 The Boeing X-51 Waverider also uses JP-7 fuel to run the Pratt & Whitney SJY61 scramjet engine, with fuel capacity of some .As with the SR-71, the X-51A design super-cools this fuel (cooled by extended subsonic flight in the stratosphere; prior to acceleration to supersonic speeds); then, when in supersonic flight, the fuel is heated by its circulation through heat exchangers which transfer to it the heat load of the interior spaces of the airframe.The fuel is then pumped through rotating mechanical parts of the engines and auxiliary mechanical equipment, providing both lubrication and cooling.