During the 1970s the name of the fighter game was maneuverability, and aircraft manufacturers needed to develop new concepts that would make their fighters more nimble than those currently in service.
The legendary F-4 Phantom II had been a workhorse in Vietnam, yet it lacked maneuverability in a dogfight. Fly-by-wire control systems developed through the various Control Configured Vehicle programs conducted by the Air Force Flight Dynamics Laboratory added flexibility to modern designs.
In addition, the use of graphite-epoxy composites that are just as strong as their metallic counterparts, but less weight and more flexible, gave manufacturers the options required to create designs that could meet the new requirements set forth by the Air Force for added maneuverability.
Through the combination of relaxed static stability offered by fly-by-wire controls and construction with the use of graphic-epoxy composite materials, the designs coming off the drawing boards exceeded the known limits of pilots and crew. In order to test a new design to the proposed limits required the use of a unpiloted vehicle capable of supersonic flight and 8G plus sustained turns with superior maneuverability.
The Air Force Flight Dynamics Laboratory Vehicle Dynamics Division teamed with NASA to create requirements for such a vehicle. While submissions from Grumman, Douglas Aircraft and Rockwell all showed great promise, in August 1975, the Air Force awarded the $17.3 million contract to Rockwell to build two 44-percent scale, remotely-piloted vehicles under the Highly Maneuverable Aircraft Technology, or HiMAT, program. Rockwell chose the General Electric J85-21 jet engine with a digital control system replacing the standard hydromechanical engine control system to power the small test aircraft. To reduce complexity the HiMAT aircraft used skid landing gear in place of traditional wheels similar to the X-15.
Rockwell constructed the aircraft with a modular design giving the capability of changing the configuration. A proposed two-dimensional, thrust vectoring, engine exhaust nozzle had been test-fit in mockup form, but never built. The highly instrumented Remotely Piloted Research Vehicles were air-launched from NASA’s venerable NB-52B Mothership and flown remotely from a ground station with emergency backup controls in the aft seat of a TF-104G Starfighter chase aircraft. The advanced technologies incorporated into the new vehicles were a close-coupled canard planform, aeroelastic design with composite structures and relaxed static stability, with a secondary objective to evaluate the smaller RPRV design in comparison with a hypothetical full-scale vehicle.
An important feature of HiMAT was the flight test maneuver autopilot system. The FTMAP linked with two ground-based computers that allowed preprogrammed maneuvers, such as a constant Mach windup turn, pushover/pullups, and thrust-limited turns, that were implemented as an outer-loop command bypassing the pilot stick. The first HiMAT, RPRV 870, was devoted primarily to envelope expansion and design point demonstrations, while its sister ship, RPRV 871, was used for research data collection. An important goal of the program was the transonic maneuverability point of 8G sustained at Mach 0.9 and 25,000 feet. The supersonic endurance point goal was to sustain a 3G turn for 3.5 minutes at Mach 1.4 and 40,000 feet.
The flight test program was carried out at NASA’s Ames Research Center/Dryden Flight Research Facility located at Edwards, Calif. From October 1981 to March 1994, Dryden Flight Research Center had been merged with Ames Research Center, becoming the Ames-Dryden Flight Research Facility, and currently named the NASA Armstrong Flight Research Center.
During an unveiling ceremony at Rockwell’s Los Angeles facility in March 1978, the company revealed the first HiMAT vehicle to a group of VIPs and media. Shortly after the official ceremony, Rockwell loaded the HiMAT on to a flatbed trailer and trucked the vehicle to ADFRF, arriving on March 10, 1978. The second airframe arrived just three months later on June 15.
The following week, on March 16, 1978, RPRV 870 performed the first fit check attached to the NB-52B Mothership. Utilizing a special adapter, HiMAT made use of the same NB-52B wing pylon that once dropped the X-15, Lifting Bodies and other test articles. Taken aloft for the first time on July 11, 1979, during a planned captive flight, this first HiMAT test had to be aborted due to telemetry and aircraft problems encountered due the mission. A second captive flight took place on July 20, with all test objectives met, the first free flight was scheduled for the following week.
The first free flight for HiMAT took place on July 27 with NASA test pilot Bill Dana flying HiMAT from the ground station. With all objectives met, the small test vehicle performed a successful landing on Rogers dry lakebed at Edwards. NASA engineers spent the next few months installing instrumentation into RPRV 870 prior to its second flight on Dec. 21. Testing proceeded well until the fifth flight on July 8, 1980, when the decoder failed 5 minutes into the flight and control switched to backup pilot in the aft seat of the TF-104G. A glitch in the latest software update prevented landing skid deployment and HiMAT performed an emergency gear-up landing on the lakebed. With minimal damage, the team repaired the aircraft and it took to the air once more on Oct. 10.
The second HiMAT, RPRV 871, joined the flight test program on June 25, 1981 making its first captive flight, and first free flight performed a month later on July 24. RPRV 870 performed the first 8G maneuver demo during its tenth flight on Feb. 3, 1982, followed by the first supersonic flight to Mach 1.2 during the next flight on May 11, with NASA test pilot Steve Ishmael at the controls. During next flight on May 14, HiMAT flew to a maximum Mach number of 1.45.
Research flights continued throughout 1982 with ship 1 making its 14th, and final, flight on Aug. 27, while ship 2 carried on until making its 12th, and final, flight on Jan. 12, 1983, with test pilot Einar Enevoldson in the remote cockpit station. The average flight time for each HiMAT flight was approximately 30 minutes. At the end of the test program, HiMAT 870 had made 14 flights with a total flight time of 11 hours and 35 minutes, while 871 made 12 flights for a total of 10 hours and 57 minutes.
HiMAT’s contributions appear to be mixed. The system’s complexity was greater than predicted, flight operations more labor intensive, and the subscale size made it restrictive in many aspects. Yet, its accomplishments were impressive, sustained 8G turns at near supersonic speed, and supersonic endurance surpassed the design goals with aerodynamics as good, or better, than predicted. One HiMAT engineer stated that Dryden engineering section disliked seeing HiMAT on the flight schedule because it took virtually the entire pilot’s office to fly a mission; two in the NB-52B, one in the RPRV ground station, two in the TF-104G safety chase and one more in a second chase aircraft as required, bringing about the joke of ‘how many pilot’s does it take to fly an unmanned aircraft?’
With the flight test portion completed, NASA placed the first aircraft in storage while the second participated in loads testing. Flexibility and strength of the composite wing structure tested to the point of failure provided data on future construction techniques of composite aircraft. After testing HiMAT 871, NASA technicians repaired the aircraft and placed it in storage with its sister ship. Eventually, HiMAT 870 found a place of honor in the Smithsonian Air & Space Museum in Washington, D.C. RPRV 871 eventually ended up on outdoor display at Ames Research Center at Moffett Field, Calif. It remained there until 2009, when it returned to Dryden, restored to its original colors, and placed on display outside of the center.
A number of contributions in fly-by-wire controls and design, manufacturing and use of advanced composites from the HiMAT program ended up in follow-on design programs such as the Advanced Design Composite Aircraft, Advanced Fighter Technology Integration, Rockwell’s Tactical Interceptor, Ground Attack & Reconnaissance, as well as the Advanced Tactical Fighter program that became the Lockheed F-22 Raptor.