Armoured decks

The armoured flight deck controversy

The British armoured flight decks and enclosed hangars, as seen in their Illustrious class aircraft carriers, were vastly superior to the American Essex class that had open hangars, and an armoured main deck that supported an elevated flight deck, according to generations of RN and RAN fixed-wing pilots. There were pros and cons. Illustrious- and Essex-class total armour was comparable in terms of total weight, and the ships were of a similar size, but the British armoured flight deck design offered better protection than the American, so the argument went.

Examples

After all, look at the way Illustrious survived numerous bombs in January 1941 in the Mediterranean and Formidable survived two 500 kg bombs in the Med on 26 May 1941 and even a couple of kamikazes off Okinawa in early May 1945, all with minimal casualties. On the other hand, Essex class carriers, such as Franklin (CV-13) reported 989 casualties and severe damage on 19 March 1945 from two 250 kg bombs while Bunker Hill (CV-17) had 650 casualties and equally severe damage from two kamikaze hits on 11 May 1945.

Even modern internet comment is replete with the mantra that while British carriers “shrugged off kamikazes,” American carriers “had to retire for repairs.” Closer examination suggests this might not be quite so.

3carriers
The WW II experiences of the classes following USS Essex (top, hangar deck armour) HMS Illustrious (flight deck armour)and HIJMS Shokaku (hangar deck armour) provide interesting comparisons.

Stuart Eadon reiterated the mantra as late as 1991, saying that in contrast to the “vulnerable” wooden decks of American aircraft carriers, the armoured decks of the British Fleet carriers protected them to the extent that they frequently returned to flight operations within hours of being hit. He cites one oft-repeated comment by an observer watching a kamikaze attacker that “literally bounced along the deck and then slid off into the sea” (Eadon p. 266). Even more recently, in 2004, another source boasts, “The immense strength of the ships stood them in good stead … In the Pacific War most of the (Illustrious class) ships withstood one or even two kamikaze strikes without having to leave station” (Bishop and Chant p. 46).

Justification

Anthony Preston succinctly explains why the Illustrious class carriers were built with an armoured flight deck and enclosed hangar. The British specifications aimed to:

“Provide as much protection for the aircraft as possible. This meant … building an armoured box with 4.5-inch (114 mm) sides and a 3-inch (76.2 mm) roof,” (Preston, p.60).

The designers expected the air group to land on and be struck down in the face of air attack. Meanwhile the ship’s AA armament “welcomed the opportunity” to shoot down the attackers, he adds. This never happened.

Interestingly, the Japanese Shokaku class was also a purpose-built large aircraft carrier constructed around the same time. It had an American-style “open” hangar, with a 100 mm armoured hangar deck over the machinery spaces, much like the Essex class.

Multiple variables

Other authors, such as Stuart Slade and Richard Worth, point to a host of critically important variables that must be taken into account before coming to a reasoned conclusion. They make a convincing assertion that the different damage reported by British and American carriers can be explained chiefly by the amount of refuelled and rearmed aircraft on deck when the bomb or kamikaze hit. Put simply: the bigger and more volatile the deck park, British, American or Japanese, the heavier the damage, regardless of whether the flight deck was armoured or not. One corollary found proven was that if a bomb penetrated the British flight deck armour, the damage was likely to be long-lasting and severe. Another was that good damage control procedures were vitally important. A third potentially confounding factor, frequently overlooked in comment, is that there were only four British fleet carriers exposed to kamikazes and they were rarely subjected to the heavy sustained attacks experienced by American carriers. Except to prove an exception, even the dodgiest statistician could never rely on such a small number as four.

Illustrious

Essex Shokaku
Launched 5.4.1940 31.7.1942 12.12.1937
Displaced 28,661 tons 27,100 tons 26,675 tons
Dimensions 227 x 29 x 8.5 266 x 28.3 x 8.6 257.5 X 26 x 8.8
Speed 30.5 knots 33 knots 34.2 knots
Crew 1200 2600 1660
Propulsion 3 shafts, 110,000shp 4 shafts, 150,000shp 4 shafts, 160,000shp
Aircraft 45-65 90-100 84
A comparison of the Illustrious, Essex and Shokaku classes (dimensions in metres) in the table above shows that they were very similar in many respects as far as gross tonnage and dimensions are concerned. They all carried defensive armour to resist bombs, shells and torpedoes penetrating the vital machinery and magazine spaces. However, Slade makes a salient point, “The question is not so much whether armour is useful … but where the designer puts it,” he says. The British Illustrious class had the flight deck as the strength deck with three-inch amour, theoretically sufficient to resist 500-lb (226 kg) bombs and six-inch (15.2 cm) shells. Instead, the American Essex and the Japanese Shokaku classes both had 3.5 inches of armour on the hangar deck below a thinner flight deck that was nevertheless strong enough to take the punishment of five- to ten-ton aircraft landing on and taxying about.
Shokaku
Shokaku had a badly deformed flight deck after the Battle of the Coral Sea, but her armoured hangar deck allowed her to live to fight another day.

Perhaps there are two important and sometimes over-riding design considerations. The first is that all ship design is essentially a compromise between highly competing variables. The second is that unless the self-damage danger of aviation fuel and ordnance is not scrupulously controlled, then aircraft carriers are extremely vulnerable to secondary fires, regardless of where the armour might be sited.


More armour, more aircraft or more speed?

The Americans favoured large air groups and instead of striking them down into the protection of an armoured box, they depended on their aircraft to keep the enemy away. They also deliberately maximised their aircraft complement to the extent that they ran a permanent aircraft deck park. There was no other place to put them, even in their larger hangars. This was acceptable in most of the Pacific, but it was considered risky to delicate aircraft in the stormy North Atlantic and in action, according to RAF-advised RN 1930s-era thinking.

The British armoured box hangar design certainly protected aircraft from the elements, but it drastically reduced the number of aircraft, by maybe 40 per cent, that could be carried in hulls of similar displacement. This might not matter when there are plenty of aircraft carriers, but as the Pacific War quickly demonstrated there is almost invariably a scarcity of these vessels in the early stages.

Lift placement

The armoured box presented design difficulties in that it limited the size and placement of aircraft lifts. The centreline lifts demanded by the armoured box prevented the transfer of aircraft between the flight deck and hangar during flying operations. The open hangar concept and deck edge lift forward of the barriers permitted limited aircraft movements, even as aircraft were being flown on and off.

Illustrious
HMS Illustrious was limited to centreline lifts chiefly because of her “armoured box” construction.
Fire in an aircraft carrier, with all its volatile aircraft fuel and ordnance, is at the forefront of every carrier captain’s mind. The British were especially sensitive to this. As Anthony Preston notes, “As part of their fire precautions the British developed the concept of the ‘closed’ hangar, in which the ventilation of the hangar was sealed off from the rest of the ship” (Preston p. 44). Hangar access, other than by aircraft lift, was via air locks.

Aviation gasoline

Therefore the explosive aviation gasoline (AVGAS) fumes, including those vented by stowed aircraft in rough weather and during evasive manoeuvres, could be trapped inside a closed hangar and blown clear of the ship in a controlled manner. Open hangars risked sucking some of these fumes back into living spaces from random pathways. On the other hand, poor damage control might jeopardise the whole ship. For instance, the flight deck-armoured Japanese Taiho, after being hit by a submarine-fired torpedo at 0810 19 June 1944 looked relatively intact, she could make 26 knots and operate aircraft, but her forward lift was out of action and aviation fuel was sloshing about in the lift well for some hours. The vapour initially was restricted to the hangar but instead of venting this dangerous fuel/air mixture overboard in a controlled manner, an inexperienced damage control team vented it back into the ship. This set up a series of such large AVGAS-fed explosions, some more than six hours later, that the flight deck split longitudinally, virtually in two, and the hangar sides blew out before the entire ship was lost.

Taiho
HIJMS Taiho had an armoured flight deck, like Illustrious, but this had little to do with the manner of her sinking. She was destroyed by internal AVGAS explosions some six hours after a single torpedo hit.

Small fires in an armoured box hangar should be more easily controlled. Certainly, with both lifts up, fire curtains down and sprinklers operating, a single aircraft fire should be extinguished, promptly, within a couple of minutes. However, if reactions are slow or something goes awry, as it frequently does, the resulting fire or contained explosion could easily warp the armoured box to the detriment of the entire ship’s structure. Once this rigid structure is warped, repair becomes difficult, if not impossible.

After bombs distorted her hull on 10 January 1941, Illustrious had her centre propeller shaft cut away, with a concomitant speed reduction to 22 knots. Essex class carriers in similar situations tend to shrug off such potentially severe damage.

For instance, Oriskany’s extremely hot magnesium-fed magazine fire on the hangar deck in 1966 killed 44 people, but did not damage her main machinery or hull structure and she was back in the fleet five months later. Oriskany and other American aircraft carriers that experienced devastating ordnance-fed fires in recent times, such as USS Forrestal (29 July 1967) and USS Enterprise (14 January 1969), did not experience an AVGAS explosion, mainly because their jet aircraft no longer used high-volatile petrol as fuel, but a safer kerosene-based mixture with a much higher flash-point. Burning kerosene nevertheless spreads a very hot fire, but at normal atmospheric pressures it does not explode. None of the three carriers concerned reported structural defects, like Illustrious and Victorious, after similar fires.

Oriskanay
USS Oriskany (CV-34) burning fiercely, 26 October 1966, from a near-full magnesium flare stowage on the hangar deck.
Many better-armoured battleships were destroyed, by torpedo, bomb and shell, but no Essex or Illustrious class aircraft carrier was sunk in WW II. The Essex class USS Franklin was perhaps the most severely damaged. Three kamikaze hits, a bomb hit and a near miss between 9 and 30 October 1944 sent her back to the USA for a ten-week repair schedule. Another two 250 kg bombs plus internal fires and ordnance explosions on 19 March 1945 earned her a six-month sojourn in the Brooklyn Naval Yard. Importantly, she sailed home and was repairable after such severe damage.
Franklin
USS Franklin (CV-13), 19 March 1945, only 50 miles from the Japanese mainland , dead in the water, on fire and with a 15 degree list. She had taken two 250 kg bomb hits through the flight deck, one igniting ammunition stowed aft. Under tow, the crew quelled the fires and the ship eventually made her way home to the USA under her own power for repairs. The war ended before her repairs were completed.

Good aircraft fuel and ordnance handling practices mitigate aircraft carrier volatility. A number of big American and Japanese aircraft carriers were lost or heavily damaged by AVGAS explosions or fires following WW II action (e.g., American: Lexington CV-2 8 May 1942,Wasp CV-7 15 September 1942, Franklin CV-13 19 March 1945, Bunker Hill CV-17 11 May1945; Japanese: Soryu and Akagi 4 June 1942, Shokaku and Taiho 19 June1944.) Big British carriers also reported AVGAS-related fires (e.g. Formidable 12 August 1942 and 3 February 1951) but the British (and the Australians) tend to be much more conservative with their AVGAS and ordnance handling.

The RN insist that no aircraft be refuelled or rearmed during launch/land procedures. The entire AVGAS supply was routinely drained and even inert gas pumped into the lines when attack was expected. The Americans and Japanese, with their larger aircraft complement and complex deck parks had a greater demand for faster turn-around times. They frequently re-armed and refuelled aircraft during flying operations.

USS Forrestal
USS Forrestal CV-59 burning during the 26 July 1967 fire. Nine 450 kg bombs exploded on the 1.5 inch (38 mm) thick flight deck, killing 134 crewmembers and injuring 161. After seven months in a dockyard for repairs, Forrestal returned to the fleet and served out more than her originally planned life until decommissioned in 1993.

RN and RAN carriers routinely plugged rockets into the aircraft’s firing circuit on the catapult, with the aircraft pointing in a safe direction. USS Forrestal, to “save time” before its 29 July 1967 fire, plugged in rockets and removed quick-release circuit-breaker safety clips even before aircraft had completed their engine start­up procedures. It was suspected that a stray voltage surge, as an F-4 switched from ground to aircraft electrical supply, fired the Zuni rocket that initiated so much destruction.

USS Enterprise

USS Enterpise fire, 14 January 1968.

USS Enterprise CVN-65, the world’s first nuclear-powered aircraft carrier, had a similar fire, 14 January 1969. This time a Zuni rocket cooked off after heating by a huffer starter exhaust. It left 28 dead and 343 injured. There were 18 explosions, some of multiple simultaneous 227 kg bombs, leaving five holes up to 20 feet (six metres) in diameter in the flight deck. Most of the damage was repaired in eight weeks at Pearl Harbor.Enterprise is expected to serve out 50 years in harness, more than her original planned life.

British warship designer and well-known author David K. Brown asserts that the British armoured deck design never lived up to its reputation and concludes that “More fighters would have been better protection than armour” (Brown p. 56). He explains how the RN originally envisaged battles in comparatively restricted seas within the range of some land-based aircraft, such as the Mediterranean. This “narrow seas” concept was proven to be far removed from reality. Similarly, the idea that a 226 kg bomb would be the biggest that British carriers would need to resist was quickly refuted in practice. When a 500 kg bomb penetrated an Illustrious class carrier’s armoured deck (e.g. Illustrious 10 January 1941; Formidable 12 August 1942) it caused severe and frequently lasting damage.

 


Ship’s girder

The main reason for this was the structural consideration that the Illustrious class had the hangar as an integral part of the ship’s girder, while the Essex class had their hangar mounted outside this important structural design element. Ship designers see a large hangar space as essentially an unwelcome large open void. If the hangar is sited within the ship’s rigid girder and exposed to thermal or explosive shock, it might force this girder to deform.

Short of virtually rebuilding the entire ship, once deformed it stays that way. The Illustri­ous class carrier HMS Victorious, for instance, took nearly eight years from 1950 to re­build her from her hangar deck level up. This means that any damage wrought by fires or explosions on the seemingly more vulnerable Essex flight deck or armoured hangar deck might well be more extensive at first glance, but such damage need not easily compromise the ship’s rigid girder and therefore should be more easily repaired.

Damage control

Damage control in an aircraft carrier is a highly complex subject. There is a grave danger that an aircraft carrier will not be sunk by the simple explosion of a bomb, torpedo or shell, but by the fire  generated from that hit. Fully-fuelled and fully-armed aircraft on the flight deck or hangar deck frequently have the potential to cause more damage than a single well-directed bomb, torpedo or kamikaze, as the 1967 Forrestal (CV-59) and the 1969 Enterprise (CVN-65) fires demonstrated. The Forrestal fire claimed nearly 300 casualties. Nine 1000-pound (454 kg) bombs detonated, tearing holes in the flight deck through which flaming jet fuel poured into the spaces below. The fire was eventually extinguished but the ship had to return to Norfolk for extensive repairs that lasted ten months. Enterprise required 12 weeks in Pearl Harbor, but Forrestal served out her time and Enterprise is still serving; unlike the Illustrious class ships that tended to require longer repair periods or even pay off after lesser abuse.

The 1930s-era Lexington (CV-2) and Saratoga (CV-3) had the flight deck as the strength deck, but 1930s Japanese designs such as the Shokaku and even the German 23,000-­ton Graf Zeppelin all had the hangar deck carrying the main armour. Ark Royal and Eagle were the first two British carriers built after the Illustrious/Implacable classes and their 1942 designers opted for an armour distribution and hangar configuration more towards American lines. The heavily armoured flight deck and enclosed box hangar sacrifices were found to be just not worth the effort. By then it was also clear that no flight deck could ever be built that would protect the ship from the larger bombs becoming available, yet still keep topweight within manageable proportions.

The Forrestals and later big American carriers all have a solid flight deck, but this is not so much armour, as a necessary platform for the very heavy aircraft (30,000+ kg) they operate. Protective flight deck armour capability is a secondary consideration.

There are two design features that help these big ships. Firstly, they are incredibly large, so the void represented by the open hangar is proportionately smaller. Secondly, they are built with considerable redundancy to be remarkably tough: e.g. Oriskanay (CV-34) fire 26 Oc­tober 1966; Forrestal (CV-59) fire 29 July 1967; Enterprise (CVN-59) fire 14 January 1969.

Carrier design trend

Successful carrier design has been led chiefly by the British and Americans. The British contributed major improvements. Since WW II these include the angled deck, mirror, steam catapult, V/STOL and ski jump.

The Americans were right to insist on their faster 30+ knot carriers. The Americans also introduced air-to-air refuelling, Airborne Early Warning, auto­throttle and then auto-land systems. It must be concluded, however, that the British armoured deck and enclosed hangar concept, as built into the Illustrious class, never demonstrated the protection its designers sought. Additionally, the sacrifice in aircraft numbers and the difficulty of repair once the ship’s girder is distorted strongly militate against it ever being considered again.

All post-WW II trends, including the four British Malta class cancelled in late 1945 and the more recent American “super” carriers, tend to follow the Essex armoured hangar deck and open hangar principle over the Illustrious “armoured box” design.

References:

Bishop, C. and C. Chant. Aircraft carriers: The world’s greatest naval vessels and their aircraft. Silverdale Books: Wigston, 2004.
Brown, D.K. Nelson to Vanguard: Warship design 1923-1945. Naval Institute Press: Annapolis, 2006.
Eadon, S. (Ed.) Kamikaze: The story of the British Pacific Fleet. Square One publications: Worcester, 1991.
Foster, W. E. Fire on the hangar deck. Naval Institute Press: Annapolis, 2001.
Freeman, G.A. Sailors to the end: The deadly fire in the USS Forrestal and the heroes who fought it. Harper Collins: New York, 2004.
Friedman, N. British carrier aviation: The evolution of the ships and their aircraft. Naval Institute Press: Annapolis, 1988.
Hoyt, E.T. The kamikazes. Harper Collins: New York, 1985.
Preston, A. Aircraft carriers. Bison Books Corp.: Greenwich, 1979.

Websites:

Slade, S. Were armoured flight decks on British carriers worthwhile? (2002)
http://navweaps.com/index tech/tech030.htm.
Worth, R. The armoured box: The war’s verdict (2002) http:// navweaps.com/index_ tech/tech030.htm.


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