There's this interesting interview with Sharon Weinberger on her book, Imagineers of War.
http://www.thespaceshow.com/show/20-aug-2017/broadcast-2969-sharon-weinberger
She says that the line between success and failure is quite fuzzy, with "failed" projects like NASP and Q nonetheless feeding on to later developments. She does however mention that DARPA lacks an internal historian, which affects its institutional memory.
Pure uninformed speculation on my part, but it may be kinda sorta perhaps possible - not proven - that if some rumours are true, a PDE powered 'Aurora' may have flown, but the acoustic effects of the PDE engines caused severe MRO problems, turning it into a very expensive and short-lived hangar queen that never saw service. In British Secret Projects: Jet Fighters Since 1950 Tony Buttler notes that
the acoustic effects of rockets on the airframe in the Fairey F.155, Saunders-Roe P.187 and Vickers Type 559 raised serious concerns. Then there's the huge exhaust ramp on NASP and many other hypersonic aircraft renderings that must have presented huge heating problems and is notably absent from the SR-72 renderings.
You don't hear much about PDEs any more, but Aerojet Rocketdyne has been investigating 'Rotating Detonation Engines':
http://aviationweek.com/print/technology/aerojet-rocketdyne-explores-detonation-engine-options
... though now they're seeking to demonstrate combined-cycle.