Today there are many rumors about just how fast the SR-71 could go. The speed limit for the Blackbird ironically had nothing to do with the airframe; it had to do with the engines.
The SR-71, unofficially known as the “Blackbird,” is a long-range, advanced, strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft. The first flight of an SR-71 took place on Dec. 22, 1964, and the first SR-71 to enter service was delivered to the 4200th (later 9th) Strategic Reconnaissance Wing at Beale Air Force Base (AFB), Calif., in January 1966.
The US Air Force (USAF) retired its fleet of SR-71s on Jan. 26, 1990, because of a decreasing defense budget and high costs of operation.
Throughout its nearly 24-year career, the SR-71 remained the world’s highest-flying and fastest operational aircraft. From 80,000 feet, it could survey 100,000 square miles of Earth’s surface per hour.
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.
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. 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.
Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Facebook Page Habubrats for awesome Blackbird’s photos and stories.
Photo credit: U.S. Air Force and Lockheed Martin