Since the last time a Pulse Detonation Engine delta-winged supersonic aircraft was observed heading to California across land was in December 1999, perhaps the propulsion system has fallen out of favor.

It could be that this propulsion won't be brought back until manned space travel beyond low Earth orbit returns to the space program. An engineer at an air show in Arizona, who works at Edwards AFB for a defense contractor explained to me that PDE was not very efficient for use on aircraft.