Firing Where the Spacecraft Is Fastest | Visual AI Learning Guide

So where do you fire? On an elliptical orbit, the spacecraft moves fastest at periapsis, the closest point, and slowest at apoapsis, the farthest. That difference is the opportunity.

0:00 / 0:00

Auto

Apollo applied the Oberth effect by placing its largest remaining burns near periapsis, the point in an orbit where the spacecraft moves fastest.

On an elliptical orbit, speed is highest at periapsis and lowest at apoapsis. A burn at periapsis therefore buys more orbital energy per kilogram of propellant.
The same principle applies to the departure and arrival geometry: burns timed when the spacecraft is moving fastest relative to the body it is leaving or approaching give the greatest velocity change for the propellant spent.
This is why burn timing, not just burn size, appears in the mission plan: the trajectory is shaped so that the critical burns occur where the spacecraft is already moving quickly.

Key point: the Oberth effect turns timing into a fuel-saving tool. The propellant saved at periapsis is propellant that stays available for the return.