Five innovative Air Force aircraft that you’ve probably never heard of

By Staff Sgt. Antonio Gonzalez
Air Force Public Affairs Agency

The history of the Air Force is rooted in finding new ways to solve problems. It’s one of creating out-of-the-box solutions and achieving greater possibilities. Drone technology, stealth aircraft and space vehicles were merely science fiction until we made them reality. The aircraft you are about to discover brought upon revolutionary new abilities for today’s Air Force and will continue to have an impact on technology in the future.



The tilt-wing XC-142A was an experimental aircraft designed to test the operational benefits of vertical/short takeoff and landing transports. Such an aircraft would permit rapid movement of troops and supplies into unprepared areas under all-weather conditions. An XC-142A first flew conventionally on Sept. 29, 1964, and on Jan. 11, 1965, it completed its first transitional flight by taking off vertically, changing to forward flight and finally landing vertically.

Tilting the wing and engines skyward permitted vertical takeoff like a helicopter and then the wing and engines were gradually tilted forward to provide the greater speed of a fixed-wing aircraft in forward flight. The engines were linked together so that a single engine could turn all four propellers and the tail rotor. In tests, the XC-142A was flown from airspeeds of 35 mph backwards to 400 mph forward. XC-142As were tested extensively by the Army, Navy, Air Force and NASA.

Lockheed D-21B


The Lockheed D-21 is an unmanned or “drone” aircraft designed to carry out high-speed, high-altitude strategic reconnaissance missions over hostile territory. It is a product of the Lockheed “Skunk Works” program that developed the A-12, YF-12A and SR-71 “Blackbird” manned aircraft in the 1960s.

Originally, the D-21 was designed to be launched from the back of a modified A-12 (redesignated M-12) carrier aircraft. The first flight of the D-21/M-12 combination took place on Dec. 22, 1964, but the first D-21 release from an M-12 did not occur until March 5, 1966. Two more launches were successful, but on July 30, 1966, a D-21 collided with the M-12 after release, destroying both aircraft and resulting in the death of one of the M-12′s crew members. No further “piggyback” launches were attempted.

A new launch system was developed using modified B-52H aircraft as carriers. The new D-21 configuration (designated D-21B) had dorsal mounting hooks for carriage under the B-52′s wing and a solid rocket booster for the initial acceleration required to start the ramjet engine. The first launch from a B-52 took place on Nov. 6, 1967, but the D-21 crashed. Several flights followed in 1968 with mixed success.

The first operational launch was on Nov. 9, 1969, but the D-21B was lost. Several successful operational missions were flown over the next two years, but the D-21 program was highly classified and details have not been released. The program was canceled in 1971, and the D-21s were placed in storage.

Tacit Blue Whale


The Tacit Blue (Whale) aircraft was built to test the advances in stealth technology. The USAF, the Defense Advanced Research Projects Agency, and the Northrop Corp. worked together from 1978 to 1985 to demonstrate that curved surfaces on an aircraft result in a low radar return signal from ground radar. With such a low radar return signal, Tacit Blue demonstrated that such an aircraft could operate close to the battlefield forward line without fear of being discovered by enemy radar. It could continuously monitor enemy forces behind the battlefield and provide targeting information to a ground command center.

The aircraft made its first flight in February 1982, and by the conclusion of the program in 1985, had flown 135 times. It had a digital fly-by-wire flight control system to help stabilize the aircraft. Tacit Blue had a single flush inlet on the top of the fuselage to provide air to its two engines.



The unmanned Boeing X-40A was the first-phase flight test vehicle for the U.S. Air Force’s Space Maneuver Vehicle (SMV) program that began in the late 1990s. The SMV program aimed at developing a new generation of small, reusable, highly maneuverable space vehicles for deploying satellites, surveillance, and logistics missions.

Designed and built by the Boeing Company in partnership with the Air Force Research Laboratory, the X-40A was produced at Boeing’s Phantom Works facility at Seal Beach, California. This test aircraft was a 90 percent scale version of what would later be designated the X-37 space plane.

On Aug. 11, 1998, the X-40A made its first successful flight at Holloman Air Force Base, New Mexico. Lifted to an altitude of about 10,000 feet by a helicopter, the X-40A was released, and it made an unpowered flight to demonstrate high-speed guidance, navigation, and control capabilities.

Following that single flight, the USAF loaned the X-40 to the National Aeronautics and Space Administration (NASA) to test the shape, guidance, and other systems of the X-37. After captive-carry flights to practice release procedures and test equipment, the X-40A made its first flight for NASA on March 28, 2001. Carried aloft by a U.S. Army helicopter to 15,000 feet before being released, the X-40A flew itself, guided by onboard systems, to a gentle landing at Edwards AFB, California. The X-40A made a total of seven successful flights in support of the X-37 program.

First manned PDE-powered flight, Jan. 31, 2008. (U.S. Air Force photo)


On Jan. 31, 2008, the modified Scaled Composites Long-EZ completed the first manned flight of an aircraft powered by a pulsed detonation engine (PDE). The flight was the result of five years of innovation and cooperative effort between the Air Force Research Laboratory (AFRL) and Innovative Scientific Solutions Inc. (ISSI). The project used the popular Long-EZ designed by Burt Rutan as a platform.

Engines normally burn fuel and air at subsonic speeds to provide propulsion. PDEs detonate the fuel-air mixture into repeated, controlled explosions. The resulting supersonic shockwaves create thrust. In this engine, the thrust is expelled through four tubes out the back of the aircraft. Remarkably, the PDE in this aircraft was made partially from off-the-shelf automotive parts.

Pulsed detonation engines are still in their infancy, but this technology may become common as it matures. PDEs are much less complicated and promise to be less expensive to operate than jet engines. Moreover, they offer a fuel savings of between 5-20 percent over traditional turbojet engines.

It’s easy to see how these experimental aircraft helped develop technologies present in our current inventory. What future technology do you think will be in our next test aircraft?