The upper stage, derived from the Shuttle's External Tank (ET) and based on the S-IVB stage of the Saturn V, was to be propelled by a single J-2X rocket engine fueled by liquid hydrogen (LH2) and liquid oxygen (LOX). The J-2X was derived from the original J-2 engine used during the Apollo program, but with more thrust (~294,000 lbf) and fewer parts than the original engine. On July 16, 2007, NASA awarded Rocketdyne a sole-source contract for the J-2X engines to be used for ground and flight tests. Rocketdyne was the prime contractor for the original J-2 engines used in the Apollo program.