Rocket engine

A rocket engine is a reaction engine that produces thrust by burning stored fuel and oxidizer and expelling the hot exhaust through a nozzle. Because it carries its own oxidizer, a rocket engine can work in the vacuum of space, where air-breathing engines cannot.

History of rocket engines

The most important early step in modern rocketry came on March 16, 1926, when the American physicist Robert H. Goddard launched the first liquid-fueled rocket from a farm in Auburn, Massachusetts. NASA says the rocket, which he later called Nell, burned gasoline and liquid oxygen, and rose 41 feet during a flight of 2.5 seconds before landing 184 feet away in a cabbage field.

Goddard placed the engine on top of the rocket, above the tanks, an unusual layout he thought would add stability. After further tests he moved the engine beneath the propellant tanks, which gave adequate stability and simplified the design. He added movable vanes in the exhaust and gyroscopes to control the rocket’s attitude on longer flights.

The National Air and Space Museum notes that Goddard kept the 1926 launch secret for about a decade. The Smithsonian had published his treatise A Method of Reaching Extreme Altitudes in January 1920, and from the early 1930s he built larger rockets in Roswell, New Mexico, with funding that aviator Charles Lindbergh helped arrange. The highest altitude any of his rockets reached was about 9,000 feet.

Goddard died on August 10, 1945. NASA reports that Germany and the Soviet Union studied his results to advance their own programs, while the US government showed little interest before World War II.

How a rocket engine works

NASA Glenn Research Center describes the basic sequence. Stored fuel and oxidizer are ignited in a combustion chamber, producing a large volume of exhaust gas at high temperature and pressure. The exhaust passes through a nozzle that accelerates it, and thrust follows from Newton’s third law of motion: the gas pushed backward produces an equal and opposite force on the engine.

The thrust depends on three things: the mass flow rate through the engine, the exit velocity of the exhaust and the pressure at the nozzle exit. The thrust equation is F = mass flow rate times exit velocity, plus the difference between exit pressure and surrounding pressure times the exit area. See the entry on Newton’s laws of motion for the principle behind it.

The narrowest part of the nozzle is the throat. The hot flow is choked there, meaning its speed is Mach 1, and the throat area then fixes the mass flow rate. The ratio of exit area to throat area sets the exit velocity and pressure, so nozzle design largely determines performance.

Performance is often summarized as specific impulse. NASA defines total impulse as average thrust multiplied by firing time, and divides it by the weight of the propellants to obtain specific impulse, a measure that works for both liquid and solid rockets.

Types of rocket engines

NASA names two main categories. In a liquid rocket, the fuel and oxidizer are stored separately as liquids and pumped into the combustion chamber, where burning occurs. Thrust can be stopped by shutting off the flow of propellants.

In a solid rocket, the fuel and oxidizer are mixed and packed into a solid cylinder. Under normal conditions the propellant does not burn, but an igniter starts the reaction, and burning proceeds until the propellant is used up. NASA notes that a solid motor is easier to handle and can sit for years before firing, while a liquid engine is heavier and more complex because of its pumps and tanks.

Common propellants include substances covered elsewhere on this site, such as oxygen and hydrogen, though the choice depends on the mission and the engine design.

Applications of rocket engines

Rocket engines launch satellites and spacecraft into orbit, and they power the maneuvers of vehicles once in space. NASA’s Glenn pages also point to rocket-powered aircraft built during and after World War II to explore high-speed flight, including the X-1A, used to break the sound barrier, and the X-15.

The X-15 was powered by a liquid rocket engine and carried a single pilot to a height of more than 60 miles above Earth. Such flights show why rocket engines suit missions that leave the atmosphere, where propellers and turbines have no air to work with.

Advantages and disadvantages of rocket engines

The chief advantage is independence from the atmosphere. Turbine engines and propellers use air as their working fluid and draw oxygen from it, but a rocket uses its own exhaust gases as the working fluid and carries its oxidizer. This is why a rocket works in space and a turbine does not.

The trade-off is that the vehicle must carry all of its propellant, which adds weight and limits how long an engine can burn. Liquid engines need pumps, tanks and plumbing. Solid motors are simpler, but once ignited they burn until the propellant is exhausted, whereas a liquid engine can be shut off by stopping the propellant flow.

Rocket engine vs. jet engine

Both are reaction engines that produce thrust by accelerating a working fluid, and both obey Newton’s third law. The difference lies in where the working fluid and oxygen come from. A jet engine takes in air from the atmosphere and uses the oxygen in it to burn fuel, so it cannot operate above the atmosphere.

A rocket engine carries both fuel and oxidizer, so it needs no outside air. In exchange, it consumes propellant very quickly, which is why rockets are used for short, high-thrust phases such as launch while jet engines serve long flights within the atmosphere.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue