In the 1980s an idea was conceived by military planners to develop a hypersonic spy plane that could reach Mach 5+, making it truly the fastest manned aircraft ever to fly. While there have been reported sightings of the prototypes, the SR-91 Aurora, if it even existed, likely never made it much best the concept stage.
The goal of the project was to develop a replacement for the aging and expensive to maintain a fleet of SR-71 Blackbirds, which cost around $200 – $300 million a year to operate. Technically known as a Special Access Program (SAP) – a “black program” that isn’t generally for public knowledge – little has been released about the SR-91. Even the name “Aurora” was only disclosed when censors failed to catch a reference to it a 1985 budget request that also discussed the state of the SR-71 and U-2 programs.
More recently in May 2006, a British Ministry of Defence report referred to the U.S. Air Force priority plans to produce a supersonic vehicle that could reach speeds of Mach 4 to 6. There has been much speculation in the nearly 15 years since that this may have been another reference to the SR-91 Aurora, and while it seems plausible this is the same program for a reconnaissance aircraft from 21 years prior, there is simply too little information to know for certain.