The Lockheed SR-71 “Blackbird” is one of those magnificent high-altitude, Mach 3 reconnaissance airplanes that was created by Kelly Johnson at Lockheed’s Skunk Works division and never had its equal. To this day, in 2019, it remains the fastest air-breathing manned aircraft ever built and seen up until 2019, one record which it set in 1976. The trick lies in those special engine nacelles that house Pratt and Whitney J-58 turbojet engines and special air inlets of that very same kind.
The J-58 was revolutionary because it was the first dual-cycle engine. It acted as a typical turbojet at subsonic and transonic speeds, and at approximately Mach 2 its airflow became predominantly ramjet-like. This dual-cycle operation gave the J-58 more than 32,000 pounds of thrust at zero feet altitude. High-temperature-resistant nickel superalloys like Inconel and Waspalloy were used in construction to maintain forces between 800°F at the inlet and 3200°F in the afterburner duct.
The inlets were extremely important for the SR-71 to attain levels of speed up to Mach 3.2. These were fitted to encase each J-58 engine, as each was equipped with a moveable conical spike that adjusted the geometry of the inlet. The spike managed supersonic flow and positioned supersonic shockwaves to the optimum for performance while preventing supersonic flow from reaching the forward compressor end. The spike translated about one and 5/8 inch per 0.1 Mach with an overall movement travel of 26 inches at the maximum position of complete retraction.
Another significant part of the nacelles were the bypass exit doors which helped in reducing the overpressure of the air that was entering the inlet. Then it expended airflow outside to manage the overpressure. These opened and closed proportionally to DPR to avoid overpressure situations. The slow air that exited through these doors was allowed to stay shut unless necessary to keep drag at a minimum level.
Such advanced technology even extended to JP-7 fuel, specially designed not to auto-ignite under the SR-71’s intense Mach 3 flight. Even though its technology has now been declassified, no one has been able to replicate the SR-71’s engine or inlets. Rumors of its follow-on, the SR-72, will use a different combination of turbojet and scramjet without the legendary spike.
The unmatched history of operational use makes the legacy of the Blackbird great. In its three-decade-long service, the SR-71 was famously able to outrun over 4,000 missiles because of its mix of high speed, high altitude, and stealth. This meant that no Soviet interceptor, such as the MiG-25, which had speeds above Mach 3.2, could ever top the sustained performance of the SR-71. While the MiG-25 could reach such speed only for a couple of minutes at a time and would thus irrevocably damage its engines, the SR-71 could maintain its Mach 3+ speed for hours.
There were huge engineering challenges with the SR-71. New types of wires had to be developed for its avionics systems, and special hydraulic fluids, and 93% of its structure had to be made of titanium. Ironically, the titanium was procured from the Soviet Union using CIA shell companies. The windows of the aircraft were of solid 1.25-inch-thick Quartz and could even heat in-flight meals because of the intense temperature.
What made the SR-71 such a difficult target was the combination of high speed and stealth. With a radar cross-section of only 22 square inches, it presented an exceptionally low target. The Soviet MiG-25, which had reportedly reached speeds between Mach 2.5 and 2.83, did not have the sustained performance capability of the SR-71. The MiG-25 couldn’t climb fast enough to intercept the SR-71, and their missiles lacked the thrust to close the distance, Soviet pilot Lt. Viktor Belenko explained after his defection in 1976.
The SR-71, indeed, stands as a testimony to human ingenuity and excellence in the engineering marvel that never fails to amaze and make one shed admiration in the aviation world.