Impulse packs were small devices integral to the function of impulse engines.

In 2265, the Starship Enterprise's starboard impulse pack was severely damaged due to an encounter with the galactic barrier. Gary Mitchell, who had gained incredible psychic powers due to his exposure to the barrier, informed Lieutenant Lee Kelso of the damaged pack, and warned him that, if they tried to activate the pack in the state that it was in, they would blow out the entire impulse deck. (TOS: "Where No Man Has Gone Before")