Big, Black & Bold

The Lockheed proposals were designated A-1 to A-12. The final twin-engined A-12 design was given a full go-ahead on 30th January 1960. Twelve A-12s were manufactured and were originally fitted with Pratt and Whitney J75 afterburning engines until the J58 engine became available. The J58 (JT11D-20) was originally designed for a US Navy attack aircraft(later cancelled), and was adapted (or redesigned) for use in the Lockheed A-12. When Lockheed were satisfied with the J75/A-12 subsonic performance, a J58 was installed in one side. After several months of testing, J58s were fitted in both sides, and supersonic testing commenced.

The Air Force then saw use for the aircraft as a long-range air defence fighter, and the YF-12 was born. The A-12 design was modified and fitted with Hughes ASG-18 Doppler fire control radar, and four nuclear-tipped AIM-47A (GAR-9) air-to-air missiles. The first such YF-12 flew on 7th August 1963. The reconnaissance version was approved for production in December 1962, when a contract for the first batch of six was signed. This type was designated SR-71 and was to be the definitive version. The first SR-71 flew on 23rd December 1964.

The Blackbird’s maximum speed has yet to be officially released, but an SR-71 holds the FAI world absolute speed and height records over a 15/25 km straight course in horizontal flight of 2189 mph at 86,000 ft, set in July 1976. More importantly, the SR-71 can maintain this sort of speed for extended periods. The operating altitude of the Blackbird is so high that the crew qualify for astronaut wings. At this height and speed, ram air friction heats the aircraft skin to 300°C, and the thermal stresses cause the aircraft to stretch by 11 inches. Almost every part of the aircraft had to be specially designed to withstand the tremendous heat. The aircraft is basically titanium (Lockheed/ TMC 8-120), the engines use a special fuel, the intake system is revolutionary, the hydraulic fluid is unique, the control system is complex, and the aerodynamics approach perfection.

Performance

The Blackbird cruises at altitudes in excess of 80,000 feet at speeds of over Mach 3. Its two J58 engines each produce 32,500 lbs static thrust (in horsepower, equivalent to the ‘Queen Mary’ for each engine), and the aircraft’s gross weight is 145,000 lbs. Cold, the aircraft is 107 ft 5 in (32. 74 m) in length, stands 18 ft 6 in (5. 64 m) high, and has a span of 55 ft 7½ in (16. 94 m). A lift-off speed of 200 kts is normal, and the take-off roll is in the vicinity of 5000 feet (1500 m). By the time the wheels are up, it has power enough to accelerate away from a Mirage Ill in the process of making a maximum speed run at low level. The aircraft climbs from sea level at 10,000 ft/min at 400 KEAS, and at altitude is still climbing at 1000 ft/min at Mach 2. 99. Once cruising at high altitude, the aircraft’s sensors can bring 100,000 square miles of the earth’s surface under scrutiny in one hour! The Blackbird carries 80,000 lbs (36,000 kg) of JP-7 fuel, giving it a range of 2400 miles (3860 km). Additionally, the aircraft has an air-refuelling capability, using a special KC-135O tanker fitted with a dual fuel system for handling special JP-7 fuel as well as normal JP-4. (In November 1979, during Kangaroo 111, a KC-135Q was observed in Australia, but was in ‘Dirty-Q’ configuration – only JP-4 being carried at the time). In April 1971, an SR-71 completed a non-stop flight of some 15,000 miles (24,000 km) in 10½ hours, with an average speed of over 1400 mph (2260 km/hr). This time and speed include descending and decelerating to subsonic speed to hook up with the KC-135Q tanker!

The aircraft descends initially at only 400 ft/min, with speeds of Mach 2. 5 at 70,000 ft. By 50,000 ft, the descent rate reaches 5000 ft/min at Mach 1. 6. The aircraft goes subsonic at 30,000 ft. In the circuit, airspeed is kept around 250 kts, slowing down to 180 kts on finals. Nose up pitch for landing flare is about 12°, and the delta wing and chine configuration (discussed later), generates enough ground effect to result in a landing sick rate of 1 to 1 ½ ft/sec – making landings (at 150 kts) exceptionally smooth. A brake chute is deployed after touch-down to reduce landing roll to 3600 ft (1070 m).

Construction

Heat was the biggest problem to overcome. The structure had to be able to withstand extremes of temperature ranging from as low as – 90°F in subsonic flight during refuelling to up to 1200°F around the engine nacelles at high speed. When an inlet malfunctions, the result can be quite dramatic. When an inlet ‘unstarts’, the thrust it produces is lost, and the aircraft yaws violently into the malfunctioning engine the crew’s heads against the side of the cockpit. Only titanium and stainless steel had the ability to withstand the expected temperature environment of Mach 3 (300°C), and because titanium weighs about half as much as stainless per cubic inch, it was chosen for 93% of the aircraft structure. Early in design development, it was found that the thermal stresses in large wing panels would warp them severely, so chordwise corrugations were pressed into the outer skin, controlling the warping satisfactorily. Millions of dollars were spent in the development of the special Lockheed/TMC (Titaniums Metal Corporation) B-120 alloy, and on the research of titanium alloy manufacturing techniques.

Problems with the fuel tank sealants and hydraulic oil were encountered. A sealant was specially developed that would function throughout the temperature range, but some drawbacks had to be accepted. Because of airframe expansion and contraction, the fuel tanks leak profusely on the ground. JP-7 has, however, a very high flash point and a lighted match will not ignite it. Hydraulic oil costing $130 per gallon was developed at Penn State University for the project. Rubber O-rings could not be used in hydraulic cylinders, so with a special chemical coating were developed for titanium pistons working in titanium cylinders.

Engine

The centreline of the basic J58 was laid down in late 1956. It was to be a 26,000 lb st afterburning turbojet to power a Navy attack aircraft with a dash capability of up to Mach 3 for several seconds. However, the Navy J58-P2 had to be greatly modified to fit the Lockheed requirement. A chemical ignition system using tetraethyl borane (TEB) for igniting the main engines and the afterburners was developed.

Ground testing the engine was not without difficulty, as there was no test facility that could provide the temperature and pressure conditions necessary. So a test stand was developed where the exhaust of a J75 engine was run through and around the test J58 to simulate the temperature environment. Other problems were encountered during flight testing. For example, under certain wing loading conditions, the nacelle would rotate into the engine and crush the ‘plumbing’. So a new engine mount was designed. The remote gearbox was also found to be moving by 4 inches relative to the engine during high transient Mach numbers, so a new joint system was installed.

Over 600 horsepower is required to turn the J58 engine up to starting RPM, and originally an engine starting cart, using two Buick racing car engines geared to a common shaft drive, was used for starting. After several years an air starting system was installed into operational hangars. The JP-7 fuel is used to cool the crew and aircraft electronics, then used as a hydraulic fluid at 600°F to control the afterburner exit flaps, before being fed into the powerplant. Greases, electrical wiring and plugs, high-temperature plastics for radomes also had to be designed.

Turboramjet

Compressor blades do not operate efficiently at supersonic speeds because of the shock waves that form on them. Inlet designs for supersonic engines aim to reduce supersonic airflow to subsonic speeds. The Blackbird system uses round mixed compression inlets 21 feet long, at Mach 3 each inlet ingesting 2800 cubic metres of air per second (equivalent to 6 million people breathing in). The inlet design acts to trap the shock wave inside the nacelle and ahead of the compressor, allowing air to enter the compressor at subsonic speeds. Only a portion of this compressed inlet air enters the combustion chamber; the remainder is channelled to bypass the engine and enter the exhaust nozzles to expand and produce extra thrust. In fact, at cruise speeds, 54% of thrust is produced by the inlets, acting as superchargers in front of the engines, and a further 28 is derived from the exhaust ejector design(convergent/divergent nozzles re-accelerating the bypass air to close to cruise Mach). The afterburning engines produce the rest of the thrust, keeping the whole system ticking over.

The inlet cones, or ‘spikes’, move almost 3 feet (1 metre) to control the position of the shock wave in the inlet. The spikes are operated by a hydraulic actuator which can provide 31,000 lbs of force under some nacelle flow conditions. If an inlet malfunctions, the result can be quite dramatic. When an inlet ‘unstarts’, or ‘pops its shock’, the thrust it produces is lost, and the aircraft yaws violently into the failed inlet, banging the crew’s heads against the cockpit side. Accelerometers were installed in the fuselage that sense the yaw and rudder input is automatically provided to correct the swing. The spikes then move all the way forward and automatically restart the inlet.

Aerodynamics

To minimise aerodynamic drag, which increases with speed, Lockheed chose a blended wing-body design. This allowed a slim, long fuselage and a thin delta wing. The lift axis of a delta wing, however, moves rearward with speed, and in most delta wing aircraft, like the Mirage Ill, a gradual up-eleven force overcomes this rearward movement. With the SR-71, this method would have caused too much drag, so the sides of the forward fuselage were broadened out to form lifting ‘chines’, acting like a fixed canard. To account for the effect of chine airflow, the inlet spikes are pointed downward and inward to the thrust line. Dual, inwardly canted vertical stabilizers mounted on the rear of the engine nacelles provide directional stability and are all-moving type control surfaces. Elevens inboard and outboard of the engines provide pitch and roll control. The control system is hydraulically powered by two independent systems. No useful lift increase was found with the use of wing flaps, so the ground effect is used to obtain satisfactory take-off and landing characteristics. The efficiency of the aerodynamics of the Blackbird can perhaps be measured by its lift/drag ratio – an incredible 11. 5 at subsonic, and around 6. 3 at supersonic!

Fuel System

The Blackbird has six fuel tanks in its fuselage and wings. Because the airframe is constantly expanding and contracting, it is impossible to keep the tanks sealed, and fuel leaks copiously from the tanks when on the ground. The JP-7 has a very high flash point and is used extensively for cooling. When the tanks are partly empty, the bottom of the fuselage remains cooler than the top, causing unique aerodynamic and structural problems. 80,000 lbs of fuel is carried and is controlled by an automatic system to keep the very critical centre of gravity in exactly the right position.

Crew

The Blackbird has a crew of two, seated in tandem cockpits: a pilot, and a Reconnaissance Systems Operator (RSO). The crews must pass a stringent physical test, comparable to that undergone by NASA astronauts, and will, in fact, operate in a very similar environment. To keep the crew compartment cool in the high temperatures a huge air conditioning unit and other special cooling systems are incorporated. Operating at incredibly high altitudes, the crew wear full space suits, as used in the Gemini programme. The air at 80,000 ft is only about 5% as thick as at sea level! It takes about 30 minutes to don the suit, aided by Physiological Support Division personnel, and portable cooling units keep the inside of the suit cool before entering the aircraft. The crew pre-breathes 100% oxygen for about 30 minutes before the flight to eliminate nitrogen in the body and reduce the chances of decompression sickness at high altitudes. Special high protein, low residue meals are provided before each mission. The suit also provides protection against wind blasts in the event of ejection, but can never provide enough protection against the scorching heat, which is sufficient to scorch a glove put up to the windscreen, which reaches 350°C at cruising Mach.

Navigation System

The Blackbird uses an astro-inertial navigation system of incredibly high accuracy, mounted behind the RSO’s cockpit in the upper fuselage. The system has the capability to track 52 stars 24 hours a day. A special data tape is installed in the aircraft prior to the mission to provide the constantly changing celestial navigation information. The system’s clock has an accuracy of ± 5 milliseconds. A star tracker mounted on gimbals takes periodic sightings, information from which is channelled to the system computer, keeping the aircraft within a mile of track.

Operation

The SR-71 entered service with the USAF Strategic Air Command in 1965. Since then, they have exceeded Mach 3 over 11,000 times, and have completed 18,000 air refuellings. Today, they still serve with the 9th Strategic Reconnaissance Wing, forming the 1st Strategic Reconnaissance Squadron. Over 30 SR-71s are believed to have been built, some of which have crashed, others now in storage. Three versions were built: the A, B and C.

The SR-71A is the reconnaissance type, while the SR-71B and C are training versions, fitted with a second pilot’s cockpit in place of the RSOs. Naturally enough, much surrounding the operation of the Blackbird is classified. However, both the Koreans and Russians have complained of overflights by SR-71s. A detachment of Blackbirds were once based at Kadena Air Force Base at Okinawa, Japan, and is believed to have been conducting flights over China. Cuba also, no doubt, has been scrutinised by the electronic eyes of the Blackbird. It is also likely that they observed the Arab/Israeli wars of 1967 and 1973, and the Greek/Turkish flare-up. SR-71s were also used in the Southeast Asian conflict, one aircraft, 61-7974, completing at least 44 communist overflights. The complete operational history of the USAF Blackbirds will probably not be released for many years to come. Even during construction, false companies were formed to hide the aircraft’s existence, yet at the height of the programme, some 9000 Lockheed employees were working on the Blackbird production line. Twelve A-12s were built, one of which was converted to the first of four YF-12 prototypes. The last YF-12 was designated YF-12C and was actually the prototype for the SR-71. Some forty SR-71As and Bs are believed to have been made. (The SR-71C is an A version converted to B configuration. )

During the YF-12 development, trials were conducted with a reconnaissance drone mounted on top of the fuselage. Thirty-eight of these GTD-21 drones were manufactured by Lockheed, to be launched at high altitudes and at speeds sufficient for ignition of the drone’s single ramjet engine. Other drones or remotely piloted vehicles are believed to have been carried semi-submerged in the Blackbird’s fuselage equipment bay. The now retired GTD-21 was forty feet (12. 2 m) in length and had a camera bay in the underside of its fuselage. The first photograph of this drone mounted on a YF-12, taken in the 1960s, was first released in May 1982 – giving some idea of the secrecy of both projects.

The A-12s and YF-12s went into storage in 1968 until their complete retirement in 1976. (Two YF-12s were operated by NASA for research purposes during this time. ) The SR-71 however, will continue to operate for a number of years until it will apparently be replaced by another Lockheed ‘Skunk Works’ project- the ‘Stealth’-technology penetrator. Far less is known about this project than the Blackbird, but it is believed to have first flown in 1977. Design philosophy has tended towards low radar, infra-red and optical signatures rather than high performance – invisibility rather than speed. Lockheed’s ‘Stealth’ is a single-seat reconnaissance/strike aircraft, and it is believed that a similar technology ‘Stealth’ bomber is being developed by Northrop. The aircraft are reputedly invisible to radar; a radar-ablating paint coating converts radar energy to heat so that the paint smokes off on exposure to radar emissions and thus does not return any signals. The reconnaissance/strike penetrator is thought to be powered by two 12,000 lb st (53. 4 kN) turbojets, giving it a maximum speed of about M = 1. 5. The engine intakes and two-dimensional nozzles are installed on top of the fuselage, providing a very low infra-red signature. Although no photographs have been released, the aircraft is believed to be roughly the same size as the SR-71. Perhaps in twenty years’ time, the story of this fascinating aircraft will begin to come to light – just as the incredible Blackbird of the 1960s is just now becoming ‘public’.