Question: When were the Essex class carriers available?
About the article "Battle of Midway", section "Impact".

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The "Nimitz"-class aircraft carriers were ordered to supplement the aircraft carriers of the "Kitty Hawk" class and "Enterprise" class, maintaining the strength and capability of the U.S. Navy after the older carriers were decommissioned.
The ships were designed to be improvements on previous U.S. aircraft carriers, in particular the "Enterprise" and "Forrestal"-class supercarriers, although the arrangement of the ships is relatively similar to that of the "Kitty Hawk" class.
Among other design improvements, the two reactors on "Nimitz"-class carriers take up less space than the eight reactors used on "Enterprise".
Along with a more generally improved design, this means that "Nimitz"-class carriers can carry 90% more aviation fuel and 50% more ordnance when compared to the "Forrestal" class.
The U.S. Navy has stated that the carriers could withstand three times the damage sustained by the "Essex" class inflicted by Japanese air attacks during World War II.
The hangars on the ships are divided into three fire bays by thick steel doors that are designed to restrict the spread of fire.
This addition has been present on U.S. aircraft carriers since World War II, after the fires caused by Kamikaze attacks.
The first ships were designed around the time of the Vietnam War, and certain aspects of the design were influenced by operations there.
To a certain extent, the carrier operations in Vietnam demonstrated the need for increased capabilities of aircraft carriers over their survivability, as they were used to send sorties into the war and were therefore less subject to attack.
As a result of this experience, "Nimitz" carriers were designed with larger stores of aviation fuel and larger magazines in relation to previous carriers, although this was partly as a result of increased space available by the new design of the ships' propulsion systems.
A major purpose of the ships was initially to support the U.S. military during the Cold War, and they were designed with capabilities for that role, including using nuclear power instead of oil for greater endurance when deployed in blue water, and the ability to make adjustments to the carriers' weapons systems on the basis of new intelligence and technological developments.
They were initially categorized only as attack carriers, but ships have been constructed with anti-submarine capabilities since .
As a result, the ships and their aircraft are now able to participate in a wide range of operations, which can include sea and air blockades, mine laying, and missile strikes on land, air and sea.
Because of a design flaw, ships of this class have inherent lists to starboard when under combat loads that exceed the capability of their list control systems.
The problem appears to be especially prevalent on some of the more modern vessels.
This problem has been previously rectified by using damage control voids for ballast, but a solution using solid ballast which does not affect the ship's survivability has been proposed. 
An armoured flight deck is an aircraft carrier flight deck that incorporates substantial armour in its design.
Comparison is often made between some of designs of the Royal Navy (RN) and the United States Navy (USN).
The two navies followed differing philosophies in the use of armour on carrier flight decks starting with the design of the RN's "Illustrious" class and ending with the design of the "Midway" class, when the USN also adopted armoured flight decks.
The two classes most easily compared are the RN's "Illustrious" class and "Implacable" class and their nearest USN contemporaries, the "Yorktown" and "Essex" classes.
The "Illustrious" class followed the "Yorktown" but preceded the "Essex", while the "Implacable" class design predated the "Essex" but these ships were completed after the lead ships of the "Essex" class.
The development of armoured flight deck carriers proceeded during World War Two and before the end of World War Two, both the USN, with USS "Midway", and the Imperial Japanese Navy (IJN), with "Taiho" and "Shinano", would also commission armoured flight deck carriers, while all USN fleet aircraft carriers built since 1945 feature armoured flight decks.
The remainder of the IJN carrier force during World War II had unarmoured flight decks just like the Yorktown and Essex classes of the USN. 
The "Independence"-class aircraft carriers were a class of light carriers built for the United States Navy that served during World War II.
This class of ship were constructed as a result of the interest of President Franklin D. Roosevelt.
With war looming, the former Secretary of the Navy noted no new fleet aircraft carriers were expected to be completed before 1944.
He proposed to convert some of the many cruisers then under construction to carriers.
Studies of cruiser-size aircraft carriers had shown the type had serious limitations, and on October 13, 1941, the General Board of the United States Navy replied that such a conversion showed too many compromises to be effective.
Undeterred, President Roosevelt ordered another study.
On Oct 25th a second study from BuShips reported that the cruiser conversion aircraft carrier would result in a vessel of lesser capability, but they would be available much sooner.
Following the December 1941 Pearl Harbor attack the need for more carriers became urgent.
In January 1942 The Navy responded by greatly accelerating construction of the 34,000 ton Essex class aircraft carriers, and ordering the conversion of a Cleveland class light cruiser then under construction to completion as a light aircraft carrier, becoming the USS "Belleau Wood" (CVL-24). 
The "Essex" class was a class of aircraft carriers of the United States Navy, which constituted the 20th century's most numerous class of capital ships with 24 vessels built in both "short-hull" and "long-hull" versions.
Thirty-two were originally ordered; however as World War II wound down, six were canceled before construction, and two were canceled after construction had begun.
No "Essex" class ships were lost to enemy action, despite several vessels sustaining very heavy damage.
The "Essex"-class carriers were the backbone of the U.S. Navy's combat strength during World War II from mid-1943 on, and along with the addition of the three "Midway"-class carriers just after the war continued to be the heart of U.S.
Naval strength until the supercarriers began to come into the fleet in numbers during the 1960s and 1970s. 
In World War II, the United States Navy produced a number of light carriers by converting cruiser hulls.
The "Independence"-class aircraft carriers, converted from "Cleveland"-class light cruisers, were unsatisfactory ships for aviation with their narrow, short decks and slender, high-sheer hulls; in virtually all respects the escort carriers were superior aviation vessels.
The "Independence"-class ships, however, had the virtue of being available at a time when available carrier decks had been reduced to and in the Pacific and in the Atlantic.
In addition, unlike escort carriers, they had enough speed to take part in fleet actions with the larger carriers.
Late in the war, a follow on design to the "Independence"-class, the "Saipan"-class, was designed.
Two vessels in this class— and —were completed after the war's end.
After very brief lives as carriers, the "Saipans" were converted to command and communication ships.
The British 1942 design light fleet carrier was a scaled-down version of their "Illustrious"-class fleet carrier.
The design could be built in a yard with little or no experience of warship construction.
Although built to merchant standards, the design incorporated better watertight subdivision.
Expected to have a lifetime of about three years, the last of the design was taken out of service in 2001.
The first eight were built as the "Colossus" but revisions upgrading the design to handle larger and heavier aircraft led to the remainder being the "Majestic" class.
In the post-war period, the Royal Navy operated a force of ten "Colossus" class carriers including the two maintenance carriers.
In all, fifteen ships were completed from the 1942 design, of which most of the "Colossus" class and all the eventually completed "Majestic"s were variously sold to Argentina, Australia, Brazil, Canada, France, India and the Netherlands.