Jet engines top out at around Mach 3. In 1990, the SR-71 set a speed record, flying at Mach 3.3 from Los Angeles to Washington, D.C., in barely more than an hour. To go faster than that, the SR-72 will use a scramjet, which takes super-compressed air, combines it with fuel, ignites it, and jets it out the back, sending the craft forward at supersonic or near-supersonic speeds. Scramjets go really, really fast, but because they need super-compressed air to work, they have problems at lower speeds. To solve this low-speed problem, Lockheed proposes a dual engine that shares a common air intake. A regular jet engine provides thrust from takeoff to Mach 3, and a scramjet takes over after that, propelling the plane from supersonic to hypersonic.