Nuclear Thermal Propulsion Systems
Contrasting with the operating principle of direct nuclear thrust systems, nuclear thermal thrust systems do not rely upon the products of a nuclear reaction for direct thrust; instead, nuclear thermal thrust systems rely upon the heat generated during a reaction to expand and expel particles for propulsion. Although no nuclear thermal propulsion system is flight tested, several extensive governmental projects developed and ground tested preliminary nuclear thermal thrusters. Two primary categories of nuclear thermal thrusters are briefly examined, with an analysis of the current and future viability for each of the developing conceptual technologies.
Active Nuclear Thermal Thruster
Figure 23: Labeled diagram of an open gas core reactor active nuclear thermal propulsion system (NASA, 2005).
Heat generated from a nuclear reactor powers the first type of nuclear thermal power systems; an initiated reaction generates heat, which is then converted into thrust by the expansion of particles. As an initiated and regulated reaction is required, these systems are types of an active nuclear thermal propulsion system. Three subsequent types of nuclear reactor cores generate propulsion for this system; solid core, liquid core, and gas core. In a solid core reaction, a propellant fluid passes through a thermal radiative system where heat from the reactor is transferred to the fluid; evaporation and expansion occurs as the temperature increases, and once the propellant reaches the approximate temperature of the reactor, the particles are expelled as exhaust for thrust (Winter, 2006). Rockets previously tested in the NERVA program used solid core propulsion systems, indicating the operability and advanced development stage of the solid core thruster (Winter, 2006). Solid core rockets have an approximate Isp range between 850 and 1000, and a relatively high thrust compared to alternate nuclear propulsion systems (Winter, 2006). Standard liquid core propulsion systems are considered too difficult to construct and maintain; an alternative, proposed by Robert Zubrin, is a Nuclear Salt Water Rocket (NSWR) (Zubrin R. , 1991). Consisting of several boron-carbide storage pipes and a single larger pipe ending in the rocket nozzle, a uranium or plutonium salt solution would flow into the larger pipe; once the solution reaches a specific level, the solution begins a fission reaction (Zubrin R. , 1991). This continual chain reaction would superheat the liquid, expanding to form a gas and expelled from the thruster nozzle (Zubrin R. , 1991). A primary concern for the viability of this system is the ability for the liquid to reach a supercritical mass; this ability is unclear at the current stage of development (McNutt, 1999). If operational, the rocket would have an Isp range between 5,000 and 100,000, with a very high relative thrust level (Winter, 2006). The final type of nuclear core for the nuclear thermal propulsion category is a gaseous core. A Gaseous Core Nuclear Rocket (GCNR) operates through the magnetic confinement of the reactor core, which is in a gaseous state; the fissile material is typically either uranium hexafluoride or uranium tetraflouride (Velidi, Guven, & Dhar, 2012; Winter, 2006). Several injection locations are spread equally around the spherical containment chamber, for the homogenous reaction of the fissile material (Velidi, Guven, & Dhar, 2012). Hydrogen serves as both the propellant and coolant, with aqueous hydrogen flowing around the spherical containment chamber (Velidi, Guven, & Dhar, 2012). Once heated by the fission reaction, the hydrogen becomes highly compressed and is expelled through a nozzle, providing propulsion to the system (A.S. & E.E., 2007; Velidi, Guven, & Dhar, 2012). Approximate Isp levels range between 3,000 and 5,000, while a relatively high thrust level is maintained (A.S. & E.E., 2007; Winter, 2006). In comparing the three types of nuclear reactor cores for nuclear thermal propulsion systems, the potential future validity of these technologies is assessed. While extensively developed and ground tested, the solid core nuclear thermal rocket is considered not relatively viable; extremely low relative Isp values and equal or lower thrust levels than the two alternative reactor core types ensures the infeasibility of the solid core propulsion system. The next reactor type, the GNCR, has significant advantages over the other nuclear reactor propulsion types. A higher Isp range than the solid core reactor and a theoretically proven conceptual design compared to the unproven NSWR concept induces a relatively high level of considered viability; however, extremely complex construction and lower Isp and thrust levels than the NSWR results in a partial recommendation for constrain viability. The last reactor core type, the NSWR, has an extremely high Isp range and a very high relative thrust level, indicating it’s overall feasibility as a long distance, high mass propulsion system. An unproven conceptual operation is the primary constraint for the NSWR system. Nevertheless, this report concludes that the NSWR system is currently the most potentially viable reactor based nuclear thermal propulsion system.
Passive Nuclear Thermal Thruster
Similar to the previous nuclear thermal propulsion systems, the radioisotope nuclear thermal thruster in the passive nuclear thermal propulsion subcategory indirectly provides propulsion through radiative heating of a liquid propellant (LeMoyne, 2006). This working fluid is held in cylindrical storage chambers; as the temperature of the fluid increases due to radiative heating, it begins to expand through pipes surrounding the reactor (Dalley, Friedman, Martinez, Allen, & Jortner, 1962). Once within the surrounding pipes, the working fluid is subjected to further temperature increases, eventually creating a highly compressed gaseous compound (Dalley, Friedman, Martinez, Allen, & Jortner, 1962). This compressed gas is then expelled from the thruster, providing propulsion to the system (Dalley, Friedman, Martinez, Allen, & Jortner, 1962). With an extremely low Isp range between 700 and 800, this propulsion method is highly inefficient; further, an extremely low conversion efficiency of approximately 1% and extremely low thrust to weight ratios indicate this system is highly unviable for the proposed mission constraints (Bussard, 1958; Dalley, Friedman, Martinez, Allen, & Jortner, 1962). Preliminary research indicates minimal development or testing with the radioisotope thermal propulsion system, and extremely low theoretical performance ranges restrict any possibility for viability. This report concludes that based upon current stages of development and conceptualization, the radioisotope thermal propulsion system is currently and foreseeably unviable.
Nuclear Thermal Propulsion Systems: Conclusion
In the preceding analyses of nuclear thermal propulsion systems, conceptual and developing propulsion systems were detailed and comparatively assessed. In the passive nuclear thermal propulsion subcategory, the radioisotope nuclear thermal conceptual propulsion system was examined; with an extremely low relative Isp range and negligible thrust production, this report recommended the radioisotope propulsion method be considered unviable within the defined parameters. The other propulsion system, the NSWR of the active nuclear thermal propulsion subcategory, has significantly higher Isp and thrust ranges allowing this technology to be considered as a potentially viable propulsion technology. Significant detracting factors of the NSWR system include negligible current or expected development, and an unproven operational concept. However, as the radioisotope propulsion method is theoretically unviable, this report concludes that of the nuclear thermal propulsion systems, the NSWR is the most viable potential technology and is given a partial recommendation for continued development and research.