Executing a Burn

This tutorial circularizes a vehicle’s orbit at its apoapsis. The script plans the burn, and the vehicle’s flight computer executes it, as when the player plans a burn in the game. The vehicle needs an engine and the propellant for the burn.

The complete script is circularize.py.

Planning the Burn

The vis-viva equation gives the speed at apoapsis now, and the speed of a circular orbit there. We plan a prograde burn of the difference at the apoapsis:

mu = orbit.body.gravitational_parameter
r = orbit.apoapsis
v1 = math.sqrt(mu * (2 / r - 1 / orbit.semi_major_axis))
v2 = math.sqrt(mu / r)
burn = vehicle.add_burn(ksa.ut + orbit.time_to_apoapsis, prograde=v2 - v1)
print(f"Circularization burn: {burn.delta_v:.1f} m/s, {burn.burn_time:.1f} s")

The flight computer works out the burn time from the vehicle’s engines, and centers the burn on its time.

Executing the Burn

We warp to 30 seconds before ignition, and turn on the auto-burn. The flight computer points the vehicle along the burn, fires the engines at the ignition time and stops once the burn is complete, and then turns the auto-burn off. An event, a server side function, waits for that:

ksa.warp_to(burn.ignition_time - 30)

print("Executing the burn")
auto_pilot.auto_burn = True
finished = conn.add_event(lambda: not auto_pilot.auto_burn)
with finished.condition:
    finished.wait()
burn.remove()

The flight computer keeps the last burn it completes, so we remove it.