Urban legend or not I don't know, but I remember a story floating around a few years back. Remember the National Aerospaceplane, weird looking hypersonic passenger a/c? Well it was cancelled amid a glare of publicity because of the billions that had been invested in it wasted when it was shut down. A Senate Defence Committee were supposed to be investigating but were taken into a classified briefing and when they came out, they stopped investigating. Legend is that the funds were a cover for the SR71 replacement.
No idea if there is an iota of truth in it but it's an amusing story that fuels the conspiracy theorists...
