It is certainly a baffling question, and a guess would be that perhaps it is a capability that the Blackbird has that
no other airplane has? There are 2 capabilities that come to mind:
1 is sustained Mach 3+ flight and
2 is the ability to launch a large object at those speeds. (The photos below are of the 5 ton D-21 drone,
1 of which led to a SR-71 crash).
Again, it is certainly odd.
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Sigh* Now I gotta put on my airplane nerd hat...
The Blackbird (all three versions) were among the coolest aircraft ever...
There were three versions of the Blackbird', with several sub-types. The SR-71 was was just of the three, obviously the most well known and longest lived one, but she wasn't the first. There was also the YF-12, a fighter version of which three were made that was eventually cancelled by McNamara in 1968.
Here's the YF-12 with an AIM-47, a predecessor of the AIM-54 Phoenix. You can tell it is a YF-12 because the 'chins' don't extend to the tip of the nose like they do for the SR-71.
The third version of the Blackbird, and the original one, is the A-12 'Oxcart'. She was a single-seat CIA aircraft that was a little shorter and a little faster than the SR-71, and since she never was a military bird (even though some were painted up as USAF birds) she kept her Lockheed designation of A-12. She only flew operationally for a year before being retired in 1968.
It was a version of the A-12, the two-seat M-21 (M was for 'Mother') with a Launch Control Officer in the back seat, that was the launch aircraft for the D-21 drone and not the SR-71. There were only 4 launches of the D-21 from the M-21 and on the last one the D-21 engine had an 'unstart' (also an issue with the SR-71 engines) and struck the tail of the M-21 launch aircraft, causing both aircraft to crash and resulted in the death of the Launch Control Officer who drowned after ejecting.
After that the D-21's were launched from B-52's with a big ass booster to get her up to speed. The D-21 flew only 4 operational missions over China and all apparently ended in failure.
So in fact the only thing the SR-71
may be unique in doing is sustained flight at ~Mach 3, she was never used as a launch aircraft for other aircraft (though did carry payloads on her back during NASA testing) and the modifications and certifications in order to get that done would be cost prohibitive.
That is a big
may with sustained Mach 3 flight anyways, it is the only one we
know of right now.
There have also been plenty of
systems tested that exceed that speed even recently, though all but one of them were unmanned. Frankly I don't know how much more useful a manned aircraft would have when doing that testing right now.
