Gaseous fission reactors are a hypothetical type of nuclear reactor proposed for use in space travel. One limitation on the specific impulse of conventional fission reactions in nuclear thermal rockets is that if the temperature is too high the reactor core melts. The solution is to create a reactor whose core is gaseous. the fission fuel is held magnetically in a the reactor so as to not touch and melt the reactor walls. This can create specific impulses of 3,000-5,000s which is good enough for fast interplanetary travel. Retaining the fission gas is a serious technical problem. One solution is to pump the fuel through quartz tubes along the wall of the reactor, the working fluid (fuel) is heated directly by thermal radiation given off by the fission gas. This "light bulb" reactor would have a very poor thrust to weight ratio and could not take off from Earth, the Moon or any other body with significant gravity, and its acceleration times would also be very long.
See also: spacecraft propulsion
Opening the Next Frontier