WASHINGTON, Aug. 26, 2009 – The F-22 Raptor and F-18 Hornet fighter jets are fast, screaming through the air at twice the speed of sound, but the SR-71 Blackbird was faster, flying Mach 3 until mechanical problems and exorbitant operating costs forced it out of service in the late 1990s. Now, the Defense Advanced Research Projects Agency is striving to build an engine that will propel a hypersonic jet at Mach 4 and faster, while also bringing new efficiencies to ships and ground vehicles. DARPA's Vulcan program kicked off this spring and aims to create the supersonic capability needed to...