Antimatter catalyzed nuclear pulse propulsion

From Academic Kids

Antimatter catalysed nuclear pulse propulsion is a variation of nuclear pulse propulsion based upon the injection of antimatter into a mass of nuclear fuel which normally would not be useful in propulsion.

Traditional nuclear pulse propulsion has the downside that the minimum size of the engine is defined by the minimum size of the nuclear bombs used to create thrust. With conventional technologies nuclear explosives can scale down to about 1/10 kiloton, but making them smaller seems difficult. Large nuclear explosive charges require a heavy structure for the spacecraft, and a very large (and heavy) pusher-plate assembly.

By injecting a small amount of antimatter into a subcritical mass of fuel (typically plutonium or uranium) fission of the fuel can be forced.

An anti-proton has a charge of -e (just like an electron,) and can be captured by an atom into the electron orbitals. However this is not a stable configuration, and the anti-proton will start to radiate away energy as gamma rays. As it does so the orbital falls closer and closer to the nucleus of the atom.

Eventually the anti-proton decays to the point where it lies inside the nucleus. At this point it annihilates with a proton. This reaction releases a tremendous amount of energy, enough that the thermal effects will cause the nucleus to explode. This releases a shower of neutrons, causing the surrounding fuel to undergo rapid fission. The reaction is even fast enough to trigger a nuclear fusion reaction if desired.

In theory, there is no lower limit to the size of this device: One anti-proton is enough to start the chain reaction. There are real-world issues relating to the lifetime of the anti-protons and their chance of reacting with the fuel that impose a lower limit on the amount of antimatter needed per reaction, and the geometry of the fission reaction imposes a lower limit on the size of the fuel as well. However these real-world numbers turn out to be entirely feasible with today's technology and infrastructure, unlike either the Orion-type system which requires large numbers of nuclear explosive charges, or the various anti-matter drives which require impossibly expensive amounts of antimatter.

Catalyzed is a mis-nomer for this process because the anti-protons used to start the reaction aren't recovered.

Several groups are actively studying such antimatter-catalyzed micro fission/fusion engines in the lab (sometimes antiproton as opposed to antimatter), after its invention at Pennsylvania State University in 1992.

Tuning of the performance to the mission is also possible. Rocket efficiency is strongly related to the mass of the working mass used, which in this case is the nuclear fuel. Although the reaction energy of a fusion reaction is about 1/10th that of a fission reaction, the LiD fuel used in these reactions is much lighter. For missions requiring short periods of high thrust, such as manned interplanetary missions, pure microfission might be preferred because it reduces the number of fuel elements needed. For missions with longer periods of lower thrust, such as outer-planet probes, a combination of microfission and fusion might be preferred because it reduces the total fuel mass.

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