Where other designs emphasised large air groups as the primary means of defence, the "Illustrious" class relied on their 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. "Illustrious"s hangar was 82% as large as 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" design, compared to 90–110 for the 27,500-ton US "Essex" class.