Lockheed's X-24C (L-301)
    Following NASA's successful tests of the supersonic X-24B lifting body in 1975, the Agency tried to secure funds for a National Hypersonic Research Facility, commonly known as the X-24C.  The aircraft would have used a similar planform to the X-24B, a 75-degree delta, but it would have been much faster.  NASA's in-house design studies suggested the XLR-99 rocket engine, which had been used on the X-15 when it set the absolute speed and altitude records.  Eventually the engine would be augmented with an experimental scramjet and the aircraft would reach speeds of Mach 8 at 90,000 feet.
     Although the official project was shelved in the early 1980's, the aviation historian Rene Francillion believes that Lockheed did fly a hypersonic testbed in 1982.
Lockheed's own X-24C studies were known by the internal designation "L-301."  The preferred configuration used the LR-105 rocket engine for its primary propulsion--the same LOX/kerosene engine used on the Atlas sustainer.  The aircraft would then switch to a hydrogen scramjet at design altitude.  The L301 was a monster of a hypersonic aircraft, coming in at over 74 feet long!  The L301 would have been launched subsonically from a B-52G, in the same manner as the X-15.  This presented some interesting design challenges, such as keeping the L-301 clear of the wake from the mothership's engines.
     Another variation on the L301 solved the air launch problem--the aircraft was a much smaller, single-tailed airplane, and it was launched from the back of an SR-71.  Mach 3 launches had been attempted with the D-21 drone, but the project's final flight ended in disaster when a drone suffered a control failure and crashed into its mothership.
     Thiis little-known project may have shaped our understanding of hypersonic flight, yet it is still highly classified.  Any further information that can come to light would be greatly appreciated.