Nuclear Fission is an attractive possibility for spacecraft propulsion as it offers a theoretical energy density of 8 x 10¹³ (10^13) J/kg, which is substantially higher than the most energetic chemical reactions which have an energy density of 1 x 10⁷ (10^7) J/kg. To put this another way the energy available from a unit mass of fissionable material is approximately 10⁷ (10^7) (i.e. 10 million) times larger than from chemical reactions.

From atomicachive.com

Nuclear fission is the splitting of an atom into several smaller fragments. This is induced by bombarding an atom of fuel (usually uranium-235 but could also be plutonium-239, uranium-233, and possibly thorium-232) with neutrons. The fissionable atom then captures the neutron and splits (decays) into two smaller atoms (isotopes such as iodine-131, caesium-137 and strontium 90) and two or three neutrons, which go on to split other fissionable nuclei resulting in a chain reaction.
The combined weight of the fission products is less than the weight of the original nucleus and following Einstein's E=mc² this loss of mass (about 0.1% of the original mass) is converted into the massive energy output of the reaction.

There are many problems with this technique, the most famous and most crucial is the safety aspect. The products of the fission are all highly radioactive and the process itself results in a substantial amount of both beta and gamma radiation. There is no safe, permanent way to dispose of the radioactive products and further safety issues arise in the mining, refining and transportation of the fissionable material. Also the consequences of incompetently operated reactors was made tragically clear with the Chernobyl disaster.

There have been three areas of proposal and experimentation to harness the power of splitting the atom for spacecraft propulsion. These are:

1. The use of fission reactors
2. The direct use of fragments from the fission reactor
3. The use of detonating nuclear bombs (!)

The use of a reactor is the most obvious and simplest way of achieving a propulsion system. This would pump the propellant fuel, likely to be liquid hydrogen, over the reactor core and then expel the superhot gas out of the rocket though a nozzle to provide the thrust.

The Four possibilities here are:

Solid Core Schematic		Gas Core Diagram
From Nuclear Rocket Technologies

Nuclear Fission Craft

Courtesy NASA

The second potential method to harness the power of nuclear fission is the direct use of fission fragments.

Nuclear fission produces highly energetic fragments and this system uses these fragments as the propellant fluid by allowing them to escape the reactor. This has a couple of key advantages, firstly the fragments are moving very fast (a low percentage of the speed of light). Secondly the fragments are ionised so directional control can be achieved by using a magnetic field, allowing the channelling of the fragments in the desired direction. This ion control technique is already in evidence in today's ion engines.
Incredible specific impulse is possible, estimated to be over 1 million seconds.

Such systems can also provide high thrust making them ideal for fast interplanetary missions, with even a possibility of interstellar flight.

It should be noted that the design of such a reactor would be extremely difficult and very expensive. The possible options are either a very large, high mass reactor, or perhaps a smaller reactor using fuels obtained from reprocessed spent nuclear fuel, which is a costly process.

The third idea for harnessing nuclear fission for space travel is by detonating nuclear bombs to provide propulsion. This idea is more commonly referred to as a nuclear pulse rocket and was conceived in 1955 under the title Project Orion, which ran through the 1950's and into the 1960's. The objective of the project was to send a manned mission to Mars.

Project Orion		Modern Pulsed Fission Concept
Courtesy MSFC

The idea was to drop several atomic bombs per second about two hundred feet behind the spacecraft and detonate them to provide propulsion, the craft would have needed to have complete radiation shielding. In the design the base of the craft had a massive (1000's of tonnes) shock plate to catch the energy from the explosions and push the craft along. This shock plate would also have to be fitted with serious shock absorbers to allow a smooth ride. Clearly such an idea would be very wasteful of energy with only a small percentage of the energy of the explosions being caught, but it still would have been far higher performance than today's chemical rockets.

There was much experimentation on the project using conventional explosives to propel small craft along, which were apparently very successful.

There is no doubt that the engineering behind Project Orion was very impressive and if it had not been for the problems of fission radiation the project could have had a major impact on the development of the space program.
There are many suggestions that it was a better idea that that of the Apollo Program, being far cheaper, more efficient and of much higher performance in virtually all aspects.

Project Orion met its end with the nuclear test ban treaty in the 60's.
Indeed generally environmental issues have all but 'grounded' the use of nuclear fission as a propulsion source, although its use in space away from the Earth is still being talked about.

For more on Project Orion check out 'Project Orion: It's Life, Death and Possible Rebirth', a very interesting article by Michael Flora.

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