Halcyon-class

The Halcyon-class Cruiser (Pronounced 'hal-see-on') (known as Human Attack Ship class C-II by the Covenant) was a United Nations Space Command cruiser starship classification designed by Dr. Robert McLees in 2510, with a Hull Classification Symbol of SCS. Their service lives were short due to a unique hull design that was costly both to construct and to maintain, lack of speed and poor armament, and concerns about their tactical viability. They were subsequently replaced by the Marathon-class Cruiser. Despite the inherent disadvantages of the design, if upgraded appropriately and handled well, they could be among the most effective, and likely the most expensive, ships in the UNSC Navy.

Background
At 1.17 (a reference to the number 117) kilometers in length and one-third the tonnage of the succeeding Marathon-class Cruiser, 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 Cortana they were somewhat of a joke within the fleet.

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 UNSC Pillar of Autumn when she survived a crash-landing on Alpha Halo.

When the Human-Covenant War erupted in 2525 a number of Halcyon cruisers were reactivated from storage and refitted.

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 ships' 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 speculation that the gun is somewhere else on the ship is proved false in the cut scene at the beginning of the mission, The Maw, where the camera circles the ship, showing the obvious barrel in the 'bump'.

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. Workers go in and out of the corridors, as seen in Halo: Combat Evolved. 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

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 MAC Cannon 
 * Six Archer Missile Pods which were far below standards at the time. Each pod would have contained several individual missiles.
 * Four Shiva Nuclear Warheads
 * Numerous 50mm point defense guns
 * One upgraded MAC 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 Auto-cannons with overlapping fields of fire for point defense against single ships.

Powerplant
Sublight thrust is provided by nuclear fusion reactions which are channeled to six adjacent exhausts at the ship's exterior stern. There are six exhausts visible, two large and four small. These engines are likely powered by deuterium-deuterium reactions. The main engine core consists of several underpowered Mark II reactors which provided 10% of modern engine output. The class was also equipped with a standard and typical Slipspace drive for translight speeds. On the refit ships, the reactor system was one of the aspects which was heavily retrofitted.