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As for the D-21 drone, its top and cruising speeds were the same as those of the M-21 mother ship, ca. Mach 3.2 at 85,000 feet. True, the D-21 was much smaller and lighter than a Blackbird, but, as with the Blackbirds, its top speed is not determined by thrust, weight, or drag, but the vehicle?s ability to dissipate the heat caused by aerodynamic friction. The D-21s were built out of the same materials as the Blackbirds, viz. ?-120 titanium and composites (whose exact composition remains unknown); therefore, the drones had the same melting point as the Blackbirds themselves and, thus, the same top and cruising speed: ca. Mach 3.2 at 85,000 feet.
Another example of this phenomenon can be seen when comparing the top/cruising speeds of the A-12s and M-21s, on the one hand, with the top speeds of the SR-71As on the other. The A-12s and M-21s had the same power plants and basic shape as the SR-71s, but were about 9 feet shorter and weighed 25,000 pounds less, yet had the same top speed as the larger, heavier SR-71s: Mach 3.2 at 85,000 feet. The reason, again, was heat. At any speed above Mach 3.2, the titanium began to soften to the point of danger due to the high heat produced by aerodynamic friction. Some Blackbird pilots claim to have exceeded Mach 3.2 for brief periods, such as when evading missiles, but Mach 3.2 was the maximum sustainable speed for the Blackbirds and the D-21s. All of the Blackbird pilots with whom we have spoken say that the Blackbird reaches Mach 3.2 at only about 80% of its throttle travel. When we ask them what would have happened if they had firewalled the throttles, they all say the same thing: The aircraft would have continued to accelerate until it disintegrated due to the titanium's softening.
John L. Little
Assistant Curator & Research Team Leader
The Museum of Flight
