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The impulse reactor was the component that creates impulse power in an impulse engine aboard a spacecraft. Typical impulse reactors for Federation starships were fusion power plants. On occasion, impulse reactors needed to be purged. (ENT: "The Seventh") A vessel's velocity could be corresponded to impulse reactor output. (VOY: "Elogium")

Y-500-class freighters were equipped with impulse reactors. According to the ghost story told by Travis Mayweather, George Webb "locked himself in engineering and overloaded the impulse reactors. He almost destroyed the ship. Then he sealed himself in an life pod and ejected it." When asked by Crewman Cutler if the captain went after him, Mayweather explained that he couldn't because "the reactors were too badly damaged." (ENT: "Strange New World")

In 2152, a cloaked Romulan mine was attached within five meters of Enterprise NX-01's impulse reactor 2. (ENT: "Minefield")

A deflector pulse created by tying in the deflector array directly into the impulse reactors has the potential output to disrupt a subspace links. An unfortunate side effect of this hook-up was that it would likely rupture every EPS conduit on the ship. (ENT: "Countdown")

In 2355, prior to the Battle of Maxia, the Chief Engineer of the USS Stargazer reported completion of the normal maintenance on the #2 impulse reactor a full day ahead of schedule. Captain Jean-Luc Picard was hopeful, with the successful recertification of the reactor, that the starship would be operating on full impulse capacity before next week. (TNG: "The Battle", okudagram remastered)

Forcing an EPS discharge through the impulse reactor would cause an explosion that could destroy the saucer section aboard a Galaxy-class starship. (TNG: "Force of Nature")

In 2371, Lieutenant B'Elanna Torres proposed to modify USS Voyager's auxiliary impulse reactor into a crude dilithium refinery, a clear violation of Starfleet protocols, after Voyager had detected what appeared on be dilithium deposits on a rogue planetoid. (VOY: "Phage")

The shuttle Alice, acquired by Tom Paris, required heavy repairs before it was spaceworthy, including the replacement of the impulse reactor, navigational array, and plasma manifolds. (VOY: "Alice")

When Voyager got caught on a "subspace sandbar" the crew tried to escape with full impulse power, however, the attempt caused the impulse reactor to start "running hot." (VOY: "Bride of Chaotica!")

When Voyager became disabled in 2378, it was necessary for the crew to restart the impulse reactors. (VOY: "Renaissance Man")