Fission fragment rocket engines are also (theoretically) extremely efficient.. using the particles that come out of the reaction directly as a high-velocity exhaust stream.

▲

drivebyhooting an hour ago | parent [-]

FFR have super high ISP but microscopic thrust. Not exactly practical.

	▲	db48x 22 minutes ago \| parent [-]
		Practical for many missions, just not for sending humans in person to other planets. For example, consider a mission to deliver a telescope out past 550AU so that it can use the whole sun as a gravitational lens. That’s more than 11× further than Pluto, but a fission fragment drive could get the telescope out there in about the amount of time it took New Horizons to fly past Pluto.