(1) British and American practice in aircraft-carrier construction differs in a manner that affects fundamentally the characteristics of the ships. British practice is to build the hangar as a part of the main structure of the ship and to restrict the hangar to a space for stowing and servicing aircraft where necessarily serious fire risks exist. American practice is to build the hangar as a superstructure with open sides, and to treat the hangar not only for stowing and servicing aircraft but as a space where aircraft can be warmed up before flight and through which access to other parts of the ship is provided.

(2) Early aircraft carriers were ships built for other purposes and later converted. The hangars were added as superstructures. British practice has grown out of the requirement that the hangar should be treated as a dangerous area. Naturally therefore the first carrier designed as such with the requirement that she should be “as small as possible” incorporated the hangar sides and deck over in the strength girder as this resulted in appreciable economy in size. On the other hand the Americans started with a converted ship having a hangar with open sides and, with their different views regarding the work to be done in the hangar, have continued this practice. In both cases development has naturally followed from the two different original premises. Some of the arguments that have been seen for and against either system are distinctly ‘after the event’ and possible never entered the heads of the first designers.

(3) An attempt is made in what follows to deal with the subject as though it were raised for the first time and above all to try and visualise conditions that may exist during the life time of the British Carriers now being designed. While experience in this war must be applied the mistake must not be made of imagining that ships which will complete three and more years hence will be worked and fought like those serving now, particularly as aircraft are short-lived compared to ships and air-sea warfare is therefore more likely to change in character than naval warfare before aircraft came into existence.

(4) The subject is dealt with under the following headings:
A.Working Aircraft.
B.Size and Strength of Ship.
C.Protection against Projectiles.
D.Fire Risk.
These are not all of equal importance. Moreover, in some minds the importance of one would far outweigh the combined importance of the other three. Nevertheless if open-sided or closed hangars won in three out of four headings that, it is submitted should be conclusive.

A.Working Aircraft.

1. One of the first requirements in a Carrier is to get the requisite number of aircraft to their objective as quickly as possible. This is best done by making the Flight Deck the main parking space. But in addition it is necessary that aircraft in the hangar should be capable of flight almost immediately after leaving the lift, i.e. should reach the flight deck with engines warmed up. This is the argument for the open-sided hangar. The Americans have probably visualised for their carriers better weather conditions than we have for ours. We are only now building our carriers so that the flight deck can be the main parking space. Further, by the introduction of the oil immersion heater, aircraft in British Carriers can take off five minutes after reaching the Flight Deck, and it is understood, in an emergency would take off immediately.

2. In cold and stormy weather when the Flight Deck can be little used and the Open hangar would be more or less closed the arguments against the completely enclosed hangar become weaker.

3. Under darken-ship conditions if work in the hangar is to be done at night the open-sided hangar becomes a closed one. This is an argument for the closed hangar that will grow in force as night flying develops. The Americans close their hangars with roller blinds. British experience with such fittings has been distinctly unhappy.

4. In a hot climate it has been found that working conditions in closed hangars are very bad. This difficulty will, it is believed, be overcome in the new British designs by arranging for work on aircraft to be done at the ends of the hangars and providing improved ventilation.

5. One disadvantage of warming up engines in an open-sided hangar through which access is obtained to other parts of the ship is the danger of carbon-monoxide penetrating into the ship. It is understood the Americans have not experienced this trouble but the possibility exists.

6. In a ship with an open-sided hangar extending out to the ship’s side the hangar becomes a general working space instead of being reserved essentially for aircraft. Bollards, fairleads, power-worked capstans and hatches interfere with the movement of aircraft.

7. Although three lifts are not in the Staff Requirements for British Fleet Carriers, two lifts and a side-elevator are provided in American Carriers. The provision of more than two lifts must be considered in a discussion on open-sided versus closed hangars. With a closed hangar, the flight deck being the strength deck, a lift amidships impairs the ship’s strength to such an extent that it is preferable not to provide it. With an open-sided hangar the same objection applies with even greater force as the lift well cuts into the hangar deck, which is the strength deck. A side-elevator can be installed in a ship with an open-sided hangar but not in a ship with a closed hangar, the hangar side being an important part of the strength girder. There are disadvantages in a lift amidships which need not enter into this discussion: the matter is dealt with extensively in the paper submitting the Fleet Carrier design of the 1943 programme. If a side-elevator is considered a valuable fitting, the open-sided hangar has an advantage in this respect over the closed hangar.

8. Summing up under this heading the open-sided hangar wins but by not very much.

B.Size and Strength of Ship.

1. In a ship with a closed hangar the hangar bulkheads transmit the main hull forces to the flight deck which is then incorporated in the strength girder. In a ship with open-sided hangars this is not possible and all the structure above the hangar deck is, from a strength point of view, redundant with the result that the ship is decidedly larger. The new ARK ROYAL has a displacement of 32,500 tons and the new American design 45,000 tons: this considerable difference is accounted for partly by the greater speed and partly by the different system of construction of the American ship. The new Light Carrier has a standard displacement of 18,000 tons which would become about 19,000 tons if the ship were built without hangar sides and without the flight deck being incorporated in the strength girder.

2. In a Fleet Carrier with an open-sided hangar and armoured flight deck the latter must have expansion joints. These joints involve a weak line of protection right across the deck and make effective support at the joint extremely difficult. Moreover, collapse of the flight deck through action damage to its supports becomes a possibility.

3. The subject of open-sided closed hangar cannot be separated completely from that of a single versus double hangar. The open-sided hangar practically necessitates a single hangar and there are advantages in a double hangar which need not be discussed in this memorandum.

4. Summing up under this Heading the closed hangar wins easily particularly in a ship with an armoured flight deck.

C.Protection against Projectiles.

1. It does not require a great stretch of the imagination to regard the hangar of an aircraft carrier as a space equivalent to one in a battleship where the magazine, shell room and gun house of the main armament are all embodied. The protection of such a space is obviously a requirement. In the latest designs of both American and British Aircraft Carriers armour is provided on the hangar roof. To leave hangar protection at that and accept open sides seems utterly illogical. The hangar is then vulnerable to splinters, bombs, blast and even gunfire from aircraft. In this respect it should be kept in mind that projectiles from aircraft may not in the future have trajectories with a small angle of the vertical.

2. If a bomb bursts in an open-sided hangar the blast effect will be less than in a closed hangar. But a burst in the hangar is more likely if the hangar has open sides. Such damage to both types of ship has been experienced in this war. [U.S.S.] ENTERPRISE (open-sided hangar) was hit by a bomb which penetrated the ship for a distance of 42 ft. causing damage which included a bulge up to 16” deep over a large area of the Hangar Deck. ILLUSTRIOUS (closed hangar) was hit by a bomb which burst in the hangar and buckled and depressed the forward end of the hangar deck; more serious still this bomb put the forward lift out of action, the armoured doors at the forward end of the hangar being open. In the new fleet carrier design the hangar is divided by a splinter-proof partition and the doors at the hangar ends will be power-operated so that they can be kept closed in a bombing attack and quickly opened afterwards. These two examples are mentioned to illustrate that fact that with neither system will damage to structure be avoided if a bomb bursts in the hangar. In both cases it is possible to provide arrangements which may keep a part of the hangar useful after such an occurrence. It is to be noted that nothing short of a completely open side will minimise blast pressure from a burst inside the hangar.

3. Again this matter brings in the question of a single or double hangar. The former practically follows the adoption of an open-sided hangar, whereas if closed, a double hangar is easily arranged. A double hangar and consequently deeper ship is preferable from the standpoint of protection against bombs.

4. The above remarks apply principally to the Fleet Carrier. In the Light Carrier no armour protection is provided. Nevertheless the sides of a closed hangar afford some degree of protection from blast and splinters in this class of ship.

5. Summing up under this heading the closed hangar wins easily.

D.Fire Risk

1. So long as naval aircraft are fitted with petrol-driven engines the danger of fire will be a most serious risk in Aircraft Carriers. The principal fire risk is liquid petrol accompanied by a spark or flame to set it alight.

2. Aircraft are serviced with aviation spirit in the hangar. Other things being equal it is safer to carry out this operation in an open space and open-sided hangar is therefore preferable to a closed one. But other things cannot be equal. In an open-sided hangar extending out to the ship’s side the hangar becomes a general working space not under magazine regulations and giving access to important compartments. The accident of fire is more likely than in a closed space under magazine regulations. There is a greater chance of fire spreading to other parts of the ship and even if this does not occur work in the important compartments below may be very adversely affected.

3. Setting aside considerations or accidents or negligence a petrol fire cannot occur unless there exists at the same time –
(a) An inflammable mixture of petrol and air;
(b) A spark, flame or a sufficient temperature to set the mixture alight.
Tests have been carried out recently in INDOMITABLE which show that under working conditions in the Hangar an inflammable, or explosive mixture of petrol or air is unlikely to be present. The following is an extract from the Report of the Admiralty Chemist:
“Although both during and after fuelling of the aircraft, particularly of the reserve tanks of Seafire aircraft in the Upper Hangar, an appreciable odour of petrol was apparent, it is observed that in all cases, both in the actual vicinity of the fuelling aircraft and in other parts of the Hangar the actual percentage of petrol vapour present in the Hangar air was very low. In no case did the petrol concentrations exceed 0.2% by volume, and in most cases was of the order of 0.05% or less thus being well below the minimum explosive limit of 1/ 5%.”

4. The above leads to the conclusion that unless the previous heading there is a greater likelihood of incendiary material reaching an open-sided hangar than a closed one. It is even possible to visualise the heroic suicide [aircraft] diving into the open hangar.

5. Summing up under this heading there is not much to choose between the open-sided and closed hangar, if anything the latter wins.