ARNOLD AIR FORCE BASE, Tenn. --
From the testing of subscale to full-scale models at speeds ranging from subsonic to hypersonic, work performed over the decades in the nine wind tunnels that comprise the Arnold Engineering Development Complex Aerodynamics Test Branch has played an integral role in protecting civilians and warfighters by providing customers with valuable risk-reduction data.
Six continuous-flow wind tunnels within the AEDC Aerodynamics Test Branch are located across two facilities at Arnold Air Force Base, Tenn., headquarters of AEDC.
The Propulsion Wind Tunnel
The Propulsion Wind Tunnel (PWT) is home to the 16-foot transonic wind tunnel (16T), the 4-foot transonic wind tunnel (4T) and the recently-reactivated 16-foot supersonic wind tunnel (16S).
PWT is devoted to aerodynamic and propulsion integration test and evaluation of aircraft models. Some of the most powerful electric motors ever built are contained within the facility.
16T has a 16-foot-square by 40-foot-long test section that can be operated from Mach numbers 0.06 to 1.60. 16S, comprised of the same test section dimensions as 16T, is capable of operating from Mach numbers 1.60 to 4.75. These tunnels are used for testing that includes aerodynamic tests and combined aerodynamic/propulsion systems.
In 2020, the aerodynamics team added large-scale mass flow assembly testing to the suite of capabilities available in 16T and 16S.
4T is a mid-sized, 4-foot-square by 12.5-foot-long test section used for testing at subsonic to low-end supersonic airspeeds, a range approximately equivalent to 160 to 1,600 miles per hour at sea level.
Planning for PWT began in early 1950 when the Air Force Research and Development Board on Facilities met with representatives from aircraft propulsion companies. They concurred that a supersonic propulsion wind tunnel with a 15-foot-diameter test section was needed.
In December 1951, AEDC’s commanding general approved a proposal design, construction and operation of PWT’s transonic circuit. The entire PWT complex was completed and accepted by the Air Force by early 1961. The project, spread out over nearly 40 contracts, cost just south of $79 million.
Utilizing the trio of tunnels currently housed within, PWT provides customers with access to capabilities such as store separation testing and testing using pressure-sensitive paint. Store separation testing is done to ensure bombs, missiles, tanks, and other stores separate cleanly from aircraft when released. Store separation and flow-field mapping is completed using the Captive Trajectory Support system. During store separation testing, a model of a store can be positioned where needed for testing in proximity to the aircraft model to determine aerodynamic loads caused by the aircraft’s flow field. Pressure-sensitive paint is used to optically determine the surface pressure across wind tunnel models, with each image pixel containing pressure data to provide millions of measurements on a model as opposed to dozens or hundreds of mechanical pressure measurements on a similarly sized model.
The PWT 1-foot transonic wind tunnel
The first test in PWT was performed in 1953 in the now-inactive 1-foot prototype wind tunnel (1T) known as “Pee Wee.” This test was on a 0.03-scale model of the Bomarc missile for the Boeing Company.
The 1T served as the predecessor for the still-active 16T, which was completed in 1956 and that same year underwent its first powered operation prior to calibration.
The PWT 16-foot transonic wind tunnel
Within AEDC’s first decade, testing on the Navy Polaris and the Air Force Titan missiles was conducted in 16T. In mid-1957, the nose cone proposed for the Air Force Atlas missile was tested in 16T. Also among the first tests conducted in 16T was a program involving a model of the B-58 Hustler, the country’s first supersonic bomber.
A significant milestone occurred in 16T in December 1959 when a liquid-fueled rocket engine was tested in a closed-circuit wind tunnel for the first time. The prior year, the GAM-72 decoy system, also known as the Green Quail, was tested in 16T. This was the first combined aerodynamic-propulsion test of a full-scale missile of this type at AEDC.
Not long after tunnel operations began, 16T was utilized in support of the national space program. In April 1959, tests supporting the NASA Mercury program, America’s first human spaceflight program, were performed in the tunnel. This work involved the aerodynamic stability study of a manned capsule.
Even 1T was used for space program testing. In 1966, the Saturn 1B and Saturn V upper-stage configurations were tested there. These launch vehicles were developed to support the NASA Apollo program. Project Apollo succeeded in landing the first people on the moon in July 1969.
In the 1970s, programs carried out in 16T included full-scale B-1 inlet testing, testing on the Air-Launched Cruise Missile and testing on a full-scale model of the General Dynamics Tomahawk.
Testing in 16T occurring in the 1980s included one to determine the effect of a pilot after bailing out of an aircraft traveling several hundred miles per hour.
In the early 1990s, tests were conducted in 16T to support the European Space Agency’s Huygens probe to the Titan moon of Saturn. These tests evaluated the performance of the parachute used to slow the probe down as it descends to Titan’s surface.
In the early 2000s, store separation was conducted on a model of the EA-18G warfare aircraft. More recently, the facility has been used to perform store separation to support the B-1B Lancer.
The PWT 16-foot supersonic wind tunnel
The first run in 16S occurred in 1960. The first user test occurred the following year. Just a year after this, RS-70 inlet testing began in 16S. This was then the largest model ever tested in a wind tunnel, as it measured 75-feet long and weighed more than 200,000 pounds with support equipment.
Scale model tests of the F-111 aircraft began in 16S in 1964. The launch vehicle for Titan III was also tested in 16S, with testing completed in 16S and 16T in December 1966.
The first propulsion test in 16S occurred in 1962 during a shakedown program on the B-70 propulsion system.
In the early 1970s, testing in support of the B-1 bomber was performed in 16S.
As national priorities shifted, usage of the 16S facility declined at it was mothballed in the 1990s. However, efforts to return the tunnel to service began several years back. The tunnel returned to operational status early this year.
The PWT 4-foot transonic wind tunnel
4T was added to the PWT in 1968. This tunnel was built to aid in the development of new weapons and to ensure stores could be safely released from high-speed aircraft.
The concept of a 4-foot transonic wind tunnel dates back to the early 1960s. Officials at Eglin Air Force Base, Florida, where store separation is managed, had determined that in the midst of the jet age, the development of new weapons that could safely separate from aircraft and strike the intended targets was critical.
In 1965, 1st Lt. Roland H.A. West, with the help of others in the U.S. Air Force Development Division and AEDC personnel, prepared and submitted a brochure which detailed the need for a new wind tunnel and the plans for its construction at Arnold AFB. In the document, West wrote the development of a 4-foot wind tunnel would provide an economical capability for conventional weapons development.
According to West, weapons of the time would need to be redesigned and new ones developed for use in subsonic and transonic speed regimes to meet then-current and projected requirements.
At the time, 16T and 1T were the only two tunnels in these desired speed ranges AEDC had at its disposal. West pointed out issues with both. He wrote that 16T was designed for testing large- or full-scale articles. The testing West had defined would not require a tunnel of this size. Other concerns including the cost to operate 16T and its availability, as the tunnel was booked for the foreseeable future which would make the rapid testing of weapons difficult. West further indicated 1T would be too small to meet testing requirements.
Planning and discussions led to the development of 4T to study the separation of stores from aircraft. The first user test in the tunnel began in January 1968 and was conducted at the request of the Air Force Armament Laboratory. It involved the testing of a Hard Structure Munition missile model at transonic speeds.
The first store separation test at 4T was conducted in April 1968. Tests were conducted to determine the dynamic and static stability characteristics of the AGM-12E missile and to investigate the separation characteristics of the missile from the inboard wing pylons of the F-105 aircraft. This test determined that the store would separate from the inboard pylon without jeopardizing the aircraft.
Within its first 10 years, 4T had been used in a number of test programs for the Air Force, Army, Navy and NASA. These tests supported the development of stores and certified their use with numerous aircraft, including the F-105, F-4, A-7 and A-10.
In the 1980s, testing was performed in 4T to determine the ability of the F-16 to jettison both a new external fuel tank and the pylons by which they are attached to the plane’s wings. The ability of the AV-8 Harrier “jump jet” to carry and deliver a new munition was also studied in 4T in the 1980s.
Testing conducted in 4T in the fall of 1999 satisfied requirements for later flight testing of the F-22 Raptor. The following year, the tunnel was used to prepare the Unmanned Combat Air Vehicle for upcoming flight tests.
In 1989, the American Society of Mechanical Engineers designated PWT an International Historical Mechanical Engineering Landmark.
The von Kármán Gas Dynamics Facility
VKF is home to the mid-sized continuous-flow wind tunnels known as Tunnel A, Tunnel B and Tunnel C.
Recommendations for what was originally known as the Gas Dynamics Facility were approved by the Air Force Engineering Development Division commanding general in early 1951. The first developmental-type test occurred in the facility a little more than two-and-half years later. The facility was dedicated as the von Kármán Gas Dynamics Facility in October 1959 to honor mathematician, physicist and engineer Dr. Theodore von Kármán.
Tunnel A is a supersonic wind tunnel with a 2D variable flex-wall nozzle. Tunnels B and C are the only operational continuous-flow hypersonic wind tunnels in the world, utilizing fixed Mach number nozzles. Both VKF and PWT wind tunnels are renowned for their flow quality and operational tempo that allows rapid acquisition of large aerodynamic databases to accelerate the pace of vehicle development.
Each VKF tunnel is equipped with a model inject system that allows continuous pitch and roll sweeps of the test articles. These systems also provide a means to remove test articles from the flow so that changes to the test articles can be performed during air-on operation.
The first test in Tunnel A was performed in June 1958, just two years after the Air Force turned the tunnel over to the Arnold Research Organization, which had been selected to manage AEDC operations, to begin shakedown operations. This initial effort involved testing for the Thor weapons system.
The tunnel was also used in the early days of AEDC to test a model of the Air Force Atlas missile.
Like PWT, VKF played a part in helping to get Americans in space and to the moon. Tests supporting the NASA Apollo program began in VKF in June 1962. Apollo command module tests were completed in Tunnels B and C in July 1966.
Years after the first men landed on the moon, Tunnel A was still utilized to aid in space exploration efforts. In the mid-1970s, a model of the complete Space Shuttle assembly was tested in the tunnel. This design of the Space Shuttle was aided by testing in facilities across Arnold, including those in PWT. Materials testing of the spray on foam insulating the shuttle’s external tanks occurred in the 80s, 90s, and early 2000s in VKF’s Aerothermal Mach 4 Tunnel C. Testing in PWT and VKF also supported conclusions made by the Columbia Accident Investigation Board in the aftermath of the ill-fated STS-107 mission.
Along with these efforts, testing on the Poseidon ICBM reentry vehicle was conducted in Tunnels A, B and C in 1967. An upgraded version of the Patriot air and missile defense system was among systems tested in Tunnel A during the 1990s.
Testing to support the Defense Advanced Research Project Agency Operational Fires program was completed in VKF in October 2020. The goal of this program was to develop and demonstrate a novel ground-launched system enabling hypersonic boost-glide weapons to penetrate modern enemy air defenses and rapidly and precisely engage critical time-sensitive targets.
One of the three remaining tunnels under the Aerodynamics Test Branch umbrella is located at AEDC Hypervelocity Wind Tunnel 9 in White Oak, Maryland. The other two are located at the National Full-Scale Aerodynamics Complex at Moffett Field in Mountain View, California. For more information on these facilities, see the article titled “AEDC expands footprint from coast-to-coast over past 24 years” at https://www.arnold.af.mil/News/Article-Display/Article/2663705/aedc-expands-footprint-from-coast-to-coast-over-past-24-years/.