What is a space rocket?

A space rocket is, in essence, a reaction engine with tanks and a payload on top. The combustion chamber and nozzle generate thrust; the tanks store fuel (kerosene, hydrogen, methane) and oxidizer (liquid oxygen in almost every case); the structure ties the whole vehicle together and the payload fairing protects the payload during the pass through the atmosphere. A guidance system, with gyroscopes and computers, steers the flight by aiming the nozzles or small auxiliary thrusters.

Gunpowder rockets have existed since medieval China, but the space rocket was born with the theory: Konstantin Tsiolkovsky formulated the equation relating velocity, mass and propellant in 1903, and Robert Goddard launched the first liquid-fueled rocket in 1926. Germany's V-2, in 1944, was the first object to pass 100 km in altitude. After the war, the Soviet R-7 placed Sputnik into orbit on October 4, 1957, and the Saturn V carried the Apollo astronauts to the Moon between 1968 and 1972.

An airplane needs air both to generate lift and to burn fuel; a rocket needs neither. That's why an airplane flies at most around 20 or 30 km of altitude, while a rocket crosses that zone in just over a minute and keeps accelerating in vacuum. The price is inefficiency: an orbital rocket burns several hundred tonnes of propellant in nine minutes to place a few dozen tonnes into orbit. The reusable launchers of the past decade try to bring back at least part of that investment to fly again.