Armoured Flight Deck - Theory

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. 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 without the armour). The deck was closed by 4.5-inch (110 mm) 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 3-inch (76 mm) 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 stowage of fuelled and armed aircraft in an unarmoured hangar was extremely dangerous:

...Captain John S. McCain of the Ranger...actually preferred a relatively small carrier, as long as it could be protected against bombing. Again and again he told the General Board that any U.S. carrier, if her flight and hangar decks were crowded with fuelled and bombed aircraft, was a potential inferno, and that friendly fighters could not guarantee the security of any flight deck... —

During the war, the British fitted immersion heaters to the oil tanks of its aircraft so minimal warm-up was required when they reach the flight deck.

US and 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. The flight deck could also possibly fuze 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, starting with the Forrestal class which had "...an enclosed..." hangar.

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.