Editing Illustrious-class aircraft carrier (section)

==Design and concept==
The ''Illustrious'' class was designed within the restrictions of the [[Second London Naval Treaty]], which limited carrier size to an upper limit of 23,000&nbsp;tons. They were different in conception to the Royal Navy's only modern carrier at the time, their predecessor [[HMS Ark Royal (91)|HMS ''Ark Royal'']], and what may be described as their nearest American contemporaries, the [[Yorktown class aircraft carrier|''Yorktown'']] and [[Essex class aircraft carrier|''Essex'']] class carriers. The ''Illustrious'' class followed the ''Yorktown'' but preceded the ''Essex'', the latter being designed after the US abandonment of the Second London Naval Treaty and its tonnage limitations.

Where other designs emphasised large air groups as the primary means of defence, the ''Illustrious'' class relied on their [[anti-aircraft gun|anti-aircraft armament]] and the passive defence provided by an [[armoured flight deck]] for survival, resulting in a reduced aircraft complement. Other carriers had armour carried on lower decks (e.g. the hangar deck or main deck); the unprotected flight deck and the hangar below it formed part of the [[superstructure]], and were unprotected against even small bombs. However, the hangar could be made larger and thus more aircraft could be carried, but the differences in aircraft capacity between these carriers and their United States Navy (USN) counterparts is largely due to the some 100-foot-longer overall length of the US designs, and the USN's operational doctrine, which allowed for a permanent deck park of aircraft to augment their hangar capacity.<ref>Friedman 1988, pp. 18–19</ref><ref>Hone, Friedman, Mandeles, ''British and American Carrier Development, 1919–1941'', p. 125: "The 1931 edition of "Progress in Tactics" included a section on foreign tactics, including operating practices. The U.S. portion mentioned that "the number of aircraft in carriers is proportionately much higher than in our Navy, largely due to the practice of storing some aircraft permanently on deck."</ref> ''Illustrious''{{'}}s hangar was 82%{{efn|Friedman, in his works on RN<ref>Friedman (1988), p. 154</ref> and USN<ref>Friedman (1983), p. 392</ref> aircraft carriers stated that ''Enterprise''{{'}}s hangar measured 546 feet × 63 feet versus 456 × 62 feet for the ''Illustrious'' class, therefore ''Illustrious''{{'}}s hangar had 82% of ''Enterprise''{{'}}s hangar capacity.}} as large as {{USS|Enterprise|CV-6|6}}{{'}}s, but ''Enterprise'' typically carried 30% of her aircraft capacity in her deck park. ''Indomitable''{{'}}s two hangars were actually larger than ''Enterprise''{{'}}s, but she carried fewer aircraft because she did not have a large permanent deck park. In 1944/45 RN carriers began to carry a permanent deck park of similar size to their USN counterparts, and this increased their aircraft complement from 36 to an eventual 57 aircraft in the single-hangar carriers, and from 48 up to 81 in the double-hangar, 23,400-ton ''[[Implacable-class aircraft carrier|Implacable]]'' design, compared to 90–110 for the 27,500-ton US ''Essex'' class.<ref>Friedman 1988, p. 145</ref>{{efn| On 5 June 1945, USS ''Bennington'' reported that her maximum hangar capacity was 51 aircraft, 15 [[Curtiss SB2C Helldiver]]s and 36 Vought F4U Corsairs, and that 52 were carried as a deck park. At that time she carried 15 [[Grumman TBF Avenger|Avengers]], 15 Helldivers, and the rest were a mix of F6F Wildcat and F4U Corsair fighters. She was prompted to utilize, and report on, her maximum hangar storage due to a typhoon.<ref>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.</ref>}}

In the ''Illustrious'' class, armour was carried at the flight deck level—which became the strength deck—and formed an armoured box-like hangar that was an integral part of the ship's structure. However, to make this possible without increasing the displacement it was necessary to reduce the overhead height of the hangars to {{convert|16|ft|m|abbr=on}} in the ''Illustrious'' class hangars and {{convert|14|ft|m|abbr=on}} in the upper hangar of the ''Indomitable'' and {{convert|16|ft|m|abbr=on}} in her lower hangar; these compared unfavourably to the {{convert|17|ft|3|in|m}} of the ''Yorktown'' class, {{convert|17|ft|6|in|m|abbr=on}} in ''Essex class'' and {{convert|20|ft|m|abbr=on}} in ''Lexington class''.<ref>Friedman 1988, PP.145-147  </ref> This restricted operations with larger aircraft designs, particularly post-war.

This armour scheme was designed to withstand 6" cruiser shellfire or 500 pound bombs (and heavier bombs dropped from low height or which struck at an angle); in the [[British Home Fleet|Home]] and Mediterranean theatres it was likely that the carriers would operate within the range of shore-based aircraft, which could carry heavier bombs than their carrier-based equivalents. The flight deck had an armoured thickness of 3&nbsp;inches, closed by 4.5-inch sides and bulkheads. There were 3-inch [[strake]]s on either side extending from the box sides to the top edge of the main side belt, which was of 4.5&nbsp;inches. The main belt protected the machinery, petrol storage, magazines and aerial weapon stores. The lifts were placed outside the hangar, at either end, with access through sliding armoured doors in the end bulkheads.

Later in the war it was found that bombs which penetrated and detonated inside the armoured hangar could cause structural deformation, as the latter was an integral part of the ship's structure.

Pre-war doctrine held that the ship's own firepower, rather than its aircraft, were to be relied upon for protection, since in the absence of radar, fighters were unlikely to intercept incoming attackers before they could release their weapons. Accordingly, the ''Illustrious'' class was given an extremely heavy anti-aircraft armament. The armament was similar to ''Ark Royal'' with twin 4.5&nbsp;inch turrets (in a new "between-decks" or countersunk design) arranged on the points of a quadrant. The guns were mounted sufficiently high so that they could fire across the decks; de-fuelled aircraft would be stowed in the hangar for protection during aerial attack. The ''Illustrious'' class were fitted with four [[HACS]] controlled High Angle Director Towers, for fire control of her 4.5 inch guns. ''Illustrious'' pioneered the use of [[radar]] to vector carrier-borne fighters onto attacking or shadowing aircraft, and a [[Fairey Fulmar]] fighter from ''Illustrious'' achieved the first radar directed kill on 2 September 1940.<ref>{{cite book|last=Thomas |first= Andrew |title=Royal Navy Aces of World War 2 |date=2007 |publisher=Osprey |series=Aircraft of the Aces No.75 |isbn= 9781846031786 |page= 24}}</ref>