Week 7: Balance Point: CG vs. CP

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A rocket that leaves the pad straight doesn't stay straight by luck — it's because of where its weight sits compared to where the air pushes on it.

History

As rocketry matured beyond simple weapons, engineers realized that enough thrust wasn't enough on its own — a rocket also needed to fly straight, which meant understanding two invisible points on its body and how they relate to each other.

Forces

Every rocket has a Center of Gravity (CG), where its weight balances, and a Center of Pressure (CP), where the aerodynamic forces from the air effectively act. For a rocket to correct itself back toward straight flight instead of tumbling, CG has to sit ahead of CP — closer to the nose.

Math

margin (calibers) = (CP − CG) / body diameter

Worked example (metric): if CP sits 40 cm from the nose, CG sits 30 cm from the nose, and the body diameter is 2.5 cm: margin = (40 − 30) / 2.5 = 4 calibers — comfortably above the safe minimum.

Safety Note

NAR guidance requires a minimum stability margin of at least 1 caliber before a rocket is considered flight-ready. We'll come back to exactly how CP itself gets calculated in Week 16.