Abstract: A propellant casting apparatus for automatically filling rocket propulsion motors with semi-solid propellant is characterized by a unique propellant filling assembly which injects a precise given quantity of propellant into the motor from the bottom. The propellant filling assembly includes a chamber having an injection cylinder arranged at one end and a fill valve at the other. With the chamber filled with propellant, the fill valve is brought into engagement with the bottom of the motor and opened. The injection cylinder includes a piston which is extended into the chamber to inject propellant into the motor via the valve. The length of travel of the piston is accurately controlled to govern the precise quantity of propellant injected into the motor.