Orbital Maneuvers: Slow Down to Catch Up

Lesson 8 of 10 · 7 min · Free, no account needed

What you will learn

A forward (prograde) burn raises the far side of your orbit; a backward (retrograde) burn lowers it.
Lower orbits are faster round, so to catch up you brake, coast, then burn forward again.
A Hohmann transfer is half an ellipse touching both orbits, with one burn at each end.

You're in orbit behind a space station. You fire your engine forward... What happens?
Burn forward and watch two laps.
So brake instead. How hard, to meet the station after exactly one lap?
Maneuvers and the Hohmann transfer
A prograde burn (along your motion) raises the opposite side of your orbit. A retrograde burn lowers it.
To change orbit cheaply, use a Hohmann transfer: burn forward, coast half an ellipse to the other orbit, burn forward again to make it round.
The trip takes half the period of the transfer ellipse.
Draw the transfer path from your ship to the outer orbit.
A burn changes the orbit everywhere except where you burn. The highest point ends up opposite the burn, half a lap later.
Game: Docking. The station is 30° ahead. Dock using as little fuel as you can.
Minus brakes, plus speeds up. You have 4 laps and 3 burns.
Code: write the docking plan as commands, then Check it.
Each brake sets how much faster your orbit is; the number of coasts sets how long you gain. Then the matching forward burn makes your orbit round again, next to the station.
Where should you burn to arrive at the marked spot on the outer orbit?
The transfer ellipse touches the outer orbit opposite the burn point, half a lap later.
Three ways to dock. Tap them to compare fuel and time.

Lessons are interactive and need JavaScript. Turn it on to play them.