Two chamber engine built for X-2 rocketplane. Engine could be throttled continuously from 1140 kgf to 6820 kgf. Upper chamber was 2270 kgf and lower chamber 4540 kgf. Duration 175 seconds at full power or 650 seconds at 1140 kgf. Bell was to have provided the engine, but their contract was cancelled due to cost/technical problems. The X-2 engine was developed by Goddard's rocket team at the Naval Engineering Experiment Station at Annapolis, Maryland. During 1942-1945 the team had developed a variable-thrust rocket motors. This required hundreds of proving-stand tests, but eventually producing a successful motor, later used on the Bell X-2 rocket plane.