The SR-71 Blackbird's Predecessor Created "Plasma Stealth" By Burning Cesium-Laced Fuel
Skunk Works needed a way to hide the A-12's radar reflecting behind, so they dumped cesium into its fuel to create a radar-absorbing exhaust plume.
BY JOSEPH TREVITHICK AND TYLER ROGOWAY SEPTEMBER 12, 2019
THE WAR ZONE



Lockheed's A-12 Oxcart spy plane, which the company developed for the Central Intelligence Agency, was a direct response to the growing vulnerability of its earlier U-2 Dragon Lady to hostile air defense networks. As such, the plane – the predecessor to the U.S. Air Force's iconic SR-71 Blackbird – was extremely high- and fast-flying, but also incorporated then-state-of-the-art features to reduce its radar cross-section. These included a combination of a stealthy overall shape and radar-evading structures, as well as the use of composites in its construction, and the incorporation of radar absorbing materials on its skin. A far less known, but still a key component of the Skunk Works plan to make the A-12 harder to spot on radar involved a cesium-laced fuel additive to dramatically reduce the radar signature of the plane's massive engine exhausts and afterburner plumes by creating an ionizing cloud behind the aircraft to help conceal its entire rear aspect from radar waves.

Even before the U-2 had become operational in 1956, the Central Intelligence Agency (CIA) had begun exploring potential successor designs. The next year, Scientific Engineering Institute (SEI), a CIA front, began laying out requirements for the new spy plane, including a need for a reduced radar signature. By 1958, proposals from Lockheed and Convair had emerged as the most feasible and the former firm's concept showed more promise when it came to what we would consider today to be stealthy features.

“From this period [the late 1950s], our studies regarding radar cross-section showed that any flyable aircraft to be operational in the period after 1963 could not avoid radar detection,” Clarence "Kelly" Johnson, famed Lockheed engineer and head of the company's Advanced Development Programs (ADP) division, or Skunk Works, wrote in a now-declassified official internal history of the A-12 program in 1968. “This did not mean that we had not gone all out on the reduction of radar cross section, as we made many very important contributions, including those of basic shape, to the whole problem on the A-12.”



An early A-12 Oxcart during flight testing.

After the CIA chose Lockheed to develop what would become the A-12 in 1959, the company continued to refine its radar-defeating features. The overall planform was designed to deflect radar waves, but there were a number of other physical additions to help improve its radar signature. These included spiked cones to shield the face of the inlets for the plane's two massive Pratt and Whitney J58 engines, chines on the outside of the engine nacelles and engine ducts, curved extensions on the leading edges of the wings, and specially canted rear vertical stabilizers. Below the surface of its chined leading edges, radar defeating saw-tooth baffles also helped deaden the aircraft's radar returns.



A look at the various physical "anti-radar" features found the A-12 from the CIA's own now-declassified history of the U-2 and Oxcart programs.
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