Fuel cell
From Halopedia, the Halo wiki
The Fuel Cell is a grey circle located behind the left wing on the Ghost in Halo 2 and [[Halo 3|3]. It provides the engines of the Ghost with it's exploding fuel. Worth noting is that an explosion can be trigged when hitting it with a single bullet from such weapons that are able to cause a Head Shot.
File:Halo 3 ghost angle.jpg
The fuel cell located above the left wing
