Comparison of armoured to unarmoured flight deck designs

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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.

[edit] Design

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;

Comparative armour thickness of some World War II era aircraft carriers
	Flight deck	Main deck	Side belt
Yorktown class (19,875 tons)	n/a	1.5 in (38 mm)	2.5 – 4 in (64 – 101 mm)
Illustrious class (23,000 tons)	3 in (76 mm)	2.5 in (64 mm)	4.5 in (114 mm)
Essex class (27,200 tons)	n/a	1.5 in (38 mm)	2.5 – 4 in (64 – 101 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)

[edit] 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^[1]. 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. 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 hanger 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.

[edit] 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) was constrained inter-war by the influence of the Royal Air Force on developing a strategic bomber force. 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, 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^[1]. 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.

[edit] 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 16ft high hanger that was 468ft long. No British carrier other than the early 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 16ft hanger heights, except the two Implacable class ships, which had 14ft, and Indomitable which had a 16ft lower hanger and a 14ft upper hanger.

[edit] 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 relied on fighters and AA to prevent the carriers from being hit in the first place. Late in the war when the USN operated many carriers together and had improved radar, this 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." ^[2] 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 hanger carriers had smaller Avgas and ammunition supplies to match. However, it should be noted that RN carriers carried far more aircraft later in the war 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 hanger, 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 hanger capacity. Early in WW2 the RN did not use a permanent deck park. 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 many more 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. ^[3].

HMS Victorious on fire following three kamikaze strikes, Okinawa, May 9, 1945

USS Franklin listing, with crew on deck, due to a bomb piercing the unarmoured flight deck, 19 March 1945.

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 Indefatigble 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."”

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); she was written off during the 1947 survey of the postwar fleet as beyond economical repair and lingered in reserve until 1956 before being towed off to the breakers. Two of her sisters fared just as poorly. 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 warped her hull. The explosion which occurred on the 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 hanger explosions, primarily due to the USN's considerable financial and material resources. Indomitable was patched with concrete for Queen Elizabeth II's Coronation Review, then immediately afterwards towed to the breakers, illustrating the degree to which the RN's depleted finances caused it scrap rather than repair ships. The RN was in a state of continual contraction after WW2, and simply did not have the resources or inclination to repair ships that it could no longer man.

By contrast, it was found after the war, the lower deck armour of American carriers made certain that the bombs and kamikaze craft which did hit, tended to do their damage outside of the ship's structure. American carriers of the Essex class also survived some of the worst kamikaze hits of the war, albeit with very high casualties. 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. It should be noted that both of these USN carriers, individually, 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. As most of the damage was outside of the ship's structure, and combined with higher quality construction of US shipyards (while the British were forced to rush the construction of the ships to get them combat ready, the numerous and capacious American yards on the East and West Coasts allowed the US Navy to build carriers at a more leisurely pace individually while producing ships collectively at a furious rate), this ensured that even significantly damaged carriers could be economically restored to good condition, as the Franklin and Bunker Hill were successfully repaired though they saw no postwar service.

The only 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.

[edit] 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.

[edit] References

[edit] See also

• Were Armored Flight Decks on British Carriers Worthwhile?