Armoured flight deck

The Comparison of armoured to unarmoured flight deck designs is often made between some of the aircraft carrier designs of the Royal Navy and the United States Navy. The two navies followed differing philosophies in the use of armour on the carrier flight decks. The two classes most easily compared, and which differ the most, are the RN's Illustrious class and Implacable class and their nearest USN contemporaries, the Yorktown and Essex classes. 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.

Design

In choosing the best design for their carriers, the British had to consider the advantages and disadvantages of hangar design. 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. A closed hangar design was the strongest structually and made for a lighter hull. The closed hangar also gave superior protection for the aircraft in the waters it was expected to operate - the "narrow seas" of Europe.

The carriers that were built with armoured decks fall into two distinct types - those with the armour at flight deck level protecting everything below (typified by British ships) and those that had the armour between the hangar and the rest of the ship as in American and Japanese carriers. The different thickness of armour, and how they were distributed, are described in the table below;

Armour thickness of some World War II era aircraft carriers

Class (standard displacement)	Flight deck	Main deck	Side belt
Yorktown class (19,875 tons)	n/a	n/a	2.5 – 4 in (64 – 101 mm)
Illustrious class (23,000 tons)	3 in (76 mm)	2 in (51 mm)	4.5 in (114 mm)
Essex class (27,200 tons)	n/a	2.5 in (64 mm)	4.75 in (121 mm)
Shōkaku class (25,600 tons)	n/a	3.9 in over machinery (99 mm)	1.8 in (46 mm)
Implacable class (23,500 tons)	3 in (76 mm)	2.5 in (64 mm)	4.5 in (114 mm)

Theory

Carrying the 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 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 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 armoured hangar was capable of being environmentally sealed for protection against Chemical weapon attack, which meant that aircraft engines could not be run-up under cover; any work requiring engines to be running had to be carried out on the flight deck. During the war, the British fitted immersion heaters to the oil tanks of its aircraft so minimal warmup was required when they reach the flight deck. Thus the RN had anticipated the operating environment of post WW2 carriers, which had to be protected against possible NBC ( Nuclear, Biological, Chemical weapons )attack, making the pre-war USN open hangar design completely obsolete. 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, which carried about twice the explosive capacity. GP bombs also caused severe hull damage if they exploded in the water close to the hull, AP bombs, much less so.

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 would still protect the ships innards — including the engine spaces and fuel storage. The flight deck could also possibly detonate 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. Most carriers with hangar deck armour also usually had wooden flight decks which were easy to repair. However, all flight decks were particularly vulnerable to burning, so after refuelling, fuel lines in the deck would be purged with exhaust gasses to reduce this danger.

Circumstances

The differences in construction were determined by doctrine that was largely driven by the different circumstances of the RN and USN. The USN had its own aircraft procurement budget and procedures, independent of the Army Air Corps. The RN's Fleet Air Arm (FAA) had been constrained inter-war by the Royal Air Force's development of a strategic bomber force - the Fleet Air Arm then being part of the RAF. As a result, in the late 1930s, the FAA did not have any modern high-performance aircraft at its disposal, indeed its first monoplane, the Blackburn Skua, only flew for the first time in 1937. It should be noted, that this situation was not unique to the RN. The USN went into to battle at Midway flying obsolete torpedo bombers, for example Douglas TBD Devastators, while the USN's more modern dive bombers and fighters only arrived in 1941, while at the same time RN procurement plans for superior aircraft were cut short by the crisis following the Battle of Britain, and the subsequent reallocation of the UK aircraft industry, which was also hampered by Luftwaffe bombing. 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. By contrast, both the USN and the RN expected to be operating within the vast expanses of the Pacific Ocean but the USN did not have to worry about operating in the Mediterranean. 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. The USN, in contrast, was able to benefit from technology transfers from the UK and the wartime experiences of the RN, which was freely shared with the USN, prior to its entry into the war, allowing it to anticipate the changes needed to prepare its carriers for coming conflict with Japan.

Aircraft Restrictions

Within the confines of ship design, and the Second London Naval Treaty to which they complied, the Implacable-type carriers had to accept lower twin hangar heights (to keep the metacentric height within acceptable limits) and size, and as a result, had some restriction on aircraft type. The Illustrious class had a single 16 ft high hangar that was 468 ft long. No British carrier other than the early (1918) HMS Argus had a hangar to match the 20 ft (6.1 m) hangar height of the American Lexington class or 17 ft 6 in (5.3 m) hangars of the Yorktown-class ships and Wasp. With their larger elevators, Eagle, Glorious, Courageous, Furious, Indomitable, Implacable, Indefatigable and Argus were the eight British aircraft carriers of the Second World War capable of striking down aircraft without folded wings. RN carriers had 16 ft hangar heights, except the two Implacable class ships, which had 14 ft, and Indomitable which had a 16 ft lower hangar and a 14 ft upper hangar.

Defences

The British approach of armoured flight decks was an effective form of passive defense from bombs and kamikaze attacks that actually struck their carriers, but the American carriers primarily relied on fighters to prevent the carriers from being hit in the first place. RN carriers such as Ark Royal or Illustrious had far heavier AA outfits than their USN counterparts, up to the introduction of the USN Essex class carriers. Ark Royal , in 1940, carried 16 x 4.5-inch guns, 32 x 40mm "Pom-pom" and 32 x 0.5 inch against 8 x 5", 16 x 28mm and 24 x .5" for Enterprise, in 1940. "In wartime, however, the US Navy found the armoured carriers fascinating. After having examined HMS Formidable in 1940, the US naval attaché commented that, were he crossing the Pacific, he would prefer her to a Yorktown, the closest US equivalent, on the basis that she might carry fewer aircraft, but she would be much more likely to get there". Late in the war when the USN operated many carriers together and had improved radar, their fighter and AA defence was reasonably effective, yet both conventional and kamikaze attacks were still able to penetrate USN defences. Yorktown and Hornet succumbed after first being crippled by bomb hits in 1942, while Bunker Hill and Franklin nearly succumbed in 1945. The larger air groups (80–100 planes, vs. 52–80 for late war British ships) allowed for a more effective combat air patrol (CAP) without reducing strike capability, improving the protection of the whole battle group and lessening the workload of the carrier escorts. Carrier fighters were able to shoot down far more kamikaze aircraft than any amount of deck armour would have protected against showing the value of absolute numbers, but in the early war period IJN aircraft had little difficulty in penetrating USN CAPs; near the end of the war, veteran American fighter pilots in superior F6F Hellcat and F4U Corsair fighters were able to defeat the young, inexperienced and ill-trained kamikaze pilots with ease and run up huge kill scores but attackers were still able to get through. (In addition to larger aircraft complements, the US Navy had larger fleets and more resources, so they could establish destroyer pickets and develop dedicated AAW ships such as the Atlanta-class antiaircraft cruisers which would have also drawn attention away from the carriers.) On the surface, the record seems balanced.

British naval historian D.K. Brown put the practical difference between American and British design philosophies in no uncertain terms: "More fighters would have been better protection than armour." but that British designs were good for the circumstances in which they were meant to be used. Yet, even the Ark Royal, Britain's newest carrier prior to WW2, never operated close to her theoretical aircraft capacity. Prior to the development of effective Radar and high speed monoplane fighters, a successful fighter defence was extremely unlikely for any navy thus calling into doubt D.K. Brown's conclusions. The benefits of flight deck armour were intended to counter these issues. Fewer aircraft meant a lower priority to attack than the more heavily-armed American carriers and the RN's operational doctrine dictated smaller airgroups, and the armoured hangar carriers had smaller Avgas and ammunition supplies to match. However, RN carriers carried far more aircraft later in the war, making use of the deck park when they adopted USN style operational doctrine. The 2nd generation RN armoured carriers, Indomitable and the Implacable class which had an additional half length lower hangar, approached their USN counterparts in the numbers of aircraft operated. Failing to account for operational doctrine, gives very misleading impressions of armoured carrier capabilities, since the USN would operate more aircraft on any given carrier due to a permanent deck park, which augmented the hangar capacity. The RN operating in harsher weather protected their aircraft from the elements and did not use a permanent deck park in the earlier part of the war. US carriers and their fighters shot down more than 1,900 suicide aircraft during Operation Kikusui (the last and largest Kamikaze attack in the Okinawa campaign), versus a mere 75 for the British, yet both forces suffered the same number of serious hits (four), on their carriers although the kamikazes made 173 Strikes against other USN targets, while the 4 USN carriers suffered a massive death toll, in contrast to the relatively light casualties on the RN carriers. .

File:HMS Victorious on fire.jpg

HMS Victorious on fire following two kamikaze strikes, Okinawa, May 9, 1945. Total casualties, 3 dead and 19 wounded.

USS Franklin listing, with crew on deck, due to a bomb piercing the unarmoured flight deck, 19 March 1945. Total casualties, 724 killed and 265 wounded.

The kamikaze threat overall was serious, but allied defences neutralized it, and many kamikaze strikes missed the deck armour entirely, or bounced off the decks of both British or American carriers. In some cases, kamikazes either struck glancing blows that did only superficial damage that was fixed within minutes or hours, or missed entirely, due to the poor training and poorer flight experience of their pilots. The majority of kamikazes that did inflict harm caused no more damage than they would have against a smaller ships. After a successful kamikaze hit, the British were able to clear the flight deck and resume flight operations in just hours, while their American counterparts took a few days or even months. The U.S.N liaison officer on HMS Indefatigable commented: "When a kamikaze hits a U.S. carrier it means 6 months of repair at Pearl [Harbor]. When a kamikaze hits a Limey carrier it’s just a case of "Sweepers, man your brooms."”

American carriers of the Essex class suffered very high casualties from serious kamikaze hits, though they did survive. USS Franklin was struck by two 250/550 kg/lbs armour-piercing bombs, one of which penetrated her armoured hangar floor and set off ammunition, killing 724 personnel. USS Bunker Hill was severely damaged by pair of kamikaze hits which killed 346 men. Each these USN carriers suffered more casualties than all the British RN armoured carriers combined, illustrating the life saving features of RN carrier design. Illustrious, which had the highest toll, suffered 126 fatal casualties and 84 wounded when hit by six 1100lb bombs on January 10, 1941.

The only Allied carrier lost to a deck hit was the American Independence-class light carrier, USS Princeton (CVL-23). Indeed, many light and escort carriers were unarmoured, with no protection on the hangar or flight deck, and thus they fared poorly against deck hits.

Post War Analysis

What was not discovered until late in the war was that the kamikaze impacts proved to have a long term effect on the structural integrity of British carriers and their post war life was shortened, as the RN lacked the financial resources to repair the damage, in contrast to the vast resources of the USN which was even able to rebuild carriers such as Franklin that had been completely gutted and the crew decimated by IJN attacks. HMS Formidable (R67) was an excellent example of this; while she weathered a severe kamikaze hit in 1945 which cratered her deck armour, the hit caused severe internal structural damage and permanently warped the hull (damage worsened in a postwar aircraft-handling accident wherein a Vought Corsair rolled off a lift and raked the hangar deck with 20mm cannon fire, causing a severe fire); but plans to rebuild her as per Victorious were abandoned due to budget cuts, not structural damage, and she lingered in reserve until 1956 before being towed off to the breakers. The Royal Navy planned to rebuild most of the armoured carriers in the early post war period:

"There seems to have been general agreement that the first ship to be modernized should be an Illustrious. Formidable was laid up and required a long refit in any case, so she was provisionally selected for modernization. Illustrious was a deck landing training and trials carrier, and could not be spared, particularly as she was needed to test the new generation of naval aircraft. This left HMS Victorious as the only other candidate. In early 1951 the other two ships of the programme were HMS Implacable, followed by HMS Indefatigable, for modernization, respectively, 1953-55 (to relieve HMS Eagle so that she could refit in 1956 with steam catapults) and 1954-57. HMS Indomitable was scheduled for a more limited modernization (1957) as the future deck landing training ship. At this time Eagle was scheduled for completion in August 1951 and Ark Royal in 1954, so that the full programme would provide the Royal Navy with five fleet carriers plus a semi-modernised deck landing training ship."

Illustrious suffered a similar battering, especially off of Malta in 1941 when hit by German dive bombers and after the war was limited to 22 knots (41 km/h) because of accumulated war damage (her centreline shaft was permanently disabled due to accumulated wartime damage); she spent five years as a training and trials carrier (1948–53) and was disposed of in 1954. Indomitable was completely refit to like-new condition, only to suffer a severe gasoline explosion onboard, which caused "considerable structural and electrical damage to the ship". Indomitable was refitted between 1948 and 1950 and served as flagship of the Home Fleet then served a tour of duty in the Mediterranean, where she was damaged by the petrol explosion. She was partially repaired before proceeding under her own power to Queen Elizabeth II's 1953 Coronation Review, before being placed in reserve in 1954. Indomitable was scrapped in 1956. The explosion which occurred on Indomitable's hangar deck, while severe, would also have caused severe casualties and extensive damage to an Essex-class carrier, several of which returned to service after hangar explosions, primarily due to the USN's considerable financial and material resources. The postwar Royal Navy could only afford to rebuild Victorious and had to abandon plans to rebuild four other armoured carriers due to cost, and to provide crews to man the postwar built carriers, such as Ark Royal, due to reductions in manpower.

Another factor is the advantage in resources that the US Navy had over the Royal Navy. The numerous and capacious American yards on the East and West Coasts allowed the US Navy to build and repair carriers at a more leisurely pace and higher quality individually, while producing ships collectively at a furious rate. The British with their strained facilities were forced to rush repairs (indeed the overloaded British shipyards had forced some vessels to be sent to the US for repairs) and some ships such as Illustrious, were forced into service even though not fully repaired. The RN was in a state of continual contraction after WWII, and simply did not have the resources or inclination to repair ships that it could no longer man.

Midway and Forrestal classes

While flight deck level armour was eventually adopted by the Americans for the Midway design, the strength deck remained on the hangar level. Midway had originally been planned to have a very heavy gun armament (8 in weapons). The removal of these weapons freed up enough tonnage to add 3 inches (76 mm) of armour at the flight deck level. While this made a great deal of sense from an air group perspective, the Midway ships sat very low in the water for carriers (due to their much greater displacement), certainly much lower than the smaller Essex-class carriers, and had a great deal of difficulty operating in heavy seas. Flight deck armoured ships almost universally (except for the Midway class as completed) possessed a hurricane bow, where the sides were sealed up to the flight deck; wartime experience demonstrated that ships with the hurricane bow configuration (also including the American Lexington class) shipped less water than ships with an open bow. Late-life refits to Midway to bulge her hull and improve freeboard instead gave her a dangerously sharp roll, and made flight operations difficult even in moderate seas. This was therefore not repeated on Coral Sea (Roosevelt had been decommissioned years earlier). After the war, most of the Essex class ships were modified with a hurricane bow and the wooden flight decks were replaced with aluminium or steel for improved resistance against the blast of jet engines, making them appear to have armoured flight decks, but in fact their armour remained at hangar level.

The supercarriers of the postwar era, starting with the Forrestal class — nearly 200 feet longer and 40 feet wider in the beam than their World War II counterparts — would eventually be forced to move the strength deck up to the flight deck level as a result of their great size; a shallow hull of those dimensions became too impractical to continue. The issue of protection had no influence on the change; the Forrestal class had 1.5" of flight deck armour, but the subsequent Kitty Hawk and all following classes do not have armoured flight decks (deck armour is of little to no use against modern anti-ship missiles).

As before, however, the USN continued to design its ships for large air groups, continuing to reason that the best defence was a good offence.

See also

• Were Armored Flight Decks on British Carriers Worthwhile?