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.

The SR-71 Blackbird’s Predecessor Created “Plasma Stealth” By Burning Cesium-Laced Fuel
Joseph Trevithick and Tyler RogowayView joseph trevithick and tyler rogoway's Articles

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., CIA

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., CIA

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.”

A J58 during a ground test., CIA
Another J58 engine running on a test stand with its afterburner engaged., NASA

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.

A breakdown of the radar cross-section reducing goals and the efforts Lockheed was undertaking to meet them with regards to the A-12 from a now-declassified company report., Lockheed via USAF

Lockheed tested mixtures using sodium, potassium, and cesium. “Cesium seemed to be the material of choice because in the gaseous state it is the most easily ionized,” Lovick wrote in his book.

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. 

In 1965, Pratt & Whitney had informed the CIA that it was looking into a new "carrier fluid" for the cesium in A-50, assumed to be dialkyl phosphate, because of a supply chain problem. The company had been purchasing the base solution from sources in the detergent industry, which were generating it as a byproduct of their own production processes. New U.S. government regulations had subsequently forced changes in detergent production that eliminated this byproduct. So, it's unclear if the composition of A-50 subsequently changed or if Pratt & Whitney just identified a new source of dialkyl phosphate in the end. 

A note about A-50 production from a 1965 CIA memo regarding the A-12., CIA

This raises separate questions about the toxicity of the A-50 additive. Though there is no indication that A-50 used a radioactive cesium isotope, the compound is toxic and it's unclear how hazardous the complete mixture may have been, before or after being burned up in the J58s, especially given that its primary component was a detergent byproduct. At the same time, jet fuels are themselves generally toxic, to begin with, and the special fuel that the A-12s ran on was certainly no exception.

It’s also not clear from the available records how much A-50 was necessary per gallon of JP-7 jet fuel to create the desired effect. The J58s on the A-12s, as well as the SR-71s, needed JP-7, which has a very low volatility and high flash point, because of the unusual combination of extremely high internal temperatures and very low external temperatures the planes experienced during high-altitude supersonic flight.

The development of the A-12, in general, where almost every step was a cutting-edge engineering feat, suffered numerous delays and, despite a first flight in 1962, it did not fly an operational mission until 1967. Between 1967 and 1968, A-12s flew 26 sorties over North Vietnam and an additional three over North Korea as part of Operation Black Shield

It's hard to tell definitively whether or not A-12s used JP-7 mixed with A-50 on any of those flights. The CIA wanted to use it on at least one Black Shield mission, referred to as BX6725, on Oct. 4, 1967, which included grabbing imagery of Chinese military facilities on Hainan Island as the aircraft exited North Vietnam into the South China Sea. At least one other mission, BX6727, followed similar tracks that offered an opportunity to collect imagery on Chinese targets.

A low-quality scan of the map of the approximate tracks for Black Shield mission BX 6725. Hainan Island is not visible, except for the name, but lies just to the northeast of where the routes began and ended in the South China Sea, to the right., CIA

However, none of the post-mission reports clearly state that the A-12s on either of these missions, or any others, used A-50. Available records also make clear that standard JP-7 did not have the cesium mixture and that personnel would only have added it when authorized to do so for a specific mission. It’s not entirely clear why, with the reported effectiveness of the mixture, the CIA would have declined to actually employ operationally in the end.

North Vietnamese air defenses, some of the densest in the world at the time, attempted to engage the A-12s during each sortie. On one flight on Oct. 30, 1967, they fired at least six Soviet-made SA-2 Guideline surface-to-air missiles—the same missiles that had shot down Gary Powers' U-2 over the Soviet Union in 1960—at an Oxcart with CIA pilot Dennis Sullivan at the controls. The pictures he took during that flight caught the contrails of the missiles heading toward his aircraft, according to an official CIA history. He saw at least three of them detonate near him and a post-flight inspection found a fragment of one of the missiles embedded in the A-12's fuselage. So, there was certainly a threat. 

Years before Black Shield, the CIA had also worked to assess the capabilities of advanced Soviet radars as part of programs codenamed Melody and Palladium. These projects involved complex efforts to generate false radar returns and then monitor Soviet reactions, thereby gauging what they could and could not "see" on their radar screens. As such, the Agency would have had at least some sense of how vulnerable the A-12s would be to detection in spite of their stealthy features.

A Soviet P-14 Tall King radar, the appearance of which had prompted the CIA to begin looking into reducing the radar cross-section of the U-2, as well as its successor, in the first place., ShinePhantom via Wikimedia

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. 

A KC-135Q refuels an SR-71, the successor to the A-12., USAF

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. 

Concerns about JP-7 mixing with the conventional jet fuel the KC-135s use to power their own engines during missions eventually led the Air Force to convert 56 of the aircraft in into the KC-135Q configuration between 1966 and 1968. These aircraft were specifically set aside to support SR-71 operations and featured body tanks that were completely isolated from the rest of the plane's fuel system to ensure everything stayed separate.

The A-50 additive would have only compounded these issues, though Lockheed's Lovick said in his book that the company actually conducted the initial tests of the additive effects using a J57. The pre-planning message regarding BX 6725 also suggests that, somewhere along the line, studies had shown tankers could safely burn A-50 in emergencies. 

An excerpt from the BX 6725 mission planning message talking about issues relating to A-50., CIA

The Air Force seems to have been unconvinced. “Tankers almost forced to burn A-50 if there is a delay by receiver,” a message dated September 1967 concerning Black Shield operations states.

"A-50 still a major logistical problem but understand the reasoning behind at least giving it a try,” the missive continued. “SAC [Strategic Air Command] still not authorizing use except in emergency."

A mention of issues related to A-50 use from a September 1967 CIA message., CIA

Whatever the case, A-50 appears to have at least remained available for use throughout Operation Black Shield. What happened to any remaining stockpiles of the additive after the A-12 program came to a close in 1968 is unclear. 

Though there’s no evidence they ever did, there’s no reason the SR-71 would not have been able to use it, if necessary, with the same potential impacts on engine life and performance. The segregated fuel system on the KC-135Q tankers, which the Air Force eventually re-engined with CFM56s to create the KC-135T variant, would have largely eliminated the concerns about the additive finding its way into the tanker's engines for any reason. KC-10 Extenders, which also supported the Blackbirds later in their service life, also had the ability to keep the JP-7 separated from the aircraft's own fuel load.

A KC-10 refuels an SR-71., USAF

In addition, even though the SR-71 featured improved electronic warfare capabilities and never overflew the Soviet Union, it still operated in areas where there were very real threats from enemy air defenses. Blackbirds also flew over North Vietnam, for instance, and certainly flew missions around the Soviet Union and its Warsaw Pact allies, which carried distinct risks.

A recently declassified picture of an SR-71 experiencing an in-flight emergency in Swedish airspace during a mission near the Soviet Union in 1987., Forsvarsmakten

SR-71s also flew missions over Egypt, Israel, Lebanon, and Syria around the Yom Kippur War in 1973, areas again with heavy, potentially hostile air defenses. Flights around North Korea, including near the Demilitarized Zone that bisects the Korean Peninsula, were risky, too, with the North Koreans attempting to shoot down one of the Blackbirds on at least one occasion in 1981. 

One of the Blackbirds also flew a mission over Libya to conduct bomb damage assessments following U.S. airstrikes on that country in 1986 during Operation El Dorado Canyon. There were also flights in the Persian Gulf targeting Iran during the 1980s, as well as routine flights over Cuba. Additional missions reportedly occured over or near The Falklands, Nicaragua, and South Africa. The aircraft could have been called upon to conduct other riskier missions if an actual large-scale conflict had broken out between the United States and a major adversary, such as the Soviet Union.

Work related to plasma stealth, as a general concept, certainly continued in the decades after A-12 operations ceased, both in the United States and the Soviet Union. The extent to which this included research and development on creating these ionized clouds via chemical additives in an engine's exhaust stream is less clear. 

There are reports that Russia may be continuing to investigate the possible value of plasma stealth, especially as it related to the development of hypersonic vehicles. There are unconfirmed reports that the 3M22 Zircon hypersonic cruise missile may have some capabilities in this regard, though the exact method of generating the ionization is unclear. 

Regardless, ensuring adequate radar-evading effects in the rear aspect remains one of the more challenging elements of designing stealthy aircraft and missiles. So, plasma stealth would certainly continue to be one possible method going about this. Concerns about impacts on the engine from using chemical additives would be largely moot in a one-time-use system, such as a cruise missile. Impacts on actual flight performance from using fuel additives would still be an issue, especially with flight vehicles that need to be flying at extreme speeds to remain stable. 

Plasma stealth could also be a valuable option for quickly improving the stealthy qualities of existing designs. As was the case with A-50, fuel additives, specifically, could provide a way of rapidly adding in this kind of capability, but only as necessary and with minimal modifications to the aircraft itself.

If nothing else, though, the development of the A-50 additive is another intriguing and very obscure part of the story of the A-12, as well as its successor, the SR-71, much of which still remains unknown to this day, and it also offers intriguing insights into the dawn of stealth technology.

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