Storm drive

A storm drive was a powerful engine which used nuclear fission to produce energy to power a starship. The destruction of a storm drive would cause a major energy release, equivalent to an atomic bomb, and would leave massive amounts of residual radiation.

There was a storm drive powering the Titanic and, without it functioning, the ship would lose orbit. In a parallel world, the engines failed and the Titanic fell on London, destroying it and poisoning all of Southern Wales. (TV: Turn Left) In the original timeline, the Doctor managed to use the heat of reentry into Earth's atmosphere to fire up a secondary storm drive and return the Titanic to orbit. The secondary storm drive was enough to keep the ship in orbit until rescue could arrive. (TV: Voyage of the Damned)