A Larger Footprint: Reentry Vehicles and Lifting Bodies

The NACA and military visionaries initiated early efforts for the X-15 hypersonic research aircraft, in-house design studies for hypersonic vehi­cles were started at Langley and Ames, and the Air Force began its X-20 Dyna-Soar space plane program. The evolution of long, slender config­urations and others with highly swept lifting surfaces was yet another perturbation of new and unusual vehicles with unconventional aero­dynamic, stability, and control characteristics requiring the use of free – flight models for assessments of flight dynamics.

In addition to the high-speed studies of the X-15 in the Ames super­sonic free-flight facility previously discussed, the X-15 program spon­sored low-speed investigations of free-flight models at Langley in the Full-Scale Tunnel, the Spin Tunnel, and an outdoor helicopter drop model.[495] The most significant contribution of the NASA free-flight tests of the X-15 was confirmation of the effectiveness of the differential tail for control. North American had followed pioneering research at Langley on the use of the tail for roll control. It had used such a design in its YF-107A aircraft and opted to use the concept for the X-15 to avoid aile­rons that would have complicated wing design for the hypersonic air­craft. Nonetheless, skepticism existed over the potential effectiveness of the application until the free-flight tests at Langley provided a dra­matic demonstration of its success.[496]

In the late 1950s, scientists at NASA Ames conducted in-depth studies of the aerodynamic and aerothermal challenges of hypersonic reentry and concluded that blunted half-cone shapes could provide ade­quate thermal protection for vehicle structures while also producing

a significant expansion in operational range and landing options. As interest in the concept intensified following a major conference in 1958, a series of half-cone free-flight models provided convincing proof that such vehicles exhibited satisfactory flight behavior.

The most famous free-flight model activity in support of lifting body development was stimulated by the advocacy and leadership of Dale Reed of the Dryden Flight Research Center. In 1962, Reed became fasci­nated with the lifting body concept and proposed that a piloted research vehicle be used to validate the potential of lifting bodies.[497] He was par­ticularly interested in the flight characteristics of a second-generation Ames lifting body design known as the M2-F1 concept. After Reed’s convincing flights of radio-controlled models of the M2-F1 ranging from kite-like tows to launches from a larger radio-controlled mother ship demonstrated its satisfactory flight characteristics, Reed obtained approval for the construction and flight-testing of his vision of a low – cost piloted unpowered glider. The impact of motion-picture films of Reed’s free-flight model flight tests on skeptics was overwhelming, and management’s support led to an entire decade of highly successful lift­ing body flight research at Dryden.

At Langley, support for the M2-F1 flight program included free – flight tow tests of a model in the Full-Scale Tunnel, and the emergence of Langley’s own lifting body design known as the HL-10 resulted in wind tunnel tests in virtually every facility at Langley. Free-flight test­ing of a dynamic model of the HL-10 in the Full-Scale Tunnel demon­strated outstanding dynamic stability and control to angles of attack as high as 45 degrees, and rolling oscillations that had been exhibited by the earlier highly swept reentry bodies were completely damped for the HL-10 with three vertical fins.[498]

In the early 1970s, a new class of lifting body emerged, dubbed "racehorses” by Dale Reed.[499] Characterized by high fineness ratios, long pointed noses, and flat bottoms, these configurations were much more efficient at hypersonic speeds than the earlier "flying bathtubs.” One Langley-developed configuration, known as the Hyper III, was evalu­ated at Dryden by Reed and his team using free-flight models and the

mother ship test technique. Although the Hyper III was efficient at high speeds, it exhibited a very low lift-to-drag ratio at low speeds requiring some form of variable geometry such as a pivot wing, flexible wing, or gliding parachute.

Reed successfully advocated for a low-cost, 32-foot-long helicopter- launched demonstration vehicle of the Hyper III with a pop-out wing, which made its first flight in 1969. Flown from a ground-based cock­pit, the Hyper III flight was launched from a helicopter at an altitude of 10,000 feet. After being flown in research maneuvers by a research pilot using instruments, the vehicle was handed off to a safety pilot, who safely landed it. Unfortunately, funding for a low-cost piloted project sim­ilar to the earlier M2-F1 activity was not forthcoming for the Hyper III.