A plane change is a manoeuvre that tilts a satellite's orbital plane — changing its inclination or node. It is notoriously expensive: rotating a low-Earth orbit by 45° costs nearly as much delta-v as launching from the ground, which is why missions go to great lengths to launch into the right plane.
Understanding Plane Change
Why rotating velocity is so dear
An altitude change works with the orbit — prograde and retrograde burns add or remove energy along the direction of motion, so every metre per second counts fully. A plane change fights the orbit: the spacecraft must cancel part of its enormous sideways velocity and rebuild it in a new direction, and for angles beyond a few degrees the arithmetic approaches "stop and start again". The sin(θ/2) formula makes small corrections merciful — trimming half a degree of insertion error costs tens of m/s — but grows without mercy: at 60° the manoeuvre costs exactly the orbital velocity. Hence the planner's hierarchy: prevent (launch into the right plane), combine (fold rotation into an existing burn at the slowest point), precess (let oblateness drift the node), and only then, reluctantly, pay.
Plane geometry as strategic constraint
The cost of plane changes explains patterns visible all over this site's trackers. Space stations receive visitors only from launches timed to their plane — a pad passes under a station's orbital plane roughly once or twice a day, defining instantaneous launch windows. Mega-constellations budget a launch per plane and rebalance within planes by phasing, never across planes by burning. Sun-synchronous satellites exploit precession permanently, their orbits designed so the free nodal drift tracks the Sun year-round. And rideshare missions to a shared orbit leave secondary payloads stuck with the primary's plane — a constraint that has pushed orbital-transfer vehicles and last-mile services into existence precisely to sell small plane adjustments that the payloads cannot afford themselves.