The Oberth effect states that a rocket burn produces a larger change in the spacecraft's kinetic energy when the burn is performed at high speed than when the same burn is performed at low speed.
- Kinetic energy depends on the square of speed: \(E_k = \tfrac{1}{2}mv^2\). A fixed velocity change \(\Delta v\) added at a higher speed therefore changes \(E_k\) by a larger amount.
- Equivalently, the same propellant produces a larger change in orbital energy when the engine fires while the spacecraft is moving fastest.
- Practical consequence: for a given mission goal, burning near the fastest point of an orbit — periapsis — requires less propellant than burning near the slowest point — apoapsis.
Key point: the Oberth effect is not a new force. It is a consequence of how kinetic energy depends on speed, and it makes burn timing a fuel-saving decision.