What are rockets?

A rocket engine works by Newton's third law: if it expels gas backward at high speed, it receives an equal and opposite thrust forward. Unlike a jet engine, which draws oxygen from the atmosphere to burn its fuel, a rocket carries both fuel and oxidizer with it. That independence from air lets it operate in the vacuum of space, but it also forces it to carry an enormous mass: in a typical orbital launcher, more than 85% of the mass on the pad is propellant.

Solid-propellant rockets, descendants of 13th-century Chinese fireworks, mix fuel and oxidizer into a single block that burns all at once; they are simple and powerful but cannot be shut off or throttled. Liquid-propellant rockets, first flown successfully by Robert Goddard in 1926, pump two separate fluids into the combustion chamber, such as kerosene or hydrogen with liquid oxygen; they are more complex but can be throttled, shut down, and relit, which makes them essential for maneuvering and landing. Hybrids combine a solid fuel with a liquid oxidizer and occupy a small niche.

Reaching low Earth orbit requires about 7.8 km/s, plus losses from gravity and drag. No single-stage rocket achieves this with a reasonable payload, so launchers are divided into stages that separate once spent, lightening the vehicle. Sizes range from sounding rockets a few meters long to giants like the Saturn V or Starship; in all of them, the Tsiolkovsky equation relates the achievable velocity to the exhaust velocity of the gases and the fraction of mass burned.