Son of Orion

NASA is working on a new propulsion system which promises a nearly 20 fold increase in ISP over chemical rockets and thrust levels in the tens of thousands of pounds by technologies used to detonate thermonuclear warheads.

Short version is that nuclear fusion at levels well below break even still generate a lot of neutrons, and these induce fission, which generates a plasma providing thrust. 

It sounds a lot like the Orion program from the early 1960s, which involved dropping nuclear bombs out the back of a spacecraft to shove it along its course:

Engineering research outfit Howe Industries is working with NASA to develop a new plasma-based propulsion system that might help solve the problem of moving around the solar system faster with bigger payloads.

Early studies suggest the pulsed plasma rocket (PPR) propulsion system could produce up to 100,000 N [22,481 lbf] of thrust within a 5,000-second impulse.

The concept has been developed from an earlier Pulsed Fission-Fusion (PuFF), but is smaller, simpler, and less expensive, according to NASA.

"The exceptional performance of the PPR, combining high [specific impulse] and high thrust, holds the potential to revolutionize space exploration. The system's high efficiency allows for manned missions to Mars to be completed within a mere two months," NASA said.

PuFF relies on fission-ignited fusion systems that have already been proved in thermonuclear weapons. But instead of a bomb, the aim is to produce a controlled jet of plasma.

These are indeed impressive numbers with the  ISP numbers on par with other high ISP propulsion systems like ion drives.

That being said, it seems to be an awfully bumpy ride.