I expect most of you out there have heard of the SR-71, nicknamed the “Blackbird’. The United States premier strategic reconnaissance aircraft until its retirement in the late 1990’s, the SR-71 still holds the official world record as the fastest air-breathing manned aircraft to fly. This it set in 1976 as 2,193.2 mph (3,529.6 km/h) or about Mach 3.3.
However, at least one pilot is on record as stating that he managed Mach 3.5 – which would be understandable because he had a Libyan SAM missile after him at the time.
Regardless, the Blackbird was an amazing aircraft. But the SR-71 did not spring out of nowhere. In fact, there were a couple of models of predecessor aircraft that set the stage for the truly remarkable Blackbird.
The first was the A-12.
Flying a couple of years before the SR-71, the A-12 was operated by the Central Intelligence Agency (CIA). A less capable reconnaissance aircraft than its successor, it was soon retired when the SR-71 came on the scene but was the first “service” aircraft regularly conducting Mach 3 missions.
This was followed by the YF-12A. But this was not a recon bird. This was an interceptor, one that would have been, I believe, the fastest combat aircraft to enter service – had it done so.
The story of the YF-12 starts in the late ‘50s. The United States Air Force was looking for a replacement for the F-106 Delta Dart interceptor to protect the North American continent. I mean of course they were, the F-106 had not even entered service at that point!
But during that period of the Cold War, especially with aircraft technology leaping along, money was almost no object. Thus, the USAF planned for aircraft replacement before they even entered service. To be fair, the USAF was actively developing a strategic bomber capable of Mach 3 at that point – the XB-70 Valkyrie – so it made sense to assume that the Soviets were planning the same.
Initially, the idea had been to adopt the North American XF-108 Rapier.
But that hit all sorts of funding and development problems and the USAF decided in 1959 to find an alternative. The service was aware of the development of the A-12 for the CIA and so when Lockheed’s legendary designer Kelly Johnson said his Skunk Works department could build a fighter variant, they jumped at it.
In 1960 they ordered three prototypes for construction from Lockheed, ultimately giving the new aircraft the designation of YF-12A. This would see the A-12 modified from a single-seat configuration to a two-seater. The pilot in the front would fly the aircraft, whilst the rear seater would operate the massive Hughes AN/ASG-18 radar that would be fitted in the aircraft’s nose.
This had been originally developed for the cancelled XF-108 and was the United States first aircraft mounted Pulse Doppler radar. This had a maximum range in excess of two hundred miles and was reported to be able to accurately detect bomber-sized targets out to one hundred miles.
Combined with two infrared search-and-track sensors this would provide the YF-12 with a “look-down, shoot-down’ capability against aircraft flying beneath the interceptor and masked by the radar signature return from the ground. We take such a feature as standard in modern fighters, but in 1960 it was a revolutionary development.
Naturally, there was a price to pay and that was in the radar’s bulk, with the AN/ASG-18 weighing 2,100lbs – just under a metric tonne. To make the most of the YF-12s long range abilities, it was to carry three GAR-9/AIM-47 Falcon long-range missiles.
These had a range of about one hundred miles, a speed of Mach 4 and an active radar seeker with a 100-lb warhead. The expected accuracy of the Falcon meant that initial plans for a nuclear warhead, such as equipped most of its predecessors, were dropped.
To carry this weapon load, the YF-12 stripped out the four mission bays which on the A-12 carried cameras and intelligence gathering equipment. These were converted to carry the three missiles with the fourth carrying the necessary tracking and guidance systems.
Because it was based on an aircraft then in production – even if at a low rate – development was fast for what was possibly the most advanced aircraft in existence at the time. Other than the electronic and payload modifications, as well as the addition of the intercept officer, only a few airframe alterations were necessary – though with the YF-12 operating in such an intense environment these had to be exquisitely designed and tested.
For example, in flight the leading edges on the aircraft ran at a temperature of 421 Centigrade / 790 Fahrenheit and the engine outlets would glow white at maximum speed.
The radar and infrared detectors meant that the A-12’s/SR-71’s distinctive chines that ran from the wing to the nose had to be cut down, giving the YF-12 a rather distinctive appearance.
Additionally, stabilization fins had to be added under the aircraft due to the altered aerodynamics. But the skill of the Skunk Works and Johnson meant these modifications were soon tested and added to the design. The first prototype took to the skies in August 1963 and the existence of the aircraft was announced by President Johnson only a few months later in February 1964.
On 30 September of that year the YF-12A was unveiled to the public. This revealing of such an advanced aircraft served two purposes.
Firstly, the importance for both the United States and the Soviet Union in demonstrating their technological superiority was a major propaganda issue. I mean, the entire race to the moon was, let us face it, largely about bragging rights. For the USA, showing their new super interceptor was logical.
But it also served to distract from the YF-12s siblings, the A-12 and the just coming into service SR-71. This way Russian intelligence would have problems keeping track of the critical recon assets, never being sure if their sighting reports and satellites were seeing the fighter or reconnaissance variants.
This subterfuge was a bonus on what was shaping up to be an incredible aircraft. Powered by two Pratt & Whitney J58s, each providing 32,000 pounds of thrust, the YF-12A had remarkable performance. On May 1, 1965, one of the prototypes set records for speed and altitude when it achieved 2,070.101 mph (3,331.505 km/h) and 80,257.86 feet (24462.57 metres). The successful flight testing went alongside promising results with the aircraft’s weapon systems.
YF-12 pilots ran simulations on intercepting Russian bombers and, rather remarkably, also began to develop tactics for dogfighting the aircraft. I have not found any solid information on what tactics they came up with, but I suspect the YF-12 would have been the ultimate “boom-and-zoom” monster!
What was certainly impressive was the YF-12 and AIM-47 combination. Seven test firings were conducted. One missile failed but the others were all successful.
The final one of these saw a YF-12 launch a missile from a height of 74,000 feet (22,500m) at a speed of Mach 3.2 at a B-47 target drone which was flying at an altitude of 500 ft. Despite not having a warhead fitted, the Falcon managed to hit the target and took a large section of its tail off!
The results were all promising enough that the USAF decided to purchase the aircraft. On May 14, 1965, they ordered 93 aircraft that would be designated as the F-12B. These would have an increased combat radius over the YF-12As, from 1,200 to 1,350 nautical miles. They would also have an improved fire control system which increased the range a bomber could be definitively detected from 100 to 125 miles.
Price was set at $18 million per aircraft, broadly about $154 million in 2021 money. With them the USAF would have interceptors second to none that could deal with any Soviet bomber incursion in short order and at speed.
But….
By 1965, the United States had other defence issues that seemed far more pressing. The war in Vietnam was beginning to ramp up, and military spending was flowing into the conflict.
Additionally, the Soviet Union seemed to be putting its nuclear deterrent eggs into the ballistic missile basket. With budget considerations and a lack of need, Secretary of Defense McNamara delayed authorizing the funding of the F-12B.
This situation continued until late 1967, when the order was cancelled, and the money funneled into improvement projects for the F-106.
It would be easy to say McNamara blundered with this choice and missed the chance to get an aircraft radically superior to anything flying. But truth be told, the F-106 did provide a perfectly adequate interceptor for the North America continental area throughout the Cold War.
Plus, there were concerns about sourcing the titanium that would be necessary for building the aircraft. Even at the height of the Cold War, most of this was purchased from the Soviet Union. They, realizing that it was being used in the new high-speed aircraft, cut back on the amount they supplied.
So it was decided that the precious material would be better used on the SR-71 program than on the F-12B. Plus it is not like the effort expended on the YF-12 project was wasted.
Both the AIM-47 Falcon and AN/ASG-18 provided the basis for more advanced systems that developed from them – the AIM-54 Phoenix and AWG-9 radar that would go onto be fielded on the US Navy’s F-14 Tomcat interceptor.
The YF-12A’s also continued to serve. One was damaged in 1966 during a landing mishap, though it then provided parts for the so-called “YF-12C”, which was in fact a SR-71 used by NASA. This aircraft joined the two remaining YF-12A’s which from 1969 were used in a joint USAF/NASA program where they were used for experiments in high speed and altitude flight.
They provided invaluable data, some of which was applied to the development of the Space Shuttle. Though one YF-12 was lost in an accident in 1971, fortunately without personnel loss, the other aircraft continued to be used until 1979.
And according to NASA, they still use the data gathered by the program for their research projects today!
If you want to see the last remaining YF-12A, you can, as it resides at the National Museum of the United States Air Force in Dayton, Ohio.
And that is the YF-12A – the ultimate interceptor. One heck of an aircraft that would have been great to see in service, but which really was taking things to the absolute limit in just about every possible way.
Links:
https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-047-DFRC.html
https://internationalaviationhq.com/2019/12/25/lockheed-yf-12-sr-71/
