The armoured flight deck debate is nothing new. It is one that has raged back and forth since the 1930s. The USN, in particular, put enormous effort into understanding the concept’s strengths and weaknesses.

 DOCTRINE DICHOTOMY

The pre-war USN was intrigued by the potential of armoured flight decks. But it was hesitant about the trade-offs it entailed.

Time and again the US Navy’s Fleet Problem exercises revealed that carriers were immensely vulnerable.

It wasn’t so much a matter of strike size. It was a matter of who got in the first strike. Even lightly damaged carriers never had a chance to launch a counterstrike.

This became the nexus of decades of debate.

Larger, protected ships (such as Lexington and Saratoga) could individually carry large numbers of aircraft while having some ability to withstand attack. But few such ships could be built under the Washington Treaty’s tonnage ceiling.

Smaller, lighter ships (such as Ranger) could be deployed in groups and get large combined strikes of aircraft into the air quickly. Individually, though, each ship was very vulnerable.

It was the same problem the RN had uncovered during its own manoeuvres.

Both navies were faced with a quandry: Would greater numbers of smaller flight decks mean enough capacity for a winning counterstrike? Or would a more heavily protected fleet carrier be able to sustain effective operations after taking a measure of damage?

“Given the tonnage ceiling, however, carriers with either armoured or unarmoured flight decks were a gamble,” Hone, Friedman and Mandeles write in American & British Aircraft Carrier Development. “Any design was risky, because the Washington and London treaties forbade navies from building an operational carrier that could be experimented with and then thrown away if the experiment failed. As a result, Navy carrier commanders were still arguing about the value of carriers with armoured flight decks as late as 1939.”

THE BIG-STRIKE

The ultimate effectiveness of the ‘big strike’ concept cannot be denied. Swarms of powerful carrier-launched aircraft simply overwhelmed Japanese defences in 1945.

Proponents of the concept were vindicated. Many felt decades of “black shoe” (gun-based) thinking had restrained the inevitable ascendancy of “brown shoe” naval aviation.

But, as Hone, Friedman and Mandales point out, things are never that simple.

The ‘big strike’ was simply not possible before 1944.

Carrier strikes represented a ‘pulse’ of offensive firepower. It took time to build up, deploy, coordinate and deliver this pulse. Then it had to find its way safely home. During this time, the enemy could well be doing the same.

In the 1930s and early 1940s, no carrier group could both defend itself and strike at the same time.

“Under actual war conditions it is quite possible that all of the carriers engaged … (will be) lost or put completely out of action,” CinC US Fleet noted of Fleet Problem XV in 1934. “With opposing air forces of equal efficiency this is by no means an impossible result of the opening movements of a naval campaign.”

The only option was to find the enemy first, and get in the first strike.

In Fleet Problem XX of 1939, USS Ranger was able to locate USS Enterprise through radio direction finding. Ranger’s aircraft ‘destroyed’ Enterprise without the latter having any time to put up an effective defence.

All this cemented the perception that carriers were inherently vulnerable. Worse so even than World War I’s battlecruisers. “Eggshells with hammers” had a new application.

The value of weapons without any real chance of surviving long in a war was a real question.

And while massed carrier forces were possible, key engineering and logistic challenges had yet to be identified and solved. Such as the provision of a large fleet train of support ships carrying aviation fuel, ammunition, replacement aircraft and crew.

Then there were the aircraft themselves. Their performance was poor. Endurance was limited - especially with bomb loads. Finding targets was immensely problematic. Not to mention navigating back to a flight deck that was nowhere near where it had been when they took off.

These challenges were only fully overcome in 1944. Aircraft such as the Hellcat, Corsair and Avenger had the necessary performance to meet the demands placed upon them. And they were well supported by the radar direction, navigation aids and an immense supply chain needed to put them on target.

THE ARMOURED OPTION

The 1930s was a time of incredible technological advancement.

And there was a real risk that the time taken between design and delivery could result in an already obsolescent ship.

A late 1930s Bureau of Ordinance (BuOrd) study predicted a minimum of 1.5in STS steel was needed to resist a general purpose 500lb bomb. But the USN was well aware its own ordinance was set to double, even quadruple, that weight.

A 1.5in armoured flight deck, it was calculated, would add 1460 tons to a Yorktown-size design. But its increase in displacement would be much greater given the need for heavy supporting structure and a broader beam needed to counterbalance the added topweight.

Studies also anticipated problems with the need of accommodating flight operations in an armoured deck: What about the holes for arresting gear, crash barriers, elevators, catapults – lights?

But there was another issue. 

Putting armour on the flight deck would leave a large gap between the ship’s horizontal protection and the side protection offered by a belt. This gap was particularly vulnerable to naval gunfire.

“The deck over the vitals of the ship would have to be, therefore, protected by a lower armoured deck unless protection against gun fire can be omitted entirely, which appears off hand to be a step which present knowledge of probabilities could hardly justify,” a Preliminary Design Office report noted.

The Royal Navy agreed. Its answer eventually resolved itself in the form of the ‘armoured box’ hangar approach. This, it considered, killed two birds with one stone: the aircraft could be protected from the elements and enemy fire – and all angles into the ship’s vitals were covered.

The USN considered t all this extra weight resulted in unacceptable air group size limitations under treaty limits. Some calculations circulated among US Naval staff warned that an armoured flight deck on a Yorktown-size ship would strip away as much as two thirds of its potential flight group.

BuOrd’s eventual recommendation was that 2.5in lower down in the ship, beneath ‘initiating’ structure, would be enough to stop a 1000lbs bomb.

CAPTAIN McCAIN

Early in 1939, Captain John S. “Slew” McCain – the commanding officer of USS Ranger and grandfather to the US Senator of the same name – proposed a force of small but fast carriers to support what he considered to be his vulnerable strike carriers.

These were to be optimised for fleet defence. And have armoured flight decks.

Everything about them was to be focused on CAP. Their 150-180m long flight decks would accommodate two catapults and three fast lifts to constantly cycle an airgroup of 24-30 through the air.

But McCain felt such fighter carriers would only be worthwhile if they they were not inherently vulnerable.  And that meant armoured flight decks. No number of fighters in the air could guarantee protection for the fleet below, he told anybody who would listen.

McCain wanted a carrier capable of keeping 500lb bombs from reaching the hangar. He also wanted the deck itself to remain intact as much as possible to sustain operations. But his chief concern was a deck crowded with armed and fuelled aircraft. The potential inferno must be prevented from spreading through the ship, he emphasised to the USN’s General Board.

Admiral King, then Commander of Aircraft, reportedly thought the concept was irrelevant. The idea was soon lost among the USN’s internal debate.

TECHNICAL CONSIDERATIONS

By 1939 BuOrd was recommending the next generation of US fleet carriers be built to resist 1000lb general purpose bombs dropped from up to 10,000ft. It noted that such a bomb would easily penetrate the superstructure and 1.5in horizontal armour of current designs – and that the hit would seriously endanger the ship.

Another reset in thinking was needed in 1941.

That year, BuOrd reported that 500lb bombs dropped from 10,000ft would need 1.9in STS armour to be defeated. A a minimum of 2.5in would crack the casing of a 1000lb semi-armour piercing bomb. But an armour piercing variant of the same weight could breach 2.88in. And then there were the new 2000lb bombs, like the German Esau used against HMS Illustrious …

By the time serious consideration was being given to the production of a large US armoured flight deck carrier, the thinking was even a 500lb bomb could penetrate 3in armour plate if dropped from a high enough altitude.

But war experience was fleshing out the dimensions needed to be applied to armour thinking.

Ships would manoeuvre to evade air attack. This made targeting them harder. So pilots - of dive bombers in particular - would fly closer to improve the odds of a hit. And if you drop the release height of a 500lb bomb to 4000ft, it would only be able to penetrate 1in.

And, yes, aircraft could - and were - carrying more or bigger bombs. But the heavier the bomb-load, the shorter the aircraft’s effective range.

Both considerations could be played to a carrier’s advantage.

Lieutenant Commander Steadman Treller
USN Observer, HMS Formidable, April 1941

The armored flight deck of the Formidable (sister ship of Illustrious) was not struck by any bomb or shell while l was in the Mediterranean. Its effect upon the design and operation of the ship as a carrier was evident in the following ways:

Reduces drastically the capacity for aircraft. Although Ark Royal and Formidable are about the same tonnage (22.000), the former, an old carrier. operates 54 aircraft but the Formidable is limited to 36.

The reduced height of the flight deck above waterline in Formidable prevents aircraft being parked on deck in fairly rough weather. This does not concern the British but would require radical changes in our carrier system. Heavy spray was taken over the flight deck in Mediterranean and on one occasion I saw a small amount of green water. In an Atlantic storm the flight deck was swept frequently by green water: the forward end sustained damage from seas and ground tackle on the forecastle was torn loose.

Aircraft elevator speed is reduced by the added weight. Although the Formidable’s elevators were not fully armored, they were heavier than Ark Royal's. Ark Royal speed, 7—9 seconds for one way trip; Formidable, 13—14 seconds.

The armored “box” formed by the flight deck, sides and armored doors must be vented in any case - witness Illustrious damage from hangar explosion. Standard orders on Formidable are (when day bombing attack is likely):
(1) Forward elevator dovm, after one up.
(2) Forward armored doors wide open, after ones shut.
(3) Lobby doors closed.
(4) Wire curtains up.
(5) Wire parties outside of hangar (in lobbies).

The British Navy is not so sure that they can armor their carriers effectively. They insist that armor is necessary but are not sure how they will obtain a satisfactory thickness.

My conclusions are that the British gave up almost 50 percent of their aircraft capacity to obtain partial protection of the carrier. The additional aircraft. particularly if they are fighters. will provide protection to the carrier. Furthermore. armoring a carrier at the expense of your main striking power is a defensive attitude.

If carriers are to be used in an enclosed sea without the support of the shore based fighters. an armored deck is necessary. but it should be thicker than 3". If carriers are to be used for normal purposes, the additional aircraft capacity is more important than armor.

DESIGN STUDIES

BuShips assessment of the HMS Illustrious action, June 1941

The fact that the existence of an armored flight deck forces the use of such heavy bombs is an important argument in favor of such a deck. On the other hand, this bomb did penetrate the flight deck and did gut the hangar by blast effect, fragments and fire. Moreover. if this bomb had struck a few feet further forward, and had had a somewhat longer delay in its fuze action, it would undoubtedly have entered a magazine and caused a disastrous explosion, since in the Illustrious the protective plating on the hangar deck is not extended over the central portion of the ship, including the magazines, protection for these spaces presumably being furnished by the lower side belt. . . .

The question is often asked as to what the results would have been if a ship of the CV 9 class had been subjected to the same attack as the Illustrious. . . . Assuming the hits to have occurred in approximately the same locations in CV 9, as in the Illustrious, there seems no reason to believe that the CV 9 would not also have survived. The bomb striking the middle portion of the flight deck would almost certainly have had its fuze action initiated at the flight deck and therefore might have detonated before reaching the hangar deck, or it might have penetrated the hangar deck and detonated after penetration with severe. but probably not fatal, structural damage. . . .

In case the bomb had detonated in the hangar of the CV 9 which has sides of light construction, there is some question as to whether the blast effect would have been as serious as in the hangar of the Illustrious. . .. Subject to confirmation by [small scale] tests, it is believed that the provision of one inch S.T.S. transverse bulkheads in the hangar will restrict the damage from blast effect in a relatively open-sided hangar to the section between two of these bulkheads, and will protect the adjacent bays from a large part of the fragment attack.

Protection for the hangar of the Illustrious was achieved by material sacrifices in other characteristics of the carrier as compared with the Essex (CV 9 class). The direct comparison is obscured by the fact that the Illustrious is of smaller displacement than the CV 9 class and that the British practice of carrier operation differs materially from ours in the number of planes operated. Even allowing for these, however, there is a material difference in number of planes carried, speed, endurance, gasoline capacity, and bomb storage between the Illustrious and the CV 9 class. . . .

RESOLUTION

The Essex class (CV9) was an evolutionary leap forward in carrier design. They were the embodiment of pre-war USN large air group doctrine. They were big, fast and responsive. They could be mass produced.

They were not ideal. The treaties that had constrained the Yorktowns (CV5) had expired. But it was not possible for a complete design reset given the urgency of the international situation.

But, by 1943, the limitations of the Essex class were already becoming apparent. New equipment, extra guns and an associated explosion in crew numbers saw the ships become cramped and overcrowded. They quickly became seriously overweight, threatening their stability.

Also, early battle experience highlighted  their light protection. While the armoured hangar deck protected the ships vitals as intended, the flight deck superstructure – in particular the gallery deck – proved far more important than anticipated. Here was the new Combat Information Centre (CIC) along with associated radar and signals rooms, command staff, ready rooms, briefing rooms and other equipment.

Repairing such unprotected facilities took much longer than anticipated. And the casualty rate among the personnel in these spaces would prove demoralising.

Even before these events unfolded, BuShips had changed its thinking. Survivability had its advantages. And simply forcing an enemy aircraft to carry a heavier bomb reduced the effective range of that aircraft …

By 1942 The US Navy’s General Board wanted ships totally unfettered by pre-war treaties and thinking. In the case of aircraft carriers, this meant extra armour no longer required an aircraft capacity trade-off. It also meant the ship could be given much more extensive – and heavier – compartmentalisation below decks.

The first hearings were conducted in March 1942. By this time the extensive battle damage reports of HMS Illustrious and Formidable were readily available. Pros and cons were assessed. Design details were thrashed out.

This eventually resolved itself into a 45,000ton ‘large’ carrier.

By July 1942, construction of the lead ship – to be named USS Midway – was approved by US Congress. In August, contracts were awarded for the construction of four CVB type carriers (Battle Carriers).

But President Roosevelt hesitated. He did not initially sign off on the contracts as he was worried about long construction times and concentration of resources.

He challenged his chief of navy, Admiral Earnst King, to justify their build over a swarm of smaller 11,000 ton light fleet carriers of a similar vein to that of the Royal Navy’s Colossus class.

Admiral Nimitz waded into the debate, expressing a lack of enthusiasm for either option: “(Midway was) unnecessarily large and unwieldy, and concentrating too great a percentage of strength in one hull … (But) the view is generally held that the 11,000 ton vessel is too small to meet the requirements of a first line carrier.”

His opinion, and that of other Pacific Theatre commanders, was speed of delivery was imperative. And that meant an evolution of the Essex design.

Admiral King, however, argued the navy needed to think beyond its immediate needs. So he found a compromise. Production of the 27,000 ton Essexes would not be slowed if the 45,000 ton CVBs were to take over the docks set aside for the now cancelled Montana-class battleships.

With the rapid supply of Essexes assured, President Roosevelt finally gave his seal of approval on December 29, 1942.

OUTCOMES

Much of the doubt over the value of the CVBs was based on the USN’s early war experience. The Royal Navy had lost one fleet carrier to surface ship guns, and two to submarine torpedoes. But the intense dive-bombing attacks survived by HMS Illustrious, Formidable and Indomitable soon challenged this thinking.

The USN’s own war damage lessons before 1945 did not implicate the flight deck. Instead, its carriers had been lost to torpedoes and fuel-vapor explosions. And the Essex’s new subdivision was regarded as more than equal to the challenge.

There was one cause for hesitation, however: The carnage wreaked upon the Japanese carriers by dive bombers at the Battle of Midway.

Then Japan changed tactics. From late 1944, it turned its aircraft into human-guided missiles. Fighters and light bombers began delivering their warheads right on to the flight deck and hangar sides.

One direct hit on an Essex would most often produce a ‘mission kill’. Many merely required a few weeks at a forward repair facility. Others needed many months of reconstruction back at mainland shipyards.

Which is why USN liaison officers aboard the British Pacific Fleet - as well as their senior officers back in Hawaii - were incredulous that two direct kamikaze hits against HMS Formidable were shrugged aside within 5 hours. There was no urgent need to return even to the Ulithi anchorage to restore the ship’s ability to operate aircraft. No ship suffered a ‘mission kill’.

But the pre-war British carriers - as predicted - had other issues.

Off Okinawa, the US Task Group 58.1 had two Essex class (27,000 ton) and two Independence class (10,600 ton) carriers. Between them they carried 280 aircraft.

Off Sakishima Gunto, the British Pacific Fleet’s Task Group 57.2 had three Illustrious class (23,000 ton) carriers and one Indomitable (24,000 ton) class. Combined, these carried about 235 aircraft.

The comparative value of strike strength and mission resilience remains a debated point even today.