Direct Nuclear Thrust Propulsion Systems
A direct nuclear thrust system uses the products of a nuclear reaction as propellant; three conceptual types are analyzed with an assessment on current development, theoretical performance, and conceptual design. While these technologies are currently theoretical, assessment of viability is vital for the continuation of current developmental strategies and research investment. An overall conclusion states this report’s recommendation for the technology that best accomplishes the mission parameters based upon theoretical application and functionality.
Figure 13: A conceptual design for a fission fragment thruster. A indicates the exhaust nozzle on the thruster, B indicates the interior fragment confinement and radiofrequency bombardment chamber, C denotes the deceleration chamber for accelerated particles, d shows the confinement electromagnets, e indicates the radiofrequency coupler, and f is the individual radiofrequency antennae.(Duckysmokton, Dusty plasma bed reactor, 2007).
Fission Fragment Thruster
Figure 22: Artist’s rendering of a fission fragment production system for capture and expulsion; a is the fissile material on a disc, b, which impacts the production system denoted by c, and breaking into the fission fragments shown in d. (Duckysmokton, Dusty plasma bed reactor, 2007).
Of the various types of direct nuclear propulsion systems, the first is a fission fragment nuclear reactor thruster. The basic operating principle behind a fission fragment thruster is the direct utilization of energy produced in a fission reaction. A possible implementation of this system is achieved through the exposure of a fission reactor core to space, with the generated interior fission fragments magnetically directed towards the exit nozzle (Chapline, Dickson, & Schnitzler, n.d.). An alternative fission fragment propulsion system uses decelerated fission fragments or decay products which are guided from the fission reactor through magnetic fields, then heated by radiofrequency induction coils and expelled from the thruster (Clark & Sheldon, 2005). While both propulsion types utilize the direct products of fission, the latter magnetically selects only the extremely high temperature ionized fission fragments for expulsion and then increases their temperature through radiofrequency induction coils, while the former utilizes the average high temperature of the reactor, which includes unreacted particles and no additional heating (Clark & Sheldon, 2005). A diagram of the latter reaction is shown in Figure 13, and the estimated Isp levels from the reaction reach approximately 1,000,000 (Clark & Sheldon, 2005). No fission fragment nuclear reactor thruster concept has been tested, but an analysis by Rodney A. Clark and Robert B. Sheldon indicates the system is within current technological limits (Clark & Sheldon, 2005).
Nuclear Pulse Propulsion or Fusion Rocket Thruster
Figure 21: Artist’s conception of a fusion rocket en route to Mars (University of Washington, 2013).
Similar in design to the fission fragment system, the fusion rocket thruster utilizes the direct products of a fusion reaction to produce thrust. Several differing theoretical concepts attempt to create the fusion reaction and utilize the produced energy as thrust; however, recent experiments at the University of Washington demonstrate the use of Magneto Inertial Confinement Fusion (MICF) for the production and direction of fusion energy (Pancotti, et al., n.d.). A plasmoid is impacted with circular metallic rings, which compress at the nozzle of the thruster chamber producing fusion conditions and expelling the energetic products, while a directed magnetic force expels the highly energetic ionized plasma (Pancotti, et al., n.d.). This reaction produces both high Isp and extremely high variable thrust levels (Pancotti, et al., n.d.). The propulsion of the solid rings towards the plasmoid both expels the fusion reaction through the thruster chamber and into the nozzle, and to create fusion conditions for further propulsion (Slough & Kirtley, n.d.). This method of momentary propulsion is a form of Nuclear Pulse Propulsion (NPP), which uses directed pulsed fusion explosions to provide thrust and propulsion to the thruster; while traditional NPP systems relied upon larger fusion “bombs”, the underlying principle is maintained (Klien, 2012). A primary advantage to this system is the external nature of the propulsion technology, reducing the risk of internal systems or personnel injury (Pancotti, et al., n.d.). This system, currently in preliminary experimental testing, is a highly promising direct nuclear thrust technology, with a comparatively high stage of development.
Antimatter Thruster
An additional type of NPP technologies is the Antimatter Catalyzed Nuclear Pulse Propulsion (ACNPP) system, which utilizes small quantities of antimatter to start a fusion reaction (Kircher, n.d.). While conceptually sound, no testing has occurred with the ACNPP design, and minimal antimatter production and little comparative advantage over the currently developing NPP systems heavily reduces the overall viability of the ACNPP system (Chakrabarti, Dundore, Gaidos, Lewis, & Smith, n.d.; Kircher, n.d.). Additionally, a direct antimatter drive is conceptually proposed, but the miniscule annual production of antimatter, approximately 10 nanograms, means the direct antimatter drive is currently and foreseeably unviable (Kircher, n.d.).
Direct Nuclear Propulsion Systems: Conclusion
For each of the three categories for direct nuclear propulsion several conceptual designs were detailed. As previously mentioned, these technologies are inherently unviable due to lack of development, implementation, or technological ability, but each of the systems described were briefly analyzed and discussed. For the conceptual designs in the antimatter category, this report concluded that they are currently and foreseeably unviable due to production and storage challenges related to antimatter particles. Additionally, the fission propulsion category contains several theoretically sound technologies, but minimal scalability, high crewmember risk, and comparatively small thrust levels indicate the limited current and future viability of this technology. Finally, this report analyzed the fusion category for direct nuclear propulsion, and the current stage of development and experimentation in addition to the relative safety of operation and controllability of the fusion reaction indicates a high probability for future viability and the recommendation for the continuation of development for the NPP system.