atw International Journal for Nuclear Power | 04.2020

More documents

Recommendations

Info

atw Vol. 65 (2020) | Issue 4 ı April FEATURE | RESEARCH AND INNOVATION 196 | Fig. 9. Orion Nuclear Pulse Rocket. | Fig. 10. Orion Flight Testing Using Putt Putts. away from the vehicle. The vehicle itself is designed such that a portion of the blast wave resulting from the nuclear detonation is intercepted by a specially designed “pusher plate” attached to the body of the spacecraft by giant shock absorbers. These shock absorbers act to moderate the spacecraft jerk (e.g. time rate of change of acceleration) resulting from the impinging blast wave such that the acceleration of the main body of the vehicle is reduced and smoothed to levels which can be tolerated by the crew. An illustration of a conceptual Orion nuclear pulse vehicle is presented in Figure 9. Such a pulsed nuclear propulsion program was actually initiated in the United States during the late 1950s and early 1960s under the project name Orion [5]. Several small scale proof of principle models called Putt-Putts or Hot Rods were actually built and flown. These proof of concept vehicles used chemical rather than nuclear explosives as the propulsive medium and after several failures, one of the vehicles finally achieved stable flight and flew to an altitude of about 30 meters as illustrated in Figure 10. Figure 11 illustrates what might be a typical pulsed nuclear rocket dynamic response resulting from a series of | Fig. 13. Closed Cycle Nuclear Rocket Engine “Nuclear Light Bulb”. | Fig. 11. Dynamic Response of a Nuclear Pulse Vehicle. | Fig. 12. Open Cycle Gas Core Nuclear Rocket Engine. nuclear acceleration pulses. Note that contrary to what might be expected, the accelerations experienced by the crew during the time period over which the detonations are occurring can be made survivable, although probably quite uncomfortable. No doubt, the crew would happily accept being put into a state of suspended animation during the thrusting phase of the mission rather than experiencing such a stomach-churning ride! Another method of achieving higher specific impulses in to employ a gaseous fissioning core to eliminate the problem of fuel melting at extremely high temperatures. As might be expected, a number of significant design challenges exist with this concept primarily with regard to designing a feasible means of transferring heat from the gaseous fissioning core to the gaseous propellant. There are basically two different reactor configurations which may be employed to construct a gaseous core nuclear rocket engine. One concept may be described as the “open cycle” configuration. An illustration of the open cycles gas core nuclear rocket concept is presented below in Figure 12. In this configuration, a fissile material is injected into the core where it is subsequently vaporized due to the high temperatures present there. The hydrogen propellant is also injected into the core, but in such a way that a stabilizing rotation is induced in the gaseous fissioning core. Chief among the feasibility questions with this core configuration is the issue of keeping the gaseous fissioning core from escaping through the nozzle at an unacceptably high rate. To be practical, the gas core rocket must maintain its gaseous core in a stable critical state, while minimizing the loss rate of fissionable material through the nozzle while simultaneously maximizing the heat transfer rate to the hydrogen propellant and allowing it only to escape through the nozzle. Such stringent requirements Feature Nuclear Rockets for Interplanetary Space Missions ı Dr. William Emrich
atw Vol. 65 (2020) | Issue 4 ı April | Fig. 14. “Nuclear Light Bulb” Simplified Flow Diagram. will be no doubt be difficult to achieve in practice. It is thought that specific impulses in the order of 2000 seconds may be possible if near 100 % uranium plasma containment is achieved. Another gaseous core concept may be described as the “closed cycle” or “nuclear light bulb” configuration. In the nuclear light bulb, the gaseous uranium is confined in closed transparent containers which allow radiant energy from the core to be transmitted through the container walls where the energy is absorbed in a seeded hydrogen propellant which flows on the outside of the container. This concept has the obvious advantage of containing 100 % of the nuclear fuel; however, it also introduces an entirely new set of design challenges. Chief among these design challenges is the problem of maintaining the structural integrity of the transparent core containment vessel in the presence of an extremely harsh temperature environment while simultaneously allowing the transmission of vast amounts of radiant energy through its walls. These design challenges were addressed in a program at United Technologies [6] in the 1960s when the company had an active program underway to develop a nuclear light bulb rocket engine. A diagram of the engine concept developed by the company is illustrated in Figure 13. The radiation (primarily ultraviolet light) emitted from the high temperature fissioning uranium plasma, passes through the containment vessel’s transparent walls and is absorbed in seeded hydrogen propellant which flows along the outside of the containment vessel. This hot hydrogen propellant is subsequently exhausted through nozzles to produce thrust. The transparent walls of the containment vessel are of particular concern in the nuclear light bulb design. The material comprising the containment vessel walls, therefore, must not only be highly transparent, but must also be actively cooled to prevent overheating and eventual vessel failure. In this design, the extremely hot fissioning uranium plasma is prevented from touching the transparent containment vessel by a vortex flow of seeded neon gas which acts as a buffer between the containment walls and the uranium plasma. The neon gas (along with some entrained uranium) is continually extracted from the edge of the reactor core where it is separated from the uranium and cooled in a heat exchanger before being reinjected back into the containment vessel. The rejected heat from the neon is used to partially preheat the main hydrogen propellant stream. The separated uranium is also reinjected back into the containment vessel thus preventing any uranium loss from occurring in the system. A schematic diagram of the nuclear light bulb concept is illustrated in Figure 14. In the United Technologies experiments with the concept yielded equivalent specific impulses of over 1300 sec. Conclusions The application of nuclear energy to space propulsion systems has long been seen as a means to enable missions to outer space that are not achievable by any currently conceivable chemical based rocket propulsion system. While nuclear rocket engines are clearly superior to chemical rocket engine in that they deliver efficiencies that are over twice that of the best chemical rocket engines, no operational nuclear rocket engine has yet been developed. It is anticipated that these engines, while deceptively simple in concept, will no doubt require a daunting amount of detailed engineering to finally develop a practical engine system. These engineering details include not only the usual thermal, fluid, and mechanical aspects always present in chemical rocket engine development, but also nuclear interactions coupled with some unique materials restrictions. None of these engineering details are expected to be insurmountable, however. Hopefully, with the mounting desire within the United States and elsewhere to send human to Mars, it may be that renewed efforts will be made in the near future to once again initiate programs to finally develop an operational nuclear rocket engine. References [1] Finseth, J. L., “Overview of Rover Engine Tests - Final Report”, NASA George C. Marshall Space Flight Center, Contract NAS 8-37814, File No. 313-002-91-059, (Feb. 1991). [2] Haslett, R. A., “Space Nuclear Thermal Propulsion Final Report”, Phillips Laboratory, PL-TR-95-1064, (May 1995). [3] Harvey, B., “Russian Planetary Exploration History, Development, Legacy, and Prospects”, Springer-Praxis Books in Space Exploration, ISBN 10: 0-387-46343-7, (2007). [4] Emrich, W., “Principles of Nuclear Rocket Propulsion”, Elsevier Inc., ISBN 978-0-12-804474-2, (2016). [5] “General Atomic Division of General Dynamics, \”Nuclear Pulse Space Vehicle Study\”, GA-5009, Vol. I thru IV, NASA/MSFC Contract NAS 8-11053, (1964).” [6] Mcl.afferty, G. H., “Investigation of Gaseous Nuclear Rocket Technology – Summary Technical Report”, United Aircraft Research Laboratories, Report H-910093-46, prepared under Contract NASw-847, (November 1969). Author Dr. William Emrich Senior Engineer NASA – Marshall Space Flight Center Huntsville, Alabama USA FEATURE | RESEARCH AND INNOVATION 197 Feature Nuclear Rockets for Interplanetary Space Missions ı Dr. William Emrich
Page 1 and 2: nucmag.com 2020 4 ISSN · 1431-5254
Page 3 and 4: atw Vol. 65 (2020) | Issue 4 ı Apr
Page 15: atw Vol. 65 (2020) | Issue 4 ı Apr
Page 59 and 60: Kommunikation und Training für Ker

atw International Journal for Nuclear Power | 04.2020

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?