Nükleer Enerjinin Roket Tahrik Sistemlerinde Kullanımı

Abstract

Bu çalışmada genel hatlarıyla nükleer enerjinin roket tahrik sistemlerinde kullanımı üzerinde duruluyor. Giriş bölümünde; roketlerin evrimi kısaca anlatıldıktan sonra, roket motorlarının sınıflandırılması yapılıyor. Bu bölümde ileri roket tahrik sistemlerinin yamsıra farklı teknik kullanan nükleer roketler üzerinde duruluyor. İkinci kısımda roket performans parametrelerinin nasıl oluştuğu gösteriliyor. Bu parametrelerin kullanımıyla tüm roketin performansı elde edilir. Sistem analizi bölümünde ise çeşitli dizayn değişkenlerinin optimizasyonu ve nükleer roketler için genel performans bölgelerinin tanımlanması yapılıyor. Burada önemli olan her bir bileşenin parametresinin bileşenin tüm araç performansı üzerine etkileriyle belirlenebiliri esidir. Nükleer roketlerde hedef düşük molekül ağırlıklı yakıtı kullanılabilir sıcaklıklara ısıtmaktır. Bu da en azından kullanılan ısı transferi proseslerini bilmeyi gerektirir. Bu çalışmada ısı transferinin nükleer roketlerle ilgili kısımları inceleniyor. Roket reaktörünün performansı, reaktör bileşenlerinin malzeme limitleriyle belirlenir. Maksimum performans, tüm bileşenlerin mümkün olabilen malzeme limitlerine itilmesiyle sağlanır. Bu nedenle nükleer roketlerde kullanılan malzemeler kısaca incelendi. Nükleer roketlerin kontrolünde reaktör kontrolüne ek olarak kısa zaman aralığında işletme koşullarına ulaşılması söz konusudur.Bu çalışmada roket kontrolü üzerinde genel hatlarıyla duruldu.

Throughout the last decade the application of neutron physics to engineering problems has resulted in many advances in nuclear-reactor technology, and while the literature abounds in contributions dealing with large stationary power reactors, nothing has yet been presented on the problems of high-power-density mobile (flyable) reactor systems. The solution of these problems is largely in the domain of the engineer, since proper heat generation and removal, fluid (coolant) distribution and flow, and structural integrity must be achieved in order to construct succesfull high-power-density mobile reactors. There is more to nuclear rockets than high-power-density reactor design. The performance requirements for nuclear rocket power plants will be determined by the performance required of the rocket vehicle itself. Accordingly, analysis of the internal and external ballistic behaviour of nuclear rockets are presented in sufficient detail to enable those just entering the rocket and missile field, as well as those now engaged in such work, to obtain a reasonable understanding of the subject. No attempt is made to include chemical rockets per se, as the literature in the field is extensive and readily available. To utilize nuclear energy to rocket engines, took place over nearly forty years period is demonstrated. In order to produce a worthwhile text in a field as large as nuclear rocket propulsion, it is necessary to exclude some areas from discussion. Therefore, characteristics of rocket behavior and rocket-motor performance peculiar to chemical rockets are excluded from discussion herein. XIV The basic difference between chemical and nuclear-powered rockets of the sort considered herein is in the method of obtaining the energy required for vehicle propulsion. The chemical rocket fills its energy needs from the combustion or decomposition of its propellants; the working fluid of nuclear rocket provides no intrinsic energy but is heated by kinetic energy of fission fragments released in the controlled fission process within a nuclear reactor. Since rockets move by virtue of the principle of conservation of momentum, it is desirable that the working fluid (propellant) of the nuclear rocket be expelled rearward at a velocity as high as is possible. This requires high-working fluid temperature and low molecular weight. One of the greatest obstacles to the human exploration of space has been the physical limit in the efficiency of chemical propulsion systems. Chemical propulsion has been a mature technology for decades, and efficiency improvements over this time span have amounted to only a few percent. This limits of chemical propulsion have forced the space exploration community to develop other strategies for overcoming the strictures imposed by gravity in their exploration pursuits. These strategies have their own limits and invariably result in increased costs and mission time. Nuclear propulsion systems generate twice the efficiency of the best modern chemical systems. This Nuclear propulsion does not face the same physical limitation as chemical propulsion, improvement provide mission planners with such an enormous leap in capability that the full range of possibilities has yet to be identified Before the application of nuclear energy to rockets can be discussed, it is necessary to understand the characteristics of rocket-motor gas dynamics and inherent features of ballistic- rocket-vehicle flight. Fundamental equations expressing the various pertinent phenomena are derived and summarized in chapter 2. Also of interest are physical and XV chemical characteristics of the best potential nuclear rocket working fluids or propellants. These are discussed, and estimates of performance are presented based upon the previously developed gas-dynamic equations. The application of method of system analysis then discussed with particular reference to the determination of the general performance regions for which nuclear rockets are of interests and to the optimization of various design variables. The relative advantage accruing from use of a particular design condition of rocket-vehicle component- performance parameter can only be determined by an analysis of its effect on complete vehicle performance. As an example, high rocket-motor chamber pressure produces high exhaust velocities and yields thrust units of high specific power output, but requires larger, more powerful and heavier propellant-pumping equipment, than that for lower-pressure use. The choice of optimum operating pressure for maximum vehicle performance can only be determined by analyzing the performance capabilities of vehicle's over a wide range of chamber pressure. The effect of changing propellant-tank pressure, vehicle initial acceleration payload weight, and many other parameters must be determined by similar methods. System analysis attempts to do this by relating generalized functional weight and performance equations for each component to the fundamental vehicle performance equations given in chapter 2. Since a nuclear reactor is, in the engineering sense, an unlimited heat source, the problem of core design reduces to the most efficient utilization of this source. For nuclear rockets, the goal is to heat a low-molecular weight propellant to as high a temperature as practicable. Analysis and design to achieve this goal require an understanding of the heat-transfer processes and of geometries that might be used to exploit these processes. In chapter 4 conventional approaches to the heat-transfer problem are presented. The discussion includes heat transfer by convection, material on core power density, fluid friction, pressure drop and system flow stability is presented. XVI The peak performance of a heat-exchanger type of rocket reactor is fixed by limitations on the reactor component structures. Maximum performance results from a design in which all component parts are pushed as close to their material limits as possible. In order to do this designer must be aware of the major problems to be overcome in each section of the reactor and must know the properties and capabilities of the materials of interest. In principle, the control of nuclear reactor is comparatively simple, for by controlling the fission process any level of power can be achieved. In practice, however, the factors which influence the fission process, such as the neutron-energy distribution, fission and absorption cross-section variations with temperature, geometrical changes in the core structure and control rod effectiveness, all tend to increase the complexity of the problem. For the particular case of nuclear rocket reactors, the additional requirements of reaching operating conditions in as short period as possible in order to conserve propellant and of in-flight thrust programming for proper guidance introduce further complications. As a consequence, a complete and a comprehensive treatment of the control problem can not be covered in a single chapter. So, the general approach to the control systems is presented. Results : Space exploration benefits from nuclear propulsion primarily in improved accessibility to targets, reduced transit time to targets, increased payload mass fraction, and, in electric propulsion and bimodal applications, dramatically increased power availability to the payload. These benefits permit major improvements in scientific return and cost-effectiveness. Greater power permits much greater data transmission rates and, when combined with larger payload fractions, many more scientific instruments. Nuclear propulsion permits accomplishment of many missions existing launch vehicles and enables many more with smaller, cheaper launch vehicles than would be required with chemical propulsion. A four point comparison between chemical and nuclear propulsion could be listed as follows. XVII 1 Nuclear systems do not compete favorably with chemical systems for missions requiring only a few hundred pounds of payload placed in low earth orbits. 2 For long-range ballistic missiles and space vehicles in low earth orbits requiring payloads of a few tons or more, nuclear systems are competitive with chemical systems and can accomplish such missions with a single stage rather than two or three stages needed by chemical systems. 3 For more ambitious missions such as moon landing and take off interplanetary exploration or establishment of stationary satellites, the substitutions of a nuclear upper stage for one or two chemical stage almost invariably increased the payload by from 60 to 100 %. Replacement of the entire chemical system by a nuclear system leads to increases of payload relative to gross weight by factors of from 5 to 10. 4 Very difficult missions such as rapid or extended space maneuvering, such as might be required to avoid high-intensity radiation zones or to accomplish rapid interplanetary travel, probably can be accomplished only by means of some from of nuclear propulsion. In corporating nuclear propulsion into exploration spacecraft present a number of unique design problems not encountered with chemical propulsions. Chief among these are effects and consequences of radiation associated with the reactor and vehicle configuration issues. Radiation-related issues include safety considerations, nuclear heating of cryogenic propellants, and impingement of radiation on the payload. Configuration issues stem from the relatively large size of nuclear propulsion systems, the low density of the preferred propellant and from radiation mitigation techniques.