The Blackbird
Unofficially referred to as the “Blackbird,” the SR-71 was a long-range, Mach 3+ strategic reconnaissance aircraft that developed from the Lockheed A-12 and YF-12A.
The first SR-71 flight occurred on December 22, 1964, and the 4200th (later 9th) Strategic Reconnaissance Wing at Beale Air Force Base, California, received the aircraft in January 1966.
The Blackbird was in a different category from anything that had come before. “Everything had to be invented. Everything.” Skunk Works legendary aircraft designer Kelly Johnson recalled in an interesting article that appeared on the Lockheed Martin website.
The SR-71 could fly at Mach 3.55
There are a lot of rumors these days regarding the SR-71’s potential speed. The maximum speed was restricted by the J58 engine’s temperature. Ironically, the aircraft speed limit was related to the engines rather than the airframe. A temperature probe located directly in front of the engine compressor provided the pilot with the temperature; they were only allowed to run as fast as the temperature reached about 427C, or 800 degrees Fahrenheit.
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-71s for over 30 years, added;
‘I don’t know of any warranty that P&W had or any other parts manufacturer for that matter.
Don’t exceed 427C
‘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.’*
*SR-71 pilot Ed Yeilding (who along with JT Vida as RSO flew the Blackbird during the speed run of Mar. 6, 1990) said that another reason they didn’t exceed 3.2 is that they were afraid they’d run out of gas before they got to Dulles airport.
The engine 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.
Check out Habubrats SR-71‘s Twitter profile, SR71Habubrats‘s Instagram profile, and Born into the Wilde Blue Yonder Habubrats‘s Facebook page for further Blackbird photos and stories.
Photo by U.S. Air Force and Lockheed Martin
