In 1996, the U.S. Navy issued a mission needs statement for a 21st-century tactical aviation sea-based platform. This document led to the establishment of the Future Aircraft Carriers (CVX) program, with the purpose of designing the platforms that will replace the Enterprise (CVN-65) and the Nimitz (CVN-68) classes starting in 2013. As conceived, this program was to start with a "clean sheet" and explore new and nontraditional possibilities, as well as evaluate more conventional concepts. With the decision in the fall of 1998 to pursue an "evolutionary" as opposed to "revolutionary" design process for what is now referred to as CVNX, the Navy has narrowed the spectrum of potential changes. But there remain significant cost- and performance-related design decisions to be made before the first CVNX keel is laid in 2006.
Cost versus Performance
When a warship is developed, tradeoff studies in cost and performance are required to balance what the fleet needs with what the engineers and accountants can provide. An example study common to almost any weapon system is that of offensive-versus-defensive capabilities. A carrier can be designed to maximize its firepower (in the form of aircraft sorties generated) at the expense of its own self-protection, or vice versa. This particular tradeoff is critical, because many design features that increase survivability can have a negative impact on the ability of the carrier to generate sorties. These performance characteristics also must be balanced against their cost.
In the course of the CVNX program, Navy decision makers have been asked to consider a variety of cost saving measures, such as decreasing flight deck size and reducing top speed. In the survivability area, they include such considerations as deleting some armor protection and reducing the number of defensive systems (weapons, sensors, and countermeasures). These measures have been characterized as "austere ship" concepts.
As a counter, design concepts with quantum improvements in survivability also are being evaluated. These include advanced armor protection and damage control, incorporation of state-of-the-art combat systems, and signature reduction. These designs are referred to as "robust" concepts.
But cost-performance analysis of these conceptual designs, even with the aid of computer simulations, is an often difficult and subjective process. As a consequence of strategy, tactics, and good fortune, the survivability concepts designed into carriers as far back as the Midway (CVB-41) have never been put to the ultimate test of open ocean combat with a capable foe. One must go back to 1945 to find actual combat survivability data for aircraft carriers. Experience is a major driver of design, and during World War II, several different classes of fleet carrier had their interpretations of "austere" and "robust" rigorously evaluated. Early conversions and experimental concepts gave way to more advanced designs, which became stronger as the hard-learned lessons of combat became understood. Of the combatant nations, the United States and United Kingdom were most able to take advantage of these lessons and incorporate improvements, so that by 1945, their carrier designs had the benefit of six years of combat experience.
But as these lessons recede into the pages of history, the cost aspect of cost-performance tradeoffs has taken preeminence, making austere concepts highly attractive. For that reason, it might be useful to review how different design philosophies (not necessarily design features) fared under intense combat conditions.
What Is Survivability?
Survivability is made up of two components: susceptibility and vulnerability. Vulnerability is a measure of how well a ship can withstand a hit. During World War II and today, this is a function of armoring, damage control, and repair. Susceptibility is a measure of how well a ship can avoid being hit. In World War II, a carrier's susceptibility was more a function of naval tactics and doctrine than of ship design. It was driven by the protection provided in turn by the air wing, the guns of the escorting cruisers and destroyers, and finally the carrier's own gun batteries, speed, and maneuverability. In the 1990s, susceptibility is greatly reduced by advances in signature management and electronic countermeasures. To compare design philosophies, susceptibility and vulnerability features of World War II carriers will be characterized as either robust or austere in the context of the threats they faced at the time.
British Aircraft Carriers
The Royal Navy (RN) started the war with six carriers, only two of which were "keel-up" designs. The first U.K. fleet aircraft carrier was the Furious, a light battle cruiser conversion. She and her sisters, the Glorious and the Courageous, carried between 33 and 48 aircraft. As battle cruisers they relied on speed to avoid threats and were not heavily armed or armored. Fast in comparison to their contemporaries when launched in 1918, this was not the case in 1939. In addition, the aircraft they carried were not as capable as their land-based counterparts—a feature that hobbled British carriers until they began using U.S. aircraft later in the war.
With these limitations, the Furious and her sisters should be classified as austere for both susceptibility and vulnerability by World War II standards. Their combat careers bear this out. The Courageous was hit by three torpedoes and sunk during the first month of the war. The Glorious was cornered by the German battle cruisers Scharnhorst and Gneisnau during the evacuation of Norway. Lacking the speed to run, unable to launch aircraft, and escorted by only two destroyers, she has the distinction of being the only fleet aircraft carrier to have been sunk by surface gunfire.
Another conversion, the Eagle was laid down as a battleship. Like her cousins in the Furious class, she was an austere ship, carrying only 21 aircraft at a top speed of 24 knots. She operated in the Indian Ocean on raider patrol and troop convoy escort duties early in the war. She was transferred to the Mediterranean in 1940, and in that high threat environment it was only a matter of time before her limited survivability features were put to the test. The Eagle was torpedoed and sunk during the relief of Malta, on 11 August 1942.
The first RN carrier designed and built as such, the Hermes carried only 12 aircraft at a speed of 25 knots. Not a true fleet carrier, her intended mission was scouting. Austere in terms of both susceptibility and vulnerability, she was hindered by her small size even during peacetime, and was relegated to China Station duty soon after commissioning. She was bombed and sunk by Japanese carrier aircraft off Ceylon on 9 April 1942. She was carrying no aircraft at the time, but it is doubtful that her small contingent of Swordfish torpedo bombers would have proved much help against the modern Japanese planes that sent her to the bottom.
The Ark Royal, the first RN ship designed and built as a true fleet aircraft carrier, could carry 72 aircraft, but in reality never carried more than 54—the result of a decision by the Royal Navy to trade an increase in susceptibility for a decrease in vulnerability. In contrast to their U.S. allies, the Royal Navy generally did not park aircraft on carrier flight decks, choosing instead to shelter them in enclosed hangars underneath an armored flight deck. This protected the aircraft from damage and heavy weather, but it greatly restricted air wing size and the ability to generate sorties rapidly. Despite this, the Ark Royal was a generally robust design, well armed and protected. Her Achilles' heel was the arrangement of her boiler exhausts, which proved fatal when she was hit by one torpedo on 13 November 1941 off Gibraltar. The hit resulted in slow flooding, which choked off the boiler uptakes and immobilized the ship. Salvage and towing failed, and the Ark Royal eventually capsized and sank 14 hours after being hit.
The Illustrious class was built in anticipation of conflict and operations within range of land-based bombers. Design tradeoffs were weighted heavily in favor of vulnerability reductions, again to the detriment of carrying and operating aircraft. To compensate, the class carried twice the number of guns as U.S. ships, plus heavy armor. They incorporated an armored "box" around a single hangar deck and were designed to carry 43 aircraft, a number later increased to 54 with the addition of an extra half hangar. This feature allowed hangar bay fires to be isolated from the rest of the carrier. These ships were robust designs from a vulnerability standpoint, but their small air wings make it impossible to consider them as truly robust in terms of susceptibility. Their combat careers show them as capable of withstanding a great deal of punishment (see Table 1), but they were forced to endure many attacks, first in the Mediterranean then in the Pacific. It is interesting to note, however, that although above-water hits from bombs or kamikazes generally put their U.S. counterparts of the Essex class out of action, the carriers of the Illustrious and the later classes were able to continue operating with only minor damage.
The next two British carrier designs were modifications of the Illustrious. To increase air wing size, the Indomitable featured an additional hangar level, which was added at the expense of some armor plating. She was followed by the Implacable class, in which the second hangar bay was lengthened and the armor replaced. These ships were robust from a vulnerability standpoint, and once their air wings were equipped with modern aircraft they approached robust in terms of susceptibility as well. The only problem was that they never carried enough aircraft, and still were not capable of operating them at the rate enjoyed by the U.S. Navy. In belated realization of this deficiency, the United Kingdom elected on its final carrier design (the Malta class) to follow the U.S. philosophy of open hangars and armored decks. But the end of the war and the accompanying reductions in military expenditures kept the Malta class from progressing beyond the design stage.
U.S. Aircraft Carriers
The U.S. Navy started World War II with seven fleet carriers, all but two being "keel-up" designs. One advantage that the U.S. Navy held over its British counterpart was greater control over the development of its naval air arm. U.S. carrier air wings were numerically larger and equipped with comparatively superior aircraft, and their flight deck procedures were more efficient. The quantum improvement in susceptibility meant that U.S. carriers were far less prone to having their vulnerability reduction features put to the test.
The first U.S. fleet carriers, the Lexington (CV-2) and Saratoga (CV-3), were battle cruiser conversions and were the largest carriers in the world until the Midway class. Because of their large air wings, speed, and self-defense batteries, they can be considered robust from a susceptibility standpoint, but mid-range or less from a vulnerability aspect. Their combat careers show mixed results. The Lexington was struck by three bombs and two torpedoes during the Battle of the Coral Sea. Damage initially was controlled, but fumes from ruptured fuel lines ignited and the resulting fires rapidly became uncontrollable. She was abandoned, then sunk by U.S. torpedoes. The Saratoga was torpedoed twice during 1942, and the resultant damage kept her out of action until 1943. After suffering additional damage in early 1945, at the close of hostilities she was deemed unfit for further service and was used as a target ship for Operations Crossroads atomic bomb tests. In this ultimate test of her vulnerability features, she survived the first test blast, but was sunk by the second test on 25 July 1946.
The Ranger (CV-4), the first U.S. ship designed and built as a carrier, was an attempt to build a smaller, more efficient ship that could operate almost as many aircraft as the Lexington on much less displacement and at lower cost. The Ranger carried 86 aircraft but was handicapped by very small growth margins. This overall austere design did not prove successful—she was too slow to operate with the fleet and could not launch aircraft in heavy weather. She participated in Operation Torch and raids into Norway, but never operated with the fast carrier task forces in the Pacific theater.
The next U.S. keel-up fleet carrier design was the three-ship Yorktown (CV-5) class. The maximum size allowable under Washington Naval Treaty restrictions, this class featured armor plating, though this was on the hangar deck, not the flight deck as with their British counterparts. Two design flaws would prove significant—their hangar ventilation was connected to ship ventilation (which would allow hangar bay fires to spread through the rest of the ship), and they had insufficient underwater protection. The Hornet (CV-8) had minor improvements compared to the first two ships of the class, including upgraded antiaircraft armament. From a vulnerability standpoint, they were mid-range ships (except for the Hornet, which approached robust), but their heavy defensive armament, high top speed, and two fighter squadrons made them robust from a susceptibility standpoint. The combat careers of these three ships are well documented. They fought in a host of large-scale sea battles and all absorbed tremendous damage (see Table 2). It is of note that the two that were sunk—the Yorktown and Hornet—received their kill shots from torpedoes. The Hornet, with increased underwater protection, took a total of 16 Japanese and U.S. torpedoes before sinking.
The Wasp (CV-7) was a small ship meant to use up the remaining carrier tonnage allowed under treaty restrictions. An austere ship based on a scaled-down Yorktown, she had even less underwater protection, a design flaw that would prove fatal. After brief service in the Atlantic, she was sent to the Pacific to fill the gap left by the losses of the Lexington and Yorktown. While operating against Japanese forces in the Guadalcanal area on 15 September 1942, she was struck by three torpedoes from the submarine 1-19. Gasoline fires rapidly became uncontrollable, and the Wasp was abandoned and scuttled by U.S. destroyer torpedoes.
The Essex (CV-9) class originally was conceived as a Yorktown modified to include better underwater protection. As war drew near and treaties became less of an issue, however, this design grew into a larger, powerful, and versatile weapon. The susceptibility versus vulnerability philosophy embodied in this class was in marked contrast to their British sisters. The Essex-class carriers depended on the capabilities of their air wings, escorts, speed, and damage control to survive and did not incorporate the heavy armor (and attendant small air wings) of the Illustrious class. Consequently, they were not hit often, but when they were, they frequently were out of action for many months. None sank, but two—the Bunker Hill (CV-17) and the Franklin (CV-13)—sustained enough damage to put them out of service permanently. Other ships of the class received varying degrees of damage from kamikazes later in the war, including the Essex, Intrepid (CV-11) (four times, twice with severe damage, plus one torpedoing), Ticonderoga (CV-14), Randolph (CV-15), Lexington (CV-16) (plus one torpedoing), Wasp (CV-18), and Hancock (CV-19).
When the Illustrious was undergoing repairs in Norfolk during 1941 her armored flight deck attracted a great deal of interest, especially among U.S. naval architects. The U.S. Navy embraced the armored deck for the class that followed the Essex, but eager to retain the rapid turnaround rates of previous designs, it rejected the enclosed hangar, placing instead 5-inch gun batteries below the flight deck on either side. What began as armored versions of the Essex class doubled in tonnage to become the Midway class. But by the time these supercarriers put to sea, the war was over.
Conclusion
Table 3 summarizes fleet carrier losses during World War II. Note that only one ship featuring robust vulnerability reduction features was sunk, and this was because of design flaws. In addition, several escort carriers, plus the light carrier Princeton (CVL-23), succumbed to attacks during the war, but these ships were by definition austere designs.
The consequences of austerity in aircraft carrier survivability, especially in the area of underwater protection, are evident. The likelihood of sea combat on the scale seen in World War II may be small, but the lessons of history weigh in strongly on the side of robust survivability features. Austere ships in that conflict—the Hermes, Ranger, Wasp, and Eagle—or ships that did not provide a balance of susceptibility and vulnerability did not pass their ultimate cost-performance tests. With the proliferation of sophisticated and lethal anticarrier torpedoes and cruise missiles, coupled with reduced numbers of escorts, the possibility of combat damage to CVNX is real. As long as the mission of the U.S. Navy's carriers requires them to go in harm's way, such austerity is not affordable.
Mr. Lillard is a civilian contractor with Whitney, Bradley & Brown, Inc., working for the CVNX Program Office in the requirements development section. His area of emphasis has been survivability requirements.