Glossary / Space
Rocket equation
The equation stating that a rocket's gain in speed depends on its exhaust velocity and on the ratio of its starting mass to its burnt-out mass.
In 1903 the Russian teacher Konstantin Tsiolkovsky showed that a rocket's change in velocity equals the exhaust velocity times the logarithm of starting mass divided by final mass[1]. Because the mass ratio enters through a logarithm, a small gain in speed demands a steep rise in fuel.
Multi-stage rockets that discard empty tanks, and ion engines with very high exhaust velocity, are ways of easing this constraint.
Related terms
- Ion engineA thruster that ionises a gas such as xenon and accelerates it electrically to high speed. Its thrust is tiny, but it uses propellant extremely efficiently.
- Hohmann transferMoving between two circular orbits by riding an ellipse tangent to both, with just two burns — a fuel-efficient way to transfer.
On the timeline
Sources
- K. E. Tsiolkovsky, “Исследование мировых пространств реактивными приборами (Exploration of Outer Space by Means of Rocket Devices),” Научное обозрение (Nauchnoe Obozrenie) 5 (1903).