Emergency thruster
From Halopedia, the Halo wiki
Summary
Emergency thrusters are standard equipment on UNSC vessels. They are usualy placed on strategic locations within hardened points of the outer hull of the vessel. They are tanks of tryhydride tetrazene and hydrogen peroxide. When the the two substances mix, they do so explosively, generating a force that literally blasts a vessel onto a new course. However the Thruster is expended in the process, and must be replaced in a ship dock before it can be used again.
The UNSC Iroquois had six of these devices on it. Two of them were used during The Battle of Sigma Octanus IV, the first during the performance of the "Keyes Loop" in order to cause several plasma torpedoes to hit a Covenant destroyer, the second time to blast the Iroquois out of the gravity well of Sigma Octanus IV. The Pillar of Autumn also had several of these, though it quoted "The left side emergency thrusters were missing". They were used during an attack on a Covenant sniper ship to avoid the fast-as-light energy beams.
