Engine contracts were let to Pratt and Whitney, Rocketdyne, and Aerojet. The engine specification called for a Lox/LH2 engine with a bell nozzle, capable of gimballing plus/minus 7 deg, producing 188,000 kgf at sea leval and 216,000 kgf at altitude. The booster engines would be equipped with a 6:1 expansion nozzle, and the orbiter with a two-position nozzle to bring the expansion ratio up to 120:1 at altitude. The engine had to throttle between 50% and 115% of the rated thrust (the latter rating for abort engine-out situations). The engine was to be equipped with a digital engine controller and be compact and reusable. References: 44.