further

See Newton’s Laws of Motion and Momentum for the physics of thrust, Gravity and Orbits for why speed keeps a spacecraft up, and Satellites, Space Telescopes and Space Exploration for what rockets carry.