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The airframe was made of titanium obtained from the USSR during the height of the Cold war. The builder used all possible guises to prevent the Soviet government from knowing what the titanium was to be used for. In order to keep the costs under control, they used a more easily worked alloy of titanium which softened at a lower temperature. They painted the aircraft dark blue (almost black) to dissipate heat and to act as camouflage against the sky.

The shape of the vehicle is designed so that the plane had a very small 'radar cross-section' — the SR-71 was an early stealth design. In addition, the geometry of the airframe is such that the engine inlets are inline with the shockwave from the nose of the aircraft. This compressed the air in a similar way to a ramjet and permitted higher performance. Indeed, at top speed more than 80% of the thrust was due to the ramjet effect. However, for this effect to operate successfully it also necessitated moveable inlet cones; incorrect positioning tended to make the engines 'unstart', a curious euphemism for when the engine's combustion is essentially blown out like a candle. Due to the tremendous thrust of the remaining engine pushing the aircraft asymmetrically along with the sudden deceleration caused by losing 50% of available power, an unstart would cause the aircraft to violently yaw to one side. This caused at least one pilot to crack his crash helmet on the cockpit canopy, although no aircraft were known lost to this event. Lockheed engineers eventually developed control software for the engine inlets that would recapture the lost shockwave and relight the engine before the pilot was even aware an unstart had occurred. The SR-71 machinists were responsible for the hundreds of precision adjustments of the forward air by-pass doors within the inlets. This helped control the shock wave, prevent unstarts and increase performance.

Due to the great temperature changes in flight, the fuselage panels were supposedly essentially loose. Proper alignment was only achieved when the airframe warmed up, due to the air resistance at high speeds, and the airframe then expanded several inches. Because of this, and the lack of a fuel sealing system that could handle the extreme temperatures, the aircraft would leak its JP-7 jet fuel onto the runway before it took off. The aircraft would quickly make a short sprint, meant to warm up the airframe, and was then air-to-air refueled before departing on its mission. Cooling was carried out by cycling fuel behind the titanium surfaces at the front of the wings (chines). Nonetheless, once the airplane landed no one could approach it for some time as its canopy was still hotter than 300 degrees Celsius. Asbestos (non-fibrous) was also used, such as in non-ceramic automotive brakes, due to its high heat tolerance.

The JP-7 jet fuel is interesting in its own right: originally developed for the A-12 Oxcart plane in the late 1950s, it has an extremely high flashpoint to cope with the heat, to the extent that a match dropped in a bucket of JP-7 does not ignite it. The fuel also contains fluorocarbons to increase its lubricity, an oxidizing agent to enable it to burn in the engines, and even a cesium compound, A-50, which disguises the exhaust's radar signature. As a result, JP-7 is said to be more expensive than malt Scotch whisky, which gives some idea of how much a single SR-71 mission would have cost.

Studies of the aircraft's titanium skin revealed the metal was actually growing stronger over time due to the intense heating caused by aerodynamic friction, a process similar to annealing.

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Ford Tri-Motor

 The skin of the SR-71 is actually corrugated, not smooth. The thermal expansion stresses of a smooth skin would have resulted in the aircraft skin splitting or curling. By making the surface corrugated, the skin is allowed to expand vertically as well as horizontally without overstressing, which also increases longitudinal strength. Despite the fact that it worked, aerodynamicists were aghast at the concept and accused the design engineers of trying to make a 1920's era Ford Tri-motor, known for its corrugated aluminum skin, go Mach 3.

The J-58 engines used in the Blackbird are the only military engines ever designed to operate continuously on afterburner, and actually become more efficient as the aircraft goes faster. Each J-58 engine could produce 32,500 lbf (145 kN) of static thrust. Conventional jet engines cannot operate continuously on afterburner and lose efficiency as they go faster.

The Blackbird's engines started up with the assistance of an external "start cart", a cart containing two Buick V-8 engines which was rolled out onto the runway underneath the aircraft. The two Buick engines powered a single, vertical driveshaft connected to a single J-58 engine. Once one engine was started, the cart was wheeled over to the other side of the aircraft to start the other engine. The operation was deafening.

 

 

The Myth And The Lore

 

The plane has developed a small cult following, given its design, specifications, and the aura of secrecy that surrounds it. Some conspiracy theorists have speculated that the true operational capabilities of the SR-71 and the associated A-12 have never been revealed. Most aviation buffs speculate that given a confluence of structural and aerodynamic tolerances that the plane could fly at a maximum of Mach 3.3 for extended periods, and could not exceed Mach 3.44 in any currently known configuration. Specifically, these groups cite the specific maximum temperature for the compressor inlet of 427?C. This temperature is quickly surpassed at speeds greater than Mach 3.3. Mach 3.44 is given as the speed at which the engine enters a state of "unstart". Some speculate that the former condition can be alleviated by superior compressor design and composition, while the latter might be solved with improved shock cones.

There is a smaller group of individuals that believe the SR-71 is already capable of Mach 4 or greater. This is supported primarily by the reconnaissance flights where the mission times and distances traveled could only be accounted for by speeds between Mach 3.6 and 4.1. It is projected by a few that later improved craft might approach speeds of Mach 4.5, and be competitive with the X-15 under specific flight conditions.

It should be noted that the SR-71's Pratt & Whitney J58 engines never exceeded test-bench values above Mach 3.6 in unclassified tests. Given the history of the plane, the advanced and classified nature of much of its original design, and most importantly, the simple fact that no SR-71 exists in a form which is immediately airworthy, it may never be known what the true design tolerances of the aircraft were, or if these tolerances were ever approached in flight. This un-verifiability undoubtedly contributes to the myths and fallacies surrounding the SR-71.

 

 

From The SR-71 Flight Manual

 

The operating envelope of the JT11D-20 engine requires special fuel. The fuel is not only the source of energy but is also used in the engine hydraulic system. During high Mach flight, the fuel is also a heat sink for the various aircraft and engine accessories which would otherwise overheat at the high temperatures encountered. This requires a fuel having high thermal stability so that it will not break down and deposit coke and varnishes in the fuel system passages. A high illuminometer number (brightness of flame index) is required to minimize transfer of heat to the burner parts. Other items are also significant, such as the amount of sulfur impurities tolerated. Advanced fuels, JP-7 (PWA 535) and PWA 523E, were developed to meet the above requirements.?

 

 

 

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