Instant expert

A rocket engine profits from one basic principle: as it propels material in one direction, it receives a kick in the other. Whether it’s a child on ice skates who throws a stone forward and finds themselves gliding backwards or a modern rocket leaving Earth’s surface, the physics of conserving momentum underwrites the drama.

“ EVEN A SLOW LEAK OR EVAPORATING WATER HAVE PROVIDED A SMALL KICK TO NUDGE A CRAFT FROM ITS INTENDED COURSE”

Does the rocket need to have a flame to be effective? Not in principle. Some smaller thrusters have simply puffed vapour away from the direction they want to travel. Even a slow leak or evaporating water have provided a small kick to nudge a craft from its intended course. But a launch-worthy rocket engine uses what engineers have sometimes called a ‘controlled explosion’ to provide an incredible thrust. This is not because it ejects heavy objects, but because it ejects a furious stream of high-speed molecules.