Background

The Sikorsky S-61 designation was assigned to the company’s first twin engine helicopter that was designed from the ground up to take full advantage of emerging turbo-shaft engine technology. This new model marked the end of reciprocating engine installations at Sikorsky and ushered in the era of the lighter and more compact turbine engines. The S-61 served all branches of the US Military as well as commercial airline service. It began its long production career as a model specifically tailored to anti-submarine warfare for the US Navy bearing the designation HSS-2. The model history described below focuses on this most important version of the S-61 helicopter series.

The threat of Soviet nuclear-armed submarines arose soon after the first Russian nuclear detonation in 1947 which immediately elevated anti-submarine warfare (ASW) to a mission of immense national importance. In 1950, at the height of the Cold War, the CIA noted an unconfirmed report that the only atomic bomb the Soviets had produced would have to be delivered by submarine. In 1955 the new Chief of Naval Operations, Adm. Arleigh Burke, asked the Committee on Undersea Warfare of the National Academy of Sciences to conduct an Anti-Submarine Warfare ASW study. That study, Project Nobska, concluded that “Confronted with quiet submarines of long endurance, a sufficiently accurate means of navigation, and suitable weapons, defense against shore bombardment by submarines becomes a huge problem.”

World War II experience in the North Atlantic demonstrated the futility of engaging submarines with surface vessels. The population of Nazi U-Boats peaked at 240 in 1943. Against this force, 875 sonar-equipped escorts could not conclusively prevail. There was a need to combine the sonar’s detection, localization, and targeting, with the speed of an aircraft.

One of the solutions was the ASW helicopter with active dipping sonar, a concept first demonstrated on a Sikorsky HOS (R-6) helicopter by the Naval Research Laboratory in 1946.

Starting in the early 1950’s, the U.S. Navy, Sikorsky and the Pacific Division of Bendix Aviation Corp. began developing the technology for helicopter ASW, combining the speed of an aircraft with underwater sonar previously based aboard ships. Using Sikorsky HSS-1 (H-34) helicopters, hunter-killer technology and tactics were developed and fielded. One helicopter carried a submersible sonar sensor and an airborne processor to detect, locate, and display the target; another helicopter carried a torpedo to address it. Space and payload limits prevented one helicopter from carrying both.

Simultaneously, the Navy recognized the need for improved helicopter engine technology and in 1953 awarded General Electric a $3M contract for the XT-58 “baby gas turbine.” The engine was to weigh 400 pounds and produce 800 HP, a two-to-one improvement over the legacy, reciprocating Wright R-1820 engine powering the HSS-1. The result exceeded expectations. The resulting engine weighed 324 pounds and generated more than 1,000 HP in a 10 cubic-foot package compared with the legacy piston engine of 62 cubic-feet.

The increase of almost four times the power/weight ratio in a compact 1/6th size permitted Sikorsky, for the first time, to truly tailor a helicopter to a mission, rather than tailor a mission to fit the payload and space remaining after the engine was installed.

In the mid 1950’s the U.S. Navy requested that Sikorsky modify an existing HSS-1N to accommodate two T58 engines in place of the R-1820. This was designated as the HSS-1F and showed the significant benefit of turbo-shaft engines over piston engines. The engines were not only lighter and smaller but provided 2,000 horsepower compared to the 1,525 available from the piston engine.

The benefits of the turbo-shaft were profound. The light weight and small size allowed the Sikorsky designers to place all of the mechanical components (engine, main gearbox, driveshaft, tail gearboxes, and tail rotor) on the top of the fuselage and freed the cockpit and cabin design to be tailored to the mission requirements. Heretofore, the major design problem had been the location of the heavy piston engine with the cockpit and cabin squeezed into the remaining space.

With this new engine, Sikorsky was able to offer the Navy, in place of the HSS-1F, the HSS-2, an all-new design beautifully tailored to the ASW mission. Sikorsky Engineering Report 61007 documenting the October 1957 HSS-2 Mockup Review reported:

“The success of the HSS-1 piston engine helicopter as an anti-submarine search and attack vehicle has prompted the Navy and Sikorsky Aircraft Division to enter into the next logical step, the development of a turbine powered modification of the HSS-1, which incorporates twin turbine engines, amphibious characteristics, all the technological and functional advantages gained through experience with the HSS-1, with capability of performing a combined Hunter-Killer ASW mission.

A new, low frequency, sonar (AQS-11) is being developed concurrently by the Pacific Division of Bendix Aviation Corp. as Contractor Furnished Equipment. This sonar together with the four (4) hour endurance HSS-2 constitute the major components of the first weapon system to meet Navy’s ASW requirements.

The Bendix dipping sonar system consisted of a sonar reeling machine, a reel of 500 feet of sonar cable, the submersible sensor, sonar data processing and display equipment, and a sonar operator control console. The HSS-2 also featured navigation and mission avionics and sensors to provide automated approach to, and maintenance of, a hover stabilized above the submersed sensor and provisions for carrying and firing torpedoes. This powerful mission suite was enclosed in an airframe with hull and sponson flotation based on the iconic Sikorsky Pan American Clipper flying boats with which Pan American Airlines had pioneered international transoceanic air travel two decades earlier.

Configuration Features

The HSS-2 was primarily intended for the ASW mission, but Marine Assault and Army Transport missions were also considered. Sikorsky Engineering Report 61003, Engineering Basic Data Report Model HSS-2, HUS-2, H34B Helicopters, 2/26/57 states:

General design objectives are:

1. Design to accomplish given missions
2. Minimum weight
3. Component life and Reliability
4. Minimum drag and cleanest external surface
5. Ease of maintenance
6. Manufacturing simplicity

The most distinctive feature of the Sikorsky S-61 series is the use of a boat hull with outboard floats (called sponsons). The hull design with a 12 degree deadrise angle was derived from Sikorsky’s extensive experience in flying boat design. The outboard sponsons significantly increased the lateral stability in the water. The main landing gear retracted into the sponsons and water take-off and landings were performed with the gear retracted. To increase lateral stability and buoyancy, inflatable floats were added to the outsides of the sponsons.

The other significant configuration item was mounting of the engines and drivetrain above the cabin. This completely separated the cockpit and cabin from the machinery and reduced noise while increasing safety compared to the prior piston engine designs. As noted earlier, this was due to the significant reduction in the size and volume of the turboshaft engines.

The S-61 (HSS-2) was Sikorsky’s first model to employ a main transmission capable of providing almost 100- to-1 reduction ratio needed for the turbo shaft engine’s high, 20,000 rpm, output speed. Prior MGBs had reduction ratios of only 10 or 12- to-one reflecting the much lower output speeds of piston engines. Higher engine output speed forced advancements in free wheel unit technology and high speed bearings.

The main rotor head featured technology previously proven on the S-58, such as basic rotor configuration and the NACA 0012 airfoil, but for the first time offered oil-lubricated rotor bearings, a departure from the previous grease lubrication scheme. Oil lubrication eliminated the need for periodic grease system maintenance. Another important feature of the HSS-2 was the fully automatic main rotor blade fold. Along with simple manual fold of the tail rotor pylon, this allowed for a convenient, compact placement aboard ship. The rapidity and reliability of the blade fold was important operationally and became more so when S-61s were operated from vessels smaller than the ASW carriers on which they were initially deployed.

Automatically stabilized flight and an automated approach to, and maintenance of, a stable hover had been developed in the previous S-58 and S-56 models. An Automatic Stabilization System, ASE, provided electronically stabilized flight. A coupler provided the autopilot functions to automatically approach and maintain a hover fifty feet above the ocean surface, into the wind, stabilized relative to the sonar cable supporting the submerged sonar transducer. A radar altimeter provided altitude; a Doppler radar system provided airspeed data for stabilization and auto approach. Earlier ASE and Coupler systems had consisted of vacuum tube circuits. For the first time HSS-2 systems were fully transistorized solid state circuits.

The HSS-2 was fitted with a two-piece personnel door on the left side, just aft of the cockpit. It also had a large siding door on the right side in the aft cabin. This greatly aided in moving equipment in and out of the cabin.

General Arrangement Drawing

Mission Equipment

The HSS-2 was equipped with a comprehensive suite of mission electronics that allowed it to track submarines in all weather, day and night. This equipment included: