Keeping in mind power is associated to high thrust at high Isp, the next issue is the question of strategy. How large should Isp be and still produce reasonably high thrust, so that mission time will be also reasonable? Do we really need GW-class reactors? Are there trade-offs between Isp and thrust?
The answer to all these questions is, it depends on mission. At fixed power (e.g. fixed by the size of the nuclear reactor) the question can be rephrased as: What is the best way to exploit this power? In an NTR maximum temperature imposed by structural limits cannot go above 2,500 or perhaps 3,000 K, even looking far into the future. Materials capable of 2,500 K are in the testing stage. Propellant temperature must be even lower and determines Isp, so that not much can be hoped for above Isp = 1,000 s. This is more than twice the Isp of current chemical rockets, but it is not enough for enabling some (manned) interplanetary missions, for instance those to Neptune or Pluto. For sustained thrust of order 2 weeks, and at Isp = 1,000 s too much propellant would be necessary. The ship would be so large and massive to accommodate the propellant that acceleration would be too low. Trip time to Neptune, for instance, would increase far beyond the 4-week round trip imagined in Section 7.1.
If nuclear thermal rockets are the baseline nuclear propulsion system, what are potential advances capable of raising Isp? Conceptually at least, to reduce propellant consumption at fixed power, either structural temperature limitations must be bypassed, or thermodynamics must be bypassed.
The first approach leads to the so-called Rubbia engine, in which the traditional direction of the heat transfer process (fission fragments ! fuel bars ! propellant) is short-circuited by direct injection of fission fragments inside the propellant. Within the same approach, a different solution is to let the fuel fission in its gaseous state (that is, at much higher temperature than when solid), and heat the propellant radiatively; this is the gas-core nuclear rocket concept.
The second choice assumes the nuclear reactor must only generate electric power, leaving the job of accelerating the propellant to the Coulomb or to the Lorentz force. This means using one of the many types of already-existing electric thrusters.
In some more detail, thermal rocket solutions, whether baseline or advanced, convert the KE of fission fragments directly, or via heat exchange, into KE of propellant particles. Because the KE of fission fragments is -102 to 103 keV 106 to 107 K!) and if magnetic confinement of fragments is unfeasible, temperatures may be kept reasonably low by "diluting" the extremely high KE of fission fragments with, or in, a much larger mass of propellant Mp, as explained conceptually in Section 7.3.1.
This strategy is best suited to a propulsion system where thrust must be "high"; it also produces 7sp of order (2-4) x 103 s at most. Solid-core reactors, where temperature must be kept below, say, 2,500 K, such as the ones tested in the US and Soviet Union, can yield Isp of order 800-1,000 s only, are capable of thrust comparable to that of chemical rockets, and fall in this class. A conceptual scheme is shown in Figure 7.7 (note the presence of a radiation shield). Acronyms typical of this class of propulsion are nuclear thermal propulsion (i.e., NTP), or nuclear thermal rocket (NTR), since the primary mode of propulsion is based on thermalization of fission products, that is fragments collide with propellant molecules and divide among them their high KE until thermal equilibrium is reached. The hot propellant then expands in a conventional nozzle.
In the second choice of strategy, the nuclear reactor is viewed only as a power source. This power may be converted into electricity by conventional thermodynamics cycles, such Stirling or Brayton, by direct thermionic or thermo-electric conversion, by magneto-hydro-dynamic conversion, or by more advanced processes. The electric power feeds an electric thruster, for instance an ion, or magneto-plasma-dynamic (MPD) thruster. Thrust is typically much lower (1-100 N) than in the first class, but Isp may reach 106 s. Hence, the acronym NEP (nuclear electric propulsion). The general scheme of an electric thruster is shown in Figure 7.8.
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