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Writer's pictureKatie B
Jul 14, 20206 min read

Case Study: SR-71 by Spencer

Updated: Jan 5, 2021

The Lockheed SR-71 Blackbird was a reconnaissance aircraft developed and used by the United States Air Force (USAF) that was first put into action in 1966 (Paul R. K., 1998) but was designed in secrecy in the late 1950s. The SR-71 was a replacement for the U-2 spy plane, which was taken out of service due to the advancements in Soviet anti-aircraft missiles “another hit a Soviet interceptor attempting to reach the American aircraft. The U-2 crashed but Powers survived the near miss and was quickly captured; the crash did not destroy the U-2 and the Soviets were able to identify much of the equipment” (Edlow and Welzenbach 1992). The U-2 spy plane was not very practical due to the fact the wingspan and shape of the aircraft meant that it needed a spotter to land and using predefined terms to tell the pilot which position to land the aircraft, this meant more people were needed to run the aircraft and it wasn't easy to get into the air “To get an outside viewpoint, another U-2 pilot literally drives behind the aircraft at high speeds while it’s landing, radioing information to the pilot in the plane to help them land” (Rob Verger 2019) The SR-71 was part of a fleet of spy planes built to go deep into enemy territory without being shot down or even detected in a time before satellites, drones and advanced technology like unmanned reconnaissance aircraft.


The SR-71 was designed to minimise its radar detection, an early attempt at stealth aircraft. The finished aircraft was painted a dark blue, almost black, to increase absorption of heat and radar in a further attempt at making it a stealth aircraft and to act as camouflage in the night sky. The dark colour led to the aircraft's nickname ‘The Blackbird’. This special paint was used for Blackbird’s wings, tail and fuselage, which contained iron ferrites, this absorbed incoming radar energy instead of reflecting it to the sender which would reveal its location “Special radar-absorbing materials were incorporated into sawtooth-shaped sections of the aircraft's skin.” (Hott, Bartholomew and George E. Pollock, 2014) . The sleek lines of the long fuselage made the plane look unique, unlike anything before it.


Heat can be potentially detrimental for the aircraft, especially flying at supersonic speeds, this is supported by research done by the UK Department for Aircraft Structure which “showed that temperatures between about 100 and 150°C gave improvements in fatigue strength whereas at higher temperatures the strength dropped rapidly below the room temperature value until at 300°C the failure was due primarily to creep” (Kiddle and Heath-Smith, 1975). This highlights that the aircraft needs to be able to withstand a lot of heat to avoid damaging the structure of the aircraft potentially putting the aircraft and aircrew in danger. Furthermore, the heat from the aircraft, jet engines and after-burners makes the plane substantially more detectable by radar. The USAF overcame this problem by making the aircraft able to absorb most of the heat generated through air resistance thus meaning its radar footprint and structural integrity were solid, this was done through smartly designed aerodynamics and also the use of iron ferrites in the paint.


The range and manoeuvrability alongside electronic countermeasures, which scramble signals and block laser or GPS guided missiles, meant that it was an invulnerable aircraft at the time “Over its operational life, the Blackbird carried various electronic countermeasures (ECMs), including warning and active electronic systems built by several ECM companies and called Systems A, A2, A2C, B, C, C2, E, G, H, and M. On a given mission, an aircraft carried several of these frequency/purpose payloads to meet the expected threats.” (Air & Space Magazine: 45. December 2014). With a radar cross-section of a small light aircraft, if the SR-71 Blackbird was found on a radar scanner it was too late for the Surface to Air Missile (SAM) Computer and missiles to lock on for a successful and accurate launch.


The Pratt & Whitney J58 was an American jet engine that was not specifically designed for the SR-71 and had previously been used in the Lockheed A-12 and YF-12 “The J58 was initially developed for the US Navy to power the planned version of the Martin P6M jet flying boat.” (National Museum of the Air Force 2010) The SR-71 had a wide speed range meaning the engine needed two modes of operation to take it from idle on the ground, to high speeds at astonishing altitudes. It was a conventional afterburning turbojet for take-off and acceleration to Mach 2 and then used a permanent compressor bleed to the afterburner from Mach 2 and above. It was an afterburning turbojet with a special compressor bleed to the afterburner which gave increased thrust at higher speeds “After passing through the turbine, the exhaust, together with the compressor bleed air, entered the afterburner.” (SR-71 Online - SR-71 Flight Manual: Section 1, Page 1-20) . The way this engine was designed allowed it to reach speeds of unimaginable magnitude, at Mach 3.3+, along with reaching revolutionary altitude even to this day (25,908 metres) (Paul R. K., 1998) which made it the highest-flying supersonic fighter jet ever made to this day.


JP-7 was a special type of fuel developed for the USAF that was usually used for supersonic or higher category aircraft and was extremely hard to ignite. The USAF suggested that Lockheed use this system which added to the complexity and price tag of the aircraft, almost coming in at $40 million for the whole fleet ($10 Billion in today’s money (Ian Webster, 2020)) (Edwards, Owen (July 2009))


In some cases, the aircraft could cost as much as $30,000 ($200,000 in today’s money (Ian Webster, 2020)) per hour to operate when all of the military factors are accounted for but was seen as justifiable because of its small specialised fleet of 32 planes (Marshall, Eliot 1990). The unique design of the SR-71 was a burden for the military’s budget as most of the parts were custom made, not accounting for its designer supersonic grade fuel which itself cost $18,000 per hour ($130,000 in today’s money Ian Webster, 2020)). This was deemed justifiable because it meant that they could collect the information they needed to during the cold war from Soviet territory.


As the Air Force’s budgets declined toward the end of the Cold War, the service could no longer justify keeping the expensive SR-71 in its operational fleet “One view is that the SR-71 program was terminated due to Pentagon politics, and not because the aircraft had become obsolete, irrelevant, suffered maintenance problems or had unsustainable program costs” (Graham, Richard (7 July 1996)). This was especially true due to the advancement of Soviet SAM missiles which could reach a much higher altitude and thus meaning the SR-71 could no longer hold its legendary status as being able to outrun every Soviet missile. The replacement of spy planes with dual role aircraft (reconnaissance and bomber capabilities) along with Unmanned Aerial Vehicles (UAVs) meant that there was no longer a need for a specific purpose aircraft “are also used for much aerial reconnaissance in the 21st century, being able to overfly hostile territory without putting human pilots at risk, as well as being smaller and harder to detect than man-carrying aircraft” (Norris, Guy (1 November 2013)). The change of focus from the USAF and the US military as a whole changed after 2001, to fighting terrorism after the 9/11 attacks. The service expected to replace the ‘Blackbird’ with satellites and other technologically advanced systems. The successor to the SR-71 is the Lockheed Martin SR-72 an unmanned, hypersonic (Mach 5 and above) reconnaissance aircraft concept (Norris, Guy (1 November 2013)).


The SR-71 was used by NASA for research purposes until 1997. The purpose of NASA using the SR-71 was to research and test at Mach 3 and above, because of the SR-71’s capabilities it was the perfect candidate for researching at supersonic speeds. “The extreme operating environment in which they flew made the aircraft excellent platforms for conducting research and experiments in a variety of disciplines: aerodynamics, propulsion, structures, thermal protection materials, high-speed and high-temperature instrumentation, atmospheric studies, and sonic boom characterization.” (Brian D. , 2014)


 

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