What is a rocket?

Short answer

A rocket is a vehicle or engine propelled by expelling the gases of its own combustion at high speed, without drawing in outside air: it carries its own fuel and oxidizer on board, which is why it works in the vacuum of space. It relies on Newton's third law, action and reaction, and is today the only means of putting objects into orbit.

Published: September 21, 2026 · Updated: September 21, 2026 · Edited by Above Ten Thousand · Editorial policy

RS-68 rocket engine test

Action and reaction

Rocket physics fits within Newton's third law: by expelling mass backward at high speed, the vehicle receives an equal and opposite thrust forward. Unlike a jet engine, which draws oxygen from the atmosphere, a rocket carries its own oxidizer, whether liquid oxygen, nitrogen tetroxide, or one mixed into a solid propellant. That's why it's the only propulsion system that works in a vacuum and the only one capable of reaching orbital velocity, about 7.8 km/s in low Earth orbit.

Types of propulsion

Solid-propellant rockets, like the strap-on boosters of the SLS or Ariane 6, are simple and powerful, but cannot be shut off once ignited. Liquid-propellant rockets, like the Falcon 9's Merlin engines or Starship's Raptors, can be throttled and restarted, which is why they dominate main stages and landing stages. Hybrids combine a solid with a liquid oxidizer, and electric thrusters, with tiny but highly efficient thrust, move satellites already in orbit.

Stages and orbits

An orbital launch vehicle is divided into stages that separate as their tanks empty, so the rocket doesn't carry useless mass. The first stage overcomes gravity and dense air; the second accelerates the payload to orbital velocity. Since 2015 some first stages have returned and landed to fly again. The same principle, in a much smaller version, powers everything from signal flares to amateur rockets, and in giant version, vehicles over 100 m tall that carry people and satellites to space.