The Marquardt and Lockheed teams determined that the unmanned drone would most suitably be powered by a modified Marquardt RJ43-MA-11 ramjet engine similar to that used on Boeing lM-99 Bomarc surface-to-air-missile during cruising flight.
Using Marquardt's propulsion system experience, it was not difficult for the Lockheed team to execute a functional reconnaissance platform in a relatively short period of time. Concurrently, the technology base that had been generated by initial flight trials of the A-12 had given the engineering team (under Johnson) considerable confidence in the aerodynamic and low-observables precedent (i.e., reduced RCS) set by the chined delta. This configuration was a given by the time initial design options were studied for the D-21.
The new project was assigned to Art Bradley under the supervision of Dick Boehme. A small team was assembled to handle engineering and a section of the Skunk Works shop at Burbank was walled off specifically to accommodate the new drone activity.
The D-21 engine was properly identified as an XRJ43-MA20S-4 and was developed from the earlier RJ43 series of ramjets optimized for use on the Boeing IM-99 Bomarc series of surface-to-air anti-aircraft missiles. The RJ43 had the ability to function as an independent external power plant on any vehicle that could reach sufficient speed to allow efficient inlet operation. lt was developed in supersonic wind tunnels at Marquardt's Van Nuys, California, test facility, flight tested on the Lockheed X-7A-3 at Holloman AFB, New Mexico, and, in its MA11 version, was deployed operationally on the Bomarc B.
The MA20S-4 engine employed in the D-21 used many MA-11 components but was modified to operate at lower pressures and higher temperatures. The S-4 was mounted internally in the D-21 and had no inlet structures of its own. Instead, it utilized the D-21's inlet system. The engine's center body and main structure were retained to house the fuel control, fuel pump, fuel injector nozzles and flame-holder assemblies. The latter was redesigned to allow for stable combustion at extreme high-altitude, high-temperature and low-pressure situations. lgnition was by a pyrophoric fluid known as triethyl borane (TEB) to allow for re-ignition in the event of flameout. The combustion chamber/exit nozzle was redesigned to provide for the much greater expansion ratio required for high-altitude cruising.The design also incorporated an ejector system for engine structure cooling.
lt is noteworthy that until the advent of the D-21, no ramjet had ever powered any craft for longer than fitteen minutes; the D-21's XRJ43-MA20S-4 routinely operated on missions lasting 1½ hours or longer.
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Propulsion system work began to accelerate at this time as well. A Marquardt RJ43-MA-3 engine was tunnel-tested at simulated "Q-12" operating conditions. Per Johnson: "We were all amazed, including Marquardt, that the engine could be shut off as long as 45 seconds and still restart, due to hot engine parts."