Understanding Apoapsis and Periapsis

The Apsides

Every elliptical orbit has two special points:

Apoapsis (Ap): The highest point, farthest from planet - You're moving slowest here - Best place to change periapsis - Also called apogee (Earth), aphelion (Sun)

Periapsis (Pe): The lowest point, closest to planet - You're moving fastest here - Best place to change apoapsis - Also called perigee (Earth), perihelion (Sun)

These points define your orbit's shape and size!

Why Apsides Matter

At apoapsis and periapsis: 1. Your velocity is purely tangential (sideways) 2. No radial component (not moving toward/away from planet) 3. This makes burns most efficient!

Burning at other points wastes energy fighting the radial component.

Strategic Rule: Always plan major burns at apsis points when possible.

The Apsis Strategy Guide

Want to change apoapsis? → Burn at periapsis Want to change periapsis? → Burn at apoapsis Want to change both? → Two burns, one at each

Want to escape? → Burn prograde at periapsis Want to land? → Burn retrograde at apoapsis

Master this, and you master orbital mechanics!