The SR-71 Blackbird

The CIA A-12 Blackbird Program

 

The A-12 started out as an USAF interceptor to replace the cancelled F-108A Rapier. In October 1962, CIA authorized the Skunk Works to study the feasibility of modifying the A-12 to carry and deploy a reconnaissance drone for unmanned overflight of denied areas. The project was codenamed TAGBOARD.

24 Mar 2020

Reducing the A-12 Blackbird's Cross Section

Reducing the A-12 Blackbird's Cross Section

Author: SuperUser Account  /  Categories: A-12 History  / 

During the spring of 1959, Kelly Johnson's Skunk Works crewwhich then numbered only 50 - had begun building a full-scale mockup of the proposed aircraft. The mockup was to be tested for its radar cross section by Edgerton, Germeshausen & Grier (EG&G) in cooperation with the Scientific Engineering Institute at a small testing facility at lndian Springs, Nevada. Lockheed objected to this site because its pylon would not support the full-scale mockup and because the facilities were in full view of a nearby highway. On I 0 September 1959, EG&G agreed to move its radar rest facility to the former U-2 resting site at Area 51 of the Atomic Energy Commission's Nevada Proving Grounds.''

When the new radar test facility with its larger pylon was ready. Johnson put the A-12 mockup on specially designed trailer truck that carried it from Burbank to Area 51. By 18 November 1959, the mockup was in place atop the pylon, and radar testing could begin. These tests soon proved that Lockheed's concept of shape, fuel additive , and non-mettalic parts were workable. But it would take more than before the OXCART achieved a satifactory radar cross section.

It was in the course of this radar testing that the OXCART received it's characteristic cobra-like appearance. Edward Purcell and Franklin Rodgers had come up with a theory that a continuallu curving airframe would be difficultto track with a radar pulse because it would present fewer reflectors or sharp angles from which pulses could bounce in the direction of the radar. To acheive the continuously curving airface, Kelly Johnson added thin, curved extensions to the engine housings and leading edges of the wings and eventually to the fuselage itself, creating what is known as a chine on each side. At first Johnson was concerned that the additions might impair the air worthiness of the plane, but wind tunnel tests determined that the chines actually imparted a useful aerodynamic lift to the vehicle. Because titatium was very brittle and therefore difficult to bend, Johson acheived the necessry curvature by combining triangular-shaped peices of titantium called fillets. These fillets were glued to the framework of the chines with a special adhesive, expoxy resin.

On later OXCART models the fillets were made from electrically resistive honeycomb plastic with a glass-fiber surface that would not melt at speed. When struck by a radar pulse. the composite chines tended to absorb the pulse rather than reflect it. A similar approach was used for the leading edges of the wings. Again electrically resistive honeycomb material was fabricared into triangular shapes, known as wing teeth, and fitted into the titanium wings. Both the metal and composite tillets and teeth were held in place with the newly developed epoxy cements.

The remaining area of concern in the A-12's radar cross section was the two vertical stabilizers. To reduce radar reflections, Kelly Johnson canted the stabilizers inward 15 degrees and fabricated them out of resin-impregnated nonmetallic materials. Once these changes were completed, the only meta! in each vertical stabilizer was a stainless steel pivot. The Air Force. which later ordered several versions of the OXCART aircraft for its own use never adopted the laminated vertical stabilizers." 

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