When I was "in", back in 1971, I met a chief warrant ET type who'd stood lots of weapons watches as duty officer on the Gun Line in the Gulf of Tonkin. One night he was on watch-officer duty in the CIC of USS Horne, a DLG patrolling on Yankee Station. The radar operator called his attention to a return that looked like a fast mover climbing out of a base on the island of Formosa. The radar operator, with little else to look at elsewhere, clicked the trackball and got a lock -- the target was doing 2400 knots, headed across the Formosa Straits toward China and climbing like a bat out of hell. "Damn", said both men, and my friend said to the radar operator, "Hit it again." This time, 3600 knots and enormous altitude. That was enough to classify the target, right there. The unknown was still accelerating -- my friend estimated that by this time he was doing about 4500 kts and still accelerating, now over China, when suddenly the vox circuit the task group's cruiser was on blared, calling the Horne and commanding "Break lock! Break lock now!" The radar operator's hand was about to descend to hit the bug again, and he looked up at my friend, who smiled and shook his head "no". The target continued on its way westward at unbelievable speed, and my friend and his radar operator had the satisfaction that they'd locked up an SR-71.
Later on I read an article about the CIA's flights over China, collecting material from Chinese nuclear-weapons tests, and the article stated that those flights were east-to-west and typically recovered in Norway after only 20 minutes, and it offered the snippet that the engine nacelles typically glowed bright orange in flight, with operating temperatures so high that liquid metal was used as the lubricant. Something to think about. And we mothballed this aircraft? I grant you the design and many of the airframes were nearly 40 years old, but still. Do you suppose we already had something better?