NASA considered reviving the Lockheed D-21 as an alternative to the Hyper-X. To satisfy that mission the ramjet (supersonic diffuser, subsonic combustion) would have been replaced by a scramjet (supersonic inlet, supersonic combustion). So far the Hyper-X project is still science fiction.

Lockheed D-21 Supersonic Drone piggybacking the SR-71 Blackbird.

Construction: Titanium Alloy Beta-120 (Titanium - 13%Vanadium - 11%Chromium - 3%Aluminum) monococque airframe with some composite plastics.
Length: 42 feet, 10 inches
Wingspan: 19 feet, 9 inches
Height: 7 feet, 1/4 inch
Speed: Mach 4 (2,500+mph) at 80,000 to 95,000 feet (engine only functions at supersonic speed)
Maximum Gross Take-off Weight: 11,000 pounds
Operational Ceiling: 95,000 feet
Maximum Unrefueled Range: 3,000 nautical miles
Armament: None

[The following is taken from the website]

[W.R.T.] the Ramjet Engine model number. All current references to this engine identify it as a Marquardt XRJ 43-MA20, the same engine as used on the Bomarc B Interceptor Missile. This is not the case and if possible, I would appreciate your help in spreading this informational fine point. The D-21 engine is properly identified as an XRJ 43-MA20S-4.

I have intimate knowledge of these engines and of the previous models from which the D-21 engine derived. The XRJ 43-MA20 was a complete, externally mounted, ramjet engine with a fixed geometry, Mach 2.35, isentropic spike inlet. The engines' integral combustion chamber/exit nozzle was designed for relatively low altitude cruise and therefore did not have a high expansion ratio nozzle. The ignition system was two pyrotechnic flares. There was no re-ignition capability. The fuel control and flame holder combustion limits would not allow operation at high altitudes. The MA20 engine, used on the Bomarc, contained an all pneumatic fuel control that maintained constant Mach Number at two selectable Mach Numbers using pressure signals from the "built in" inlet. This engine could function as an independent external power plant on any vehicle that could reach sufficient speed to cause efficient inlet operation. It was developed in supersonic wind tunnels at Marquardt's Van Nuys, California test facility, flight tested on the Lockheed X-7A3 at Holloman AFB, Alamogordo, NM and deployed operationally on the Bomarc B

The MA20S-4 engine, used in the D-21, used many of the MA20 components, but modified to operate at lower pressures and higher temperatures. The S-4 was immersed in the body of the D-21 and had no inlet structure (because it used the D-21 inlet system). The engine center body and main structure remained, to house the fuel control, fuel pump, fuel injector nozzles and flame holder assembly. The flame holder system was redesigned to allow for stable combustion at extremely high altitude, high temperature and low pressure. Ignition was by TEB (TriEthylBoran) 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 cruse. The design also incorporated an ejector system to provide for engine structure cooling.

The pneumatic fuel control computer was modified to function at much lower pressures and higher temperatures. The input pressure signals were modified to accept air data from the D-21 inlet and to maximize full power operation limited only by the D-21 inlet conditions and mission parameters. The fuel flow schedule, pumps, controls and injector nozzles were redesigned to permit accurate flow control and injection at the much lower air flow requirements of extreme high altitude.

If it would be possible to display both of these engines on a dolly, side by side, they would not look at all alike. They have common roots (in the Marquardt RJ43 ramjet family) but the similarity ends there. So far, attempts to display these engines have been stopped by the environmental concerns of some authorities. Much of the center body construction is a Magnesium Thorium alloy. It may not be possible to overcome this caution, but I would like to see such a display as a means to educate aviation enthusiasts about the success of ramjets, as used in the US.

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Project Tagboard & Project Senior Bowl description --- long version, so long I haven't read it!