Why was production of Lockheed's SR-71 Blackbird halted, given that it was an advanced supersonic jet that was ahead of its time?
Why did they stop building it?
Because they built 32, and that was as many as was needed at the time. 32 spy planes was plenty for the USAF/CIA to use, so building more would have been a waste. By the time those airframes may have needed to be replaced after 30 years or so, the SR-71 had been withdrawn from service and there was no need to build any more.
I think the question you're really trying to ask, though, is:
Why was it withdrawn from service?
And why wasn't it replaced when it was withdrawn?
The answer to those questions is a bit more complex, but essentially comes down to "Because it wasn't really needed any more."
The SR-71 was designed and built in the 50's and 60's. That means it was designed shortly before the first space flight, and built shortly afterward.
When designed, there was a huge need for a fast, "impossible to shoot down" reconnaissance aircraft. The SR-71 suited that - it could dodge missiles just by outrunning them, and by the time any adversary scrambled a fighter and got to the SR-71's altitude, the SR-71 would be a couple of hundred miles away and possibly even over a different country.
So what happened?
Why maintain a fleet of very high maintenance planes and keep them on high alert, then fly them over your rival's airspace? That's expensive, time consuming, difficult, risky, and (quite understandably) tends to upset your rival. It was, in short, an expensive way to annoy Russia and China. Okay sure, it got some reconnaissance too, and did its job (as an aircraft) beautifully, but when times are tense the last thing you need is to antagonise each other.
As an aircraft, then, it was a huge success... but as a geopolitical pawn, and as a military asset? It's a little more questionable. It certainly performed a vital role, but it had negative downsides from doing that.
Fast forward a few years, and we developed reconnaissance satellites with a whole bunch of advantages
- With a few of them in the right orbit, you can have a satellite over an interesting part of the world pretty much 24/7. Even without that many, you can pass over an area pretty regularly.
- Less obvious. Okay, your rival does not like you launching them - but once in space they're just "there", there's no antagonising "Oh look, an SR-71 flying over us"
- They're "fire and forget" for the most part. Once in space, you just use it: you don't need to keep a bunch of pilots and aircraft on the ground ready to fly
- It's less obvious what you're looking at, and when... if an SR-71 flies over the base that contains your new secret weapon, you're gonna notice. But when a satellite flies over for the 18th time today? Not so much
- No planning and preparation time (which, for an SR-71, was a huge undertaking). You just start taking pictures, or wait for a satellite to get to the right place, then start taking pictures.
They have disadvantages, too
- For 24/7 coverage, you need quite a lot of them, and they aren't cheap
- You have to replace them occasionally (and/or boost them into higher orbits)
- If you upgrade your camera, it's a lot harder to replace the one on a satellite than an SR-71
But overall, a satellite is a more reliable, less antagonistic way to spy on your rivals. They're not perfect, but you know you've got permanent spying capability with no need to plan your reconnaissance ahead of time.
In short: It wasn't needed anymore.
For several reasons:
First off, the SR-71 was a plane which was never intended. An earlier Blackbird incarnation, the single seat A-12 had been in service for several years with the CIA under the codename Oxcart. When the veil of secrecy was lifted and the USAF discovered it, the Air Force demanded to be responsible for strategic aerial reconnaissance. Internal politics between the DoD and the Johnson administration facilitated this.
The Air Force expressed interest in the Blackbird for two missions: first, as a long range interceptor and fighter escort for the B-70 bomber project in parallel development at the time and as a high speed reconnaissance aircraft with expanded surveillance and electronic countermeasures onboard. Both roles would require a two person crew in order to operate the systems, leading to a longer aircraft with a second crew station.
The interceptor version of the Blackbird resulted in the YF-12A prototype first flown in 1963. It featured a two person crew as well as cropped nose chines and a conical nose radome for the AN/ASG-18 fire control radar and the recognizance bays were refitted as weapons bays to carry the AIM-4 Falcon air to air missile. The program was cancelled in favor of the XF-108 Rapier aircraft, itself cancelled along with the XB-70.
While the A-12 finished off its service life and was quietly retired to Palmdale and Area 51, the SR-71 would have an outrageously successful career with the USAF despite mixed feelings with the program amongst the top brass. Gen LeMay hated the Blackbird and felt the Air Force should spend its program dollars on bombers, not on a high maintenance speed demon for reconnaissance.
The SR-71 program officially retired in the late 1990s but all throughout its career, Lockheed attempted to sell derivatives of the aircraft for various defense programs. Lockheed submitted Blackbird derivative designs for the F-X fighter program, which became the F-15. And early incarnations of Lockheed's proposal for the Advanced Tactical Fighter, which became the F-22, were based upon the Blackbird.
The SR-71 was a special plane with capabilities beyond what are commonly known. It was also incredibly expensive and was limited by high maintenance costs and special equipment requirements which were housed at only limited facilities. If for instance a blackbird was forced to land at a facility other than one of those handful of authorized and certain equipment was not available to service that plane before it cooled down, it would be required to do a few million dollars in repairs and the plane would be grounded until that work was completed.
At the time of construction, the number of planes needed, plus a few spares were built. No additional units were ever ordered because the equipment to make them and their parts was also very expensive. As no other crafts shared those parts, such as the engines, there was no economical justification for keeping that equipment either, so by the 1970's most of that was dismantled and without enormous expense, no more could be made. Parts were exchanged between planes, sometimes, to keep the fleet operational well beyond its expected life cycle. Because of part deformations through the extreme flight conditions encountered, exchange was not always possible, so sometimes when something like a bolt wore out, a replacement would need to be fabricated because the stock threading no longer even fit, so a bolt might cost many thousands of dollars to replace.
Even with these constraints, the ability to quickly respond to the need for threats meant the SR-71 was kept in service well beyond intended or economical life cycle, until obviously other platforms became available and relieved the need. Those platforms are simply better or a way would have been found to keep the beast in the air. Nostalgia or just pure love for a great design was not enough to justify keeping it active.