AGM-136 Tacit Rainbow
The requirement was for a low-cost air-launchable system to aid in the destruction of enemy air defense networks. The proposed unit would combine elements of cruise missiles and UAVs, it would be launched in large numbers by heavy bombers, fighters, or possibly mass ground launch systems. The missiles would fly in advance of manned aircraft up to 450 km (280 mi) to pre-programmed target zones and patrol there until enemy radar sources were detected which would then be destroyed. This extended patrol time on target ("loiter time") was the key feature of the new system, a Persistent Anti-radiation Missile (PARM) as opposed to a HARM.
The project was started by the DoD in 1982, but moved to the control of the USAF Aeronautical Systems Division in 1984 as a joint Navy/Air Force project. The majority of the system was designed and developed by Northrop with Texas Instruments providing the seek head and Boeing providing a system that allowed it to be launched from B-52 bombers. The first test air-launch was on July 30, 1984.
The unit was 8 ft 4 in (2.54 m) long and 5 ft 2 in (1.575 m) in span with a body diameter of 27 in (686 mm), flight and control surfaces deployed after launch. It massed around 431 lb (195 kg) including the 40 lb (18 kg) warhead. Power was provided by a Williams F121 turbofan, producing 310 N (70 lbf) of thrust from the 0.9 m, 22 kg unit. Some sources state that production units would have used a 1,200 N (270 lbf) variant of the Williams International WR-24. Achieved speed and range are uncertain, low subsonic speed is probable and all sources indicate a range much lower than the hoped-for 450 km (280 mi). Each unit was to cost around $200,000, up to thirty would have been loaded in a single B-52.
The Naval Research Advisory Committee reported in 1989 that the project was not progressing well. In 1991 a DoD audit found numerous management problems. The program was canceled in 1991 (FY 1992), without any production units and at a total cost of around $4 billion. It was only the second post-Vietnam military project to be canceled after completing testing but before production.
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Below is a list of museums which have a Tacit Rainbow in their collection:
- Museum of Aviation, Robins Air Force Base, Georgia 
- National Museum of the United States Air Force, Wright-Patterson Air Force Base, Ohio 
- U.S. Naval Museum of Armament & Technology, NAWS China Lake, California