Here's What You Need to Know: The premise behind Tacit Rainbow was to eliminate or suppress enemy air defenses using a loitering munition equipped with an anti-radiation guidance system.

In the 1980s, a controversial radar-hunting weapon system known as the AGM-136 Tacit Rainbow blurred the lines between missile and drone, leading to a loitering anti-radiation missile that could suppress or destroy enemy air defenses like no other weapon before it or since.

Amid a slew of bad press, underwhelming performance, and a crumbling Soviet threat, the Tacit Rainbow program was unceremoniously canceled in 1991, but as the world returns to a focus on Great Power Competition, it may be high time that we see Tacit Rainbow, or at least an updated successor, make its triumphant return.

Using Anti-Radiation Missiles in Warfare

Put simply, anti-radiation missiles home in on the radar air defense systems use to detect and engage incoming aircraft, making these missiles a valuable weapon in the initial days of a conflict when America’s warfare doctrine calls for establishing air superiority. As radar arrays come online to detect incoming planes, specially equipped aircraft can fire high-speed anti-radiation missiles (HARM) like the AGM-88E AARGMto follow the radio waves back to their source and destroy it.

The traditional approach to engaging enemy radar arrays, commonly known as Wild Weasel operations, involves sending fighters armed with anti-radiation missiles into contested airspace to wait for radars to come online to engage them. They then fire their radar-hunting missiles and hope the array stays online long enough for their weapon to find its target… all without getting shot down in the process.

Of course, not all anti-radiation missiles are designed to be launched from aircraft to take out land-based radars. There have also been air-to-air and even surface-to-air anti-radiation missiles developed around the world for different applications, and in fact, Tacit Rainbow itself began development as a surface-launched munition meant to take out these enemy radar arrays without having to put piloted aircraft in harm’s way.

Wild Weasels were an endangered species over Vietnam

The premise behind Tacit Rainbow was born out of the Vietnam War, where the very concept of Wild Weasel mission sets developed out of necessity, thanks to the introduction of Soviet Surface-to-Air missile platforms in the nation’s North. Throughout the conflict, pilots would literally use their fighters, sometimes jets as dated as F-100 Super Sabers, to bait enemy air defenses into powering on to give their wingmen’s anti-radiation missiles a good target. It was an effective means of suppressing air defense systems, but it was also incredibly dangerous. 

In May of 1966, the first modified F-105 Wild Weasel fighters, replacements for the far less capable F-100s, arrived near Vietnam, with 11 positioned in Korat Royal Thai Air Force Base, and seven others deployed to the 355th Fighter Wing at Takhli Air Base, both in Thailand. Within six weeks of arriving, all seven Wild Weasels assigned to the 355th had been shot down. Within six more weeks, the number of operational SAMs in the region had ballooned to more than 100.

At the time, flying 100 missions was considered a full combat tour, earning you a ticket home, but Wild Weasel pilots were tasked with such dangerous missions that reaching that figure seemed all but impossible at times.

“By your 66th mission you’ll have been shot down twice and picked up once,” F-105 pilots would commonly joke throughout the war. 

By the late 1970s, the Wild Weasel mission was being passed on to the F-4G, which was a modified version of the previous and capable F-4E Phantom II. These Wild Weasels were the most capable in the job yet, with a top speed of better than Mach 2.2 and the ability to carry 18,650 pounds of ordinance, but make no mistake, the immense danger associated with Wild Weasel missions remained as prevalent as ever.

Marrying missiles and drones: The birth of Tacit Rainbow

While the use of drones in combat is often seen as a characteristic of the Global War on Terror, drones and remotely piloted aircraft were already in service to the United States and other nations decades before Predators began prowling the skies over the Middle East. In fact, pilotless aircraft date back almost as far as piloted ones do, with the first credited to Sopwith Aviation in 1917. By the 1940s, B-17 Flying Fortresses were being converted into remotely piloted drones to collect data around nuclear explosions, and by the early ’70s, Lockheed’s supersonic D-21 reconnaissance drone was flying (unsuccessful) missions over China. 

There was no doubt that pilot-less aircraft could be a benefit to America’s defense apparatus by the early 1980s, and with the ever-present threat of war with the near-peer Soviet Union looming, Wild Weasel missions weren’t looking any safer. As a result, the Defense Advanced Projects Research Agency, or DARPA, began exploring the idea of combining the two concepts into a single radar-hunting UAV, or unmanned aerial vehicle.

Some of you may already be shouting at the screen that a marriage between ballistic missiles and drones had already occurred in the form of cruise missiles, which originated with Germany’s V-1 during World War II. You’re right, but DARPA’s new weapon was to be more drone than missile in comparison. Rather than flying like an aircraft to a target, as is common for cruise missiles, they wanted a munition that could loiter over a target area for extended periods of time, waiting and watching for enemy radars to come online before plunging itself into any target that appears. 

At the time, DARPA was using a two-word naming convention for their programs, with “tacit” serving as the first word and a seemingly random noun as the second. Thus, their new weapon system was dubbed, Tacit Rainbow.

How it was meant to work

The premise behind Tacit Rainbow was to eliminate or suppress enemy air defenses using a loitering munition equipped with an anti-radiation guidance system. Importantly, the weapon wouldn’t have to actually destroy enemy radar to be successful in its mission. In sufficient volume, Tacit Rainbow missiles could force an enemy to leave their air defenses offline to avoid their destruction, leaving American aircraft to operate freely in the area. 

In July of 1981, Northrop was chosen as the program’s sole contractor, with a development deadline of July 1983 and a target budget of $110.9 million (or about $339.1 million in 2021 dollars). However, by 1986, the Air Force had determined that they needed an air-launched variant of the weapon as well, which increased total anticipated costs by $160.6 million ($407.29 million in 2021). At that point, Raytheon was also awarded a $29.8 million ($75.57 million) contract as a second-source producer of the weapon.

Throughout this time, Tacit Rainbow remained shrouded in absolute secrecy under “Special Access” security protocols and was only reduced to Top Secret status around the same time the program was adjusted in focus. 

“The Tacit Rainbow was to be launched from B-52G and F-16 aircraft, cruise to a designated position, loiter, search for preprogrammed threat emitters, and attack those emitters.”

“Report on the Audit of the Acquisition of the Tacit Rainbow Anti-Radiation Missile System” via the Department of Defense Inspector General.

Deciphering the intended capabilities of Tacit Rainbow

The first Tacit Rainbow test launch took place on July 30, 1984, with at least 30 others of varying sorts to follow via multiple platforms, including bombers, fighters, and ground-based rocket launchers. The weapon carried a 40-pound WDU-30B blast fragmentation warhead and it may have leveraged the same or similar antenna and RF seeker found in the AGM-88 HARM missile. Unlike the AGM-88, however, Tacit Rainbow couldn’t be easily defeated by simply turning off the targeted radar array. If it lost its target, the weapon would simply switch back to loitering mode and begin its hunt anew. 

Even today, decades after Tacit Rainbow was revealed to the public in 1987, many details about the weapon’s legitimate capabilities remain difficult to ascertain. However, some particulars can be gleaned through the very public discourse regarding its budgetary overruns, alleged political corruption, and its direct involvement in Soviet-American arms reduction talks that took place in 1990 and 1991. Other details have been revealed through the steady release of documents over the years, including the Joint Tactical Autonomous Weapons System Program Office’s Tacit Rainbow program overview (created while the program was ongoing) and the Inspector General’s 1991 audit of the program’s acquisition process.

Interestingly, some details about the capability offered by Tacit Rainbow were released not through formal documentation, but rather through an Op-Ed penned by Georgia Senator Sam Nunn and then-acting Air Force Secretary James McGovern in the Washington Post in 1988. In the article, both officials refuted previously published allegations about Nunn’s undue influence over the embattled program (the original article, “Missile Project Close to Nunn’s Heart” by Jack Anderson and Dale Van Atta, is no longer hosted anywhere online). Finally, basics regarding its size, shape, and operation are all available from the National Museum of the U.S. Air Force, where a single AGM-136A Tacit Rainbow used in airworthiness tests remains on display. 

The weapon itself measured 8 feet, 4 inches long, with a body diameter of 2 feet, 3 inches, and a wingspan of 5 feet, 2 inches. It was powered by a Williams International F-121 turbofan that produced 70 pounds of thrust, propelling a total weight of 430 pounds. This combination led to a platform that could remain airborne for a claimed 80 minutes as it loitered over an area waiting for radar arrays to come online.

This 80-minute loitering time, if achieved, could allow a pack of Tacit Rainbow missiles to loiter over a target area for an hour and twenty minutes, engaging radar arrays as they came online or simply forcing the enemy to keep them offline. In either case, it would allow more than sufficient time for friendly aircraft to accomplish their missions inside the target area. 

Although it began as a ground-based weapon system, Tacit Rainbow’s focus soon turned to the skies, with plans to deploy the weapon in large numbers from B-52s, or in smaller numbers from Navy A-6E Intruders, and Air Force EF-111s and F-16s.

“Tacit Rainbow searches hostile territory for radar emitters while entering a target area. If radars are not active, the missile will enter a loiter pattern over the target area. Once a signal is detected, Tacit Rainbow attacks. Should the target be turned off, the missile returns to loiter in the target area, searching for it or other radar emitters.”

Air Force description of Tacit Rainbow upon its unveiling to the public in 1987.

The weapon’s total range has still yet to be released, though its specific inclusion in correspondence between Soviet Minister of Foreign Affairs Eduard Shevardnadze and U.S. Secretaries of State and Defense at the time, James Baker and Dick Cheney, respectively, does offer some insight. 

While Tacit Rainbow was never intended to carry nuclear payloads, the Soviet military believed it could be modified to do so, and as such, wanted it included in the limitations the two nations sought to place on air-launched cruise missiles. Baker countered and won in a letter listing his reasons it shouldn’t be included, third of which addressed the issue of range–which he characterized as greater than 373 miles but less than 500.

“Third, on your concern about range. I am advised that its range is less than 800 km. As you know Tacit Rainbow only became an issue when we considered accepting your proposal for a 600 km ALCM range threshold. Under our preferred position of 800 km, Tacit Rainbow was not an issue.”

-Secretary of State James Baker III in a letter to Soviet Minister of Foreign Affairs Eduard Shevardnadze dated May 19, 1990.

Some unconfirmed sources claim Tacit Rainbow offered a total range of only 280 miles, though if true, that figure may represent the weapon’s actual (and seemingly poor) performance in testing, rather than its intended performance on paper.

“They don’t know ordnance engineering.”

On June 30, 1988, one year after the program was unveiled to the public, the Los Angeles Times published one of numerous stories to come about the cost overruns and program mismanagement that seemed to pervade every facet of Tacit Rainbow’s development. 

According to the report, Tacit Rainbow was already a year behind schedule and ten times more expensive than originally intended, placing a great deal of the blame on Northrop’s shoulders and seemingly shining a light on a conflict between the Air Force and Navy, who were working together on the effort under congressional recommendation. 

“At one point, development costs at Northrop were running $20 million per month,” one official said. “Even the Air Force wanted to terminate Northrop’s contract.”

“Paisley Halted Experts’ Attempt to Kill Missile : Navy Weapons Scientist Who Opposed Troubled Northrop Project Was Later Eased Out of Job” by Ralph Vartabedian in the Los Angeles Times, June 30, 1988

The article also claimed then-Assistant Navy Secretary Melvyn R. Paisley exercised undue influence over the program, going so far as to force seasoned experts out of their jobs at the China Lake Naval Weapons Center for expressing concerns about its troubled development. Those experts, the article contends, could see that Tacit Rainbow was not living up to expectations as early as 1985, with many calling for Northrop’s contract to be terminated in favor of soliciting new bids from defense contractors.

“There was an awful lot of dumb engineering by Northrop,”  said Dr. Frank Cartwright, a former senior adviser at China Lake during Tacit Rainbow’s development. 

“They don’t know ordnance engineering.”

Other allegations of corruption reached Senator Nunn, prompting the aforementioned Op-Ed in his defense, but the trouble for Tacit Rainbow was only beginning. The Navy soon pulled out of supporting full-scale production of the weapon, and an issue of “Inside the Pentagon” printed in November of the following year indicated that the Navy remained hesitant to support production even amid renewed pressure from the Pentagon. 

In February of 1991, the New York Times reported that the Defense Department was canceling Tacit Rainbow as a part of its FY1992 budget proposal, and the following month, a damning report from the United States General Accounting Office would serve as the first post-mortem analysis into the program’s failure. 

The GAO report was the first official document to be released that put Tacit Rainbow’s failings in clear, easy-to-understand terms. To put it bluntly, the weapon simply couldn’t do what it was designed to do. In fact, it could barely find a target at all. 

“Tacit Rainbow was unreliable in its flight test program and did not demonstrate its readiness to begin production. In over one-half of the 16 flight tests, the missile did not hit the target because of guidance system failures and other performance problems, and only 2 successful flight tests occurred out of the last 10 attempts,” the report explained.

Could the U.S. use a new Tacit Rainbow?

In hindsight, it’s clear that the Tacit Rainbow program was too riddled with failures and rampant politicking to have manifested in a successful weapon system, but the concept behind the weapon remains as viable today as ever. 

Perhaps influenced by Tacit Rainbow, the United States would go on to deploy drones into Iraqi airspace at the onset of the Gulf War in 1991 to tempt Iraqi commanders into activating their air defense radar systems. Once online, these systems became easy prey for fighters armed with anti-radiation missiles following behind, but as many stories out of the Gulf War clearly show, surface-to-air missiles remained a prevalent concern.

In 1995, the Pentagon took another swing at a loitering air defense suppression munition with the ADM-160 MALD (Miniature Air-Launched Decoy)

Today, the United States Navy is in the early stages of production on the AARGM-ER, an anti-radiation missile with an operational range of (potentially) around 120 miles. This is just far enough to keep the aircraft employing these weapons out of range of being shot down, but with an increased focus placed on anti-stealth defenses in nations like Russia and China, Wild Weasel F-35s would still face daunting risks in a large scale fight. 

However, in the years since Tacit Rainbow was canceled, drone and missile technology have both improved dramatically. 

It stands to reason that a modern version of the Tacit Rainbow concept, leveraging the low-observability and low-cost of attritable drone programs like the Kratos Valkyrie and even the hypersonic approach and subsonic loitering capabilities from programs like the Army’s Vintage Racer, could wreak havoc on even the most advanced enemy air defenses. 

During Tacit Rainbow’s era, the enemy radar systems had to stay online until the missile reached its target for success, but the AARGM-ER’s targeting apparatus, made up of a digital passive radar emission detector, active millimeter wave radar, and a combination of INS and GPS systems, allows for continued pursuit of radar arrays even after being powered down or potentially, while moving. It goes without saying that a loitering munition equipped with the same targeting capability could be much more effective than even Tacit Rainbow’s unrealized goals.

Even without a hypersonic approach to a target area, a large volume of inexpensive loitering anti-radiation missiles like Tacit Rainbow could overwhelm surface-to-air missile systems without the extended presence of Wild Weasel fighters braving not just inbound SAMs, but anti-aircraft fire and any number of other threats that could arise in a large-scale peer-level conflict. Instead, Wild Weasel could become a phrase more commonly associated with a class of inexpensive swarms of loitering weapon systems, rather than fighters at all. And because these weapons are much smaller than traditional fighters and produce much less heat from their engines, they’re also less likely to be shot down while they wait. 

Like Vintage Racer, the next generation Tacit Rainbow could lob sub-munitions at targets as they appear, saving itself for further missions, or it could potentially fly armed with its own warhead to engage a final target itself after its submunitions were all expended, not unlike the intent behind the otherwise utterly insane Project Pluto. 

Then again, Tacit Rainbow’s development continued for years before the public became aware of its intent or its troubles. Maybe a spiritual successor has already managed the same feat, coming to fruition behind a curtain of classified funding. 

Only time will tell.

Alex Hollings is a writer, dad, and Marine veteran who specializes in foreign policy and defense technology analysis. He holds a master’s degree in Communications from Southern New Hampshire University, as well as a bachelor’s degree in Corporate and Organizational Communications from Framingham State University.

This article first appeared at Sandboxx News.

Image: U.S. Air Force/Flickr