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Lockheed A-12: The CIAs Blackbird and other variants

Lockheed A-12: The CIAs Blackbird and other variants


A thermal thicket


The innovative use of shape and materials to produce as stealthy a vehicle as possible was equaled by the necessary use of exotic materials and manufacturing techniques. The best frontline fighter aircraft of the day were the early Century-series jets, like the North American F-100 Super Sabre and McDonnell F-101 Voodoo. In a single bound, the A-12 would operate at sustained speeds and altitudes treble and double respectively those of such contemporary fighters. The technical challenge facing the Skunk Works team was vast and the contracted timescale in which to solve them was incredibly tight. Johnson would later remark that virtually everything on the aircraft had to be invented from scratch.

Operating above 80,000ft, the ambient air temperature was often below -60 degrees C and the atmospheric air pressure just 0.4 pounds per square inch; but cruising in afterburner at a speed of a mile every two seconds, airframe temperatures soared from between 245 and 565 degrees C. However, during the subsonic air refueling phase of a mission, the airframe would be subjected to steady-state temperatures of -65 degrees C. Thermodynamic considerations therefore were fundamental.

Sustained operation in such an extreme temperature environment meant lavish use of advanced titanium alloys, which accounted for 85 percent of the aircraft’s structural weight; the remaining 15 percent was comprised of composite materials. The decision to use such materials was based upon titanium’s ability to withstand high operating temperatures; it also weighs half as much as stainless steel but has the same tensile strength — high-strength composites were not available in the early 1960s. The particular titanium used was Beta-120/Ti-13V-11Cr-3A1, which can be hardened to strengths of up to 200 Ksi. But using this advanced material wasn’t without problems — titanium is not compatible with chlorine, fluorine or cadmium. For example, a line drawn on sheet titanium with a Pentel pen will eat a hole through it in about 12 hours — so all Pentel pens were recalled from the shop floor. Early spot-welded panels produced during the summer had a habit of failing, while those built in the winter lasted indefinitely. Diligent detective work discovered that to prevent the formation of algae in the summer, the Burbank water supply was heavily chlorinated. Subsequently, the Skunk Works washed all titanium parts in distilled water. As thermodynamic tests got underway, bolt heads began dropping from installations; this, it was discovered, was caused by tiny cadmium deposits, left after cadmium-plated spanners had been used to apply torque. As the bolts were heated to temperatures in excess of 320 degrees C, their heads simply dropped off. The remedy: all cadmium-plated tools were removed from toolboxes.

This sequence of three images was taken from a once-lost cine tape. The first shows a solid titanium ingot being pressed into an engine nacelle ring. The nacelle was an integral part of the wing, acting as a chordwise beam and torque tube, transmitting aerodynamic loads from the outer wing section forwards and redistributing them to the forward and aft wing boxes. The inboard wing sections were in forward and aft box sections, separated by a 3ft 3in-wide compartment that provided support for the main undercarriage. Finally the inboard section of the right engine nacelle is depicted — the rear of the intake spike mount is already in position. (Lockheed Martin)

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.

Article 133 (60-6939) was the final A-12 off the line. Note how both outboard wing sections hinge upward to allow installation of and access to the J58 engines; and the Q Bay behind the cockpit that provided the pressurized compartment within which the camera was installed. (Lockheed Martin)The Pratt & Whitney J58 continued to be developed throughout its life, eventually generating 34,000lb of sustained thrust whilst cruising in afterburner. Three of the six unique compressor bypass tubes can clearly be seen. (Pratt & Whitney)

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).

Undergoing stress testing, note the wedges of dielectric material installed into the serrated wing leading edge to reduce the aircraft’s RCS. Note also the chordwise corrugations pressed into the inboard section of the wing to minimize the effects of thermodynamic expansion. (Lockheed Martin)The successful execution of Oxcart’s mission depended upon the air refueling support supplied by specially modified USAF KC-135Q tankers flown by the 903rd Air Refueling Squadron, based at Beale AFB, California. (Lockheed Martin)


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.

Development of a durable fuel tank sealant was an ongoing problem. Cruising at high Mach the airframe expanded due to thermodynamic heating. Upon descending to air refuel, the airframe cooled — a process that was considerably speeded-up when cold fuel was pumped into the tanks from the KC-135Q tanker at 5,000lb per minute! The pounding taken by the silicon-based sealant invariably led to it cracking, causing fuel to leak from numerous gaps.


Air Inlet Control System (AICS)


The A-12 also boasted a unique, highly efficient air inlet system that supplemented thrust via three components: an asymmetric mixed-compression, variable-geometry inlet; the J58 engine; and a convergent-divergent blow-indoor ejector nozzle. The AICS regulated the massively varying internal airflow throughout the aircraft’s entire flight envelope, ensuring that the engines received air at both the correct velocity and pressure.

To satisfy the J58’s voluminous appetite for air during operations at ground idle, taxiing, and take-off, the center-body spikes were positioned fully forward, allowing an uninterrupted flow to the engine compressor. Supplementary flow was also provided through six forward bypass doors; additionally, a reverse flow was set up through exit louvers on the spike’s center-body and a set of variable-area “inlet-ports” that were regulated by an external slotted-band, which drew air in from two sets of doors. The task of operating these doors and positioning of the electrically operated, hydraulically actuated spike was controlled by the pilot. Operating together, the forward bypass doors and the center-body spike were used to control the position of the normal shock wave, just aft of the inlet throat. To optimize inlet efficiencies, the shock wave was captured and held inside the converging-diverging nozzle, just behind the narrowest part of the “throat,” thereby achieving the maximum possible pressure rise across the normal shock.

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