EC-121 Warning Star
Manufacturer: The Lockheed Aerospace Company
Airframe: Modified Lockheed Constellation
Engines: Four Wright R-3350 Cyclones
Power output: 3,400 horsepower per engine
Length: 116 ft. 2 in
Wingspan: 126 ft. 2 in.
Height: 27 ft. 0 in.
Search Radar: AN/APS-20 (replaced by AN/APS-95 in later variants)
Height finder: AN/APS-45 (replaced by AN/APS-103 in later variants)
Nicknames: Warning Star, Super Constellation, Constellation, Super Connie, Connie
Primary function: Airborne Early Warning
First manufactured: 1953
Maximum weight: 145,000 pounds
Maximum speed: 290 mph (368 mph for some models)
Maximum altitude: 18,000 feet
Range: 1,000 to 4,000 miles depending on variant and payload
AN/APS-95 - 250 nm
AN/APS-45 - 120 nm
AN/APS-103 - 160 nm
AN/APS-20 - Unknown.
Armament: Six tons of electronic surveillance equipment
Crew: Typically 17 (26 according to some sources, and 31 were reportedly killed when North Korea shot down an EC-121 over the Sea of Japan in 1969)
2 Weapons Controllers
2 Flight Engineers
1 Radio Operator
6 Radar Operators
2 Radar Technicians
Although United States military forces had been experimenting with radar mounted in fighters and bombers for early warning since World War II, the EC-121 was the first plane designed and built specifically for the task. The Warning Star began development under the Navy designation PO-1W. It was successfully tested in NATO exercises resulting in large orders for the aircraft from both the United States Navy (its variant carried the designation WV-2) and the Air Force. First delivery occurred in 1955, and the last active planes were retired in 1978. During that time EC-121s saw duty as an extension of the DEW Line, patrolled the North Atlantic, saw duty in Vietnam, and according to one source, supported the Apollo Space Program.
As stated above, the EC-121 was the first dedicated AWACS airplane deployed by the United States. In October, 1967 an EC-121 patrolling the skies over Vietnam guided a US fighter to a successful intercept of a North Vietnamese Mig 21, the first successful use of an airborne control platform for a combat operation. EC-121s were used in the Vietnam War from 1965 through 1971.
Because it was the pioneer of early warning and control, the EC-121 went through many modifications during its many years of service, and was used to test many experimental radar and electronic surveillance systems. The engines were upgraded, digital data processing systems and relay systems for sending real-time information directly to ground installations were later added. Although most Warning Stars were equipped with two fixed radomes (one above and one below the fuselage), it eventually became the test-bed for the rotating radome design later incorporated into the E-2 Hawkeye and E-3 Sentry.
http://www.fas.org/man/dod-101/sys/ac/ec-121.htm (This may not be an original source, I found the exact text of this page on several other sites.)