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.
However, design and production problems forced the Marine Corps to replace it with 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, had 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.
HR2S-1 "the Deuce"
Military production versions of the CH-37C were designated by the Marine Corps as HR2S-1. Since the HR2S-1 was the second version of the transport helicopter, a '2' was inserted in its designation to indicate this. The Marines, being big on card playing and the '2' card being called a 'Deuce', they nicknamed the HR2S-1 "the Deuce".
Designed to fulfill a 1950 Marine Corps requirement for a series of large transport helicopters, the Sikorsky HR2S-1 was first flown on December 18, 1953 and for the next ten years would hold the distinction of being the largest helicopter flying outside of the Soviet Union. Capable of carrying twenty-six fully equipped combat troops or a varying array of cargo, the HR2S-1 was a single-rotor layout powered by two-engines fitted on nacelles alongside the fuselage (this design left the fuselage clear for load stowage). The helicopter was also fitted with "clam-shell" nose doors for straight loading of vehicles as well as a 2,000 lb capacity winch.
As in landing craft, several types of helicopters would be required to execute effectively the several operations of lifting cargo, vehicles, and personnel. Also helicopters would be needed for reconnaissance, casualty evacuation, pathfinding, and the exercise of command and control. For these operations there was seen a definite requirement for a "family" consisting of HR2Ss for heavy equipment and large personnel loads and a need for the HUSs and HRSs in lifting lighter loads of equipment and troops. While the Marine Corps had considerable numbers of the lighter helicopters, the shortcoming was in the quantity of the heavier transport helicopter-the one most essential to any significant landing operation.
A comparison was made between the 1954, helicopter lifting capability and that which was programmed for 1957 - the time when all nine Marine transport helicopter squadrons would be equipped with the HR2S - showed that it would take seven hours in 1954 to land the assault elements of one battalion landing team (BLT) with one MAG (HR) consisting of three 15-plane HRS squadrons. By 1957, the increased lifting capability of the HR2S would permit the same size MAG to land a complete Marine division in approximately 15 hours. The comparison used the "K" series Table of Organization (T/0) with supplies sufficient for three days operations. The radius of assault for the HRS helicopter group was 15-20 miles whereas the HR2S MAG was figured at a radius of 50 miles. An average load for the HRS was computed at 1,300 pounds and at an amazing 8,000 pounds for the HR2S.
In a close analysis of the HR2S-landed division, however, it was determined that the number of helicopters was still inadequate. It was felt that the minimum assault force should consist of four battalion landing teams landed simultaneously with additional support provided on the second wave. Also, it was calculated that sufficient helicopters would not be available for providing support for tactical operations ashore while concurrently executing the ship-to-shore movement. These deficiencies could be remedied by increasing Marine Corps transport helicopter units to a total of 12 squadrons with a combined strength of 180 aircraft. In addition, it would be necessary for the helicopters to be capable of carrying an emergency payload of 35 passengers or 12,500 pounds for the initial assault and for heavy lifts. The increase of 45 HR2Ss would meet the initial lift requirement and provide tactical support ashore during the early phases of the assault. In the case where helicopters were needed in operations ashore during the early phase, the overall time to land the complete division would then be on the order of 12 to 14 hours.
Resupply requirements of the division, combined with the total requirements necessary for lifting a Marine aircraft wing, were examined next with the view of determining the capability for landing a division-wing team with the 180 HR2Ss. By allowing 217 trips per day for resupply of the division, the wing could be moved ashore with 30 days supply in a period of 50 hours, provided the wing equipment was helicopter transportable. This period would be increased to 70 hours should one MAG of HR2Ss be employed to support operations ashore after the initial landing.
The T-56 [gas turbine engine) growth potential of the HR2S would provide an aircraft capable of the performance of 12,000 pound payload, 100 nautical mile radius, 130-150 knot speed. However, that would be about the growth limit of the HR2S. But it was false optimism in 1954 to believe that the HR2S could be modified to have the capability to lift 12,500 pounds. The importance of obtaining a payload of 12,500 pounds could not be over-emphasized as it would then be possible to helicopter-lift the most crucial heavy pieces of division property: the 155mm howitzer and the two and one-half ton truck.
General Shepherd submitted his request to the CNO for the additional number of helicopters on 23 October 1954 . "The validity of the concept outlined in [the letter of 17 July 1951]," he stated, "has been borne out by events which have since transpired. It now appears that we are ready for - in fact, obliged to take - the next step in logical progression toward development of our helicopter capabilities . . . . " The general continued, "I propose that each of the nine Marine helicopter transport squadrons be provided with 20 HR2S aircraft " at the earliest practicable time. " He further pointed out to the CNO that this would represent an increase from 135 helicopters in the present program to a total of 180.
The requirements for a medium helicopter were taken up by a board which convened later at HQMC in January 1955. The HR2S-1 was a large aircraft which would require a much larger, level landing area than the HRS. Open level areas capable of receiving a squadron of HR2Ss were comparatively rare in many types of terrain. One of the advantages of the medium size helicopter was its ability to land in almost any type of terrain. An organization with only large helicopter transports would not have the flexibility in the selection of landing zones that was enjoyed by the HRS squadrons.
The board, in making its recommendation stated that "each Marine aircraft wing [should] contain one group of three squadrons of 20 large [HR2S] rotary-winged transports, and one squadron of 15 medium [HRS/HUS] rotary-wing transports." The total number of 180 HR2Ss was reaffirmed by the board as the appropriate number of heavy transport helicopters.
On 24 May 1955, the Commandant officially announced his decisions on the recommendations made by the Smith Board. In matters relating to the helicopter program, General Shepherd not only approved the idea of adding medium helicopters to the aircraft wing organization, but increased the number from one medium squadron to two such units per aircraft group. By his action General Shepherd thereby approved for planning purposes the first additional expansion to the helicopter program since its initial massive enlargement in 1951.
On 1 April 1955 the Commandant requested that "the need for a vehicle to rapidly shuttle supplies to the forward elements, to execute tactical movements of small units, and to evacuate battle casualties points to the use of a utility helicopter such as the HUS." On 16 June 1955 the CNO approved an increase in the total number of helicopters. He approved an operating program for Fiscal Year 1959 of 180 HR2S helicopters and 45 HUSs. In 1955, the US Marine Corps received its first HUS-1s as an interim type, ostensibly until the HR2S (later H-37) entered squadron service. However, the HUS lasted far longer in USMC service, and in much greater numbers, than the HR2S ever did. Fifty-five production versions ordered by the Marines were delivered by February 1959.
The Commandant on 19 October 1955 informed the CNO that information then available to him indicated that actually there were two versions of the HR2S being considered for initial production, and that both fell considerably short of meeting the specifications set forth by BuAer. Three problem areas in particular were of concern: the combat radius had been reduced two-thirds and the ability to hover out of ground effect had decreased to approximately half the altitude specified. Ground effect is encountered when a helicopter is hovering at a height above the ground of less than its rotor diameter. While the two foregoing problems were directly related to an excess in weight, the third difficulty involved the inability of the helicopter to automatically fold its blades. These shortcomings severely restricted its operational use.
In view of these problems, the Commandant recommended that the CNO restrict deliveries of the HR2S to 15 aircraft and that production and delivery of the HUS-1 be accelerated to the extent necessary to provide the Marine Corps with an operating inventory of 90 HUSs by the end of 1957. These two recommendations of General Shepherd were made to afford a longer interval of time for the development of the HR2S. In the interim, the HUS would partially fulfill the urgent lift requirements of the Marine Corps.
Shortly thereafter, on 23 November 1955, the Commandant again modified his recommendations concerning the desired operating strengths for both the HR2S and the HUS. In his correspondence with the CNO, General Shepherd mentioned it had been discovered through informal discussions with BuAer and Sikorsky Aircraft, that two of the factors affecting the actions which he recommended he previous month had changed considerably. Mainly, these factors centered around the fact the turbine version of the HR2S had now been delayed two years and that the results of a recent weight reduction conference on the HR2S revealed it was possible to accomplish sufficient reductions in weight to provide improved performance of the first production models. In view of this, General Shepherd requested that the recently curtailed delivery rate of the HR2S be increased from 15 to 60 helicopters by the end of 1958. He also favored an increase in the numbers of HUSs, since both the Army and Navy versions of the S-58 (H-34 and HSS-1) were proving to be a highly satisfactory aircraft. In fact, it had been reported to the Commandant that the Army was increasing the number of seats in its H-34s from 12 to 18 and that the Army aircraft was consistently carrying loads ranging from 3,750 pounds to 4,000 pounds with over an hour's fuel on board.
Realizing 60 HR2Ss was far from the original number of 180, General Shepherd desired that the CNO make a further compensatory acceleration in the HUS procurement which would provide for an operating strength of 140 helicopters by the end of 1958. In reply, a review of the procurement program for the HR2S was made by the CNO and presented to the Commandant on 12 April 1956. the Fiscal Year 1957 budget cycle and to overall procurement of the HR2S-1. It is interesting to note that the total number of HR2Ss had declined in a series of actions from a total of 158 aircraft in June 1955 to only 34 by November the same year.
In June 1955 the FY 1957 HR2S-1 procurement submitted to OB&R [Office of Budget and Review] consisted of sixty (60). OB&R review, and as agreed to by BuAer because of deficiencies uncovered in the HR2S-1, resulted in reducing the quantity to thirty (30) [for FY 57] . This quantity (30) was submitted to OSD [Office of the Secretary of Defense] for review. OSD review, again as a result of the helicopter's mechanical deficiencies, resulted in eliminating both the thirty (30) HR2S-1 in the FY 1957 program and the thirty-six (36) in the FY 1956 program. In late October the Marine Corps requested that the number of HR2Ss be held to a maximum of fifteen (15). Accordingly, total procurement was further reduced and this procurement program, as thereby finalized, was incorporated in the President's budget. Subsequently, in November, the Marine Corps requirement for an operating strength of sixty (60) was received. However, it was impossible to incorporate this revision in the Budget at that late date.
Also in the reply, signed by Vice Admiral Thomas S. Combs, Deputy Chief of Naval Operations, was the statement of views relative to future procurements of the HR2S-1 . He indicated that present planning contemplated the purchase of 12 additional helicopters, thereby increasing the over all HR2S-1 total to 46. The last 12 were necessary in order to provide for sufficient FY 1958 follow-on re-order lead time. It was felt that if and when the HR2S-1 demonstrated, by actual test, it could satisfactorily meet BuAer specifications, the procurement program would be accelerated and would be designed to meet the Marine Corps operating requirements. However, Admiral Combs stressed, "until this circumstance occurs it is considered only prudent to restrict procurement to that level which will provide an adequate test quantity and a minimum production line which can be accelerated."
It was pointed out that irrespective of procurement planning, Marine Corps requirements would never be met until the HR2S-1 actually proved its capability to perform its designed mission. In conclusion Admiral Combs said, "It is therefore considered that present HR2S-1 procurement is sound as present conditions permit. The CNO is fully aware of the Marine Corps' need for the HR2S type helicopter and will take action to meet this need as soon as possible." Admiral Combs' letter firmly placed the number of HR2Ss at approximately one-fourth of the desired 180.
Disappointing as it was, the Marine Corps' overall helicopter program was far from bankrupt. This turn of events did, however, establish a trend in which the Marine Corps began to adopt the light, but more trouble-free, helicopter as its main assault transport. The prospects of obtaining the smaller HUS-1 appeared to be brighter at this time due to the developmental problems in the HR2S program and the fact the HUS was a much less expensive aircraft. Resistance to the reduction in quantity of the HR2S was only a natural reaction since Marine Corps planning for the execution of its new concept was based on using the larger helicopters as the main assault transport. Although the numbing agent to this stinging blow had been provided earlier in the year in the form of CNO approval for procurement of nearly 140 HUSs, it did, nevertheless, subsequently require the reorientation of the entire helicopter program.
The UH-34 was the prime vehicle in the 1st MAW at the start of the Vietnam war and through most of the following year. In midsummer 1965, a detachment of CH-37s was deployed to give a heavy-lift capability to the wing. The obsolescent CH-37 was a valuable addition and stayed in Vietnam until early 1967 when the first echelon of CH-53s arrived.
The US Navy used an AEW version, called HR2S-1W. Two variants (called HR2S-1W) fitted with a large diameter radar scanner in the nose for early-warning duties were procured by the Navy in 1957. A similar experiment in which some CH-37s were fitted with a bulbous, dielectric radome for use as Marine Corps radar patrol aircraft proved unsuccessful.
Three world records for helicopters were established during the period 9-11 November 1956 when a HR2S with a Marine pilot at the controls carried a 11,050 lb payload to an altitude of 12,000 ft, carried 13,250 lbs to over 7,000 ft, and set a three kilometer speed record of 162.7 mph.
Initially procured in 1956, the Sikorsky (model S-56) CH-37A/CH-37B, with a crew of three, was a large medium lift transport helicopter. Clam shell doors in the nose provided access to a cargo compartment that could accommodate two jeeps or a 105mm howitzer. The Mojave could carry 26 troops or 24 litters in the MedEvac role. The CH-37 had a single five-bladed main rotor and a metal four-bladed tail rotor. The CH-37 was powered by two Pratt & Whitney R-2800-54 2100 hp pistons engines and had a speed of 131 mph (114 knots).
Production of the S-56 ended in May 1960, but Sikorsky was engaged until the end of 1962 in converting all but four of the H-37A's to H-37B (later CH-37B) standard. Improvements in this version included the installation of Lear auto-stabilisation equipment and the ability to load and unload while the helicopter was hovering.
In February 1963, the Department of the Army quickly and quietly snatched up a large group of soldiers from a variety of locations and set them down at Fort Benning, Georgia, where they became part of the innovative 11th Air Assault Division (Test) (11th AAD). The idea was to create a combat force "freed from the tyranny of terrain" by exploiting the capabilities of Army aviation.
Initially, the medium lift helicopter available to the division was the venerable CH-37, Mojave. The then brand new CH-47, Chinook, did exist; but very few of these "Hooks" were available, and they were experiencing the "teething" problems normally associated with new equipment. Both helicopters could carry a howitzer as an internal or external load. In the continental United States, Redlegs favored the former approach. After all, an inadvertently dropped howitzer sling load makes a terrible mess on the ground and might even damage a howitzer.
One early experiment in ammunition resupply occurred during an 11th AAD Artillery firing demonstration at Fort Benning. In a cooperative effort involving aviation, support command, and field artillery elements, soldiers trucked ammunition to a small airstrip. There, they broke it down into a variety of helicopter loads and moved it to the firing battery via a combination of internal and external shipments using both the CH-37 and CH-47 helicopters. The demonstration clearly showed that ammunition could be loaded very quickly into a helicopter using various pieces of cargo handling equipment. Unfortunately, unloading at the battery position relied exclusively on the strong backs and legs of battery personnel. Sling-loading ammunition using a nylon mesh sling was the way to go.
On 31 May 1965 the 3d Battalion, 319th Artillery, as part of Task Force SURUT, participated in the largest air assault conducted in Vietnam to that date. The task force, consisting of the 319th reinforced by a cavalry troop, an engineer platoon and a composite platoon made up of volunteers from the support battalion, secured a landing zone (LZ) and guided in CH-37 Mohave helicopters carrying the howitzers. Up to this point in the war, the Mohaves had been doing yeoman duty as all-purpose aircraft. So smoothly and efficiently did this initial move go that three hours later these same howitzers mounted preparation fires on another LZ for Task Force DEXTER, a reinforced infantry element of the 173d Brigade. This was the first such operation ever conducted in actual combat by a US Army unit-one that had been in Vietnam less than 30 days.
It was used for aircraft recovery in Vietnam. That the CH-37 did not see more extensive service in Vietnam is primarily the result of its replacement in the Army inventory by the turbine-powered Sikorsky CH-54 Tarhe, a machine that weighed slightly less than the CH-37 but which could carry nearly four times as many troops or five times as much cargo. The last CH-37 was withdrawn from Army service in the late 1960s. The CH-37 was replaced by the CH-47D Chinook, but of course the CH-47 did not have CH-37's heavy-lift capability.
During the period July 1957 through November 1966 there were 264 CH-37 mishaps reported, to include accidents, incidents, forced landings, and precautionary landings. Of these, 29 were major accidents and four were minor accidents. There were 47 incidents, 89 forced landings, and 95 precautionary landings. Review of the major accidents indicated that pilot factors accounted for 48.3% of the major accidents, while materiel failures or malfunctions were responsible for 37.9%. Weather, supervision, and maintenance were responsible for the remaining 13.8%. Many incidents, forced landings, and precautionary landings are recorded with cause factors not reported. This results from reporting units failing to provide supplemental information as it becomes available. Reports of this nature, without supplemental data, are of little use to the aviation safety program and all units reporting similar mishaps are urged to follow through with supplemental data as soon as it becomes available.
|Join the GlobalSecurity.org mailing list|