Remembering the legendary speed and secrets of the SR‑71A Blackbird

October 9 marks 25 years since the SR-71A Blackbird last soared through the skies. On this occasion, it's worth remembering this incredibly fascinating aircraft, which indirectly came about thanks to the USSR, from where some construction elements reached the USA. The aircraft's speed was so impressive that the fuselage heated up to as much as 500 degrees Celsius, and the crew wore special suits with an automatic cooling system.

Our subject's story can actually begin with the advent of the jet engine when the race for ever-increasing speeds reached a completely new level. By the 1950s, the rapid development of aviation and missile technology (including anti-aircraft missiles) made it necessary to look for new solutions, not just to effectively deliver a nuclear bomb to enemy territory but also first to determine where it might be best to drop it.

The Americans initially tried to fly at altitudes allegedly too high for Soviet air defence to intercept their U-2 reconnaissance aircraft (incidentally also very interesting and still in service).

In 1960, this was painfully verified by the Soviet S-75 anti-aircraft system, whose missile shot down one of the aircraft (piloted by Francis Gary Powers). Thus, a different path was chosen: speed. At that time, extreme speed was sought to minimize flight time in a threat area, aiming to ensure safety for increasingly expensive machines and their pilots. One of the many, and possibly the most successful, "children" of this concept was the SR-71A Blackbird.

The SR-71A stems directly from or can actually be said to be a developmental version of the Lockheed A-12 aircraft. The Lockheed A-12 was a superfast (reaching speeds over Mach 3, or 3,600 km/h), high-altitude (flights above 25,900 metres) reconnaissance aircraft.

Its development for the CIA began in 1959 and, in a sense, did not end for a long time, even though the proper A-12's career was very short: test and combat flights were conducted only between 1962 and 1968, and the aircraft was decommissioned due to improvements in quality and access to satellite data. In the meantime, a two-seater training version was designed, and a fast fighter project, YF-12, was created for the USAF. At the same time, for the cooperation between the USAF and the CIA, the M-21 aircraft was developed as the carrier of the fast (Mach 3.3, or over 4,000 km/h) D-21 reconnaissance drone. All these programs were closed and influenced by accidents (including fatal ones).

The SR-71A began development in 1960, with its prototype initially meant to be the reconnaissance-bomber version YF-12, called RS-12. Over time, the strike function was abandoned, and the reconnaissance variant entered service in 1966. Many significant changes were introduced in the aircraft.

The SR-71A was slightly larger than its predecessor, with a longer nose section and enlarged fuel tanks (36,400 kg, plus the ability to refuel in the air). The electrical system also changed, but most importantly, new equipment was introduced. The AINS astro-inertial navigation system, initially developed for AGM-48 Skybolt missiles, was implemented. New onboard and external communication systems were also introduced.

The value of the Blackbird was primarily due to its reconnaissance equipment and performance. The former was expanded by adding a GA-531 SLAR side-looking radar system and an ERS passive electronic reconnaissance system. The ERS system allowed for detecting emissions at a distance of up to 500 km on both sides of the aircraft, and upon detection, the system automatically tuned to the emission, tracked it, and recorded data on a cassette tape (up to 30 minutes).

In one of the compartments, the Fairchild F489 camera with a wide-angle lens for terrain mapping could optionally be installed (film 200 metres long allowed photographing a 14 km-long strip). Data collected this way was used to record objects detected in other ways and to provide exact locations.

Other compartments could house, among other things, two high-resolution panoramic cameras Hycon HR-308B (1,800 frames on a route of up to 3,700 km, with each "photo" corresponding to about 6.5 square km). While these cameras had lenses placed obliquely, the likewise used Itek HR-9085 took pictures with the help of a mirror system. The equipment could also include two HRB-454 infrared scanners (mounted obliquely).

Of course, newer equipment appeared over time, such as the new OBC panoramic camera (using ISO 8 sensitivity film), which was installed instead of the nose SLAR radar (which, over time, started to be replaced by the CAPRE radar and then the higher-performing ASARS). This rich set of equipment magnificently complemented satellite data and data collected in other ways.

But that was not what made the aircraft famous years ago. Primarily, it was an incredibly fast airplane. Its cruising speed reached Mach 3.2 (4,000 km/h) at an altitude of 24,100 metres, and the maximum was probably much higher (rumours indicate even more than Mach 3.3; at this altitude, it would be over 4,000 km/h), making it faster than the Soviet high-speed fighter MiG-25.

To prevent the aircraft from disintegrating in flight, 85 percent of the structure was made from titanium (interestingly, mostly imported from the Soviet Union). The aircraft's skin panels were slightly corrugated, making them more resilient and less prone to cracking. High speed means significant friction, and friction leads to high temperatures (e.g., the cockpit windshield heated up to 315 degrees Celsius) - to compensate for the expansion of the aircraft's elements during flight, they fit quite loosely, ensuring a seal-only flight. Some reports indicate that even the fuel system of a "cold" aircraft leaked, but this seems to be just an urban legend.

Attempts were made to reduce radar visibility in the aircraft, achieving a modest result for such a large aircraft (17-metre wingspan, 33-metre length, 5.8-metre height, empty weight of 30,600 kg) with an RCS of 10 square metres. The JP-7 fuel was enriched to reduce the visibility of the exhaust. The J58-1 engines had afterburner thrusts of as much as 151 kN (version JT11D-20J), and proper work was ensured for each of them by adjusting air intakes.

The crew, in turn, had to use specialist suits, more akin to space suits than aviation suits. Such a complex and advanced machine could not have been cheap. Each of the 32 SR-71As built cost American taxpayers an average of $23 million today, equating to over $220 million per unit. The operation wasn't cheap either: in 1990, one hour of the flight cost $200,000 of that time. Versions B (training) and C (alternative training version) were also developed. The aircraft set several speed records (especially average ones).

The Blackbird was used by the USAF from 1966 to 1990. The main operator was the 9th Strategic Reconnaissance Wing from California. The aircraft mainly operated over the USSR (though not only, for instance, during the Yom Kippur War in 1973, flights were conducted over the Middle East), although there were also flights over Poland (for instance, in December 1980).

In 1989, the aircraft fell victim to the end of the Cold War (in practice, the withdrawal concluded a year later). The arguments included, among others, high operating costs, the development of alternative reconnaissance means, and outdated communication systems. The aircraft briefly returned to service with the 9th Reconnaissance Wing from 1995 to 1997, mainly due to flights over Serbia. The USAF eventually retired the machine due to costs, while from 1990 to 1998, NASA had a pair of aircraft for testing.

Twelve machines (and one crew member) were lost during accidents. The high speed paid off: Soviet missiles or fighters never caught up with the Blackbird, which cannot be said, for example, about the newer light stealth bomber F-117.