An armoured flight deck is an aircraft carrier flight deck that incorporates substantial armour in its design.
Comparison is often made between the carrier designs of the Royal Navy (RN) and the United States Navy (USN). The two navies followed differing philosophies in the use of armour on carrier flight decks, starting with the design of the RN's in the late 1930s and ending with the design of the , when the USN also adopted armoured flight decks. The two classes most easily compared are the RN's Illustrious class and and their nearest USN contemporaries, the and es. The Illustrious class followed the Yorktown but preceded the Essex, while the Implacable-class design predated the Essex but these ships were completed after the lead ships of the Essex class. The development of armoured flight deck carriers proceeded during World War II, and before the end of World War II both the USN, with , and the Imperial Japanese Navy (IJN), with and would also commission armoured flight deck carriers, while all USN fleet aircraft carriers built since 1945 feature armoured flight decks. The remainder of the IJN carrier force during World War II had unarmoured flight decks just like the Yorktown and Essex classes of the USN.
Design In choosing the best design for their carriers, the British had to consider the advantages and disadvantages of hangar design. The Illustrious class were also built within the limits of the Second London Naval Treaty constraining them to 23,000 tons displacement. There was a choice between open or closed hangar and the position of the armour. The placing of the strongest deck affected the strength of the hull. The further apart the deck and the keel, the stronger the design. If the flight deck was placed above the main deck then it had to be built to allow for movement with expansion sections.Brown, DK Nelson to Vanguard 2006 A closed hangar design was the strongest structurally and made for a lighter hull. The RN carried this concept one step further and designed the armoured flight deck to also act as the strength deck without any underlying plating, thus achieving an armoured flight deck on the lowest possible displacement.
The carriers that were built with armoured decks fall into several distinct types – those with armour at the flight deck level protecting the hangar against bombs; those that had armour at the hangar-deck level, to help prevent bombs penetrating into the vitals of the ship against bombs; and those with armour for the lowermost levels of the ship, with the armoured deck only about 3 ft (1 m) above the waterline, intended to protect boilers, machinery, and magazines against both bombs and plunging shells. Frequently these types of decks were used in conjunction with each other; the Illustrious- class vessels had armoured flight and hangar decks (as well as even more heavily armoured sides linking the two to produce a complete armoured box), while the American Essex class had armour at both the hangar (first) deck and fourth deck levels, the latter about below.Friedman (1983): pp. 141, 142, 220. The different thicknesses of armour, and how they were distributed, are described in the table below.
+ Armour of some World War II era aircraft carriers Ship or ship class(standard displacement)First serviceentry date Flight deck Hangar deck Protective deck Side belt (17,800 tons)1939 (19,875 tons)1937 HMS Ark Royal (22,000 tons)1938Friedman, 1988, pp.124, 128. The flight deck was designed with .75 in of Ducol steel. The enclosed hangar design required a deck of approximately this thickness to ensure adequate hull integrity, so the limited armour protection was a by-product of the enclosed hangar design. (23,000 tons)19404.5 in (114 mm) (25,600 tons)1941 (27,200 tons)1942 (29,770 tons)1944 abreast machinery (23,500 tons)1944 (64,800 tons)1944 (45,000 tons)1945
Theory Armour at the flight deck level would protect the hangar deck and the aircraft stored there from most bombs. The armour of the Illustrious class was intended to protect against 1,000 pound bombs.British and Empire Warships of the Second World War, H T Lenton, Greenhill Books, In the Illustrious class, the armoured flight deck extended for about two-thirds of the length of the ship, bounded by the two aircraft lifts (which were outside the armour). The deck was closed by armoured sides and bulkheads, forming an armoured box. The bulkheads had sliding armoured portals to allow access between the hangar and the aircraft lift. There were lateral strakes of main deck armour that extended from the base of the hangar side-wall to the top of the main side belt. The latter protected the machinery, magazines and aircraft fuel and weaponry stores. The RN's closed and armoured hangars were capable of being environmentally sealed for protection against chemical weapon attack. The armoured design meant that it would have to be attacked with armour piercing (AP) bombs, which have much less blast effect than higher-capacity general purpose (GP) bombs carrying about twice the explosive amount. GP bombs also caused severe hull damage if they exploded in the water close to the hull; AP bombs, much less so.
The USN open hangar design allowed large numbers of aircraft to be warmed up while inside, theoretically reducing the time required to range and launch a strike, but storage of fuelled and armed aircraft in an unarmoured hangar was extremely dangerous:
During the war, the British fitted immersion heaters to the oil tanks of their aircraft so minimal warm-up was required when they reached the flight deck.Brown D K, Nelson to Vanguard p.56
American carriers after the Lexington-class (which were converted battlecruiser hulls), and the earlier Japanese carriers, had their armour placed at the hangar deck, essentially treating the hangar spaces and flight deck as superstructure – making these areas very vulnerable to the blast from GP bombs and other explosions, which in turn caused massive casualties in comparison to RN designs. A bomb that struck the flight deck would likely penetrate and explode in the hangar deck, but the armour there could still protect the ship's vitals – including the engine spaces and fuel storage. A modestly armoured flight deck could also possibly initiate detonation of light bombs prematurely, which would reduce the chance of them going through the hangar deck. Such a design allowed for larger, open-sided hangar bays (improving ventilation but making the ship very vulnerable to chemical weapon attack) and the installation of deck-edge elevators. USN carriers with hangar deck armour only usually had wooden decking over thin mild steel flight decks which were easy to repair. The USN moved the structural strength deck to the flight deck,Friedman, US Carriers, p.255. starting with the whichFriedman wrote in British Carrier Development: "When the flight deck becomes the strength deck, the aircraft hangars are then enclosed within the hull structure. The terms, 'enclosed' and 'open' hangars do not, strictly speaking, refer to a hangar that is closed or open on the sides, but rather to whether the hangar is, respectively, below or above the strength deck." had "...an enclosed..." hangar.Friedman, US Aircraft Carriers, p. 268.
Aviation fuel delivery and stowage systems were extremely vulnerable. The Royal Navy stowed aviation fuel in cylindrical tanks, that in turn were surrounded by seawater. RN aviation fuel lines were purged with carbon dioxide when not in use. The USN used a similar system, which was further improved after the two navies began exchanging information in 1940. Pre-war USN and IJN carrier designs used a fuel stowage system which was not as secure as that used by the RN, but allowed for much greater stowage capacity. Several USN and IJN carriers were lost due to aviation gas fume explosions.
Doctrine and design The Royal Navy had to be ready to fight a war in the confines of the North Sea and Mediterranean Sea, under the umbrella of land-based enemy air forces. The Royal Navy, with its extensive network of bases and colonies in the Pacific Ocean, had also to be ready to fight in the vast expanses of the Pacific, as did the USN and the IJN, but the USN and IJN did not have to worry about operating in the Mediterranean. The differences in construction were determined by doctrine that was largely driven by the different approaches to the same tactical problem: How to destroy the enemy's aircraft carriers while surviving the inevitable counter strike. Prior to WWII the RN and USN both recognised that the dive bomber could disable the flight decks of enemy aircraft carriers:
The RN was thus faced with designing a carrier that would be survivable under the conditions to be expected in the Atlantic, Mediterranean, and Pacific Oceans, and before the development of effective naval radar; these conflicting demands resulted in the development of aircraft carriers whose decks were armoured against 500 lb armour piercing bombs and 1000 lb general-purpose bombs. The RN considered that an unarmoured carrier would be unlikely to be able fly off more than one deck load of strike aircraft prior to being attacked, so the armoured flight deck carriers accepted a reduction in hangar capacity to the equivalent to one deck load of aircraft.Friedman, British Carrier Aviation, pp. 156–159 USN, IJN, and some RN Fleet carriers such as Ark Royal had sufficient aircraft capacity to allow for two ranges, each equal to a full deck load of strike aircraft. The RN and IJN limited their aircraft carrier's aircraft capacity to the capacity of their hangars and struck down all aircraft between operations. The USN, typically, used a permanent deck park to augment the capacity of their aircraft carrier's hangars. The use of a permanent deck park appeared to give USN carriers a much larger aircraft capacity than contemporary RN armoured flight deck carriers. The flight deck armour also reduced the length of the flight deck, reducing the maximum aircraft capacity of the armoured flight deck carrier, but the largest part of the disparity between RN and USN carriers in aircraft capacity was due to the use of a permanent deck park on USN carriers.Hone, Friedman & Mandeles, 1999 p125.USS Bennington, Action Report, OPERATIONS IN SUPPORT OF THE OCCUPATION OF OKINAWA INCLUDING STRIKE AGAINST KANOYA AIRFIELD, KYUSHU. 28 May to 10 June 1945, p.18. On June 05 1945, USS Bennington reported that her maximum hangar capacity was 51 aircraft, 15 SB2Cs and 36 F4Us, and that 52 were carried as a deck park. At that time she carried 15 TBMs, 15 SB2Cs and the rest were a mix of F6Fs and F4Us. She was prompted to utilize, and report on, her maximum hangar storage due to a typhoon.