A few pounds of nuclear fuel can produce many megawatts of power for several years. Provided appropriate safety precautions are taken, NASA believes that space nuclear power systems are fully acceptable. All rockets use a principle called the reaction engine. Chemical rockets already run at temperatures that approach the limits of the materials in the combustion chamber and nozzle.

Chemical rockets have a low maximum velocity increment, which means that their exhaust velocities are not high enough to impart very high speeds to the rocket. A nuclear rocket such as this would be inherently safe and environmentally benign: contrary to popular belief, a nuclear rocket need not be strongly radioactive when launched. The spacecraft, with its nuclear thrusters, would be launched as a payload atop a conventional chemical rocket. Then, once the payload was in high-Earth orbit, the nuclear reactor would start up. For a given amount of propellant mass, the nuclear rocket will travel three times faster than the chemical rocket. The intense neutron and gamma ray radiation fields produced by the operating reactor are c

Rocket propellant efficiency is measured in specific impulse (Isp). It is a measure of the number of seconds that one pound of propellant can produce one pound of thrust. The most efficient chemical rocket is the space shuttle design where oxygen and hydrogen undergo the most basic combustion reaction producing H2O. The specific impulse of water is 462 (Damon, 1995, p. 21)

Stern, D. P. (1999). Far-out Pathways to Space: Nuclear Power.

Corliss, W. R. (1960). Propulsion Systems for Space Flight.

Someday, in exploring the outer planets of our solar system, humankind will want to do more than send probes that merely fly rapidly by them. In time, we will want to send spacecraft that go into orbit around these gaseous giants, land robots on their moons, and even return rock and soil samples back to Earth. Eventually, we will want to send astronauts to the planet's surface and to their moons (Powell, 1999). Nuclear thermal propulsion can provide an effective power source for a manned trip to Mars or an unmanned or manned trip to other planets in our solar system. A chemical rocket would be too large to supply enough power to make the return trip. Fusion power, photovoltaic, and even a solar wind powered sail have been considered. These technologies would require a great deal of research in order for them to become feasible and have a high chance of proving to be unacceptable or just impossible.

NASA Website (2001). "Space Nuclear Power Technology".

The intense neutron and gamma ray radiation fields produced by the operating reactor are clearly the main difficulty in using a nuclear thermal reactor on a manned mission. Shielding the crew from the reactor during the "propulsive burn" can be accomplished by the combination of a tungsten and LiH shield. Further, reduction in the neutron dose to the crew can be accomplished by incorporating a few meters of liquid hydrogen in a tank between the crew and engine. This tank for example might contain the 15% contingency liquid hydrogen and would be the last tank to be used. After the full power burn of the engine, the radiation from the reactor will be only gamma rays and within a few days, the intensity will have dropped by over three orders of magnitude.

http://www-spof.gsfc.nasa.gov/stargaze/Snucfly.htm

Nuclear rockets could impart a maximum velocity increment of up to about thirteen miles per second compared to only six miles per second from a chemical rocket.

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Approximate Pages = 9 (250 words per page double spaced)