Nuclear Propulsion Systems
The most powerful energy known to humans is that stored in the nucleus. After fission or fusion of atoms, the end product can have a smaller mass than the initial atoms (fission processes require heavy nucleons such as Uranium, while fusion processes requ
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Nuclear propulsion Systems 4.1 Overview The most powerful energy known to humans is that stored in the nucleus. After fission or fusion of atoms, the end product can have a smaller mass than the initial atoms (fission processes require heavy nucleons such as Uranium, while fusion processes require very light nucleons such as Helium). This mass defect is directly transferred into energy according to Einstein's famous equation E = mc2• Nearly all gained energy is released as heat. While fission or fusion transfer only part of the nuclear binding energy into heat, matter-antimatter annihilation (for example proton-antiproton or even hydrogen-antihydrogen) can release all nuclear energy. Although this would represent the highest energy density achievable, production and storage of antimatter is at a very early development stage. Figure. 4.1 compares the typical energy stored in the chemical reaction between hydrogen and oxygen, fission of Uranium 235, fusion between Deuterium and Tritium, and ProtonAntiproton annihilation processes. The energy stored in nuclear propellants is an enormous 107 _109 times higher than that of optimally performing chemical propellants. A propulsion system utilizing nuclear energy must be therefore capable of achieving nearly any specific impulse close to the speed of light. On the other hand, nuclear processes use very small quantities of matter. If nuclear reaction products are used only, the resulting mass flows and thrust levels would be very low. In order to achieve high thrust levels, a working fluid/gas (or the complete spacecraft mass itself as we will see in one concept) needs to be coupled with the gained nuclear energy compromising the specific impulse. Also material constraints (melting temperature of heating chamber, ... ) need to be considered, which may be relaxed by using magnetic bottles to confine the hot plasma. In principle, all combinations between specific impulse and desired thrust level can be engineered. This offers the possibility for undertaking, e.g. manned solar system exploration in acceptable time frames. In fact, nuclear propulsion is the only choice of mature and developed technology that can offer high specific impulse and high thrust levels simultaneously today. A nuclear rocket was developed near completion during the 1960's in the United States NERVA program. Renewed interest in manned Mars missions may revive efforts in nuclear propulsion despite present environmental and political considerations. M. Tajmar, Advanced Space Propulsion Systems © Springer-Verlag Wien 2003
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4 Nuclear Propulsion Systems PrOlon·Antlprolon Annihilation
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Fig. 4.1. propellant energy densities for chemical and nuclear propulsion
4.2 Fission propulsion Nuclear fission is achieved by bombarding a nuclear core with neutrons, which are not affected either by electrons or protons. The neutron then deforms the core, creating a metastable nucleon. Electrostatic repulsion causes the nucleon to split int
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