S-56 / H-37 Mojave / HR2S
The Sikorsky S-56 was developed to meet a US Marine Corps requirement for a large assault helicopter. The S-56 had two nacelles on either side of the fuselage that held both the piston engines and retractable landing gear. The prototype first flew in 1953. The S-56 was delivered to the Marine Corps as the HR2S. The US Army designated their version the H-37 Mojave. Later versions could lift 11,000 lbs of cargo or 36 fully armed troops.
Seeking alternatives to the World War II amphibious landing, the Commandant had tasked his amphibious commanders to, "Compose a special board to propose ... concepts and principles ... to wage successful amphibious warfare in the future." On 16 December 1946 the special board submitted an advanced report to the Commandant recommending that parallel programs be initiated to develop a transport seaplane and a transport helicopter. The Board recommended the Vertical Assault Doctrine, which rested on the development of a heavy lift helicopter able to carry 20 Marines.
When the U.S. Marine Corps viewed the results of the first nuclear weapons effects test, the July 1946 Operation CROSSROADS, with significant concern. Despite the Navy and Marine Corps’ proven formula of amphibious warfare success during the lately concluded Second World War, the outcome of the two nuclear weapons detonations in Bikini Atoll was that large groupings of amphibious shipping and their support warships that made victory possible in almost every theater of World War II were now frighteningly vulnerable to the new weapon of mass destruction.
The solution was dispersal, spreading shipping, landing craft, support warships, and Marines out along the target coast to defeat the nuclear threat. However, that solution simply caused another problem: how to bring enough Marines to the decisive point to defeat the defenders. If the landing forces were dispersed, a well-disposed defender could annihilate the smaller Marine formations in detail before they could join inland to overwhelm the defenders or capture key terrain.
There were technological solutions seemingly close at hand—the helicopter and large flying boats. A special board appointed by the 20th Commandant of the Marine Corps, General Lemuel C. Shepherd Jr., looked at the dispersal problem in some detail. The board members, Colonels Merrill Twining and Edward Dyer and Lieutenant Colonel Samuel Shaw examined various means of keeping amphibious assault forces out of atomic danger while quickly concentrating Marines to defeat a defending force.
General Shepherd’s special board surveyed this scene and recognized that current helicopter technology was not going to put a lot of Marines on the beach very quickly, and that the flying boat was a longer term option, if it ever was an option at all. They spoke with the helicopter manufacturers, especially with Igor Sikorsky and one Frank Piaseki of Philadelphia, Pennsylvania. Piaseki had some original ideas about large helicopters, and, together, the two men convinced the board members that a helicopter capable of a 5,000 pound payload was possible.
While the Navy’s Bureau of Aeronautics (BuAer) had the mandate to develop the large flying boat program, the Marines took upon themselves the task of sponsoring a large helicopter program through the same Bureau, as BuAer had the authority for the procurement of aircraft for the Marine Corps. As it turned out, the 5,000 pound payload helicopter was a bit ambitious for the technologies of the late 1940’s, so an interim capability of 3,500 pounds was sought for the Marine’s new assault helicopter.
However, design and production problems forced the Marine Corps to settle for a medium lift helicopter called the HUS-1. The demise of heavy lift as the backbone of the Vertical Assault Doctrine began with the fact that the technology of a heavy lift helicopter was not feasible until 1955, by which time the Marine Corps had reoriented to become a medium lift force. The Marine Corps did eventually develop the CH-53E heavy lift helicopter; however, never with the intention of replacing the medium lift force. In an ironic twist of fate the medium lift replacement MV-22 has encountered long delays in testing and production reminiscent of the HR2S-1. Hence, the CH-53E has filled the major role as an interim helicopter in the Doctrine of Operational Maneuver from the Sea (OMFTS).
Navy Research and Development Plan, Operational Requirement Number AO-17501 (Rotary Wing Assault Helicopter) listed requirements were: " ... develop a rotary wing assault craft capable of transporting combat equipped troops (or the equivalent in combat equipment) from transport vessels to beachheads in support of landing operations . . ." and "that 20 combat equipped troops be transported with the weight of each man computed at 225 pounds. " The assigned functions in AO-17501 for the helicopter were to "operate from a CVE or larger carrier, or between carriers and suitable equipped transport ships, carrying assault troops with their initial requirements in supply, communications and organic weapons . . . ." Two of the main features listed were that it be multi-engine equipped and of an overall dimension compatible with movement on the elevator of the CVE-105 class carrier. Although the operational requirement did not assign a model designation, the twin-engine assault helicopter would subsequently bear the Sikorsky S-56 trademark and the Navy designation of XHR2S-1.
Immediately after the S-55 had entered production, Sikorsky began working on the design of a larger helicopter, intended as an assault transport for the Marines. A twin-engine solution was chosen, and to save cabin space, it was decided to house the two large radial engines in outboard nacelles, from which two drive shafts linked up directly with the reduction gear assembly which drove the big five-blade metal rotor. The H-37's innovative engine arrangement gave the craft an unobstructed cargo bay of nearly 1500 cubic feet, large enough to carry three Jeeps, twenty-four stretchers, or up to twenty-six fully-equipped troops.
Late in 1950, in response to BuAer's request, the helicopter manufacturers competing for the assault transport helicopter presented their proposals. Thereafter, in March 1951, the bureau selected two aircraft companies to build the helicopter, McDonnell and Sikorsky. Sikorsky Aircraft Company submitted two different designs. One, a basic helicopter referred to as XHRS-A, and a second design, a compound helicopter somewhat similar to McDonnell's although the propulsion for the main rotor was "conventional" wherein it did not propose the use of rotor blade tip burners. Both the basic and compound designs could be powered by reciprocating or gas turbine engines, depending upon BuAer's desires.
The proposed XHRS-A had twin engines located in wing-mounted external nacelles which transmitted their power to a single main transmission. The design called for a main rotor with five blades and a torque-compensating tail rotor of four blades; both rotors were of all-metal construction. Sikorsky claimed that the XHRS-A could carry 36 combat-equipped troops or an alternate amount of cargo in the 1,250-cubic-foot cabin. Loading and unloading of vehicles the size of jeeps could take place through clamshell doors which opened in the nose. This feature, however, restricted the helicopter's performance since it could not be flown with the doors open which delayed the loading / unloading operation, thus extending the time on the ground. Other features were : automatic blade folding, retractable landing gear, and a form of automatic pilot (automatic stabilization). The helicopter measured almost 88 feet in length and 20 feet high with the blades spread. Cruising speed was listed at 140 knots.
The first HRS-A aircraft was estimated to be available within 18 to 20 months from date of contract. The straightforward "pure" helicopter, the XHRS-A, was a much less complicated aircraft. It appeared to involve fewer problems of development, logistics, and maintenance in the field and was one which could be built in the shortest time. Therefore, BuAer awarded Sikorsky a contract for five experimental aircraft realizing that even with the simplest design there would be unforeseen problems and delays in the program.
The compound design was designated by Sikorsky as the XHRS-B. The XHRS-B had essentially the same fuselage design with identical engines and transmission facilities. Increased performance over the XHRS-A was proposed by the addition of foldable outer wing panels extending beyond the engine nacelles and the incorporation of standard propellers on the front of the engines. These additional features of the HRS-B were proposed as a logical future development of the XHRS-A basic helicopter.
The awarding of dual contracts for the same operational requirement (AO-17501) appeared justified in view of the complexities involved in both McDonnell 's and Sikorsky's proposals. The two-phase program was established in order to provide the Marine Corps with maximum protection in the event one of the designs failed to materialize. In this case, progress in the development of the assault transport helicopter was planned to provide two helicopters in logical sequence with the XHRH going beyond existing requirements. The procurement provided for the development of equipment to satisfy future requirements by taking advantage of technological progress beyond that incorporated in the HRS-A.
The first HRS-A (later designated by Sikorsky as its S-56, and by the Navy as the XHR2S-1) had been given a priority of 1B and was predicted to make its initial flight during May 1953, after which a period of experimentation would follow before a production contract would be granted.
When first publicly unveiled in January 1954, the Sikorsky S-56 was introduced by its manufacturer as a "giant, twin-engine transport", that represents a "tremendous advance in the art of designing and building helicopters". In fact, the S-56 was the biggest, fastest and most powerful production helicopter in the free world until the introduction of Boeing Vertol's turbine-driven Chinook in 1961, and it remains the largest piston engine helicopter.
The S-56, a very complex and advanced design helicopter, incorporated into it many new features, such as 'clamshell' nose loading doors, similar to the Bristol 170 fixed-wing freighter, and retractable main landing gear. It was also the first helicopter with a hydraulic main rotor blade fold and pylon fold system, automatic stabilization equipment (ASE), and the first twin-engine rotorcraft in US military service. After a decade, the Marines ended up with the versatile Sikorsky HUS/UH-34 series as the 3,500 pound payload helicopter and the monstrous Sikorsky HR2S/CH-37 as the 5,000 pound payload helicopter. Both aircraft utilized piston engines, so their performances at higher weights and temperatures was always problematic. It was not until the mid-1960’s that designers included compact gas turboshaft engines in their designs, finally manufacturing helicopters capable of consistently meeting Marine requirements first established in 1946.
The last S-56 was delivered in May 1960, but Sikorsky was engaged until the end of 1962 in a conversion program of all H-37A, with the exception of four copies, in H-37B ( later CH-37B).Improvements included the addition of an Automatic Stabilization Equipment (ASE), which allowed instrument flight and autopilot, increased capacity of engine oil tanks, replacement the rear loading door by a sliding side door and the outriggers moved from both sides of the fuselage to a single plane placed on the right side of the tail rotor pylon.
Ten machines, type CH-37B and CH-37C were purchased by Keystone Helicopters for certified load transport up to approximately 4,540 kg (10,000books). To increase the payload, these helicopters were simplified, disassembling unnecessary covers, removing the heating system, locking the train in the open position and disassembling the associated hydraulic systems, disassembly of the opening mechanism and closing the front doors, decommissioning ASE (Automatic Stabilization Equipment) stabilization system and disassembly of any external lifting system not required.
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