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17959 refueling from a KC-135Q, showing off the "Big Tail" modification that gave the plane its nickname.
The SR-71 Blackbird's interchangeable nose, allowing the plane to swap between electronic and optical sensors as conditions dictated, was one of the reasons the plane won out in a fly-off, known as NICE GIRL, with the CIA's similar A-12 OXCART. Still, the USAF began to see a need to upgrade the Sled, and in 1974 a program was initiated to add a 24" optical bar camera to an unused area of the forward chines (the structures on either side of the fuselage), as well as a 13' 9" extension to the tail which added 49 cubic feet of space for rearward-facing electronic counter-measure (ECM) equipment.
An SR-71's SLAR (sideways-looking airborne radar) nose
The tenth SR-71A, tail number 61-7959, was selected to be modified. The aircraft had been delivered to the test fleet at Palmdale in '74, and thus the modification and test program would not adversely affect the USAF's operational Blackbirds. Modifications were carried out between April and November of 1975, and included adding an adapter unit to the existing tail structure, rerouting the fuel dump line to the top of the new tail, and adding new air conditioning for the added electronics. The new tail could articulate up and down by 8.5°, allowing it to clear the runway during landings, then be moved down so as to not foul the drag chute. During the test program, pilots also found the tail articulation could also be used to trim the plane, a handy bit of happenstance.
'Big Tail' SR-71 959 being taxied. The tail is in the 'up' position. Note the sedan for scale.
After stress and vibration testing, 959 was rolled out for a high-speed taxi test on 20 November 1975, followed by its first post-modification flight on 3 December. The flight was limited to just 29,000 feet, and was subonic, while the crew carried out tail deflection and fuel dump tests. The second test flight on 11 December was supersonic, with the tail remaining in the 'up' position. Flights 3-5 were carried out at increasing altitudes and speeds, with the 6th flight on 28 January 1976 reaching Mach 3 at 75,000'. Four additional flights were carried out by the pilot only, proving that the system could be managed by one crew member, even under less than optimal conditions (the automatic system failed twice, forcing the pilot to control the tail manually).
959 in flight
The plane was then handed over the USAF for operational testing, with Tom Pugh and Bob Riedenauer as pilots and William Frazier and John Carnochan acting as RSO (Reconnaissance Systems Officers). A total of 23 flights were carried out between 5 May and 29 October 1976, testing out various combinations of optical cameras and ECM systems, including two flights with an F-4 Phantom acting as an aggressor. Two of the final missions were flown with four optical bar cameras, one each in the nose, chine bays, and the extended tail.
Big Tail backing away from a tanker, showing streaks of JP-7 from the refueling receptacle as well as from the joints in the skin over its back.
The Big Tail modifications proved to be effective, but the USAF chose not to apply them to the rest of the Blackbird fleet. Contracting defense budgets made the cost unpalatable, and operational experience had shown the SR-71 was basically untouchable by Soviet missiles. 61-7959 was retired to outdoor storage at Lockheed's Palmdale facility, though it was used as a spare parts store for the operational Habu. In 1991 the plane was disassembled and moved to the USAF Armaments Museum at Elglin AFB in Florida, were it was restored and placed on display.
The SR-71 is an imposing sight, even at rest.