A catapult seat ejection system for ejecting a pilot and seat from an aircraft is disclosed in which the thrust of the catapult is controlled to prevent injury to the pilot due to acceleration forces. The catapult comprises a main catapult tube; a piston within the tube driven by gas pressure from the burning of two solid propellants cartridges; and four shuttle vent valves powered by gas from a plurality of electrically ignited squibs controlled by an electronic control system. The catapult is initially driven by only one of the two solid propellant cartridges and the electronic controller continuously monitors acceleration. If acceleration is too great the controller fires squibs with a control signal to open the shuttle vent valves to vent the gasses from the catapult and thereby reduce the pressure and acceleration of the seat. If the acceleration is too low and the vent valves are closed, and the sensor fires the second cartridge with a second control signal.
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