USS Sealion Was The Navy’s Unique Helicopter-Accommodating Submarine

A veteran of the Pacific War, this Balao class submarine was ideally suited to experiments with helicopters loaded with Marines.

byThomas Newdick|
U.S. Homeland photo


From the very early days of naval aviation, there have been attempts, some more successful than others, to operate fixed-wing aircraft, chiefly floatplanes, from submarines. By the time the helicopter had become an established part of air warfare, soon after World War II, the idea of any kind of aircraft-carrying submarines was essentially dead. But that didn’t stop the U.S. Navy from operating at least one helicopter from a submarine, during a remarkable series of trials aboard the USS Sealion in the mid-1950s.

By now, the helicopter had proven its worth in Korea and was increasingly becoming indispensable for maritime operations, too, its key missions including search and rescue, utility transport, and anti-submarine warfare. There was another mission that the helicopter was quickly making its own — amphibious assault. In November 1956, the Suez Crisis fiasco in Egypt saw British commandos flown by helicopter from the decks of aircraft carriers as part of an amphibious invasion force. It was the first time that such an operation was carried out and it would prove hugely influential.

A U.S. Marine Corps Sikorsky HRS-1 helicopter landing aboard the submarine USS Sealion.  , PUBLIC DOMAIN

In the same year in the United States, meanwhile, experiments using helicopters for the amphibious assault mission were also taking place. One of these aimed to examine whether it was possible to deliver a small team of Marines from a submarine at sea to the shore, where they could carry out reconnaissance missions. Inserting an advance party of reconnaissance troops was nothing new in itself, of course, and it was part of the existing submarine mission set. But using a helicopter to get them to their objective was new.

The submarine chosen for the exercise was the Balao class USS Sealion. This had originally been commissioned in March 1944 and was famously responsible for being the only submarine to sink a Japanese battleship during World War II, the Kongo, which was sent to the bottom of the sea in the Formosa Strait in November 1944.

Decommissioned at the end of the war, Sealion was then adapted as a troop-carrier submarine, losing its torpedo armament and being outfitted with accommodation for up to 123 troops or cargo. Meanwhile, above the deck, a large watertight cylindrical housing could carry additional equipment, such as an LVT, or tracked landing vehicle. This kind of installation was the precursor to the Dry Deck Shelters used on special mission submarines today. By late 1948, Sealion was operating as a submarine transport, SSP-315, with Marines embarked. A change in hull classifications in January 1950 saw Sealion reclassified a transport submarine, becoming ASSP-315, which was changed again to APSS-315 in October 1956.

USS Sealion after conversion as a submarine transport, SSP-315. Note the large stowage chamber aft of the conning tower., U.S. NAVY

During the mid-1950s, Sealion took part in maneuvers involving the Army, Marines, Underwater Demolition Teams, and Beach Jumper units, at some point losing the watertight deck housing. Presumably, by this point, the utility of being able to disembark a single LVT, or similar, had been discredited, but it did leave a considerable amount of flat deck space immediately behind the conning tower.

Surprisingly little information is available on the helicopter trials that took place on the Sealion, although photos indicate they made use of the ‘flight deck’ provided after the removal of the LVT’s container. Several of these photos record that the drills took place in or around May 1956.

USS Sealion during exercises with Marine scouts of the 2nd Marine Division in 1956., U.S. NAVY

The helicopter used has been reported as a Sikorsky HRS-3, the Marine Corps version of the popular Sikorsky S-55. Powered by a 700-horsepower Wright R-1300-3 piston engine, this version was able to carry eight to 10 troops. However, at least one image shows an earlier model, the HRS-1, as used in Korea, which utilized the 600-horsepower Wright R-1340-57.

With no hangar, there was no way for the submarine to submerge with a helicopter on board and even in rough seas or poor weather, operations would likely have become very difficult, if not impossible. The limited available photos do not seem to reveal any additional support equipment to fuel the helicopter, or even secure it, suggesting these tests were designed just to study the basic feasibility of flying troop-laden helicopters to and from submarines.

Since there are no apparent records of helicopters operating from U.S. Navy submarines after this drill, we can probably safely assume that it wasn’t a success. At best, the rotary assault capacity offered would have been far less than even a modest-sized surface warship adapted for helicopter operations. Indeed, in 1958, the Navy began experiments with its first true helicopter carrier, the converted Essex class aircraft carrier USS Boxer (LPH-4).

Six UH-34 Seahorse helicopters in front of the landing platform helicopter USS Boxer., U.S. Navy

There was, however, at least one interesting footnote to the story of the helicopter-carrying submarine.

On April 26, 1956, a month before an HRS-3 was being trialed aboard Sealion, another Balao class submarine, USS Corporal, was exercising with a Navy Sikorsky HSS-1 Seabat anti-submarine helicopter, part of a trials squadron that was attempting to track the submarine off the southern coast of Florida.

When the crew of the HSS-1 realized they were losing oil from the main rotor and were too far from the coast to recover on land. After issuing a mayday call, the Corporal surfaced to provide help, expecting to pick up survivors once the helicopter ditched. After making visual contact, the respective helicopter and submarine commanders agreed to attempt an emergency landing on the Corporal. Improvising all the way, the crews managed to safely recover the helicopter, although it was initially not even clear if the hull was wide enough to support the landing gear.

The U.S. Navy HSS-1 Seabat helicopter on the USS Corporal in 1956., U.S. NAVY

Between them, USS Sealion and Corporal had proven that it was possible to operate helicopters from submarines, whether planned in advance or in an emergency. But it was destined to be a brief cameo only and, today, the closest most submarines get to a helicopter is at the opposite end of a winch cable during personnel or cargo transfers.

As for USS Sealion, the submarine was decommissioned again in 1960 before returning to service the following year, again in a transport role. In 1962, the sub helped enforce the naval blockade against Cuba, during the Cuban Missile Crisis. For the remainder of the decade, Sealion continued to work with Marine Reconnaissance teams, SEALs, and Underwater Demolition Teams, finally being classified as an amphibious transport submarine, LPSS-315, before being decommissioned for the last time in 1970. 

Carrying demolition equipment, U.S. Navy frogmen leave the submerged USS Sealion during a training exercise at St. Thomas, Virgin Islands in the early 1960s., U.S. NAVY

With such an illustrious period of service, from the war in the Pacific to the clandestine special forces missions of the Cold War, it’s perhaps not surprising that Sealion’s helicopter operations are today little known.

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