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The most advanced member of the Blackbird family
The SR-71, the most advanced member of the Blackbird family that included the A-12 and YF-12, was designed by a team of Lockheed personnel led by Clarence “Kelly” Johnson, then vice president of Lockheed’s Advanced Development Company Projects, commonly known as the “Skunk Works” and now a part of Lockheed Martin Corp.
The Blackbird design originated in secrecy during the late l950s with the A-12 reconnaissance aircraft that first flew in April 1962 and remained classified until 1976. President Lyndon Johnson publicly announced the existence of the YF-12A interceptor variant on Feb. 29, 1964, more than half a year after its maiden flight. The SR-71 completed its first flight on Dec. 22, 1964.
Today the Blackbirds remain the world’s fastest and highest-flying production aircraft ever built. The Blackbirds were designed to cruise at Mach 3.2, just over three times the speed of sound or more than 2,200 miles per hour and at altitudes up to 85,000 feet.
SR-71 limiting speed was Mach 3.55
Today there are many rumors about just how fast the SR-71 could go. The J58 engine temperature limited the top speed. The speed limit for the airplane ironically had nothing to do with the airframe; it had to do with the engines. Right in front of the engine compressor was a temperature probe that reported the temperature to the pilot; when the temperature was around 427C, 800 degrees Fahrenheit, that’s as fast as they were allowed to go.
However, a cold blast from frigid weather could make a difference in reducing the temperature. This would enable the airframe to go faster without harming the engines.
David Peters, former SR-71 Blackbird pilot, explained to me;
‘A little clarification on the speed thing. You are absolutely correct on the 427C. The issue is that I have been limited to less than Mach 3 on a few occasions because the outside air temperature was quite above standard and 427C came up at about 2.95. On other occasions like the Murmansk deal I got above 3.4 (3.49 on the one occasion) and wasn’t close to 427C. The actual limiting airspeed is around 3.55 that is where the spike being at full retraction loses the intercept on the shock wave and can no longer position it correctly in the inlet. ‘Also the overflow of the shock starts to go over the wing and interfere with the flight controls. So the limiting speed as configured would be about 3.55 so long as you don’t exceed 427C.’
Mike Relja, who worked on in with the SR-71‘s for over 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 6th [Mar. 6, 1990] speed run Ed Yeilding stated that Don Emmons gave them permission to fly the max limit of 3.3 M, they asked Lockheed if they could exceed that number and Lockheed said no if the aircraft had an unstart above 3.3 it may go out of control and come apart hard to keep the pointy end forward. Also, no fuel flow limits established above 3.3 M were ever tested.
‘P&W did give them clearance to exceed 427 CIT for 30 minutes to a limit of 450 CIT but that wasn’t needed they stayed at the book limit of 427 CIT.’*
The engines maker Pratt & Whitney would not warrant or guarantee anything beyond 427; after that, all bets are off the engine could come unglued or you could shed turbine blades. The SR-71 crewmembers were too responsible to risk and exceeded the temperature limit. They wanted to keep the warranty on the engines, the J58’s.
*SR-71 pilot Ed Yeilding (who along with JT Vida as RSO flew the Blackbird during the speed run of Mar. 6, 1990) told me that another reason they didn’t exceed 3.2 is that they were afraid they’d run out of gas before they got to Dullas airport.
Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Twitter X Page Habubrats SR-71, Instagram Page SR71Habubrats and Facebook Page Born into the Wilde Blue Yonder Habubrats for awesome Blackbird’s photos and stories.
Photo credit: U.S. Air Force
