Halcyon-class light cruiser

"A dozen Covenant superior battleships against a single Halcyon-class cruiser. Given those odds, I'm content with three... make that four kills."

- Cortana

The Halcyon-class light cruiser, known as Human Attack Ship class C-II by the Covenant, is a United Nations Space Command cruiser starship classification designed by Dr. Robert McLees in 2510, with a hull classification symbol of C. Their service lives were short due to a unique hull design that was costly to construct and maintain with low maneuverability, poor armament, and concerns about their tactical viability. They were replaced by the. Despite the disadvantages of the original design, the heavily modified proved to be one of the most effective, and costly, ships deployed in the war.

History
At just over 1.17 kilometers in length and one-third the tonnage of the succeeding Marathon class, the Halcyons were the smallest of vessels to be designated and structured as cruisers. Compared to later ships, the original Halcyons were sorely underpowered and under gunned: their Mark II fusion engines provided but a tenth of the output of modern reactors and their armament consisted of a mere six Archer missile pods and a single MAC gun. According to the AI Cortana, these ships were considered laughable within the UNSC fleet.

When the Human-Covenant War erupted in 2525 a number of Halcyon cruisers were reactivated from storage and refitted. With the right upgrades, a Halcyon-class cruiser could be a formidable foe. During the Fall of Reach, the upgraded was able to inflict damage far beyond its tonnage, engaging and destroying Covenant frigates, a  and a Covenant supercruiser. Later, in orbit above the Foreunner construct known as Halo, the Autumn destroyed four additional CCS-class cruisers under the guidance of the AI Cortana. While much of this success was due to the tactical genius of Captain Jacob Keyes and his crew, the upgraded Autumn was highly resilient and capable of unleashing tremendous artillery barrages.

The ship's main Magnetic Accelerator Cannon was enhanced to fire in three round bursts. While each individual round was weakened, this firing pattern typically ensured at least one projectile would penetrate a Covenant vessel's shields. Damaged, the targeted ship could then be overwhelmed with Archer missiles, nuclear weaponry, or further bombardment once the Autumn recharged its main batteries. Unfortunately, the ship and nearly all of its original crew were destroyed in an attempt to prevent the firing of the Halo array, and to prevent the release of the parasitic life-form known as the Flood. The detonation of the Autumn's fusion reactors destroyed not only the ring, but the majority of the Covenant flotilla which had pursued it from Reach.

By 2557, several Halcyons remained active during the postwar period, likely to fill a "cruiser gap" left by the destruction of many Marathon-class ships. The UNSC Infinity, the largest and most advanced ship in the post-war Fleet, had two rotating escort groups; one was comprised of seven Halcyon-class light cruisers following the vessel's commissioning.

Layout
Halcyon-class cruisers are composed of five hexagonal and octagonal sections mounted onto each other. The bridge is slung underneath one of the forward sections and eight docking bays are distributed throughout the ship. There are also slots for eighteen lifepods on the ship's hull. Because of the low number, these were likely used only as a last resort if Pelicans or other modes of transportation were not available. Lastly, the MAC cannon is visible as a bump on the bow.

The most noteworthy design feature was a series of internal cross bracings and honeycombs that made the ships surprisingly resistant to battle damage; ships of the class were able to remain operational despite sustaining breaches to all compartments and losing 90% of their armor. The durability of the design was demonstrated by the when she survived a crash-landing on Alpha Halo after taking a tremendous beating at Reach and over the ring. However, while the cruiser was intact, it was damaged beyond repair and could never fly again, although some systems were still online such as the fusion core, the engines, and the self-destruct. Another example of the ship's durability was during the Fall of Reach when the ship took on a Covenant supercruiser. Having wrecked havoc with Reach's defense fleet, this formitable Covenant warship breached the Autumn's hull with two piercing beam attacks, one of which severely damaged its reactors. Not only did the Autumn continue its engagement with limited loss of functionality, it destroyed the supercruiser after a protracted battle.

Corridors on the Halcyon-class were segmented by several retractable containment bulkheads. In the event of an emergency, these bulkheads could be closed and affected sections sealed from the remainder of the ship. A potential application of this was during a hostile boarding action, whereas the atmosphere in the sealed sections could be vented via commands from the bridge, asphyxiating enemy troops.

Armories were located throughout the ship, holding a selection of small arms and their requisite ammunition. Eight docking bays allowed a number of Pelican dropships, Longsword Interceptors, Warthogs and Scorpion Tanks to be carried.

A number of service corridors ran the length of the ship. Although normally restricted to personnel, they could be accessed by lifts in main engineering or connecting passageways from the docking bays. If necessary, these expansive corridors could permit passage to Warthogs and some sections were large enough to accept Pelicans. The corridors served a secondary purpose of lessening the ship's considerable mass, thereby increasing acceleration.

Halcyon-class cruisers are constructed in space and are not rated for atmospheric deployment; however, R7 thrust couplings allow them limited atmospheric operation capabilities, as demonstrated by the Pillar of Autumn which was capable of achieving liftoff from Aszod, supported by a series of rocket pods.

Armament
Known elements of the Halcyon-classes' original armament consisted of the following; With the outbreak of the Human-Covenant War, Halcyon-class ships were put through extensive refits. At least one ship, the Pillar of Autumn, had the following armament:
 * One Magnetic Accelerator Cannon 
 * Six Archer missile pods which were far below standards at the time. Each pod would have contained several individual missiles.
 * Four Shiva-class nuclear missiles
 * Numerous M910 50mm point-defense guns
 * One upgraded Magnetic Accelerator Cannon capable of firing 3 rounds in quick succession.
 * 300 Archer missile pods which were arranged in thirty columns and ten rows. Each pod contained 26 individual devices, equaling 7800 missiles.
 * Four Shiva nuclear warheads. One of these missiles was disguised as a Longsword fighter, which could be remote controlled.
 * Forty 50mm MLA autocannons with overlapping fields of fire for point defense against single ships.

Power plant
Sublight thrust is provided by nuclear fusion reactions which are channeled to two primary and twelve secondary adjacent exhausts at the ship's exterior stern. There are six exhausts visible, two large and four small. These engines are powered by reactions between hydrogen isotopes, including deuterium. The main engine core consists of a three underpowered Mark II Hanley-Messer reactors which provided 10% of modern engine output as of 2552.

On the refit ships, specifically the Pillar of Autumn, the reactor system was one of the aspects which was heavily retrofitted. The Autumn was installed with an experimental reactor architecture which used a set of primary and secondary reactors to greatly boost its power generation capabilities.

The class was also equipped with a standard Slipspace drive for translight speeds.

Ships of the line

 * - After refitting, the Autumn was selected as the SPARTAN-IIs' chariot for Operation: RED FLAG, due to its new resilience and firepower. Destroyed by John-117 in the Battle of Installation 04 through wildcat destabilization of the engines, as a means to destroy Installation 04 itself.
 * Numerous Halcyon-class cruisers were part of Admiral Preston Cole's fleet.
 * At least seven Halcyon-class cruisers, (seemingly refit like the Pillar of Autumn), escorted the in the summer of 2557.
 * Numerous Halcyon-class cruisers were part of Admiral Preston Cole's fleet.
 * At least seven Halcyon-class cruisers, (seemingly refit like the Pillar of Autumn), escorted the in the summer of 2557.

Trivia

 * "Halcyon" is derived from the Greek word "Alcyone", which means "marked by peace and prosperity".
 * The scrapped hulls of several Halcyon-class cruisers can be seen in the Halo: Reach campaign level The Pillar of Autumn and the multiplayer map Boneyard.

List of appearances

 * Halo: The Fall of Reach
 * Halo: Combat Evolved
 * Halo: Combat Evolved Anniversary
 * Halo: The Flood
 * Halo 2
 * Halo: Contact Harvest
 * Halo Wars
 * Halo Wars: Genesis
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * The Impossible Life and the Possible Death of Preston J. Cole
 * Halo Legends
 * Origins
 * Odd One Out
 * Halo: Reach
 * Halo: Fall of Reach
 * Boot Camp
 * Invasion
 * Halo 4