A primary consideration with nuclear thermal propulsion is the radiation environment surrounding the engine during and after operation. Also, after placing the reactor in space, the orbit is planned to be high enough to prevent an accidental reentry. These safety restrictions would also be very important for any type of space reactor, whether it were for space power, nuclear electric propulsion, or nuclear thermal rockets. Several engines have during the 1960s been designed, built and tested. Nevertheless, none of those engines ever flew. For instance, the characteristics of the alpha 2 engine developed in the USA for the NERVA Program are presented below. This engine was designed on the basis of a so called “solid core engines” using high temperature carbide components such as UC and HfC. The characteristics of the ALPHA2 Engine are a thrust of 71.7 kN and a specific impulse of 860 s. It is possible to improve the efficiency of these engines by increasing the temperature of the core. The evolution of the specific impulse of thermo-nuclear engines related to the core temperature of nuclear core technology is presented thereafter. For instance, a specific impulse of about 3000s could be reached with self contained gas core engines. The main challenge related to the development of these engines is the development of light weight materials which should be both heat resistant and radiation resistant. The current technology development performed in the field of fusion reactors for power production have shown important progresses in this domain. In consequence, this technology can be considered to be available in 50 years Nevertheless, the main topic related to thermo-nuclear engines is the safety aspect du to the use of highly contaminant power sources. As a consequence, the use of this technology for space transportation systems operating from the Earth, or in low Earth orbit such as OTV should be considered with a lot of care. This technologys' benefits are discussed in the lunar transportation section. 8.6.12 Materials The performance of engines or transportation system are directly related to the materials used for their manufacturing. For instance, the AV achieved with a given engine and mass of propellants depends on the mass of the launcher and therefore of the mechanical resistance of the materials. On the other hand, light weight heat resistant materials allows to increase the temperature of the combustion chamber or to avoid costly cooling systems and so to increase the efficiency of the engine. Structural Materials Fiber reinforced composite materials demonstrate higher specific mechanical resistance and stiffness than conventional metallic alloys. Some improvement of their characteristics and knowledge will be achieved in the following use: • Organic matrix composites: These materials have been developed for many years and are already use for manufacturing of space transportation systems. Their performances could be increase in the following ways: • increasing of mechanical resistance by improvement of the fiber characteristics as, for instance, very high resistance carbon or aramid fibers, • increasing their mechanical behavior at high temperature by using polyamide organic matrix, For instance, the use of filament wounding Kevlar reinforced organic matrix composites for the manufacturing of case for solid propellant rocket engines for military missiles have allow a important reduction of the mass of the engine. The adaptation of this technology for tanks of liquid rocket engines (i.e. Shuttle one) or to the case of solid rocket boosters (i.e. Shuttle, Ariane V) would increase the mass of payload for those transportation systems. This would required the development of large capacity manufacturing and non destructive testing facilities - Metal matrix composites: those materials have been developed for many years. But at the opposite of organic matrix composites, almost no application of those composites has been developed for space transportation systems. Compared to monolithic metallic alloys MMC present, depending on the matrix and reinforcement present: • increased mechanical resistance at room temperature, • higher stiffness, • improved mechanical behavior at high temperature,
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