The UK has just released more data from the Ministry of Defence's unidentified flying object (UFO) files, as reported here in the Daily Telegraph. It's a fascinating if (as always) inconclusive report, which ends with the MoD's long-suffering UFO desk officer being advised curtly to "drop this subject" after linking early-1990s sightings of airborne phenomena to press reports about the possible existence of a classified high-supersonic replacement for the SR-71.

Coincidentally, NASA forum poster Blackstar (who has been outed on the Secret Projects forum as space historian Dwayne Day) just released a number of documents from declassified CIA records, concerning what would have been an even earlier follow-on to the SR-71:  the McDonnell Model 192, codenamed Isinglass.

Kept completely secret until the early 2000s, more than 30 years after it was canceled, Isinglass was a rocket-boosted manned glider that was intended to be shot to 230,000 feet before entering a Mach 20 (13,000 mph) glide across the Soviet Union. It was primarily built from diffusion-bonded titanium and would have been powered by a then-revolutionary hydrogen/oxygen rocket, Pratt & Whitney's XLR-129.

Eight of the vehicles were to be built, entering service in the early 1970s, and followed by a more versatile Isinglass II. The papers suggest two reasons for the cancellation, in early 1967:  problems with heating of the camera window, preventing the acquisition of imagery, and opposition from the USAF and National Reconnaissance Office, who favored developing electro-optical satellites to gather timely imagery. The USAF continued development of the XLR-129 after the cancellation - demonstrating performance in ground tests that the Shuttle Main Engine did not match reliably for a decade.

Isinglass' existence does not prove Aurora exists, of course. But it does suggest that the technical challenges involved were not as tough as people have suggested, and it does show how big programs are kept secret for decades after they were cancelled.