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Leo Rich, protege and successor to "Kelly" Johnson at Lockheed's famed "Skunk Works", regarding the "beads on a rope" contrail reported a few times behind a high speed high altitude mystery plane, remarked that it would require cryofuels. That's all he said. The "beads on a rope" contrail suggested to some that a pulsed ramjet was at work. Some have suggested that the "Flying Dorito" (which was later exposed and discontinued by the Clinton administration) may have been the Aurora (I don't think the "Flying Dorito" was supersonic though)...

Aurora - Top-Secret Hypersonic Spy Plane
The outside world uses the name Aurora because a censor's slip let it appear below the SR-71 Blackbird and U-2 in the 1985 Pentagon budget request. Even if this was the actual name of the project, it would have by now been changed after being compromised in such a manner... On 6 March 1990, one of the United States Air Force's Lockheed SR-71 Blackbird spyplanes shattered the official air speed record from Los Angeles to Washington's Dulles Airport. There, a brief ceremony marked the end of the SR-71's operational career. Officially, the SR-71 was being retired to save the $200-$300 million a year it cost to operate the fleet. Some reporters were told the plane had been made redundant by sophisticated spy satellites... Lockheed's Skunk Works, now the Lockheed Advanced Development Company, is the most likely prime contractor for the Aurora aircraft. Throughout the 1980s, financial analysts concluded that Lockheed had been engaged in several large classified projects. However, they weren't able to identify enough of them to account for the company's income.