The most advanced member of the Blackbird family
The SR-71 was the most advanced aircraft in the Blackbird family, which also included the A-12 and YF-12. It was developed by a Lockheed team led by Clarence “Kelly” Johnson, then vice president of Lockheed’s Advanced Development Company Projects—better known as the “Skunk Works,” now part of Lockheed Martin.
The Blackbird program began in secrecy in the late 1950s with the A-12 reconnaissance aircraft. The A-12 made its first flight in April 1962 and remained classified until 1976. President Lyndon Johnson publicly revealed the YF-12A interceptor on February 29, 1964, more than six months after its maiden flight. The SR-71 followed with its first flight on December 22, 1964.
The Blackbirds remain the fastest and highest-flying production aircraft ever built. They were designed to cruise at Mach 3.2—more than three times the speed of sound, or over 2,200 miles per hour—and operate at altitudes of up to 85,000 feet.
The SR-71’s limiting speed was Mach 3.55
There are many claims about the SR-71’s maximum speed, but the aircraft’s limit was actually determined by the temperature of its J58 engines. The airframe itself was capable of flying faster; however, a temperature probe positioned ahead of each engine’s compressor monitored the heat and relayed the reading to the pilot. Once the temperature reached approximately 427°C (800°F), the aircraft was not permitted to accelerate further.
In extremely cold conditions, however, the frigid air could lower the engine temperature. This allowed the SR-71 to fly faster without risking engine damage.
David Peters, a former SR-71 Blackbird pilot, clarified the aircraft’s speed limitations:
The SR-71’s limiting speed depended on both inlet temperature and Mach number. On several occasions, the aircraft was restricted to less than Mach 3 because the outside air temperature was significantly above standard, causing the 427°C limit to be reached at approximately Mach 2.95.
During other flights, including the Murmansk mission, the aircraft exceeded Mach 3.4—reaching Mach 3.49 on one occasion—without approaching 427°C. The actual limiting airspeed was approximately Mach 3.55. At that speed, with the spike fully retracted, it could no longer maintain the intercept with the shock wave or position it correctly within the inlet.
Additionally, the shock wave began to spill over the wing and interfere with the flight controls. Therefore, the aircraft’s limiting speed in its configured condition was approximately Mach 3.55, provided the 427°C limit was not exceeded.
Mike Relja, who worked with the SR-71 for more than 30 years, added:
“I don’t know of any warranty that P&W had or any other parts manufacturer for that matter.”
Keeping the warranty on the engines
On the March 6, 1990, speed run, Ed Yeilding stated that Don Emmons had authorized the crew to fly up to the established maximum of Mach 3.3. The crew asked Lockheed whether they could exceed that limit, but the company refused. Lockheed’s concern was that an inlet unstart above Mach 3.3 could cause the aircraft to become uncontrollable and potentially break apart, making it difficult to keep the aircraft flying safely. In addition, no fuel-flow limits above Mach 3.3 had ever been tested.
Pratt & Whitney had authorized the crew to exceed the 427°C turbine inlet temperature limit for up to 30 minutes, provided the temperature did not exceed 450°C. However, this allowance was unnecessary because the crew remained within the standard 427°C limit.
Ed Yeilding, who flew the SR-71 during the March 6, 1990, speed run with JT Vida serving as reconnaissance systems officer, later explained that fuel considerations also influenced the decision not to exceed Mach 3.2. The crew was concerned that the aircraft might run out of fuel before reaching Dulles Airport.
Pratt & Whitney would not warrant or guarantee the J58 engines beyond 427°C. Above that temperature, the engines could suffer catastrophic damage, including disintegration or the loss of turbine blades. The SR-71 crew considered exceeding the temperature limit an unacceptable risk. By remaining within 427°C, they preserved the warranty coverage on the J58 engines.
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Photos by U.S. Air Force
