From Halopedia, the Halo wiki
Emergency thrusters are standard equipment on UNSC vessels. They are often the reason a UNSC ship is able to escape a confrontation unscathed. These thrusters are used only in dire situations; for instance, to dodge enemy fire where the standard propulsion systems would not be able to do so. They consist of strategically placed tanks on the ship's outer hull, containing trihydride tetrazine and hydrogen peroxide. These substances, when mixed, produce significant explosive force to blast the ship 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 real-world term for this reaction would be a hypergolic propellant, which is used in some of today's modern spacecraft.
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 "the portside emergency thrusters were missing". However, this is a discrepancy, because they are later used twice.
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