HH-53C Jolly Green was developed by Sikorsky as a variant of the heavy helicopter CH-53 Sea Stallion. It entered service with the U.S. Air Force in the late 1960s. It was designed to carry out CSAR "Search and Rescue" missions in combat zones with high operational effectiveness. Compared to the previous version, it was equipped with an aerial refueling probe, additional fuel tanks, reinforced armor, and the option to mount extra defensive armament. Particular attention was given to improving the radio systems, which had to coordinate operational activities with other supporting aircraft-such as tankers or attack and support helicopters-needed to safely locate downed or missing pilots. Robust, reliable, and featuring long flight endurance, it was widely used during the Vietnam War.