Schematic drawing of the Ames Supersonic Free-Flight Tunnel.
Model, sabot, and cartridge assembly prepared for firing in the Ames Supersonic Free-Flight Tunnel.
In addition to its stable of low-speed dynamic model free-flight facilities, NASA has developed specialized ground-based free-flight testing techniques to deter-mine the high-speed aerodynamic characteristics, static stability, and dynamic stability of aircraft, Earth atmosphere entry configurations, planetary probes, and aerobraking concepts. The NASA Ames Research Center led the development of such facili-ties starting in the 1940s with the Ames Supersonic Free-Flight Tunnel (SFFT).10 The SFFT, which was similar in many respects to ballistic range facilities used for testing munitions, was a blow-down facility
specifically designed for aerodynamic and dynamic stability research at high supersonic Mach numbers. Test speeds extended from low supersonic speeds to Mach numbers in excess of 10. In this unique facility, the model under test was fired at high speeds upstream into a supersonic wind tunnel airstream (typically Mach 2). The 1-foot by 2-foot test section of the tunnel was 18 feet long and used the Ames 12-Foot Low-Turbulence Pressure Tunnel as a reservoir. Windows for shad-owgraph photography were along the top and sides of the test section. The free-flight models were launched from guns about 35 feet downstream of the test section. The launching process included the use of special sabots that transmitted the propel-ling forces of the gun to the model during launch, after which the sabots aerodynamically separated from the model. The model’s flightpath down the test range achieved high test Reynolds numbers.
Aerodynamic data were derived from motion time histories and measurements of the model’s attitudes during the brief flights. Dynamic stability characteristics could also be observed for the test article. Optical approaches for data reduction are required, because surface-mounted sensors and telemetry electronics would be destroyed, along with the model, at the terminal wall of the range. The development of the test technique and the associated instrumentation required years of work by the dedicated Ames staff. For example, the small research models had to be extremely strong to withstand high accelerations during the launch (up to 100,000 g’s) yet light enough to decelerate for drag determination while meeting requirements
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for moments of inertia. Launching the models without angular disturbances or damage required extensive development and experience. Another major issue facing the researchers was data contamination caused by oblique shock waves in the test section. To resolve this problem, the upper and lower walls of the test section were diverged to allow the flow to expand steadily and avoid discontinuous compression at the shocks.
View of the Physical Research Laboratory at Langley in 1949 showing the ill-fated Langley Free-Flight Facility in the foreground.
The 100-foot-long, 8-foot-diameter facility was equipped with viewing windows but suffered poor flow characteristics during model flights and was closed before meaningful research results were obtained.
The Supersonic Free-Flight Tunnel was completed in late 1949 and became operational in the early 1950s.11 The unique testing capabilities of this Ames facility provided valuable information on supersonic drag, lift-curve slope, pitching moment variations with angle of attack, aerodynamic damping in roll, and the location of center of pressure of test specimens.12
Efforts to develop a high-speed, free-flight apparatus had also taken place at Langley in the late 1940s at the Langley Physical Research Laboratory. The test apparatus consisted of an 8-foot-diameter tank with a length of 100 feet in which models would be propelled from a compressed-gas gun at speeds between 500 and 1,000 mph. The test mediums for the facility included air, Freon, and mixtures of the two gases. Unfor-tunately, early assessments of the free-flight test data obtained in the facility indicated severe choking and unacceptable aerodynamic contamination of results. In 1949, Langley moved its high-priority 11-Inch Hypersonic Tunnel to the same control-room site occupied by the flight facility and terminated the free-flight activities. The facility was then demolished, without producing significant technical output.
In 1958, free-flight aeroballistic testing evolved considerably with the advent of the Ames Pressurized Ballistic Range (PBR). This facility had a 203-foot-long, 10-foot-diameter test section with 24 orthogonal shad-owgraph stations and could be operated at pressures ranging from 0.1 to 10 atmospheres (providing an impressive range of achievable Reynolds number variations). Models were launched from an arsenal of guns (powder and light-gas of various sizes) into this test section. Aerodynamic testing was typically performed at velocities up to 10,000 feet/second, whereas some ablation studies were conducted up to 22,000 feet/second.
Some of the major programs and missions supported in the PBR were X-15, Mercury, Gemini, Polaris, Apollo, Viking, and NASA’s Aero-assist Flight Experiment (AFE). The PBR was last operated in 1987.
Sketch of the Ames Hypervelocity Free-Flight Aerodynamic Facility. Equipped with a shock tube for extreme Mach numbers, the facility has provided extensive supersonic and hypersonic free-flight data in a variety of investigations.
View of the Hypervelocity Free-Flight Aerodynamic Facility during recent tests in support of the NASA Orion project.
As light-gas gun technology continued to evolve and velocity capabilities continued to increase, NASA brought online its most advanced aerobal-listic testing capability, the Ames Hypervelocity Free-Flight Aerodynamic Facility (HFFAF), in 1964.
This facility was initially developed in support of the Apollo program and utilized both light-gas gun and shock tube technology to produce lunar return and atmospheric entry conditions (24,600 mph relative model velocities). At the center of the facility was a 75-foot-long test section that could be evacuated to subatmospheric pressure levels or backfilled with various test gases to simulate flight in various planetary atmospheres. The test section was con-figured with 16 orthogonal shadowgraph imaging stations and a high-precision, spatial reference wire system. At one end of the test section, a family of light-gas guns (ranging in size from 0.28 to 1.50 caliber) was used to launch aerodynamic models (at speeds up to 13,400 mph) into the test section, while at the opposite end a large shock tube could be simultaneously used to produce a counterflow-ing airstream (the result becounterflow-ing relative model velocities approaching 24,600 mph or Mach num-bers of about 30). This counterflow mode of opera-tion proved to be very challenging and was used for only a brief time from 1968 to 1971. Throughout much of the 1970s and 1980s, this versatile facility was operated as a traditional aeroballistic range, using the guns to launch models into quiescent air (or some other test gas), or as a hypervelocity impact test facility. From 1989 through 1995, the facility was operated as a shock tube–driven wind tunnel for scramjet propulsion testing. In 1997, the
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HFFAF underwent a major refurbishment and was returned to an aeroballistic mode of operation with extended light-gas gun capabilities and powder-gas guns that had been previously used in the PBR.13 It continues to operate in this mode and is NASA’s only remaining aeroballistic test facility.
In its current (circa 2009) operating configuration, the HFFAF utilizes a suite of guns (powder and light-gas) to propel models into the aerodynamic test section, wherein the shadowgraph imaging stations are used to capture the model’s flight time history. The resultant trajectory record is then used to extract critical aerodynamic parameters for the configuration being studied. In addition, infrared cameras can be posi-tioned at various stations to record model surface temperature distributions at different points along the model’s flight path. From this information, heat transfer rates and transition to turbulence locations can be inferred. Some of the major programs and missions that have been supported in the HFFAF include Apollo, Viking, Pioneer Venus, Galileo, Shuttle, International Space Station, National Aero-Space Plane (NASP), Mars Science Laboratory, and the Crew Explorations Vehicle (CEV/Orion).14 In addition, the HFFAF has been used frequently for fundamental aerodynamics testing, material testing, and sonic boom research.15
A one-quarter-scale free-flight model of the Lockheed XFV-1 VTOL tail sitter airplane being prepared for hovering tests in the return passage of the Ames 40- by 80-Foot Tunnel.
The test of the 250-pound model was the only subscale free-flight test conducted in the facility. Note the propeller-guard assembly for the counter-rotating propellers, the flight cable providing power and control inputs, and the wingtip-mounted stabilization lines.