The Solid Propellant Motor

A solid motor is the oldest rocket and still the simplest: fuel and oxidiser mixed together in one casting, lit at one end, burning until it runs out. It has no pumps, no valves, no plumbing and almost nothing to go wrong — which is exactly why it is used for boosters, ejection seats and every application where the engine sits unattended for years and must work on the first command. What it cannot do is throttle, restart or stop. Once lit it delivers its entire impulse and nothing will interrupt it. The design freedom is therefore entirely in the GEOMETRY of the propellant grain: because burning proceeds perpendicular to the exposed surface everywhere at once, the shape of the hole down the middle determines how the thrust varies with time, and that shape is chosen before the motor is ever cast.