A sole HR2S-1 was then modified for U.S. Army trials as the "YH-37" and ultimately accepted under the finalized "H-37" designation. These helicopters were procured in 94 examples with initial batch models designated as "H-37A". Army versions differed some from the USMC mounts at the main rotor hub and at the vertical tail fin. With new cargo doors, reinforced fuel stores, and an automatic stabilization system introduced, ninety of these original A-models were upgraded to the "H-37B" standard.
The United States Navy took on the helicopter in only two examples for the Airborne Early Warning (AEW) role and designated them as "HR2S-1W".
Following the 1962 reworking of American military aircraft designations, the Army's H-37A became the "CH-37A", the H-37B became the "CH-37B", and the USMC's HR2S-1 became the "CH-37C".
The HR2S-1 saw first deliveries in mid-1956 to both the USMC and Army and it was during this time that the United States military saw itself committed to actions centered around the Vietnam War (1955-1975). The Mojave was shuttled to the theater in limited numbers and were used primarily in the equipment recovery role. Crews were sent to pick up downed aircraft in enemy territory with regularity while other missions ranged from artillery relocation to fuel and personnel deliveries. The CH-37 was used alongside another heavy-hauler - the awkward-looking CH-54 "Flying Crane" - which succeeded CH-37s in the war. The USMC elected the CH-53 "Sea Stallion" as the official replacement for its Mojaves. Sea Stallions were introduced in 1966.
By the end of the decade, the Mojaves were all but out of frontline service. They also did not see export for they proved a limited design - large, heavy, and expensive to obtain and operate in any useful numbers. Additionally, the machine relied on a piston-driven engine arrangement at a time when the shift to more powerful turboshaft engine designs was obvious.
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