H-H(X) / CH-53A Stallion
In early 1958, in response to a request from the Office of the Secretary of Defense, BuAer conducted a study of the feasibility for a single VTOL aircraft development to satisfy the requirements of the Navy, Marine Corps, Air Force, and Army. When the study had been completed it showed conclusively that it was feasible and practical to develop a pressure-jet convertiplane/compound helicopter which would meet the requirements of all services. At the time, however, each service had their own ideas on the issue. The Army indicated that it wanted to proceed unilaterally with the development of a 6,000-pound payload, gear-driven, tandem helicopter being produced by Vertol as they felt that a pressure-jet convertiplane, as proposed by BuAer, would not be suitable for its mission. Later, the Air Force indicated an unwillingness to pursue such a development as it needed an aircraft with an extensive range capability for search and rescue purposes.
The Department of Defense (DOD) reluctantly authorized the Army to proceed with its program but agreed that the Navy-Marine Corps' position of developing a pressure-jet convertiplane was feasible and technically sound and authorized the Navy to proceed with its research and development. The existing operational requirement (AO-17501) under which the HR2S had been developed, was revised by the Marine Corps to reflect the desired characteristics for such an aircraft to replace the HR2S which was scheduled to be phased out in the 1964-1965 period. The Commandant submitted the document to the CNO on 26 November 1958. On 16 March 1959, it was promulgated as Operational Requirement Number AO-17501-2, with Developmental Characteristic Number AO-17501-1 (VTOL Assault Transport) as Appendix Number One.
The operational requirement stated that the VTOL aircraft should be capable of carrying a payload of 8,000 pounds outbound to a distance of 100 miles at a cruising speed of 200 knots and return with a 4,000-pound payload. A maximum airspeed of 250 knots was also specified. By 27 January 1961, the Air Force and Army had shown a renewed interest in a VTOL aircraft and through a series of DOD actions an agreement had been reached wherein all services consented to participate in the development of a prototype VTOL transport. BuWeps, the DOD-appointed manager for the tri-service aircraft, then issued a revised statement of requirements which specified the same payload but extended the aircraft's radius to approximately 250 miles and increased the cruising airspeed to 250-300 knots and the maximum airspeed to 300-400 knots. However, for the Marine Corps mission, the requirement stated that the fuel load could be reduced so that the maximum gross weight would not exceed 35,000 pounds so long as a 100-mile nautical radius of action could be flown.
By August 1961, the Navy recognized that the four-engine tilt-wing aircraft, the design which had now been selected for the tri-service evaluation instead of the compound helicopter, would be unsuitable for Navy or Marine Corps use and withdrew from the program. As a result of the evaluation of all proposals, the design development contract for five XC-142A airplanes was awarded to Vought. The contract was signed in early 1962 with first flight specified for July 1964. If this prototype program were successful, an airplane based on the prototype experience could be developed.
Long before this time, however, the CMC and CNO had recognized that any production aircraft resulting from the high-speed VTOL program would not reach the fleet in time to replace the HR2S. In view of this, and at the Commandant's urging, the CNO issued on 27 March 1961 a revised developmental characteristic (AO-17501-3) for a medium assault transport helicopter with essentially the same requirements as the convertiplane (AO-17501-2) but with a cruising airspeed of only 150 knots. The gross weight was also to be limited to a maximum of 35,000 pounds.
Since it had been determined that such a short time existed before the new helicopter was needed in the fleet, a replacement aircraft would again have to be a development of an existing model. The initial competition was therefore between three major helicopter manufacturers: Kaman, Sikorsky, and Boeing-Vertol. The Kaman Aircraft Company had shown an interest in competing for the contract but dropped out before submitting a formal bid. Vertol proposed that it could meet the requirements of AO-17501-3 by modifying its Army HC-1B Chinook, an enlarged version of its 107, or HRB-1. Sikorsky, on the other hand, based its design for the large helicopter on a revision of its jet-powered S-64 Flying Crane, an aircraft being built completely from company funds for future sale to West Germany.
The general description of the proposed transport helicopter revealed that it was to utilize a six-bladed single main rotor and a 16-foot diameter tail rotor. The cabin measured 30 feet long, 6 1/2 feet high, and 7 I/2 feet wide with a rear loading ramp. It featured a watertight hull, seats for 30 combat equipped troops, tricycle retractable landing gear, twin turbine engines, automatic blade folding, and required a crew of two pilots and a crew chief. The aircraft had an overall length of 88 feet, a gross weight of 32,000 pounds, and an empty weight of approximately 19,000 pounds. The cruising speed at the designed gross weight was listed at 150 knots with a maximum airspeed of 171 knots at sea level.
Request for proposals on the large transport helicopter were sent to the competing manufacturers by BuWeps on 7 March 1962. Sikorsky and Vertol replied in May, and on 24 August 1962 BuWeps announced the Sikorsky Aircraft design as the winner. Not only had Sikorsky submitted the lowest bid, but there was a decided preference based on technical, production, and maintenance aspects of the Sikorsky proposal. The first aircraft was to be delivered during May 1964 with fleet deliveries beginning the following year.
The original designation of H-H(X) was given the assault helicopter (H-Helicopter, H-Heavy, (X) -Experimental). It was later designated by Sikorsky as its S-65 and by the Navy as the CH-53A.
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