Week 10: Burnout Velocity

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Powered flight ends the instant the motor runs out of propellant. The velocity a rocket has at that exact moment — burnout velocity — determines almost everything about the rest of its flight.

History

Postwar decades brought steady refinement to solid rocket motor manufacturing, making motor performance far more consistent and predictable — which is exactly what let engineers (and, eventually, hobbyists) start doing reliable performance math instead of just guessing.

Forces

This week's idea is the impulse-momentum theorem: the total impulse a motor delivers — force applied over time — equals the change in the rocket's momentum.

Math

J = F·dt = m·Δv

Worked example (metric): a motor with a total impulse of 10 N·s on a rocket with a mass of 0.1 kg gives a burnout velocity change of Δv = J/m = 10/0.1 = 100 m/s.

Safety Note

Matching a motor's impulse class to the airframe's mass isn't just about performance — it's a safety requirement. Too much impulse for too light an airframe risks structural failure or unsafe flight speeds.