CIA’s Predecessor To The SR-71 Blackbird Tested Electron Guns To Hide From Radars

Concerns about advanced Soviet radars prompted the development of systems that could generate invisible radar-absorbing fields around the aircraft.

byJoseph Trevithick|
Airborne Radar photo


The Central Intelligence Agency initiated the Oxcart program, which led to Lockheed's A-12 spy plane, the predecessor of the U.S. Air Force's SR-71 Blackbird, specifically because of concerns that the iconic U-2 Dragon Lady was becoming too vulnerable to Soviet and other hostile air defenses. The aircraft's primary advantages were in its ability to fly extremely high and fast, but it was apparent from the beginning of its development that those capabilities might not be enough to defend against existing and emerging threats at the time. 

As a result, the A-12 itself featured then-state-of-the-art stealthy shaping together with radar-evading structures that made heavy use of composites and that were coated in radar-absorbing materials. The CIA undertook further efforts to explore more novel means of reducing the radar cross-section of the planes, including the development of a cesium-laced fuel additive intended to shield its rear aspect from radar waves using a concept called "plasma stealth," which you can read about in greater detail in this past War Zone feature. This same principle also led to the development of powerful electron guns that the A-12 could carry inside its fuselage to create similarly radar-absorbing fields in other directions, according to various declassified documents now available online via the CIA Records Search Tool, or CREST.

The CIA's search for a U-2 successor began in 1956 and in two years the Agency had determined the proposals from Lockheed and Convair represented the most viable designs. Stealthy features became the deciding factor.

"On 28 August 1959, Mr. Bissell [Dr. Richard Bissell] told me to come east for the 19th time in this competition," Clarence "Kelly" Johnson, the famous Lockheed engineer who for years ran the company's Advanced Development Programs (ADP) division, or Skunk Works, explained in a previously classified 1968 official internal history of the A-12. "He told me that we had won the competition, subject to our proof of low radar cross-section between that period and 1 January 1960."

A scale model of an A-12, mounted upside down at Area 51 for tests of its radar cross-section circa 1959., CIA

Bissell was the CIA’s Special Assistant for Planning and Coordination and was in charge of the Oxcart program. He had threatened to cancel the entire project if the competing companies couldn't make significant progress in reducing the radar cross-sections of their designs. Ed Lovick, another Skunk Works engineer, who Johnson credits with coming up with the cesium fuel additive idea, says that development saved the A-12 in his own book, Radar Man: A Personal History of Stealth.

The A-12's engine nacelles and ducts also had chines on the outer edges and chined leading edges of the wings with saw-tooth-shaped baffles underneath the surface to further reduce the aircraft's radar returns. Curved wing extensions on the leading edges and canted rear vertical stabilizers, as well as spiked cones covering the inlets for the two huge Pratt and Whitney J58 engines, also helped deflect incoming radar waves.

A diagram showing some of the A-12's "anti-radar" features., CIA

Lockheed made good use of radar-absorbing composite materials in all of these areas, except for the spikes over the engine inlets. Literally on top of all of that was a layer of "iron paint," also referred to as "iron ball paint" because of the small iron balls mixed in, which also helped reduce the aircraft's radar signature. That paint's special blend, which Lockheed also used on the later SR-71, reportedly cost $400 per quart in the 1960s.

A description of radar cross-section reducing efforts Lockheed and the CIA had undertaken, including fuel additives and electron guns, according to Lockheed's own 1968 internal history of the A-12., Lockheed

In spite of all of this, the CIA's fears about the potential vulnerability of manned reconnaissance aircraft persisted. The infamous Soviet shootdown of Gary Powers' U-2 on May 1, 1960, further fuel those concerns and ended spy plane flights over that country. 

The introduction of new Soviet radars, particularly the P-14 Tall King, was also a major driving factor. The CIA, together with other U.S. Intelligence agencies and elements of the U.S. military, expended considerable effort in the 1950s and in the early 1960s, as the A-12 was still in development, gathering details about these radars through programs codenamed Melody and Palladium. Palladium involved a particularly complicated set of ruses to produce false radar returns and then observe the response from examples of these radars positioned in Cuba, allowing analysts to try to work out what radar operators were and weren't seeing on their screens, and thus how the A-12's own radar cross-section would fare against those sensors.

"In April 1963 we were directed to rebuild the aircraft chines to change the optimum radar cross-section at S-band to favor better performance against the 'Tall King,'" Johnson wrote in his 1968 history. "This was an expensive and (as it finally turned out to be) undesirable change."

A P-14 Tall King radar., ShinePhantom via Wikimedia

Work on the cesium fuel additive, eventually known as A-50, offered one possible method of reducing the radar cross-section of the rear aspect of the aircraft. This was already a problematic area in the radar-evading work because of the jet's massive exhausts and the radar reflective plume from the J58s at full afterburner while flying at above Mach 3. 

At its most basic, the general concept of plasma stealth involves using some means to create a cloud of ionized particles, or plasma, which is capable of absorbs electromagnetic radiation, such as radar waves, so they can't reflect back. Burning cesium in the super-heated exhaust stream would do just that at the back of the plane.

The obvious problem was that the exhaust stream only pointed rearward. A-50 could not produce a similar ionized cloud toward the front aspect of the aircraft, which would be most exposed as the aircraft approached the target area at the beginning of its reconnaissance pass. This is also when the aircraft would be most vulnerable.

One option the CIA considered, codenamed Emerald, was to install devices elsewhere in the aircraft that would create "a seeded plasma electric arc," similar to the effect of adding cesium in the exhaust stream, but in other directions. Another idea, codenamed Kempster, was to install electron guns that would emit electrically-charged particles to produce a similar effect.

A section of a 1963 status report on the Oxcart program mentioning both the Emerald and Kempster programs., CIA

Westinghouse Research Laboratories, a division of the Westinghouse Electric Corporation, and General Electric subsequently got contracts to develop these electron guns. Westinghouse eventually took the lead in the Kempster program.

The Kempster program sought devices that would be small enough to fit inside the chines on either side of the A-12 near the wing fairings, where they could help conceal the radar reflections from the engine inlet spikes. The goal was to have the electron guns project cones of radar-absorbing particles that would extend out approximately 300 inches, or 25 feet, from the chine, according to one 1964 report.

Details about the proposed Kempster installation concept as of 1964., CIA

Electron gun technology was already well established at the time and it was, and still is, used in an array of different commercial products, including televisions and monitors that use cathode ray tubes, and manufacturing processes. A typical electron gun design uses a hot cathode at one end of a container to create a stream of electrons that then pass by electrodes and anodes that focus them into a beam and accelerates them, respectively. 

The generation of the electron beam takes place in a vacuum region, which then creates the challenge of how to get it out into the ambient environment. One established method involves simply placing a medium, typically a sheet of some kind of metal foil, also known as a "window," at one end that the electrons can pass through. 

A-12 ripping down the runway at Groom lake. , Public Domain

Westinghouse used this method to create what it referred to as windowed and window tube designs. The problem with this method is that the metal strips energy from the beam as it passes through, potentially reducing its power, and cannot be too thick or it will block the beam entirely. Unfortunately, keeping the metal barrier sufficiently thin makes it fragile. This might not necessarily be a major problem in an industrial or similar setting, where workers might be more readily able to replace the window quickly. A system intended to work reliably on an aircraft flying more than three times the speed of sound is a completely different matter.

The focus ultimately shifted from these windowed designs to another method that allows the beam to leave the gun through an open orifice at one end and uses vacuum pumps to remove gases, such as ambient air, from the internal sections. This type of pumped system is notably heavier and bulkier than a windowed design. Beyond the electron gun itself, the complete system also needs a substantial power source and cooling systems to prevent dangerous overheating. 

A very basic diagram showing the basic elements of a pumped electron gun, at left, and a windowed one, at right. The arrows how the path of the electron beam., University of South Carolina

Getting a pumped device to work within the weight and space constraints of the chines, an area of the aircraft that would also experience serious physical strains and high temperatures when the aircraft was flying at Mach 3, presented challenges for Westinghouse. The company initially bought an industrial example straight from W. C. Heraeus in Hanau, West Germany and modified it for the Kempster program.

This system worked well enough, but was far too big to fit inside the A-12's chines. Flight testing of the Heraeus gun on one of the aircraft, known as Article 126, required its installation in the aircraft's main payload bay, or Q bay, with the beam pointing straight down. Had this configuration become operational, it would have limited the space inside the bay for cameras or other equipment.

A low-quality scan of the modified Heraeus gun configured to fit inside the A-12's Q-bay., Westinghouse

"In conclusion, the undersigned believes that, even if [redacted] is successful in [the] operation of its KEMPSTER A, there is little chance that it will ever be used operationally, because of the great weight, 250 lbs or more, per gun and potentially high-power requirements," according to a 1963 CIA progress note on the program. The CIA also redacted the name of the author of that brief report. It is not entirely clear from the available CIA sources what systems were developed as part of the Kempster A phase and how they differ from those pursued as part of a second line of effort known as Kempster B.

Portions of a 1963 CIA progress report on the Kempster program., CIA

Regardless, after working with designs based on the Heraeus gun, Westinghouse subsequently developed its own compact pumped "C-Gun" design, which did fit -inside the chines. A basic control panel was installed in the cockpit to allow the pilot to turn the system on or off, or to set it to a cool down mode, as well as select whether to project the beams from the right, the left, or both electron guns.

A diagram showing Westinghouse's C-Gun electron gun design., Westinghouse

At least two flight tests took place in 1964, with an A-12 known as Article 131 carrying the electron guns, according to CIA reports. "The KEMPSTER equipment shutters opened but power failure occurred," according to a cable about a test flight on Dec. 11, 1964. The system worked as intended during another test flight six days later, according to another cable, but all further details are redacted.

Westinghouse built at least four prototype C-Guns, enough to equip two aircraft at any one time, but there is no indication that these were ever employed operationally. A now-declassified internal CIA history of the U-2 and A-12 programs says "this project proved unsuccessful," but offers no further details.

One issue may have been that the electron guns produced x-ray radiation as a byproduct of their operation, both in flight and during pre-flight checkouts on the ground. With regards to the Heraeus gun, there were concerns that this could impact the film in the cameras in the Q-bay.

This was, of course, a less pressing concern with regards to the chine-mounted C-guns, but there was still the matter of potentially dangerous x-ray exposure, especially to ground personnel during pre-flight checkouts. "Means were employed to assure safe conditions for all personnel concerned," according to a 1965 Westinghouse report.

As of October 1965, Westinghouse and General Electric were both still receiving funding from the CIA and National Reconnaissance Office for work on both Kempster A and Kempster B. The work on Kempster B was described at the time as a "theoretical study."

A discussion about Kempster funding in 1965., CIA

The CIA's U-2 and A-12 history, which the Agency first published in 1992, blames the U.S. Air Force's unwillingness to share electronic countermeasures technology with the Oxcart program as the reason for developing Kempster in the first place. The Air Force feared that A-12 operations would give the Soviets too much opportunity to analyze the countermeasures, according to the CIA.

The CIA did ultimately develop an electronic warfare package for the A-12 and that may well have sealed the fate of efforts to develop improved electron guns under Kempster B. It certainly appears to have offered sufficient protection, combined with the aircraft's speed and high altitude flight profile, that the Agency also does not appear to have ever employed the A-50 cesium fuel additive operationally, either.

The CIA did not even use A-50 during missions over North Vietnam in the 1960s where the threat of hostile fire was very real. On Oct. 30, 1967, CIA pilot Dennis Sullivan flew an A-12 on a mission over that country during which North Vietnamese air defenders fired at least six SA-2 Guideline surface to air missiles at his aircraft. Sullivan saw at least three of them detonate near him and contrails from the launches were also present in some of the pictures taken during the mission, according to an official CIA account of the sortie. The missiles came close enough to leave fragment lodged in the Oxcart's fuselage.

The CIA retired the A-12s in 1968. There's no evidence that the Air Force made any use of the work the CIA did under the Kempster program to help protect its SR-71s, which had begun flying missions in 1966, either. Those aircraft also utilized an advanced electronic warfare countermeasures suite, in addition to a stealthy shape and radar-absorbing features developed from those on the A-12, as their primary means of protecting themselves from enemy air defenses.

A contemporary comparison of the electronic countermeasures systems on the A-12 and the SR-71 from the mid-1960s. The A-12's capabilities in this regard are entirely redacted. , CIA

The basic concept of plasma stealth and using other particles to create radar wave-absorbing fields is said to have continued in both the United States in the Soviet Union for years after the Kempster program came to an end and the CIA retired the A-12s for good. As with the electron guns for the Oxcarts, information on most of these projects is limited. Most recently, the topic of plasma stealth did resurface in unconfirmed reports that the Russian 3M22 Zircon hypersonic cruise missile might use this principle, or something similar, to help conceal it from hostile radars. 

Without knowing the exact reason why the CIA deemed Kempster to be a failure, it's hard to say whether or not electron guns could ever provide sufficiently useful radar-evading capabilities to outweigh their complexities compared to other stealth and electronic warfare technologies. Regardless, the project is definitely another interesting and little known piece of the A-12's history, which ties directly into the story of the SR-71, much of which is still unknown.

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