A nuclear saltwater rocket is a concept for an advanced spacecraft propulsion system, proposed by Doctor Robert Zubrin. It uses water as propellant and uranium tetrabromide (a water-soluble salt) as fuel. The uranium used is enriched, between 20% and 90% 235U. This enriched uranium salt is dissolved in the water and stored in neutron-attenuating boron tubes to keep the fuel storage below critical mass. The rocket works by injecting the fissile saltwater into a reaction plenum, creating a critical mass, which leads to a runaway nuclear reaction that heats the water, flashing it to steam and accelerating it to thousands of meters per second. A conservative configuration using 20% 235U and managing to fission 0.1% of the fissionable fuel would yield an exhaust velocity of 69 kilometers per second, according to figures posited by Zubrin. This yields a specific impulse of around 7000 seconds, making the engine 15.5 times more efficient* than the Space Shuttle's main engines, which are in turn just about the most efficient chemical rockets possible. A ship equipped with this sort of engine, having a mass ratio of 5, which is far less than any modern spacecraft, would have enough delta-v to take off from Earth, do a Hohmann transfer to Pluto and back twice, then land on Earth again. Impressive, no?