Faster than Superman: Lockheed multisonic UAVs

The Lockheed Skunk Works has earned its place in 20th-century US aerospace history for building America's most reliable reconnaissance aircraft, most notably the U-2 Dragon Lady (which was shot down over the USSR in May 1960 and also set the stage for the 1962 missile crisis by photographing Soviet SS-4 "Sandal" missiles in Cuba) and the SR-71 Blackbird, the fastest-ever US military aircraft. However, one aspect of Lockheed's aerospace history that some aerospace historians tend to overlook is the fact that generations before NASA and the US Air Force undertook development and flight testing of hypersonic air-breathing technology, the Lockheed Skunk Works undertook development, testing, and deployment of unmanned air vehicles able to travel at high supersonic speeds (Mach 3 to 4), most notably the D-21 Tagboard. Recently, in late 2018, I finally got to see the D-21 in person at the March Field Air Museum. Therefore, I thought it would be appropriate to give an overview of multi-sonic UAVs developed by the Lockheed Skunk Works.

Top: Lockheed X-7 ramjet-powered test vehicle

Bottom: Lockheed Q-5/AQM-60 high-speed target drone

Lockheed's foray into aerospace vehicles able to travel faster than Superman (at speeds exceeding Mach 2.5) started in December 1946 when the US Army Air Force (US Air Force after September 18, 1947) issued a requirement for an unmanned experimental vehicle to investigate air-breathing flight at speeds of the Mach 3. Lockheed immediately responded with a proposal for an unmanned Mach 3 technology demonstrator, the L-171, which was 15 feet long and was to be powered by a Marquardt RJ37 ramjet, which would sustain itself at speeds of Mach 2.3-3.0 for 3 minutes. The performance estimates for the X-7 were rather remarkable, because the L-171 itself was designed several months before Chuck Yeager broke the sound barrier in the Bell X-1 on October 14, 1947. The Air Material Command favorably responded to Lockheed L-171 proposal and assigned it the MX-883 designator in January 1947. The original L-171 design was judged too small to achieve its desired performance targets, so Lockheed refined its MX-883 design to produce a larger test vehicle with better performance. The MX-883 was initially designated PTV-A-1 by the USAF (the prefix "PTV" stood for "Propulsion Test Vehicle"), but later redesignated X-7 in 1951. The first flight of the X-7 took place on April 26, 1951, and a total of 130 test flights were conducted until July 20, 1960. All X-7 test vehicles were launched from B-29 and B-50 motherships, relying on solid-fuel rockets for the boost to Mach 4, and a ramjet for sustained Mach 4 cruise; one X-7 test flight reached an altitude of 106,000 feet, while another reached Mach 4.31 (2,881 miles per hour; 4,636 km/h). The X-7 later evolved into a dedicated high-speed target drone, the Q-5 Kingfisher (redesignated AQM-60 in 1963), which had one Marquardt RJ43 ramjet and two Thiokol XM45 solid-fuel rockets. The Kingfisher was intended to simulate America's anti-aircraft missiles, including the Nike Ajax, Nike Hercules, and Bomarc, and it . Despite being the first air-breathing supersonic drone built in the US, the Kingfisher proved faster than the missiles it simulated, embarrassing the Air Force establishment and some sectors of the US aerospace industry, and by the mid-1960s, the Kingfisher program was axed.

Upper left: D-21 on display at the March Field Air Museum in Riverside, California
Upper right: D-21B being boosted to high supersonic speed by a solid-fueled rocket booster, late 1960s.
Bottom: Lockheed M-21 (s/n 60-6940) carrying a D-21 drone.

The Lockheed D-21, however, stands out as the best-known high-speed UAV ever developed by the Lockheed Skunk Works. In October 1962, the Lockheed Skunk Works proposed a Mach 3+ reconnaissance UAV under the company designation Q-12 for long-range reconnaissance missions or spy missions over enemy territory that were deemed too dangerous for the A-12 or SR-71. Like the Blackbird, the D-21 was made from titanium and radar-absorbent composites. For spy missions, the Q-12 was to be launched from the back of a modified A-12. The Q-12 was later designated D-21 in late 1963 to avoid confusion with the A-12 designator, and two A-12s on contract were modified as launch platforms for the D-21 and designated M-21 (M=Mother). The D-21, codenamed Tagboard by the CIA, utilized a Marquardt RJ43 ramjet for sustained high-supersonic flight, and the drone's fuselage is reminiscent of a stovepipe. The D-21 would be launched from the M-21 at high altitude, coast to Mach 3.8 with the ramjet and scour an enemy target with a Hycon high-resolution camera, eventually releasing a hatch with the onboard camera over a predetermined overwater area to be retrieved by a mid-air recovery system.

Captive carry flights of the D-21 began in December 1964 and continued into 1965; the first free flight of the D-21 occurred on March 5, 1966, and two additional launches followed. On the fourth launch, on July 30, 1966, a D-21 hit the tail of the M-21 mothership shortly after separation, causing both aircraft to crash and the death of one of the M-21 crewmembers. Due to the risks associated with the D-21/M-21 launch procedure, Lockheed opted for launching D-21 drones from the underwing pylon of a B-52H. Given the slower release speed from a B-52H, all D-21s under construction were equipped with a large solid-propellant rocket booster for acceleration to Mach 3+ speeds and designated D-21B. Two B-52Hs were modified as D-21 motherships, each carrying two D-21s. Twelve D-21B test launches were conducted from September 1967 to July 1969, and when the last two of those launches were deemed successes, the Air Force cleared the D-21 for operational missions over the Lop Nur nuclear weapons test site in Xinjiang, China, under codename SENIOR BOWL. Four D-21 overflights of the nuclear weapons test site were conducted from November 1969 to March 1971, but none were successful; the first mission ended in mishap when the D-21 veered off course and crashed in Siberia, the second and third missions completed their flights, but the hatches with the cameras weren't recovered, while the final D-21 mission ended crashed in Yunnan province. Because of the poor success of Operation Senior Bowl and Richard Nixon's finalization of his diplomatic opening to the People's Republic of China, the Tagboard program was cancelled in July 1971. Several D-21 drones that were built but not used in the Tagboard and Senior Bowl programs are now on display at museums in the US, including the one I saw at the March Field Air Museum. (In an interesting footnote, the D-21 that crashed in Siberia after veering off course during its first operational mission was used by the Tupolev Design Bureau as a basis of a planned reverse-engineered drone, the Voron [Raven], which never left the drawing board. More info on the Voron drone can be found in Gordon and Rigmant [2005, pp. 325-327].)

References:

Gordon, Y., & Rigmant, V., 2005. OKB Tupolev: A History of the Design Bureau and its Aircraft.  Hinckley, UK: Midland Publishing.  

Miller, J., 2001. The X-Planes: X-1 to X-45. Hinckley, UK: Midland Publishing.

Peebles, C., 1999. Dark Eagles: A History of Top Secret U.S. Aircraft Programs (Revised ed.). Novato, CA: Presidio Press.