One test undertaken studied thermal effects on sheets of large titanium wing panels. When a 4ft×6ft element was heated to the computed heat flux expected in flight, it resulted in the sample warping into a totally unacceptable shape. This problem was resolved by manufacturing chordwise corrugations into the wing outer skins. At the design heat rate, the corrugations merely deepened by a few thousandths of an inch and on cooling returned to their original shape. Johnson recalled he was accused of “…trying to make a 1932 Ford Tri-motor go Mach 3,” but added that “…the concept worked fine.” To prevent this thin titanium outer skin from tearing due to differential expansion rates when secured to heavier sub-structures, the Skunk Works developed standoff clips; these provided structural continuity while creating a heat shield between the adjacent components.
General layout
The exterior of the A-12 is characterized by an aft-body delta wing with two large engine nacelles, each mounted at mid-semi-span. Two “all-moving” vertical fins were located on top of each nacelle and canted inboard 15 degrees from the vertical to reduce the aircraft’s radar signature. A large, aft-moving inlet spike or center-body protruded forward from each engine nacelle, which helped to regulate mass airflow to the two Pratt & Whitney J58 engines. The fuselage was a titanium structure of semi-monocoque construction with a circular cross-section. The fuselage sides then flared out creating sharply blended chines (the resultant cross-section resembling a two-dimensional flying saucer) which reduced radar returns when illuminated by radar from the side. The fore and aft fuselage bodies were joined during construction at fuselage station (FS) 715. To further reduce its RCS, the A-12’s wing leading and trailing edges together with the chines were fitted with wedges of RAM, as described earlier. The primary reconnaissance-gathering system on the A-12 was a high-resolution camera which was located in a large pressurized compartment behind the pilot, referred to as the Q Bay.
Engines
The specified power plant for the A-12 was two Pratt & Whitney JT11D20 A engines (designated J58 by the US military). Known as JJ engines, each developed 32,500lb of thrust (as both the YF-12 and SR-71 designs were heavier than the A-12, their modified YJ engines produced 34,000lb of thrust). This high bypass ratio afterburning engine was the result of two earlier, ill-fated programs: Project Suntan together with Pratt & Whitney’s JT9 engine that lost out to General Electric’s J93 to power the North American XB-70 Valkyrie. So Pratt & Whitney then reduced the engine’s size by 20 percent and offered it under the J58 designator for the Vought F8U-3, which in turn lost out in competition against McDonnell’s F4H-1.
Although relatively conventional, the original single-spool high pressure ratio turbojet was rated at 26,000lb in afterburner and had already completed 700 hours of full-scale engine testing, with results being very encouraging. As testing continued, however, it became apparent that due to the incredibly hostile thermal conditions of sustained Mach 3.2 flight, only the basic airflow size (400lb per second of airflow) and the compressor and turbine aerodynamics of the original Navy J58 P2 engine could be retained (and even these were later modified). The stretched design criteria, associated with high Mach number and its related large airflow turn-down ratio, led to the development of a variable cycle (later known as a bleed bypass) engine; a concept conceived by Pratt & Whitney’s Robert Abernathy. This eliminated many airflow problems through the engine by bleeding air from the fourth stage of the nine-stage, single-spool axial-flow 8.8:1 pressure ratio compressor and channeling this excess air through six low compression ratio bypass ducts. It was then reintroduced into the turbine exhaust, near the front of the afterburner, at the same static pressure as the main flow; this reduced exhaust gas temperature (EGT) and produced almost as much thrust per pound of air as the main flow, which had passed through the rear compressor, the burner section, and the turbine. Scheduling of the bypass bleed was achieved by the main fuel control as a function of compressor inlet temperature (CIT) and engine rpm. Bleed air injection occurred at a CIT of between 85 and 115 degrees C (approximately Mach 1.9).
Hydraulics
Four independent systems (designated A, B, L, and R) supplied hydraulic power to the A-12, thereby facilitating operation of the control actuators, landing gear, and other equipment. The A and B systems operated in parallel, supplying hydraulic pressure to the flight controls, specifically the seven actuating cylinders on each outboard elevon, the three on each inboard elevon, and the two cylinders operating each rudder. A dual servo unit, one for each movable flight control surface, controlled system pressure and return of fluid to the actuating cylinders.
The L and R systems supplied hydraulic power to the left and right inlet spikes and the forward and aft bypass doors on each nacelle. The L system also served the normal brake system, landing gear, main gear inboard doors, nose wheel steering system, refueling door and fuel probe receptacle latches. The R system supplied hydraulic power to the alternate braking system, alternate nose wheel steering system, landing gear retraction system, and backup system for closing the main landing gear inboard doors. Each system was serviced by its own hydraulic reservoir and fixed-angle, variable-volume piston pump. The left engine drove the A and L system pumps, while the B and R pumps were driven by the right engine.
Fuel system
The extremely high airframe temperatures encountered by the A-12 during high-Mach cruise ruled out the use of JP-4 as its fuel source, as it had to be carried in “wet” tanks. Instead, a bespoke fuel was designed specifically for the A-12 and known as PF-1 (later known as JP-7). It was developed by Pratt & Whitney, in partnership with Ashland, Shell, and Monsanto, and remained stable despite the high temperature environment, being used first as a hydraulic fluid to activate the main and afterburner fuel nozzles before being injected into the fuel burners at over 350 degrees C and 130 psi. Such high fuel-burn temperatures presented the design team with yet another problem, because standard electrical plugs couldn’t ignite the fuel. This was overcome by developing a unique chemical ignition system involving the chemical triethylborane (TEB). Extremely flash sensitive when oxidized, a small tank of the substance was carried onboard the aircraft and used to start or restart the engines and afterburners on the ground or in the air. To ensure that the system remained inert when not in operation, gaseous nitrogen was used to pressurize the TEB tank and power the piston that injected it into the burner cans during the ignition process, regardless of engine operating conditions. As fuel was burnt, gaseous nitrogen was also used to pressurize and render inert the fuel tanks to prevent them from being crushed as the aircraft descended to lower levels to either air refuel or land.