DARPA stands for "Defense Advanced Research Projects Agency."

-Among its projects is the Vulcan engine, a ramjet-scramjet capable of speeds exceeding Mach 6 on Lockheed Martin's HTV-3x.

"The HTV-3x in an image taken from a video"

-DARPA's main "hypersonic" vehicles are:
   -HCV, or Hypersonic Cruise Vehicle.
   -HWS, or Hypersonic Weapon System.
   -The HTV, perhaps the most recently published, has the HTV-2 version and the HTV-3x currently    in testing.

-It begins testing on a B-52, which takes it to its maximum altitude. Then a vehicle detached from the previous one, a Pegasus type, carrying it in its bow, launches it, taking the HTV-3x to greater heights and speeds.

-At that moment, the Vulcan engine comes into operation, which is a mixture of ramjet and scramjet, also known as a CVC or Constant Volume Combustion engine.