The SR-71 Blackbird's Predecessor Created "Plasma Stealth" By Burning Cesium-Laced Fuel


Lorenzo von Matterhorn
Jan 31, 2009
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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.
12 Sep 2019

Excerpts from a very long and interesting column with much interesting material left out here:

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.

With the exception of the inlet spikes, these added features made heavy use of composite, radar-absorbing materials. Lockheed also developed a special "iron paint," sometimes referred to as the "iron ball paint" because the mixture contained tiny iron balls, to help absorb radar waves. The special blend, which was also applied to the SR-71, reportedly cost $400 per quart in the 1960s.

These features all had a notable impact on reducing the aircraft's radar cross-section. However, there was one aspect of the plane's radar signature that still proved difficult to manage, the exhaust outlets for the J58s and the giant plume from the engines at full afterburner, which was necessary to propel the A-12 to its blistering top speed of well over Mach 3.

“To overcome the afterburner problem of a large radar cross section return from the aft quadrant, we proposed the use of [a] cesium additive to the fuel,” Kelly Johnson wrote in his A-12 history. “This was first brought up by Mr. Ed Lovick of ADP and its final development was passed over to P&W. It was eventually a basic part of our cross section reduction methods.”

The exhaust pipes were sixty inches in diameter, so they returned large amounts of energy at all frequencies of interest and over large angles to the rear,” Lovick, who also worked on the SR-71 and the F-117 Nighthawk stealth combat aircraft, wrote in his own book, Radar Man: A Personal History of Stealth. “We knew that the only way to prevent such echoes was, in effect, to close the apertures.”

Lockheed initially experimented with various metallic mesh screens, but quickly abandoned those efforts, according to Lovick. He says that Dr. Richard Bissell, the CIA’s Special Assistant for Planning and Coordination, who was managing the program, was so worried about this particular issue, he had considered calling for the scrapping of the entire development of a U-2 successor. That’s where the cesium additive, which eventually became known as A-50, came in an idea that Lovick claims saved the A-12 program.

The basic principle behind this is a concept known as “plasma stealth.” In the simplest terms, this involves creating a cloud of plasma, or ionized gas, around some or all of an object. The plasma then absorbs electromagnetic radiation, such as radar waves, preventing them from reflecting back. There are multiple ways to generate the required plasma Lovick’s idea was to inject an alkali metal, via a fuel additive, into the extremely hot exhaust streams, where the heat would turn it into an ionized gas.

The final additive mix was 30 percent cesium metal and 60 percent dialkyl phosphate, according to Lovick. However, he says in his book that the testing of the additives, which included flight tests at Area 51, was finished by 1965, but it’s unclear if this only refers to Lockheed’s portion of the work before the project passed to Pratt & Whitney.

Given that the A-12’s speed and its electronic warfare package seemed sufficient to protect it over North Vietnam, the CIA may have continued to withhold use of A-50 for operational security reasons. Using it could have exposed its existence and given the Soviets and their allies time to develop countermeasures, rendering it less effective when it might have been absolutely necessary.

Not using it outside of an emergency situation where planners deemed to be absolutely critical to a mission’s success may have also been a product of the additive's impacts on the J58 engines. Typically, jet aircraft try to avoid ingesting particulate matter, especially alkali metals that could melt and fuze to internal components, which generally lead to extensive maintenance or potentially catastrophic accidents in flight. Beyond that, adding additional chemical components to the already highly sensitive JP-7 mixture could have had impacts on the fuel’s performance.

A CIA memorandum from 1964 offers evidence of some of these issues in noting that A-12s required a modification to their fuel systems so that pilots could increase the flow when using fuel mixed with A-50. A separate message regarding BX 6725 also specifically notes concerns about degraded engine life due to protracted use of A-50-laced JP-7, saying that "all operational considerations have been applied to reduce additive burning to a minimum."

The bigger issue at the end of the day may simply have been the Air Force's concerns about flying KC-135A tankers with loads of JP-7 mixed with A-50. Standard Black Shield missions involved A-12s launching from Kadena Air Base in Japan, then immediately linking up with one of the tankers. The spy planes would then fly their mission and link up with a tanker again, typically over Thailand, before heading back to base.
If the A-12 arrived at the refueling point late, or there was some other issue with the rendezvous, the KC-135As ran the risk of having to burn the JP-7 in order to stay aloft or make it back to base themselves. The tankers could have used the JP-7 in an emergency, but doing so required a complete purge of the tanker's fuel system and J57 engines afterward.

Radar Man: A Personal History of Stealth

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