The operation of a solar-thermal rocket is presented in Figure 12.1. Unlike a chemical rocket, the Sun powers solar-thermal rockets. Using either a Fresnel lens or a parabolic mirror, sunlight is concentrated to superheat propellant, which is then vented to produce thrust. The concentrator is the key feature that distinguishes this technology from other propulsion technologies, heating the propellant with the equivalent of up to 1,000 Suns (at 1 AU). Recall that exhaust velocity and thrust are closely related to the propellant's temperature—and a solar-thermal system can make the propellant very, very hot.
spacecraft acceleration direction FIGURE 12.1 The solar-thermal rocket
Although very simple in concept, implementation is not so straightforward. One technical challenge is the heat exchange with the propellant. As one cannot directly heat a moving fluid in a vacuum, it must be done indirectly. In indirect heat exchange, the sunlight warms a material, or "heat exchanger," that subsequently transfers the heat to the propellant.
The fuel-heating chamber must be constructed of high-temperature materials. For optimum performance, propellant temperatures are well in excess of 1,000 degrees Celsius.
Although the working fluid (propellant) in Hero's device, steamboats and coal-stoked locomotives was water, optimum performance for solar-thermal rockets is achieved if a lower molecular mass propellant is used. Current prototype solar-thermal rockets use hydrogen propellant. If we replace hydrogen with water propellant, the exhaust velocity would decrease from about 10 kilometers per second to about 3 kilometers per second.
Although the use of hydrogen as a propellant results in high exhaust velocities (typically about twice those of the best chemical rockets) there are significant drawbacks to employing this fuel on long-duration space missions. Liquid hydrogen is difficult to store in the space environment and, due to its very low boiling point of-252 degrees Centigrade, it tends to evaporate or "boil off." Even the cold temperatures of space are too warm to prevent hydrogen from turning into a gas. Hydrogen gas is notoriously difficult to store. Its low molecular weight and atomic size allows it to slip through very small cracks, making it virtually impossible to completely contain. As an alternative, some solar-thermal rocket systems are being designed to operate using methane, paying the performance penalty associated with its higher molecular weight and subsequent lower exhaust velocities, in preference to the long-term storage problems posed by hydrogen fuel.
In terms of thrust, solar-thermal rockets are intermediate in performance between chemical rockets and ion rockets. In part, because of the requirement for massive solar concentrators, no solar-thermal rocket will ever lift off from a planetary surface. But accelerations of 0.01 Earth gravity are possible in the space environment.
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