Fusion missile

Fusion missiles, also known as torch missiles, are a class of space weapons used by the UNSC from the mid-25th century onwards. Early fusion missiles were sometimes called fusion rockets. Unlike most missiles, which are powered by chemical rockets, fusion missiles are built around a rudimentary, single-use fusion drive. This allows them to achieve prodigious velocities and strike at long ranges, but also makes them far rarer and more expensive.

Ballista fusion missile
The Ballista was the first class of fusion missile introduced during the UNSC-CMA Cold War. Previously, equivalent missiles had used disposable nuclear thermal rockets, but the maturing of fusion engine technology had finally made it viable to contemplate the use of low-grade, fusion rockets compact and light enough to fit on a missile. Still, the Ballistas were large and bulky devices.

In many ways, the Ballista failed to meet the expectations set upon it. While conceptually, the miniature fusion torch was intended to grant the missiles with a far higher delta-v than conventional missiles, allowing them to pursue targets over long distances if need be, the early micro-fusion drives were too inefficient for extended maneuvering to be viable in most situations. This was largely due to the technological limitations of the time as well as the corners that had to be cut in design and manufacturing of the drives to make them cost-effective. Equipping a missile with an engine with the endurance of a ship-grade drive would be far too costly for a disposable munition.

Ballista missiles' tactical utility mainly came from the high acceleration granted by the fusion torch. However, their ability to significantly change their vector is limited following the initial acceleration burn, which limits their utility against enemy ships and (inaccurately) earned them a reputation as glorified dumb-fire munitions. As a result, they were mostly used against predictable targets, such as space stations or large ships at short to medium ranges.

The Ballistas were used as high-velocity payload delivery systems and missile buses, carrying smaller independent warheads over long distances to their targets. Depending on the use case, they were also equipped with nuclear or kinetic "shotgun" payloads intended to release a cloud of small metallic projectiles meant to shred targets.

Scorpio fusion missile
A successor to the Ballista, the Scorpio featured an enhanced fusion torch with thrust vectoring and an overall enhanced maneuvering capability. However, their expense still limited their deployment, and the Scorpio is generally seen as an intermediate stage between the Ballista and the more substantial design overhauls of the Hyperion and the Bident.

Hyperion fusion missile
The Hyperion missile is a large fusion missile developed as a successor to the Ballista lineage, albeit with a far superior maneuverability. The Hyperion series mostly uses nuclear, rather than kinetic, payloads, particularly models developed after the onset of the Human-Covenant War. Entering service in the later years of the Insurrection, various successive models of Hyperion missiles were in use during and after the war. The endurance, overall size, and cost of the Hyperion meant that it proved more useful in long-ranged attacks than fleet engagements, something emphasized with the final M4122 model's inclusion of a slipspace drive.

Bident fusion missile
A semi-parallel development to the Hyperion missile, Bident missile features a downscaled fusion torch and is optimized for maneuverability over speed. Improvements in drive technology and manufacturing meant they could be mass-produced at a lower cost than before.

Bident missiles are most commonly equipped with a nuclear-pumped X-ray laser warhead, one of the UNSC's two main forms of vacuum-enabled nukes alongside Casaba-howitzer shaped charges.