RogerBW's Blog

Aircraft-carrying submarines seem, superficially, like a really good idea. Unfortunately in practice they haven't really worked.

The idea of a submarine that could carry aircraft is as old as the idea of the aircraft carrier itself; the first aircraft carried on a submarine was probably a Friedrichshafen FF.29 floatplane, wedged onto the bows of the SM U-12 in January of 1915. Submarines of this era naturally spent most of their time on the surface, and the plan was to get the plane some way towards its target to give it more fuel reserve when heading back to its base after the attack. As soon as U-12 had left the harbour at Zeebrugge, her captain realised that the swell would swamp the plane, and ordered an immediate launch. Similarly, in 1916 the British submarine E-22 was fitted with rails for a pair of Sopwith Schneier floatplanes; when they were floated off in the North Sea, the floats broke apart in the rough water.

Experiments were carried on at a slow pace between the wars. The Americans flirted briefly with the idea, fitting a steel pod aboard USS S-1 and experimenting with various small floatplane designs, but there was no political will to take this further.

When the Washington Naval Treaty banned guns larger than 8" calibre on submarines, HMS M-2 had her 12" turret (fitted above the main hull, forward of the conning tower) removed and a waterproof hangar installed in its place; this was equipped with a custom-designed folding-wing scout aircraft, the Parnall Peto. The intended deployment was ahead of a fleet, searching for enemy battleships; in effect, the aircraft would "act […] as an additional and very powerful periscope" (report to the Admiralty, 1930). M-2 also had a steam catapult fitted, to get the floatplane up to flying speed without the need to move it into the water. After the flight, the Peto would land on the water and be craned back aboard the submarine.

Sadly, M-2 was lost with all hands during an exercise in 1932: she radioed that she was about to dive, and that was the last thing heard from her. Considering that the conning tower hatch and the hatch between hangar and main hull were both open when the wreck was found, the most plausible theory seems to be that the hangar door was opened too early during a practice launch; water rushing into the hangar weighed down the submarine, and the ballast tanks were unable to float her high enough to let it drain out. When water overflowed from the hangar into the main hull, the first thing it hit was the main fuseboard.

During the Second World War, the French built the "undersea cruiser" Surcouf, briefly the largest submarine ever constructed at a surfaced displacement of 3300 tons, intended to seek out surface ships and engage them in combat with guns and torpedoes. As part of this mission, she carried an MB.411 floatplane in a hangar aft of the conning tower, for scouting and fire direction; this would be craned to and from the surface of the sea. Surcouf took over two minutes to dive to 40 feet from a start on the surface (other submarines of the era took less than half a minute), making her vulnerable to air attack, and was plagued with mechanical problems from the start; a hugely complex system of buoyancy vents made her very hard to control when submerged. She was useful for moving high-ranking Free French officers across the Atlantic, but with ongoing technical difficulties and an ill-trained crew was something of a blundering solution in search of a problem. She was almost certainly run down in the dark by an American merchant ship off Panama (it's possible she was first bombed by aircraft of the US Army Air Force); she was lost with all hands, and the wreck has never been located. To be fair, the aircraft was not instrumental in her loss.

Meanwhile, the Japanese worked on their own aircraft-carrying submarines; the I-25 carried the E14Y1 Glen aircraft that made the sole manned attack on American soil during the war, and the I-400 class was the one that beat Surcouf's size record, at 6,670 tons displacement. Each of the three built carried three Aichi M6A Seiran floatplane bombers, probably the first attempt to use submarine-launched aircraft for other than scouting purposes. These craft were designed for strikes against the Panama Canal; but Japan surrendered before the attack could be launched.

There were many other ideas and plans that never made it to hardware; for example, the conversion of obsolete Regulus cruise-missile-carrier submarines to carry aircraft. The Regulus missile was a big beast, after all, held in a deck canister and got up to flying speed with solid-fuel rockets; surely an aircraft could be designed to be launched the same way, then recovered from the sea?

This proposal mutated with time, and the final form was a submarine aircraft carrier, holding three Convair Sea Darts; they would be brought to sea level by a deck lift just aft of the sail, and either transferred into the water by crane or (in rougher seas) catapulted aft along the submarine's deck. This was never seriously worked on; the lift would have needed a huge hole in the pressure hull, something to which submarine designers are curiously averse, and supporting its weight when loaded with an aircraft might also have been a problem.

The surface appeal (sorry) of the submarine carrier is obvious: while an aircraft carrier has a lot of striking power, it's also a big target that can't readily manoeuvre or defend itself, and it's hard to hide from the enemy (whether that enemy is using battleships, spotter planes, airborne radar or satellite observation). But a submarine is, by design, hidden; it could pop up, launch a strike, and vanish again.

Well, up to a point. It's absolutely no use for deterrence; it has to stay hidden at all times, because it can't benefit from the defences of an escort fleet, and therefore it has to be used to be effective. Even the biggest submarine carriers have had relatively tiny air wings, because space on a submarine is always at a premium. So that probably means a platform that can only be used effectively to make a nuclear strike.

The submarine carrier can't run combat air patrols, because the constant launching and landing requires it to stay on the surface for too much of the time. Its only defence is to be underwater, at which point it can't launch or recover aircraft at all.

So it pops up, launches a few aircraft (perhaps one or two per catapult, say two or four planes) and submerges again while they drop their nuclear (or maybe even conventional) bombs on the target. What next? They have to locate and get back to the submarine, and either land on the sea to be craned aboard (every historical example) or trap onto a really huge deck, far bigger than has ever been seriously contemplated. (The Skyhook system developed by BAe to allow Harriers to be operated from small ships would provide one possible alternative to this, but even that will take time if there are more aircraft than skyhooks.) An alert enemy will be aware that there was no carrier spotted before the attack and will presumably chase the planes with his own aircraft; the submarine's commander is then faced with the choice of abandoning his aircraft and pilots or having his boat bombed on the surface.

What aircraft might go aboard such a submarine? A modern strike fighter is big, and the bigger the empty space that can be filled with aircraft the more vulnerable the submarine is to flooding. On the other hand, something smaller will be distinctly less capable.

However, one approach that has been tried with a little more success is the conversion of torpedo or missile tubes on existing submarines to carry UAVs. Without the size needs of a human pilot, and with the acceptability of expending the drone rather than recovering it, this seems rather more practicable. The US Navy has expressed a continuing interest, but as far as I'm aware no full-scale hardware has been built; the closest was the Cormorant project from Lockheed Martin, designed to be launched from a Trident missile tube 150 feet below the surface, but cancelled in 2008.

(Much of my material for this post comes from the excellent Strike from Beneath the Sea, by Terry C. Treadwell. Treadwell is distinctly more optimistic about this approach than I, but has no answers to the questions of tactics.)