On the topic of classification, I've often wondered whether the whole "spy plane" thing was genuine or whether it was a McGuffin put up in the 1980s?
Aurora seems to have been assumed as an SR-71 replacement without much evidence of that fact other than the hypothesis that high speed = reconnaissance aircraft and that led logically to the leap Aurora = Blackbird replacement.
I think the recon mission is the one that makes the most sense for an "Aurora".
Putting it into the context of its time, when satellites weren't as present/persistent as today, having an asset that could be deployed in hours and different trajectories offered its advantages.
Nuclear strike with such a platform wouldn't really offer much over contemporary ICBMs.
Conventional strike, as a Prompt Global Strike predecessor? Again, I don't think that's something that would make much sense. If you were to use such an important platform to conduct a single conventional strike, the implications would be that said target is extremely important and is a well defended one. If you are striking this sort of enemy, you are most likely going to face a strong retaliation that will, in all probability, devolve in a nuclear exchange.
Then again, even as a recon platform this thing would only be able to take a snapshot of a certain location at any given time. Things may or may not happen during an overpass.
It would be a one trick pony. Extremely expensive at that, but pretty much unstoppable.
Maybe such a platform was meant to substitute for satellites in case of all out war. If satellites started to get taken out, having this sort of recon asset would make sense.
And maybe that's why it's still classified today. Not because of the technology involved, but because it's a back-up plan, something whose true worth would come out only at desperate times.
And of course given the laws of probability there can't be nine secret hypersonic spy planes out there! As Flateric would succinctly put it, there is a lot of BS here. Maybe they are all methane-powered?
If I were a betting man, I would say just two hypersonic aircraft.
And would hazard one each for Lockheed (70s-80s higher perfomances and costs) and General Dynamics (late 80s-90s lower performances and costs).
And I would also hazard to say that the X-37B might be a successor, overlapping some of the capabilites an "Aurora" might have had.