Unfortunately, the easiest way to make a more efficient engine, nuclear-electric propulsion, is controversial.

ECONOMIST: Galileo and Jupiter

While the ship will not travel at warp speed, with an ion propulsion engine powered by a 1.5GW nuclear reactor, it can travel at a constant acceleration so that the ship can easily get to key points of interest in our solar system.

MSN: Spaceship Enterprise in 20 years? Beam me up!

Every night, it shuts down its engine for eight hours to recharge its batteries with its nuclear generator.

NEWYORKER: The Martian Chroniclers