In the early years of naval aviation the question of aircraft carrier armor occupied the intellectual effort of numerous naval engineers. Two main schools evolved, one arguing that armor on a carrier should be below the hangar and the other insisting that it be part of the carrier's flight deck.
Warship Armor: A Primer
From the mid nineteenth century up until the introduction in the anti-ship missile in the late 1940's, the primary means of doing damage to another naval vessel involved striking it with a torpedo, bomb, or shell from a naval gun. Naturally the desire to keep vessels afloat meant that naval designers had to improve the protection accorded to various parts of warships in order to give them the greatest chance of survival.
The aforementioned offensive weaponry typically struck a contemporary naval vessel in one of three ways. The torpedo usually plowed into the hull between the waterline and keel causing major flooding in vital areas. Bombs generally fell from a high altitude and plunged through decks until exploding in the bowels of the target vessel. And shells fired from enemy ships followed a parabolic trajectory that could be steep or shallow - leading to plunging strikes similar to those coming from bombs in the former case, and more horizontally oriented strikes akin to being struck by a bullet from a gun in the latter situation.
Protecting against these different threats required armor set in different areas of the vessel. Vertical belts of armor between the topmost hull deck and extending below the waterline protected against shallow torpedo strikes and much naval gunfire. Anti-torpedo bulges below the waterline protected against that threat. And a horizontal deck was situated as high up on the hull of the vessel as possible to ward off bomb strikes and plunging fire from naval guns.
Aircraft Armor Conundrum: Multiple Flat Decks to Select From for Armor Placement
Aircraft carriers posed a particular challenge to naval engineers beginning in the 1920's because unlike traditional warships, aircraft carriers are effectively floating airports. Most of their topmost deck is flat as a necessity for supporting aircraft takeoffs and landings. While some hybrid types of carriers existed, for the most part they were known as flattops for a reason: aside from a conning tower set to the side of the flight deck (and even this was dispensed with in some designs, particularly early Japanese carriers) they presented a long, wide, flat profile when viewed from above.
Naval engineers thus had more flexibility in locating the primary armored deck. A comparison of armored to unarmored flight deck designs indicates that there are two optimal locations for the armored deck: either as part of the flight deck or as part of the floor of the hangar.
While the former might seem to be an obvious choice at first glance, it came with a significant drawback. The armored deck inevitably weighed a lot and necessitated significant vertical reinforcement in order to be viable. In traditional warships this posed little problem - the side armor could come right up to the edge of the deck armor forming a well-supported armored citadel. But in aircraft carriers, hangar capacity has always been an important variable, and an armored flight deck supported by side armor meant that the hangar deck would be quite constricted. Effectively, an armored flight deck meant that an aircraft carrier's air wing would be smaller than that embarked on a similarly sized carrier lacking armor at the flight deck level.
Comparison of Armored to Unarmored Flight Deck Designs in the US and Royal Navies
The two leading powers at sea during the Second World War rapidly became the United States Navy and the Royal Navy of the British Empire. Each built numerous aircraft carriers, but each also came up with a different answer when conducting its own comparison of armored to unarmored flight deck designs.
The United States chose to dispense with flight deck armor and locate the primary armored deck at the level of the hangar. Carriers like the USS Enterprise (CV-6) of the Yorktown Class as well as the series of ships known as the Essex Class (including USS Franklin (CV-13) and USS Bunker Hill (CV-17) as the fifth and ninth members) were designed with their armor under the hangar. This maximized the airwing they could take on at the expense of their survivability in the event they sustained battle damage.
The British Empire instead selected an armored flight deck as the core of its modern carrier designs like those of the HMS Formidable and the HMS Illustrious. Their airwings were restricted, but their crews safer.
The best means of conducting a comparison of armored to unarmored flight deck designs involves examining the historic performance of aircraft carriers hewing to one design choice or the other. USS Franklin (CV-13), USS Bunker Hill (CV-17), HMS Illustrious and HMS Formidable all took significant battle damage that can be compared in order to assess the relative performance of vessels with armor at the level of the flight deck versus those with their armor at the hangar deck level. A qualitative comparison of armoured to unarmoured flight deck designs cannot be perfect due to the differences in other important factors such as a vessel's mission and damage control capabilities, but in general certain trends can be identified via a careful assessment of available evidence.
The British Experience: Less Punch, Better at Crew Protection
The British answer to the comparison of armored to unarmored flight deck designs was borne in large part due to particular British Naval concerns. The Royal Navy was expected to operate in a wide variety of naval environments in order to protect the interests of its empire. Often its carriers would be needed to operate close inshore where they would be more vulnerable to land based strike aircraft. They had to be able to survive being struck by heavy bombs delivered by these bombers and still perform their mission. In analyses by British naval designers, the ability to withstand bomb hits and keep the aircraft and crew relatively safe was more important than mounting overwhelming aerial firepower.
In retrospect, the British carriers clearly played their role as intended. HMS Illustrious was one of the early vessels equipped with a thick armored flight deck, and she was asked to operate in the Mediterranean- well within the range of German and Italian bombers. She was struck by six bombs in one mission and reported sunk by ebullient German authorities only to survive and be back in service in a year and a half. Two kamikazes hit her late in the war and failed to penetrate her flight deck.
HMS Formidable- a later design than HMS Illustrious- took a kamikaze hit that gouged a ten foot by two foot hole two feet deep in the flight deck only to have it filled and ready to handle aircraft less than six hours later. Other kamikaze strikes hit her deck, but none did any greater damage. All in all, she lost only eight crewmembers killed and fewer than fifty wounded due to these attacks, which is incredible when the kinetic and thermal forces of an exploding plane crashing from altitude are considered.
The US Experience: Carriers That Dish it Out but Lose the Crew
The United States, in contrast to the British Empire, considered the number of aircraft it could squeeze into its carriers to be of vital importance. American carrier task forces were anticipated to operate at length far from home without significant reinforcement or support from land bases. As such, US carrier operations tended towards emphasizing self-sufficiency and annihilating opponents as quickly as possible to prevent their striking back. In the opinion of US strategists, the "Sunday Punch" of its carriers would outweigh the risks of leaving their flight decks relatively unprotected.
USS Franklin (CV-13) and USS Bunker Hill (CV-17) demonstrate the downside to this strategy. In March of 1945 USS Franklin was struck by two ~500 pound armor piercing bombs that readily penetrated her unarmored flight deck and exploded in the hangar where aircraft were being armed and fueled for strikes on Japan. The hangar deck turned to an inferno and caused aircraft up on the flight deck to explode as well. All but two men in the hangar at the time were killed. In all over seven hundred men died and more than two hundred were wounded by these two bombs (out of a crew of around 3000), and the Franklin was very nearly lost.
Bunker Hill was hit in the same year by two kamikaze strikes accompanied by two bomb strikes, one of which fully penetrated the flight deck. Just as with the Franklin, massive fires broke out among the armed and fuelled aircraft on deck and in the hangar and in the ensuing maelstrom the ship was nearly lost and almost 400 crewmen were killed.
Conclusion, Sources, and Image Credits
Granted, these examples are somewhat cherry-picked. British carriers were fewer in number and relatively less exposed to kamikaze strikes than their American counterparts. Whether the British carriers would never have been hit at all had their airwings been larger is debatable. And whether the Franklin or Bunker Hill would have been damaged more severely if they had been equipped with smaller airwings is also unknown.
But a straight comparison between the battle damage suffered - especially by the Franklin and the Formidable - show that the lives of crewmembers were better protected by the armored flight deck. Eight men dying in a major kamikaze strike compared to 700+ dying due to two mid-sized bombs is a strong indicator of what the respective US and UK naval priorities were during the Second World War.
Sources, Image Credits, and Further Reading
Richard Collier, The Desert War, 1977 Time Life Books
Janes Fighting Ships of World War II, 1946 Janes Publishing Company, 1994 reprint edition
Arthur Zich, The Rising Sun, 1977 Time Life Books
All images courtesy of United States government, accessed via wiki commons
For further reading, check out this discussion of the modern aircraft carrier battle group. You might also be interested in the recent moves by China to deploy an aircraft carrier. Finally, for another example of the dangers facing poorly armored warships, here's a description of The Battle of the Denmark Strait.