For 24 years, from 1966 through the 1980s, US leaders from field commanders to the President of the United States relied on data gathered by SR 71 Blackbird reconnaissance aircraft. Flying missions around the globe at speeds above Mach 3 and altitudes of 85,000 feet (26,000 m) or more, Blackbirds were a vital tool of strategic military decisions as their advanced photographic and electronic sensor systems collected intelligence for the Air Force and other federal agencies.
The whole program of the SR 71 started as soon as the Lockheed U2 started to become vulnerable for the Soviet missiles. The U2 was more of a glider based design which flew at normal operating speeds and at heights up to 70,000 feet which were easy for enemy aircrafts to attack. This was an easy target for missiles too as the speeds of U2 was less too. Hence,a new aircraft was needed, that would fly faster and higher and would outrun every surface-to-air missile. This led to the birth of SR-71 Program. The SR-71 is a delta wing, twin engined, and high altitude reconnaissance aircraft with a maximum operating ceiling of 85000 feet at 3.2 Mach!
The aircraft remains a technological marvel. Practically every area of design required new approaches or breakthroughs in technology. To withstand high temperatures generated by friction in the upper atmosphere during sustained Mach 3 flight, the Blackbird required an array of specially developed materials including high temperature fuel, sealants, lubricants, wiring and other components. Ninety-three percent of the Blackbird's airframe consists of titanium alloy that allows the aircraft to operate in a regime where temperatures range from 230 degrees Celsius at its midsection to 510 degrees Celsius near the engine exhaust. The cockpit canopy, made of special heat resistant glass, must withstand surface temperatures as high as 340 degrees Celsius.
A new engine design was needed too. Turbo jet engines are in-efficient at very high speeds and Ramjet engines are not efficient at low speeds. So, both of these technologies were combined to allow for a "Variable Cycle" engine that works in all the three speed ranges. Two Pratt & Whitney J58 turbojet engines with afterburners, each supplying more than 35,000 pounds of thrust, are housed in wing with diameters larger than the fuselage itself. Virtually every part of these complex power plants had to be fabricated from special materials to meet the demands of triple-sonic flight.
A moveable spike in each inlet controls airflow, retracting at speeds above Mach 1.6 to capture more air for the engines. At sub-sonic speeds and below 30,000 feet, the spikes would be in locked position and the air needed for the turbines are fed through the bleed openings. Above the speeds of 1.6 Mach, the spikes open up and are in forward position and open up 1-5/8 inches per 0.1 Mach number. The air which is let in will be compressed by the compressors and then they split two ways. One stream is fed to the core, which is the turbine; the other stream is bypassed for the after burners. The excess air is dropped out of the system through the bleed outlets. As the speeds increase and it reaches speeds nearing 3.0 Mach, the acceleration of the aircraft itself would heat up the air too much and then compression of the air and another round of compression within the turbine would increase the air temperature tremendously. To compensate the high temperature of the air flowing through the turbines, the air: fuel mixture that is fed to the turbine is reduced to prevent the turbine blades from melting. Thus the SR-71's turbojet components produce far less thrust and the Blackbird flies most of the times with more than 80% of the thrust generated by the bypassed stream of air that is ignited in the afterburner stage and generating huge thrust as it expands through the nozzle.
This meant that the jet needed a new fuel too. The fuel not only was needed to act as a source of energy for the turbines, but also as a coolant/heat sink for the engine and aircraft accessories, air conditioning systems and TEB (Tri Ethyl Borane) tanks and control lines that actuated the afterburners. Engine oil is cooled by the main engine fuel through a heat exchanger and cooling pipes. The fuel was also used in most of the engine hydraulics too. This meant that the fuel needed to possess properties such as high thermal stability and should not break down at high temperatures and form coke deposits or damage the fuel tanks.
This criterion led to the development of JP-7(Jet Propellant 7) grade fuel. It had a huge thermal stability and a high flash point. The fuel was so stable at normal temperatures that, if you drop a lit match into a can of JP-7 fuel, the match would go off; but would not ignite the fuel. This led to another problem. The engines do not work without the fuel. This new fuel does not ignite at normal temperatures. Adding a servo motor to assist in air compression to ignite the mixture also would not solve the problem. Thus a new compound, Tri Ethyl Borane (TEB) was used. This chemical ignites at the first contact with air and it proved to be a good starter for the SR-71's advanced engines. Hence, every aircraft has a small tank of TEB containing up to 600cc to start the engines. This is the only limiting factor for the aircraft for long range operations because the SR71 has to turn off the after burners to refuel and the small size of the TEB tank limited its range as TEB was used for igniting the after burners too would last for 16 ignitions/re-ignitions.
Overall, the entire design of the plane was too far ahead in its technological achievement at that time and even new factories had to be built to manufacture them since normal tools could not be used with titanium based structures. Every aircraft that was built till this date was hand made and each aircraft had a unique response of its own. All this meant that the SR 71 which would be spying on an enemy had to just push its throttles full forward as soon as it sees a SAM alert.
Though the Blackbird was decommissioned from service in the 1990's, researchers are looking into using the SR-71 design along with aero spike engines to use this as an Reusable Launch Vehicle for space programs.
All these things remind me about the quote at the entrance to the SR-71 Hangar at Kadena Air Base in Japan.
"Though I fly through the valley of death, I shall fear no evil for I am at 80,000 feet and climbing"
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