The Lockheed D-21 (Project Tagboard) was 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-12, and SR-71 "Blackbird" manned aircraft in the 1960's. The D-21 ramjet-powered reconnaissance drone was powered by a Marquardt RJ-43-MA-11 ramjet. Cruising at Mach 3.3 at an altitude of 90,000 feet, the D-21 had a range of over 3400 nautical miles. The D-21 was guided by an inertial navigation system on a pre-programmed flight profile.
Mindful of the Gary Powers U-2 shootdown aftershocks and the inevitable political sensitivities concerning manned overflight of large expanses of denied territory, the Lockheed ‘Skunk works’ designed a tri-sonic, air-launched, reconnaissance vehicle designated the D-21 (code-named Tagboard). Originally, the D-21 was designed to be launched from the back of a modified A-12 (redesignated M-12, that is, M=mother, D=daughter, 12 vs 21) carrier aircraft.
Under Project TAGBOARD, Kelly Johnson submitted a written proposal for a feasibility study regarding the Q-12, as he called the drone. It was accepted and Johnson drew up plans for a ramjet-powered vehicle that would be launched for a dorsal pylon atop the fuselage of a modified A-12-type aircraft. The major modification to the OXCART design included provisions for a launch control officer (LCO) seated in a compartment behind the cockpit. The drone rested on a pylon mounted on the aircraft’s centerline between the twin vertical stabilizers.
On 28 February 1963, Johnson received approval to produce 20 drones and two launch aircraft. As construction began, Johnson saw the two vehicles as “mother” and “daughter.” To avoid confusion with the single-seat reconnaissance jet, he designated the mothership M-21 (with “M” for “mother” and reversing the numerals). The drone became the D-21 and the mated combination was called MD-21. The “D” in D-21 stood for “daughter” while the “M” in M-12 stood for “mother.” The basic launch design for Tagboard probably evolved from the Convair proposal for Oxcart dubbed Fish that called for launch of a small, manned, ramjet-driven (Mach 4.2) vehicle by a supersonic B-58 Hustler.
The M-21 had generally the same performance and handling characteristics as the A-12. To stay within the aircraft’s capabilities while maintaining optimal attachment loads on the D-21, maneuvering loads were limited to no more than 2.0g for the mated configuration. Most rolling maneuvers for the MD-21 were prohibited to prevent adverse side loads on the D-21. The only aileron maneuver allowed for the mated configuration consisted of a steady turn, performed within the airplane’s capabilities.
The D-21 was almost 43 feet long with a nearly 20-foot wingspan, and about seven feet tall. As a result of shape and materials, it had a small RCS. Maximum gross takeoff weight was 11,000 pounds. Powered by an internally mounted Marquardt XRJ43-MA20S-4 ramjet engine, it had a design cruise speed of Mach 3.35 at 80,000 to 90,000 feet and a range of 1,440 miles.
By June 1963 the engineers mated a D-21 to its launch aircraft. The launch platform was a modified A-12 called the M-12, the predecessor to the SR-71. Built primarily from titanium, the D-21 had a range of 1,250 nautical miles, cruised at Mach 3.3 and could reach an altitude of 90,000 ft.
Once released from the M-12 by a Launch Control Officer (LCO) riding in the M-12, the drone flew its sortie independently. The D-21 inertial navigation system (INS) was programmed to fly the desired track and flight profile and execute camera on and off operations, allowing it to satisfactorily execute the perfect photo-recce sortie. After completing its camera run, the drones’ INS commanded the auto-pilot system to descend the vehicle to its ‘feet-wet’ film collection point. The entire palletized camera unit then ejected and parachuted towards the surface. As the drone continued it descent, barametrically activated explosive charges would destroy the vehicle. A C-130 equipped with a Mid-Air Recovery System (MARS) would retrieve the camera unit containing its valuable film and fly it to a base for processing and analysis.
By 1966 the program had progressed and was ready to perform vehicle separation. The mission profile called for the M-12 to fly at Mach 3.2 and commence a slight pull up at 72,000 ft, then push over to maintain a steady 0.9 g. With controllability checks completed and its ram-jet burning, the LCO initiated vehicle separation by throwing the switch that fired off a blast of compressed air from a cylinder fitted in the M-12’s pylon.
The first flight of the D-21/M-12 combination took place on December 22, 1964, but the first D-21 release from an M-12 did not occur until March 5, 1966. This pioneering work achieved its first successful separation on 3 July 1966, and the following launch was successful.
The flight profile required the M-21 pilot to attain a speed of Mach 3.12 at an altitude of 72,000 feet and commence a gentle pull-up before pushing the nose over to maintain a steady force of 0.9 g. With the drone’s ramjet operating, the LCO initiated pneumatic separation of the drone. On its second free flight, the drone attained a speed of Mach 3.3 and altitude of 90,000 feet. Although it demonstrated the planned performance characteristics, the D-21 still suffered a few technical problems. Additionally, the launch maneuver was risky for the flight crew.
On 31 July 1966, Johnson’s worst fears were realized during the fourth launch attempt from the M-21. After the drone lifted off the pylon, its ramjet unstarted. The D-21 slammed down onto the aft launch pylon of the M-12. The impact caused the M-12 to violently pitch-up, exposing the large underside chine area of the aircraft to the immense pressure of a Mach 3.2 airstream, which quickly ripped the M-12 in half. The M-21 pitched up and broke apart, sending debris plummeting toward the Pacific Ocean. Miraculously, both crewmen survived the aircraft’s disintegration. Although both crewmen ejected from the stricken craft,upon entering the water the LCO drowned before rescue forces arrived.
As a result of the tragedy, Johnson cancelled further use of the MD-21. No further "piggyback" launches were attempted. The remaining D-21 drones were modified to the D-21B configuration and two B-5H aircraft were configured as launch platforms under Project SENIOR BOWL.
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