During the early 1960s, the USAF Flight Dynamics Laboratory (FDL) devised new and unconventional shapes for hypersonic vehicles. The shapes - FDL-5, FDL-6, FDL-7 and FDL-8 - were designed with the intention of sustained hypersonic flight (both gliding and powered) and re-entry. Even at hypersonic speeds, they were capable of lift-to-drag ratios as high as 3:1. The tail and fin arrangements were all different, but they all shared one characteristic: They were all 75-degree triangles.
In the late 1960s, Lockheed and the USAF Flight Dynamics Laboratory built a full-scale mockup of a hypersonic research vehicle using the FDL-5 shape. This configuration used a stabilization technique called "compression sharing" and featured flip-out wings to reduce its landing speed and retractable farings in front of the split windshield. Fuel for initial acceleration was to be carried in two conformal tanks that fitted around its nose like a collar.
The USAF FDL projects paved way for future projects involving orbital, transatmospheric and hypersonic flight. These projects include the Transatmospheric Vehicle (TAV) and the Maneuvering Re-entry Research Vehicle (MRRV).
