Making interplanetary travel time practicable for manned (and unmanned) missions means new propulsion systems and new ways of generating power must be explored. To make space-ships reasonably small, that is, to save propellant mass substantially, Isp must at least double.
In any conventional (chemical) rocket Isp depends the temperature (T) of burnt gases in the rocket chamber and on their mean molecular weight (MW) as given in equation (7.3).
The large Isp of liquid H2/O2 rockets is the result of the low molecular weight (about 9 or 10) of combustion gas, rich not only in H2O (MW = 18), but also in excess H2 (MW = 2). Chamber temperatures are lowered by adding extra H2, but the ratio T/MW turns out higher.
So, increasing Isp means either raising T or lowering MW, or both. The first choice is constrained by structural material limits: the mechanical strength of almost all materials diminishes with increasing temperature. That is why liquid rocket thrust chamber walls are cooled to a temperature less than, say, 1,000 K.
If feasible, higher gas temperatures would be welcome, because they raise Isp. However, the adiabatic flame temperatures of the best liquid propellants combinations do not exceed 3,500 K (and are accompanied by severe cooling problems). Some propellant combinations may exceed 3,500 K a little, but in that case at least one of the propellants is solid. When one of the propellants is solid the rocket is called a ''hybrid rocket'' (see Section 4.25). Hybrid rockets have become of great interest after the sub-orbital flights of Burt Rutan's "SpaceShipOne", but also have a thrust/volume smaller than all-liquid rockets, and the gain in Isp over that of H2/O2 is negative or marginal.
So, in thinking about raising Isp, the obvious question one would ask is how to reach higher temperatures. Now, temperature really means internal energy. With chemical propellants the internal energy is that of chemical bonds. Chemical energy is nothing else than the potential energy of the fundamental electro-weak force, that is, of the Coulomb forces acting among electron shells (—) and nuclei of atoms and molecules (+). The number of fundamental forces in nature is just three, gravitational, electro-weak (including Coulomb) and nuclear, also called the "strong" force. Thus the quest for higher temperatures producing higher Isp should really become a quest for energy alternatives, and there is not much choice here: discarding gravity, the only option is drawing on the nuclear energy binding together nucleons (neutrons and protons) inside the atom nucleus.
This means fission, fusion (including antimatter annihilation, an extreme form of fusion), or relaxation of metastable nuclei. By analogy with combustion, the material fissioned, fused or relaxed is still called a nuclear "fuel", or simply the fuel.
Following this approach means that the energy source, or energy conversion stage, is separate from the propulsion stage and its propellant. In chemical propulsion instead the energy source is the heat release by chemical reactions between the propellants themselves. The nuclear energy source may be a nuclear reactor, or a fusion reactor. Then the heat released from the source must be transferred to a fluid/ propellant. This fluid may be exhausted as in a conventional rocket, or used in a thermodynamic cycle to produce electric power. In any event, how to transfer energy from nuclear source to propellants/fluid is a crucial item, shaping different concepts differently (see [Bruno, 2005, 2008]).
This chapter will focus on propulsion systems using fission, leaving fusion to be discussed in Chapter 8 for missions outside our Solar System. In fission the nuclei of
atoms of properly chosen materials (fuels such as U, 9Pu and others) are broken apart (fissioned) by neutrons. The neutrons needed are produced by these materials, but their fissioning effect becomes efficient only when a "critical" mass of material is assembled. Using the electronvolt (eV) as energy unit, fissioning 235U yields 160 MeV per fission fragment, to be compared to a fraction of an electronvolt in combustion. In more common units, fission heat release per unit propellant mass, J, is vastly larger that of H2/O2 propellants in a rocket (about 1.35 x 107 J/kg). In fact, as any energy release process, nuclear reactions convert fuel mass into energy according to E = mc2; the energy per unit mass, J, available in fission is of the order of 8.2 x 1013 using 235U, almost 107 times larger than in combustion, as illustrated graphically in Figure 7.4. Note that in this figure energies are plotted on a logarithmic scale!
The theoretical foundations of nuclear reactors can be found in [Glasstone, 1955]. Fission physics for propulsion applications can be found in [Hill and Peterson, 1970; Bussard and DeLauer, 1958; Lawrence et al., 1995]; recent basic fission engineering is in [Turner, 2005] and details will not be discussed here. Still, it is important to emphasize that release of nuclear energy in a reactor is unlike that by an atomic bomb. No nuclear power generator can explode like an atomic bomb, since the critical mass (a few kilograms of U in a sufficiently dense volume) is
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