In December 1957, the US Navy gave the go-ahead to a new programme for a very high performance helicopter with advanced technology, to replace the outdated S-58 (HSS-1). Sikorsky was approached again and submitted a project for a big twin turbine aircraft with a boat-type hull and retractable landing gear for amphibious operations. The aircraft had all-weather capability, a good choice of weapons loads and four hours' endurance. The project was designated S-61 and the HSS-2 prototype flew on 11 March 1959. The prototype was followed by seven pre-production aircraft (YHSS-2) which successfully completed service trials in 1960. The US Navy ordered the first ten S-61B/HSS-2 for delivery starting in September 1961. The helicopters were later redesignated SH-3A Sea King. One of the first production models set up a world speed record of 339 km/h on 5 February 1962.

The main rotor of the medium-tonnage S-61 was of the articulated type, with five interchangeable blades which could be folded automatically by hydraulic actuators. The tail boom could also be folded for stowage on board ship. The all-metal, semi-monocoque boat-type hull was amphibious, the twin mainwheels retracting into two sponsons. In the SH-3A version 255 were produced, while ten more, ordered as HSS-2Z and subsequently redesignated VH-3A, were assigned to the special American Presidential Department for personnel transport and evacuation services in case of emergency. Nine of the SH-3As were transformed into RH-3As with minesweeping equipment and three were used by the USAF for missile site support and drone recovery. Another 12 SH-3As were converted into the HH-3A for battlefield rescue work, and were fitted with two Emerson TAT-102 turrets mounted at the rear of the two sponsons, and an in-flight refuelling probe.

In April 1962, the USAF leased three HSS-2, transformed into 27-seat transport aircraft for services linking the Texas Towers radar installations. Another three S-61As were purchased for this purpose. Sixteen S-61A-4s with 31 seats were acquired by the Royal Malaysian Air Force and nine by the Danish Air Force for rescue work. In response to a Japanese naval specification, Mitsubishi obtained a license to produce the S-61; three were purchased directly from Sikorsky and by February 1972, 43 locally-produced aircraft had been delivered.

From 1966, the SH-3A was superseded by the SH-3D, which had a 1419shp T58-GE-10 turbine and new electronics. The first SH-3D delivered in June 1966 was one of six ordered by the Spanish Navy. This was followed by another four for the Brazilian Navy and 73 for the US Navy. The new variant was also built under license by Agusta from 1967, following an Italian naval order for an ASW helicopter to replace the old Sikorsky SH-34. An initial batch of 24 was built for the Italian Navy and 20 for the Iranian Navy (three in the VIP version); the Italian SH-3D is identical to the American model, apart from the installation of a Teledyne Doppler radar and a search radar on the left side of the nose.

In Britian, the Royal Navy also chose the S-61 to replace its old Wessex. Westland acquired the license to build the SH-3 in 1959, and the British specification called for some modifications. Thus a pair of 1521shp Rolls-Royce Gnome H-1400 turbines were adopted, plus other British equipment including an Ekco all-weather radar (easily recognised by the dorsal radome), Plessey dipping sonar, Marconi Doppler navigation radar, etc.

The first British-built Sea King flew on 7 May 1969 and the first squadrons were formed the following August. Westland has built over 200 Sea Kings, including the Sea King Mk.1, 2 and 5 for the Royal Navy, 15 for the Indian Navy (Sea King Mk.42), 22 for the German Navy (Mk.41), 11 for the Norwegian Air Force (Mk.43) and 12 for the Australian Navy (Mk.50). More recently, Westland has designed and built a tactical helicopter called Commando, which is directly derived from the Sea King, and the first examples flew in 1973. Commandos have been supplied to Egypt and Qatar and, as the Sea King HC Mk.4, to the Royal Navy.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

Designed to combine the hunter/killer functions in one airframe, the Sikorsky HSS-2 was the subject of a US Navy contract awarded on 23 September 1957. This called for an all-weather anti-submarine helicopter with 'dunking sonar' equipment and able to carry up to 381kg of offensive weapons. The S-61 design had watertight, hull-retractable landing gear in the stabilising floats, and was powered by two General Electric T58 turboshaft engines driving a five-bladed main rotor. The prototype flew on 11 March 1959 and seven YHSS-2 trials aircraft followed; the type was redesignated SH-3 in September 1959. The initial production version, the SH-3A Sea King, began to reach fleet squadrons in September 1961 and later conversions included the HH-3A VIP transport flown by the Executive Flight Detachment, Washington. The essentially similar CH-124, assembled by United Aircraft of Canada, was supplied to the Canadian navy. The Royal Norwegian air force acquired S-61A helicopters without ASW equipment for rescue duties, and the Royal Malaysian air force acquired S-61A-4 Nuri helicopters equipped to carry 31 troops or operate in the SAR role. One hundred and sixty-seven HSS-2 (now withdrawn), HSS-2A SH-3D HSS-2Bs (SH-3H) and a further 18 SAR-configured S-61 As were built under licence by Mitsubishi, in a programme which was completed in 1990.

More fuel and the replacement of the original 932kW T58-GE-8B engines by 1044kW T58-GE-10s led to redesignation as the SH-3D, first delivered in June 1966 and ordered by Argentina, Brazil and Spain, as well as by the US Navy. Since 1969 Agusta has been building SH-3Ds under licence in Italy as the Agusta-Sikorsky ASH-3D Some 105 ASH-3As with ASW equipment removed were redesignated SH-3G for utility duties, while further conversions which have been made since 1971 are of the SH-3H version, with updated ASW and electronic surveillance equipment. While production of the AS-61 and ASH-3D, ASH-3TS (Transporto Special, a VIP version) and ASH-3H have now ceased, Agusta claims it could re-open the line in 36 months. The Italian firm did recommence building AS-61R (HH-3F) search and rescue helicopters to meet an order for two from the national civil protection service and 13 for the air force. These will have upgraded radar, LORAN, FLIR and navigation systems, modifications that will be retrofitted to the air force's existing 19 AS-61Rs. Agusta is also the exclusive overhaul and repair agent for Europe and the Mediterranean.

The SH-3H is the standard version now in service with the US Navy, with approximately 150 earlier aircraft modified to this standard. Supplemented by small numbers of the surviving SH-3G utility version, which has had all the SH-3H's anti-submarine equipment removed, SH-3Hs serve regularly on board the Navy's carriers and at shore bases, but are being steadily replaced by the SH-60 Seahawk and Ocean Hawk. Eight of the SH-3s supplied to Spain have been modified to SH-3H standard, and three of these are now equipped with a Thorn-EMI Searchwater radar in an external inflatable radome (similar to that fitted to Royal Navy Sea King AEW.Mk 3s) for shipboard airborne early warning duties.

Acquired originally for re-supply of its radar stations, the US Air Force CH-3B was essentially a de-navalised SH-3, but the CH-3C that was ordered in November 1962 introduced a number of major changes, including a rear loading ramp. Allocated the company designation S-61R, the prototype flew on 17 June 1963 and the first CH-3C delivery was made on 30 December 1963. An engine change from 969kW T58-GE-1s to 1119kW T58-GE-5S produced the CH-3E in February 1966. Some were later converted to HH-3E standard for the USAF Aerospace Rescue and Recovery Service, being provided with armour, self-sealing fuel tanks, retractable inflight-refuelling probe, rescue hoist and 12.7mm machine-guns for defensive purposes; this was the 'Jolly Green Giant' as used in Vietnam. These aircraft have now all been retired, replaced by the HH-60. The US Coast Guard operated the HH-3F Pelican, which had advanced electronic equipment for SAR duties, but lacked the self-sealing tanks, armour and armament of the HH-3E. These have now virtually given way to the HH-60J Jayhawk.

Civil versions for passenger operations were developed initially as the non-amphibious S-61L which, with a lengthened fuselage to seat up to 30 passengers, was first flown on 6 December 1960 and was FAA-approved on 2 November 1961. It was followed by the essentially similar S-61N that was, however, an amphibious version with a sealed hull and stabilising floats that housed retractable landing gear; it was first flown on 7 August 1962 and is still in widespread civil use. Westland built versions, still in low-rate production, are discussed separately.

Orlando Helicopters now provides extensive spares support for the S-61 and is investigating ways to re-engine existing aircraft to run on alternative petrol, propane and alcohol fuels.

D.Donald "The Complete Encyclopedia of World Aircraft", 1997

Developed to meet a 1957 US Navy requirement for a high-performance ASW helicopter with boat-type hull and retractable landing gear, and all-weather capability. Designated S-61 (HSS-2), the prototype first flew on 11 March 1959; seven preproduction aircraft (YHSS-2) successfully completed service trials in 1960. US Navy ordered initial 10 S-61B/HSS-2 for delivery starting September 1961. Aircraft redesignated SH-3A Sea King in 1962. Also produced under licence by Agusta, Mitsubishi and Westland. VERSIONS S-61A: Amphibious transport, generally similar to the US Navy's SH-3A. Accommodation for 26 troops, 15 litters, cargo, or 12 passengers in VIP configuration. General Electric T58 turboshaft engines standard, but Rolls-Royce Gnome H.1200 offered as alternative.

S-61A-4: Export version for Malaysia; first ordered on 26 October 1970, known locally as Nuri.

S-61B: Initial production version with amphibious capability.

S-61D: Export version of SH-3D.

S-61D-4: For Argentine Navy; ordered in 1971.

S-61F: Experimental high-speed version with stub wings and auxiliary turbojets.

S-61L: Non-amphibious commercial version with modified landing gear, rotor head and stabiliser. First flight of prototype 6 December 1960; FAA certification 2 November 1961.

S-61L Mk II: Improved version with more powerful 1,118 kW (1,500 shp) CT58-140-2 turboshaft engines; individual cargo bins; enhanced vibration damping. Accommodation increased to 30 passengers.

S-61N: Similar to S-61L, but with sealed hull and stabilising floats (as on SH-3A) for amphibious operations. First flight 7 August 1962.

S-61N Mk II: Improved version with more powerful CT58-140-2 engines; individual cargo bins; enhanced vibration damping. Accommodation increased to 26 passengers.

S-61R: Development of S-61B; introduced many design changes, including rear loading ramp and new landing gear.

AS-61N1 Silver: Licence-built version of S-61N, with slightly shorter fuselage and greater range, by Agusta in Italy.

AS-61R Pelican: Licence-built multipurpose SAR version by Agusta in Italy.

ASH-3D/TS: Licence-built VIP transport version by Agusta in Italy.

ASH-3D: Licence-built multirole naval version built by Agusta in Italy.

CH-3B: Version of S-61A operated by USAF for missile site support and drone recovery duties.

CH-3C/E: Transport version of S-61R for USAF.

CH-124 (formerly CHSS-2): Anti-submarine helicopter similar to SH-3A, delivered to the Canadian forces in May 1963.

HH-3A: Modified version of SH-3A for Search and Rescue duties, with T58-GE-8F turboshaft engines, two electrically powered minigun turrets, high-speed refuelling and dumping system, rescue hoist, upgraded avionics, external auxiliary fuel tanks and armour installation.

HH-3E: Version of S-61R for US Aerospace rescue and recovery service.

HH-3F: Version of S-61R for US Coast Guard.

RH-3A: Conversion of nine SH-3As for mine countermeasures duty with US Navy.

SH-3A (formerly HSS-2) Sea King: Initial amphibious ASW version for US Navy; powered by 932kW General Electric T58-GE-8B turboshaft engines.

SH-3D Sea King: More powerful ASW development of SH-3A for US Navy, with 1,043kW T58-GE-10 engines and an additional 530 litres of fuel. First delivered in 1966.

SH-3G: US Navy conversion of 105 SH-3As into utility helicopters. Six equipped with minigun pods for SAR missions.

SH-3H: Multipurpose version of SH-3A and SH-3G with two T58-GE-10 turboshafts; later converted for ASW and anti-missile operations, including lightweight sonar, active and passive sonar buoys, magnetic anomaly detection equipment and radar.

UH-3A: Utility version with T58-GE-8B turboshafts.

VH-3A (HSS-2Z) : Passenger transport version of SH-3A, used on VIP and emergency evacuation for US President and other key personnel.

VH-3D: Passenger transport version of SH-3D.

Westland Commando/Sea King: Licence-produced UK versions.

CUSTOMERS: Sikorsky built 794 S-61s between 1959 and 1980 for US and foreign armed forces, as well as for commercial, mostly offshore, operators.

DESIGN FEATURES: Five-blade main and tail rotors. All-metal fully articulated oil-lubricated main rotor. Flanged cuffs on blades bolted to matching flanges on all-steel rotor head. Main rotor blades are interchangeable and are provided with an automatic powered folding system. Rotor brake standard. All-metal tail rotor. Non-folding blades on S-61L and S-61N. Fixed stabiliser on starboard side of tail section.

FLYING CONTROLS: Rotor brake standard.

STRUCTURE: Boat hull of all-metal semi-monocoque construction. Single-step. Tail section folds to reduce stowage requirements. All-metal main and tail rotors.

LANDING GEAR: Amphibious. Landing gear consists of two twin-wheel main units, which are retracted rearward hydraulically into stabilising floats, and non-retractable tailwheel. Oleo-pneumatic shock-absorbers. Goodyear mainwheels and tubeless tyres size 6.50 x 10 type III, pressure 4.92kg/cm². Goodyear tailwheel and tyre size 6.00 x 6. Goodyear hydraulic disc brakes. Boat hull and pop-out flotation bags in stabilising floats permit emergency operation from water. Non-retractable landing gear on S-61L.

POWER PLANT: (SH-3D): Two 1,043kW General Electric T58-GE-10 shaft-turbine engines. Three bladder-type fuel tanks in hull; forward tank capacity 1,314 litres, centre tank capacity 530 litres, rear tank capacity 1,336 litres. Total fuel capacity 3,180 litres. Refuelling point on port side of fuselage. Oil capacity 26.5 litres.
(S-61L+N): Two 1,118kW General Electric CT58-140-2 turboshaft engines. Two bladder-type fuel tanks in hull; forward tank capacity 796 litres, rear tank capacity 757 litres. Total fuel capacity 1,553 litres. Additional 924 litre tank optionally available for S-61N.

ACCOMMODATION: Pilot and co-pilot on flight deck, two sonar operators in main cabin. Dual controls. Crew entry door at rear of flight deck on port side. Large loading door at rear of cabin on starboard side. Crew of three: pilot, co-pilot and flight attendant on S-61L. Main cabin accommodates up to 30 passengers. Standard arrangement has eight single seats and one double seat on port side of cabin, seven double seats on starboard side and one double seat at rear. Rear seat may be replaced by a toilet.

SYSTEMS: Primary and auxiliary hydraulic systems, pressure 105kg/cm², for flying controls. Utility hydraulic system, pressure 210kg/cm², for landing gear, winches and blade folding. Pneumatic system, pressure 210kg/cm², for blow-down emergency landing gear extension. Electrical system includes one 300A DC generator, two 20kVA 115A AC generators and 24V 22A battery. APU optional.

ELECTRONICS AND EQUIPMENT: AlliedSignal AQS-13 sonar with 180° search beam width. Hamilton Standard autostabilisation equipment. Automatic transition into hover. Sonar coupler holds altitude automatically in conjunction with Ryan APN-130 Doppler radar and radar altimeter. Provision for 272kg capacity rescue hoist and 3,630kg capacity automatic touchdown-release low-response cargo sling for external loads.

ARMAMENT: Provision for 381kg of weapons, including homing torpedoes.

Jane's Helicopter Markets and Systems

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- The S-61L land-based prototype made its maiden flight on 6 December 1960.

- S-61Ls entered service with Los Angeles Airways on 1 March 1962.

- A heavylift version of this helicopter, nicknamed the 'Payloader', carried cargoes as heavy as 4990kg.

- S-61 production by Sikorsky came to an end after two decades on 19 June 1980.

- Military S-61s served in 30 countries, plus with the US Air Force, Navy, Marine Corps and Coast Guard.

- S-61s were licence-built by Westland in Britain and Agusta in Italy.

- Sea Kings were heavily involved in Vietnam, rescuing many downed USN pilots during the long conflict.

- A small number of RH-3A minesweeper variants entered service in 1964.

- The Royal Canadian Navy was the first export customer, ordering 41 of the type.

- Aeronautiche Giovanni Agusta acquired a licence to assemble Sea Kings for the Italian air force and navy.

- Argentina is unique in that it operates both Sikorsky and Agusta built examples.

- The SH-3 has been replaced by aboard USN carriers by the SH-60F Sea Hawk.