English: This 35-second video taken in 1991 shows the NASA SR-71 Blackbird being refueled during a test flight by a U S Air Force tanker.

Two SR-71A aircraft were loaned from the U.S. Air Force for use for high-speed, high-altitude research at the NASA Dryden Flight Research Center, (now Armstrong) Edwards, California. One of them was later returned to the Air Force. A third SR-71 on loan from the Air Force is an SR-71B used for training but not for flight research.

Developed for the U.S. Air Force as reconnaissance aircraft more than 30 years ago, SR-71 aircraft were the world's fastest and highest-flying production aircraft.

These aircraft could fly more than 2200 miles per hour (Mach 3+ or more than three times the speed of sound) and at altitudes of over 85,000 feet. This operating environment made the aircraft excellent platforms to carry out research and experiments in a variety of areas--aerodynamics, propulsion, structures, thermal protection materials, high-speed and high-temperature instrumentation, atmospheric studies, and sonic-boom characterization.

Data from the SR-71 high-speed research program was used to aid designers of future supersonic or hypersonic aircraft and propulsion systems, including a possible high-speed civil transport.

The SR-71 program at Dryden was part of the NASA overall high-speed aeronautical research program, and projects involved other NASA research centers, other government agencies, universities, and commercial firms.

One of the first major experiments to be flown in the NASA SR-71 program was a laser air-data collection system. This system used laser light instead of air pressure to produce airspeed and attitude reference data such as angle of attack and angle of sideslip. These data were normally obtained with small tubes and vanes extending into the air stream, or from tubes with flush openings on the aircraft outer skin. The flights provided information on the presence of atmospheric particles at altitudes of 80,000 feet and above where future hypersonic aircraft will be operating. The system used six sheets of laser light projected from the bottom of one of the two "A" models. As microscopic-sized atmospheric particles passed between the two beams, direction and speed were measured and processed into standard speed and attitude references. An earlier laser air-data collection system was successfully tested at Dryden on an F-104 testbed.

The first of a series of flights using the SR-71 as a science camera platform for the NASA Jet Propulsion Laboratory was flown in March 1993. From the nosebay of the aircraft, an upward-looking ultraviolet video camera studied a variety of celestial objects in wavelengths that are blocked to ground-based astronomers.

The SR-71 was also used in a project for researchers at the University of California-Los Angeles (UCLA) who were investigating the use of charged chlorine atoms to protect and rebuild the ozone layer.

The SR-71, operating as a testbed, was used to assist in the development of a commercial satellite-based instant wireless personal communications network, called the IRIDIUM system, under a NASA commercialization assistance program.

In addition, the SR-71 was used in a program to study ways of reducing sonic boom overpressures that are heard on the ground much like sharp thunderclaps when an aircraft exceeds the speed of sound. Data from this study could eventually lead to aircraft designs that would reduce the "peak" of sonic booms and minimize the startle affect they produce on the ground. Instruments at precise locations on the ground recorded the sonic booms as the aircraft passed overhead at known altitudes and speeds. An F-16XL aircraft was also used in this study. It was flown behind the SR-71 to "probe" the near-field shockwave while instrumentation recorded the pressures and other atmospheric parameters.

The aircraft was also used to evaluate a new concept for space propulsion called the Linear Aerospike Rocket Engine, which was planned for use in the X-33 advanced technology demonstrator for a next generation reusable launch vehicle.

The last SR-71 flight was made on Saturday October 9, 1999, at the Edwards AFB air show. The aircraft used was NASA 844. The aircraft was also scheduled to make a flight the following day, but a fuel leak grounded the aircraft and prevented it from flying again. The NASA SR-71s were then put in flyable storage, where they remained until 2002. They were then sent to museums.