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Description
The ominous growl of the slow moving Lockheed Super Constellation disrupted the sultry autumn evening in 1967 at Korat Royal Thai Air Force Base (RTAFB). The United States Air Force (USAF) maintenance personnel and Thai cleaning staff paid scant attention to the EC-121 as she taxied onto the Constellation portion of the ramp and took her place among the parked EC-121Ds and EC-121Rs. From all outside appearances she was just another EC-121D assigned to Detachment 1 of the 552d Airborne Early Warning Wing. Yet the crew that climbed down the air stairs wore patches denoting assignment not to Air Defense Command, as one would expect, but to the fighter-owning Tactical Air Command. The aircraft was assigned to Detachment 2 of the Tactical Air Warfare Center and was a one-off modified EC-121K known as Rivet Top. Until recently her mission of direct support to fighters over North Vietnam has been shrouded in the mystery associated with classified signals intelligence (SIGINT) operations, but now her story can be told in full.
Genesis of a Program
Originally designated Sea Trap, Rivet Top evolved out of a series of studies in early 1966 designed to counter the SA-2 surface-to-air missile (SAM) system that was being used to great effect in North Vietnam. In May 1966, Headquarters USAF revised the mission of the program from merely locating SAM sites and directing strikes against them to also providing warning of SAMs and fighter aircraft hostile to USAF strike aircraft in the area. The updated plan was staffed through headquarters as a formal project re-named Rivet Top and approved in November 1966. (1) The final evolution of the Rivet Top plan centered on a concept called Airborne Tactical Air Coordination Center, where intelligence collection and command and control functions were fused on a single airframe.
Work on the design and development started immediately using a single EC-121K originally delivered to the U.S. Navy as Bureau Number (Bu No) 143184. Modifications designed by E-Systems and managed by the Big Safari Program Office proceeded briskly and the aircraft was delivered for operational test and evaluation in March 1967. The EC-121K gained a USAF serial based on her prior Navy Bu No, being carried on Air Force books as 57-143184. (2) The original plan called for the Tactical Air Warfare Center to test the Rivet Top aircraft for ninety days in the U.S. followed by a combat evaluation in Southeast Asia. (3) Though her conversion work was completed in a relatively short amount of time, 57-143184 was far from a simple aircraft when she rolled off the modification line at the Greenville, Texas, plant of E-Systems.
Originally delivered to the U.S. Navy as a WV2, 57-143184 was similar to the USAF airborne early warning EC-121D aircraft operated by Air Defense Command. However, 57-143184's airframe and interior received few major alterations, reducing the amount of flight test required for fielding the system. Previously in 1958, the Navy had modified eleven of 57-143184's WV-2 brethren for the SIGINT role and designated the new machines EC-121Ms. (4) E-Systems took advantage of this work and many of the subsystems that flew on Rivet Top were developed for the EC-121M program. The WV-2's ventrally-mounted AN/APS-20 surveillance radar was disabled and the antenna turned into a large receiver for electronic intelligence (ELINT) purposes. This modification, similar to that used in the Navy's Big Look EC-121M aircraft, allowed the Rivet Top crew to develop precise location data against the SA-2's Fan Song radar via accurate direction-finding (DF) cuts. The new aircraft also benefited from the fitting of the EC-121M Brigand system which allowed the Rivet Top crew to accurately DF early warning radars. (5) Unfortunately, both systems required up to five minutes to develop an accurate location of the radar. While this timeline was effective against early warning radars that tended to stay active for tens of minutes at a time, it did not work well against the Fan Song radar, which would stay on the air for much shorter periods. (6)
To allow 57-143184 to track fighter aircraft, the QRC-248 and AN/APX-49 Identification Friend or Foe (IFF) systems, also fitted to USAF EC-121D... |
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