Not all helicopters use the same fuel, and neither do airplanes. However, some types are fairly standard, such as aviation gasoline for older propeller-driven aircraft and models with piston engines. Also, the most common type of fuel used in helicopters and airplanes is A-1 jet fuel (typically US jet fuel). Of course, military aircraft operate under very different parameters than their commercial counterparts, so modern models often use a fuel known as JP-8 or Jet Propulsion 8. This fuel contains additives such as icing inhibitors and antioxidants that improve the jet’s performance in extreme environments, as military models sometimes have.
When the sophisticated SR-71 Blackbird was introduced, it required a completely different fuel for its incredible capabilities. The aircraft was first put into service in 1966. in January and was one of the world’s fastest aircraft capable of exceeding Mach 3. It was not only the extreme speed that made it impressive, but also the height at which it could fly: an extraordinary ceiling of about 85,000 feet. Accomplishing such feats puts the plane under considerable strain.
In operation, it produced temperatures so high that it could ignite the JP-8, rendering conventional jet fuel useless with the Blackbird. So a new fuel was specially created for him. Blackbird fuel would be known as JP-7. It doesn’t seem like a huge departure from the existing fuel already used by many different military aircraft, but it had unique properties that made it safe for the Blackbird.
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Difference from JP-7 jet fuel
Front view of the SR-71 aircraft on display in the museum. – Chicago Photographer/Shutterstock
As pilot Richard Graham (via YouTube) examines the spy plane in the cockpit, he explains that the fuel flow gauges “measure in thousands of pounds per hour.” Engineers typically base fighter jet design around the concept of huge, fuel-guzzling bursts of speed when needed, rather than the practical, functional level of speed more typical of conventional commercial flight. Therefore, engines that simply drink through fuel reserves are far from a unique feature of the Blackbird. In order for this particular plane to fly at all, the JP-7 had to be designed specifically for it.
Developing this new fuel required a delicate balance. Lowering its flash point too much would have the opposite problem that bad JP-8 had and it wouldn’t have the necessary chemical reaction to start the engine at all. The SR-71A Flight Manual (via Scribd) clears up some of the difficulty: “This requires fuel with high thermal stability to prevent it from breaking down and depositing coke and varnish in the fuel system channels.” It continues: “Other elements are also significant, such as the amount of sulfur impurities tolerated. The advanced fuels, JP-7 (PWA 535) and PWA 523E, have been developed to meet the above requirements.”
An inspired fuel ignition solution was the injection of triethylborane, which is highly effective in ensuring that the fuel can both achieve and maintain a steady flame to significantly improve engine performance.
The fuel and engine that powered several extraordinary aircraft
Pratt & Whitney J58 engine on display. – Greg Goebel/Wikimedia Commons.
The Blackbird’s raw power and performance required another essential component: a massive engine. The Pratt & Whitney turbojet J58 was the engine that went into production in 1964. In order to use the Blackbird, which first flew that same year, it had to withstand the heat as much as its fuel could. The use of superalloy Waspaloy was one of the materials that allowed the engine to do this. Pratt & Whitney also equipped it with a clever system that helped it maintain the Blackbird’s massive speed without overdoing it.
The National Air and Space Museum explains, “By opening the bypass valves, air was bled from the fourth stage and six ducts directed it around the compressor rear stages, combustor, and turbine. The bled air re-entered the turbine exhaust around the front of the afterburner, where it was used for thrust and cooling.” The engine’s power was only about a fifth of the thrust the jet used at three times the speed of sound itself.
Its predecessors in the family, the A-12 and YF-12 interceptors, also used the J58 turbojet, and while the A-12 is technically the only aircraft faster than the SR-71, it’s still a surprising technological innovation that took the most famous Blackbirds to those heights. The US Air Force retired the SR-71 Blackbird in the late 1990s, but it remains an aviation icon.
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Read the original article on SlashGear.