From Halopedia, the Halo wiki
The fusion rocket is a type of UNSC warship ordnance that uses a scaled-down fusion drive for thrust. While unguided, they can reach immense velocities in seconds because of their low mass. Only one-third of human vessels carry such weapons and there are never more than three fusion rockets aboard a vessel because of the prohibitive cost of building the fusion propulsion system. Additionally, these rockets proved to be inefficient against Covenant energy shielding, and are considered impractical because of their lack of a guidance system. The UNSC typically used these to lay waste to relatively stationary targets, such as orbiting space stations.
- Halo Encyclopedia (2009 edition), page 315 (2011)