This past election my son asked why would someone want to run for President. My first answer was so I could find out about all the secret aircraft that the US has/had at Area 51.

Yes, the Valkyrie is in the fourth building, along with the Air Force One collection and a number of research aircraft such as the X-3 and X-24.

So the XB-70 retirement, on grounds of being too expensive, too complex and obsolete, was actually a cover story for top-secret Aurora. However the reason Aurora has never reached the public domain, despite being replaced by a system that regularly features in USAF press releases, is that it was retired, on grounds of being too expensive, too complex and obsolete. Have I got that right?

Current Location:
https://live.staticflickr.com/65535/...727dfb6d_c.jpgR&D Aircraft by Mark Carlisle, on Flickr

Former Location
https://live.staticflickr.com/749/21...0793c5a9_c.jpgR&D Trio by Mark Carlisle, on Flickr

Original:
https://live.staticflickr.com/4307/3...b22cbe2d_c.jpgNorth American XB-70A Valkyrie 62-0001 by Mark Carlisle, on Flickr

This has all entered the tinfoil hat territory methinks.
Developing something that big and complex could never be done in secret these days or in the 90s.

Indeed.

https://thespacereview.com/article/576/1

Six blind men in a zoo: Aviation Week’s mythical Blackstar

https://www.tmcnet.com/usubmit/2006/03/20/1472209.htm

Blackstar A False Messiah From Groom Lake

Of course there were projects ISINGLASS, Rheinberry and QUARTZ….

It is perhaps worth mentioning that while the airframes were smaller than something XB-70 sized, OXCART, Have Blue, Bird Of Prey, Tacit Blue, Senior Trend, various MiG's, various helos, some very large and complex space based sensors, and now a 6th gen fighter jet were all flown in secrecy by the USA and some of these were large and complex programs. OXCART was over 100,000 pounds, quite complex and 15 were built. Not a small program.

All those programs are now in the public domain, most have been for decades, and many of the airframes are in museums. If you want to know how to build a Mach 3+ aircraft, not only is there plenty of information to get you started on Wikipedia, but at least one museum has a pair of J58 engines outside the airframe that you can photograph and measure to your heart's content. Some of these aircraft did some pretty provocative things back in the day, but you can read all about it in Ben Rich's autobiography and watch interviews with the aircrew on YouTube.

In that context, why would another program from the same era, that was also retired due to cost/obsolescence, and who's successor is very much in the public domain, still be under wraps?

For the answer to that question have a look at the two links posted by ORAC, the first of which is a particularly good read.

You have made several references to the X-37 being a "successor" to some type of Aurora program, but perhaps it is not. A just in time air launch for transonic/suborbital recce or strike is a different profile to a spacecraft that spends months or years in orbit. As for the "very much in the public domain", I note that many specifics of the X-37 flights are quite vague, only citing "spectacular success", without saying what they actually did.

As for some programs being declassified and in museums and others not, there can be a whole variety of reasons for that. I do not believe we know all about every past program, whether they were successful or not, nor should we.

A number of nations have very well-documented transonic and suborbital strike capabilities, though the preference for the latter is generally to launch from a submarine. As pointed out in the first of those articles, using this capability for reconnaissance is a really bad idea because your adversary has no way of knowing what's attached to the missile until it's too late.

An interesting side effect of the emergence of multi thousand satellite constellations is that nuclear strikes from orbit become much easier to set up and a lot less easy to detect.beforehand. When satellites numbered in the hundreds, putting up a bunch of orbital bombs was pretty conspicuous.. Now, who can tell among the many thousands whizzing around in low orbits?

https://cimg6.ibsrv.net/gimg/pprune....9e3ce79de0.jpg
https://cimg7.ibsrv.net/gimg/pprune....467c1d207e.jpg
https://cimg8.ibsrv.net/gimg/pprune....32bfd3d730.jpg
Some pics taken at Edwards circa 1967 - 68 Memorial Day Air Show I think. Made a good sun shade on a hot sunny day.