The faster a rocket throws mass out, the faster it can move forward. The rocket equation, also called the Tsiolkovsky rocket equation after Konstantin Tsiolkovsky (1857-1935), describes the motion of a vehicle that accelerates itself by expelling part of its mass. Its delta-v is the greatest change of speed a rocket can get by expelling some of its mass, if no outside forces act. As of November 2026, the highest exhaust speed is around 4.5 km per second for liquid-propellant engines and about 2.5 km per second for solid-propellant rockets. Non-chemical systems such as ion engines can reach about ten times more. Specific impulse (Isp) is a measure of a rocket engine's efficiency: the thrust produced per weight of propellant used each second. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum.

The convergent-divergent (CD) nozzle at the heart of a modern rocket engine was developed in the 19th century by Swedish inventor Gustaf de Laval, who was working on a more efficient steam engine. The narrowest part of a rocket nozzle is called the throat. There the hot exhaust is choked: its Mach number equals 1.0. The area ratio from a nozzle's throat to its exit sets the exhaust's exit velocity and exit pressure. The thrust of a rocket engine depends on the mass flow rate through the engine, the exit velocity of the exhaust, and the pressure at the nozzle exit. A Space Shuttle main engine made 375,000 pounds of thrust at sea level but 470,000 pounds in a vacuum, burning liquid oxygen and liquid hydrogen at a mixture ratio of 6 to 1.

Liquid propellants are fairly dense, so their tanks can be relatively small and light, and that gives a high mass ratio, which is part of why large launchers use liquid-fuel stages. Most of a rocket's mass before launch is fuel and oxidizer, which together are called propellant. A rocket in airless space must carry its own oxidizer, which weighs far more than the fuel, so the rocket first has to lift not just its payload but also the much greater weight of its propellant. The propellant needed to launch a payload into Earth orbit is usually at least 20 times more massive than the payload itself. A solid rocket cannot be shut off once it is lit, but a liquid rocket's thrust can be controlled with valves.

At a 100-mile-high orbit around Earth the orbital velocity is 17,478 mph. Launched from the Moon or Mars, a rocket would need a different orbital speed because those worlds have different radius and gravity. As the height of an orbit increases, the speed needed to stay in orbit decreases, so a spacecraft in a lower orbit must travel faster than one in a higher orbit. To leave Earth for good, a spacecraft must reach escape velocity, which for a planet does not depend on the mass of the rocket: it is about 11.2 km per second for Earth. It depends on the planet, so leaving the Moon takes less speed than leaving Earth, and Jupiter's is many times Earth's.

Reaching space and reaching orbit are different, because orbital flight is a combination of altitude and horizontal velocity. Aerodynamic forces matter little for satellite launchers: the rocket gets out of the atmosphere as quickly as possible, then gains the sideways velocity it needs to stay in orbit. A rocket launched toward the east gets a boost from the speed of Earth's spin, and sitting on a launch pad near the equator, a rocket is already moving at over 1,650 km per hour with that spin. A rocket that flew straight up would fall back to Earth when it ran out of fuel, so rockets tilt to the side as they climb. Launches from the US east coast (Kennedy Space Center) are suitable only for low-inclination orbits, because major population centers lie under the path needed for high-inclination launches.

In 1955 the U.S. Air Force hired Rocketdyne to build an engine of 1 million pounds of thrust to launch future satellites. After Sputnik and the creation of NASA the F-1 was pushed on to 1.5 million pounds, and it first fired as a prototype in 1961. The F-1 engine had combustion instability: pressure swings in the burning propellants made hot spots and vibrations strong enough to destroy it, and in 1961 it caused a test engine meltdown. Engineers solved it by adding copper baffles to the injector plate. The Saturn V's first stage used five F-1 engines with a total thrust of 7.5 million pounds. Von Braun, who helped develop the German V-2 and later came to the United States, played a significant role in the development of the Saturn rockets that took people to the Moon between 1968 and 1972.

The Saturn V's first stage engines burned for about 2.5 minutes, taking the rocket and its payload to an altitude of about 38 miles. After the second stage, the third stage burned about 2.75 minutes to reach an orbital speed of about 17,500 mph. It was shut down with fuel left, stayed attached in Earth orbit, then its J-2 engine reignited to send Apollo toward the Moon. Apollo 11 lifted off on a Saturn V rocket on July 16, 1969, and two hours, 44 minutes after launch, the Saturn V's third stage fired a second time for five minutes, 48 seconds. At launch the Saturn V was the largest and most powerful launch vehicle the United States had ever built.

The Space Shuttle had three major parts: the Orbiter that carried the crew, a large External Tank holding fuel for the main engines, and two Solid Rocket Boosters that gave most of the lift in the first two minutes of flight. Staging: several minutes into the ascent most launchers discard weight, and often light a second engine on an upper stage. The first stage can be retrieved, as with the Space Shuttle's boosters, or thrown away, as on the Apollo moon rockets. The Space Shuttle was reusable. Its orbiter landed on a runway, and its solid rocket boosters were recovered from the sea.

Around 400 B.C., a Greek named Archytas is said to have flown a wooden pigeon, held on wires and pushed along by escaping steam, an early use of the action-reaction principle that rockets rely on. In 1232, at the battle of Kai-Keng, Chinese defenders drove back Mongol invaders with "arrows of flying fire", simple solid-propellant rockets, the date NASA gives for the first use of true rockets. In the 1500s a German fireworks maker, Johann Schmidlap, invented the step rocket: a big rocket carried a smaller one that kept climbing after the big one burned out. The science of rocketry began with a book Isaac Newton published in 1687, usually called the Principia, which stated three principles of motion that govern objects on Earth and in space.

In 1914, Robert Goddard received two patents, one for a liquid-fuel rocket and one for a multistage rocket. In 1915 he proved by experiment that a rocket can make thrust in a vacuum. On March 16, 1926, he launched the world's first liquid-fueled rocket in Auburn, Massachusetts. The German V-2 was the world's first large-scale liquid-propellant rocket vehicle and the first long-range ballistic missile, and it is the ancestor of today's large rockets. In 1949 the "Bumper-WAC", a small WAC Corporal missile riding on a German-made V-2, became the first human-made object to enter space, climbing to 393 kilometers (244 miles). As his rockets got bigger, Goddard moved his tests to Roswell, New Mexico.

Sputnik was launched on an R-7 rocket that Soviet designer Sergei Korolev's team had built as an intercontinental ballistic missile, to put a scientific satellite into orbit before the end of 1957. On October 4, 1957, the Soviet Union launched Sputnik, the first human-made satellite. Sputnik sent out a radio "beep-beep" signal that people around the world could hear. It fell back into Earth's atmosphere and burned up on January 4, 1958. Reactions in the United States to Sputnik included shock and amazement and led to Congressional hearings that ultimately created NASA in 1958. In April 1961, Yuri Gagarin of the Soviet Union was the first person in space. From the start, the plan was that Gagarin would not land inside his Vostok spacecraft after his April 12, 1961 orbit.

As of November 2026, NASA's Space Launch System (SLS) first flew on November 16, 2022, on the uncrewed Artemis I test flight, making it the most powerful rocket NASA had launched. SLS uses its four RS-25 engines and twin boosters together: in about eight minutes the core stage burned 735,000 gallons of liquid propellant for two million pounds of thrust, while the boosters made more than seven million pounds. At launch the SLS Block 1 makes 8.8 million pounds of thrust, 15% more than the Saturn V.

As of November 2026, Ariane 6, Europe's rocket, comes in two versions: Ariane 62 with two boosters, which can launch about 10.3 tonnes into low Earth orbit, and Ariane 64 with four boosters. Its upper stage uses the restartable Vinci engine and can fire up to four times, dropping passengers into different orbits on one flight, then deorbit itself to avoid becoming space debris. The Falcon 9 medium-lift rocket can carry payloads of up to 22,800 kg into low Earth orbit. The first stage of a Falcon 9 rocket can land upright after launch, so it can be flown again: SpaceX landed the first stage of a Falcon 9 upright on a landing pad near its Florida launch site while the second stage went on to launch communications satellites.

Words to know

• propellant: fuel and oxidizer
• impulse: efficiency measure
• nozzle: engine's exhaust outlet
• baffles: copper injector fix