In the 1960s the Cold War was heating up and after the shot down of Gary Powers by a Soviet missile, while he was flying in the U-2, the USA needed a new kind of airplane.
The SR-71 was developed as a black project, a highly classified project by Lockheed Corporation from the Lockheed A-12 reconnaissance aircraft during the 1960s by Lockheed’s Skunk Works division. This airplane was developed by the American aerospace engineer Clarence “Kelly” Johnson and the main feature was the reduced radar cross-section. At one point, a bomber variant of the aircraft was under consideration, before the program was focused solely on reconnaissance. Mission equipment for the reconnaissance role included signals intelligence sensors, a side-looking airborne radar, and a photo camera; the SR-71 was both longer and heavier than the A-12, allowing it to hold more fuel as well as a two-seat cockpit. The SR-71 designation has been attributed to lobbying efforts by USAF Chief of Staff General Curtis LeMay, who preferred the SR (Strategic Reconnaissance) designation over simply RS (Reconnaissance, Strategic). The aircraft was introduced to operational service in January 1966. This airplane is a long-range, high-altitude, Mach 3+ strategic reconnaissance aircraft. It was able to achieve Mach 3.2 at 85.000ft and its standard evasive action was simply to accelerate and outfly a surface-to-air missile that was launched. A total of 32 aircraft were built but 12 were lost in accidents with none lost to enemy action.
Kelly Johnson, one of the preeminent aircraft designers of the twentieth century, and his Skunk Works team had a track record of delivering “impossible” technologies on incredibly short, strategically critical deadlines. The U-2 was but one example. The group was known for its unfailing sense of duty, its creativity in the face of a technological challenge, and its undaunted perseverance.
This new aircraft was in a different category from anything that had come before. “Everything had to be invented. Everything,” Johnson recalled. He committed Skunk Works to succeed in its toughest assignment to date: to have the innovative, challenging, envelope-bursting aircraft flying in a mere twenty months.
The SR-71 Blackbird had two in tandem cockpits and the reconnaissance systems officer operating the surveillance systems and equipment from the rear cockpit and directing navigation on the mission flight path. Finished aircraft were painted a dark blue, almost black, to increase the emission of internal heat and to act as camouflage against the night sky. The dark color led to the aircraft’s nickname “Blackbird”. Because of its small radar cross-section, this aircraft was able to make it hard for a missile to acquire and track the aircraft on radar and had time to evade a surface-to-air missile successfully.
SR-71 Blackbird was made mostly out of titanium, 85% of the structure being made out of this material, and much of the rest was built out of polymer composite materials. To control costs, Lockheed used a more easily worked titanium alloy which softened at a lower temperature. The challenges posed led Lockheed to develop new fabrication methods, which have since been used in the manufacture of other aircraft. Lockheed found that washing welded titanium requires distilled water, as the chlorine present in tap water is corrosive; cadmium-plated tools could not be used, as they also caused corrosion. Metallurgical contamination was another problem; at one point, 80% of the delivered titanium for manufacture was rejected on these grounds. The high temperatures generated in flight required special design and operating techniques. Fuselage panels were manufactured to fit only loosely with the aircraft on the ground. Proper alignment was achieved as the airframe heated up and expanded several inches. Because of this, and the lack of a fuel-sealing system that could handle the airframe’s expansion at extreme temperatures, the aircraft leaked JP-7 fuel on the ground prior to takeoff.
The outer windscreen of the cockpit was made of quartz and was fused ultrasonically to the titanium frame. The temperature of the exterior of the windscreen reached 600 °F (316 °C) during a mission. Cooling was carried out by cycling fuel behind the titanium surfaces in the chines. On landing, the canopy temperature was over 572 °F (300 °C).
The red stripes featured on some SR-71s were to prevent maintenance workers from damaging the skin. Near the center of the fuselage, the curved skin was thin and delicate, with no support from the structural ribs, which were spaced several feet apart.
The Blackbird’s tires, manufactured by B.F. Goodrich contained aluminum and were filled with nitrogen. They cost $2,300 and would generally require replacing within 20 missions. The Blackbird landed at over 170 knots (200 mph; 310 km/h) and deployed a drag parachute to stop; the chute also acted to reduce stress on the tires.
Titanium was in short supply in the United States so the special agencies made up phantom enterprises in various countries in order to acquire the much-needed material from the enemy, USSR.
The SR-71 had a radar cross-section (RCS) around 110 sq ft (10 m2) duet o its design. Drawing on early studies in radar stealth technology, which indicated that a shape with flattened, tapering sides would reflect most energy away from a radar beam’s place of origin, engineers added chines and canted the vertical control surfaces inward. Special radar-absorbing materials were incorporated into sawtooth-shaped sections of the aircraft’s skin. Cesium-based fuel additives were used to somewhat reduce exhaust plumes’ visibility to radar, although exhaust streams remained quite apparent. The SR-71 featured chines, a pair of sharp edges leading aft from either side of the nose along the fuselage. These were not a feature on the early A-3 design; Frank Rodgers, a doctor at the Scientific Engineering Institute, a CIA front organization, discovered that a cross-section of a sphere had a greatly reduced radar reflection, and adapted a cylindrical-shaped fuselage by stretching out the sides of the fuselage. After the advisory panel provisionally selected Convair’s FISH design over the A-3 on the basis of RCS, Lockheed adopted chines for its A-4 through A-6 designs.
Aerodynamicists discovered that the chines generated powerful vortices and created additional lift, leading to unexpected aerodynamic performance improvements. The engine had a lower consumption at higher speeds and was made by Pratt & Whitney. At the front of each inlet, a pointed, movable cone called a “spike” (inlet cone) was locked in its full forward position on the ground and during subsonic flight. When the aircraft accelerated past Mach 1.6, an internal jackscrew moved the spike up to 26 in (66 cm) inwards, directed by an analog air inlet computer that took into account the pitot-static system, pitch, roll, yaw, and angle of attack. Moving the spike tip drew the shock wave riding on it closer to the inlet cowling until it touched just slightly inside the cowling lip. This position reflected the spike shock wave repeatedly between the spike center body and the inlet inner cowl sides and minimized airflow spillage which is the cause of spillage drag. The air slowed supersonically with a final plane shock wave at the entry to the subsonic diffuser. Downstream of this normal shock, the air is subsonic. It decelerates further in the divergent duct to give the required speed at the entry to the compressor. The capture of the plane’s shock wave within the inlet is called “starting the inlet”. Bleed tubes and bypass doors were designed into the inlet and engine nacelles to handle some of this pressure and to position the final shock to allow the inlet to remain “started”. The SR-71 was powered by two Pratt & Whitney J58 (company designation JT11D-20) axial-flow turbojet engines. The J58 was a considerable innovation of the era, capable of producing a static thrust of 32,500 lbf (145 kN). The engine was most efficient around Mach 3.2, the Blackbird’s typical cruising speed. At take-off, the afterburner provided 26% of the thrust. Air was initially compressed (and heated) by the inlet spike and subsequent converging duct between the center body and inlet cowl. The shock waves generated slowed the air to subsonic speeds relative to the engine. The air then entered the engine compressor. Some of this compressor flow (20% at cruise) was removed after the fourth compressor stage and went straight to the afterburner through six bypass tubes. The air passing through the turbojet was compressed further by the remaining five compressor stages and then fuel was added to the combustion chamber. After passing through the turbine, the exhaust, together with the compressor bleed air, entered the afterburner.
Originally, the Blackbird’s J58 engines were started with the assistance of two Buick Wildcat V8 internal combustion engines, externally mounted on a vehicle referred to as an AG330 “start cart”. The start cart was positioned underneath the J58 and the two Buick engines powered a single, vertical drive shaft connecting to the J58 engine and spinning it to above 3,200 RPM, at which point the turbojet could self-sustain. Once the first J58 engine was started, the cart was repositioned to start the aircraft’s other J58 engine. Later start carts used Chevrolet big-block V8 engines. Eventually, a quieter, pneumatic start system was developed for use at main operating bases. The V8 start carts remained at diversion landing sites not equipped with the pneumatic system.
After retirement in the mid-1990, aircraft were distributed to static display locations, with a number kept in reserve storage but it was reactivated a few years later. The final retirement from USAF took place in 1998 but NASA operated two last airworthy Blackbirds until 1999.
Throughout its life SR-71 broke some records like an “absolute altitude record” of 85,069 feet (25,929 m) or an absolute speed record of 1,905.81 knots (2,193.2 mph; 3,529.6 km/h), approximately Mach 3.3, and still holds this records. Also, it holds the “speed over a recognized course” record for flying from New York to London—distance 3,461.53 miles (5,570.79 km), 1,806.964 miles per hour (2,908.027 km/h), and an elapsed time of 1 hour 54 minutes and 56.4 seconds—set on 1 September 1974, while flown by USAF pilot James V. Sullivan and Noel F. Widdifield, reconnaissance systems officer. On its retirement flight from Los Angeles to Washington in 1990, to its final resting place in the Smithsonian Air & Space collection, the plane flew coast to coast in 67 minutes.
Its successor was proposed duet o the time that satellites need to arrive in the proper position in order to take pictures and on 1 November 2013, media outlets reported that Skunk Works has been working on an unmanned reconnaissance airplane it has named SR-72, which would fly twice as fast as the SR-71, at Mach 6 and it would be an Unmanned aerial vehicle (UAV).
You can still see this piece of engineering in museums across the United States, but also in the American Air Museum in Britain, Cambridgeshire, England.