Propulsion system with movable thermal choke

High thrust at sea level and high Isp at high altitudes with low ambient pressure are obtained in a vehicle with an engine of fixed geometry including an exit of non-changing cross section, by incorporating a series of propellant injectors in a divergent section of the engine, spaced apart along the engine axis at points of different engine cross section. At takeoff, the injectors at sites with a large cross section are used, and as the vehicle climbs, these injectors are successively closed leaving injectors activated at sites of successively smaller cross sections. The injectors establish thermal choke points, and the area ratio of the cross section at the exit to the cross section at the thermal choke point thus increases as the vehicle climbs from low to high altitudes.