The X-43A, or "Hyper-X" was built specifically to test scramjets, or supersonic ramjets, which burn fuel in air travelling at supersonic speeds.

BBC: Mach 10 scramjet prepares for launch

The U.S. Air Force is currently test-flying an optionally piloted supersonic drone over the Gulf of Mexico based on a modified F-16 fighter jet.

FORBES: Pilot-Optional Flight Is Closer Than You Think