| Rocket Fundamentals The thrust of a rocket is the simple product of its exhaust velocity (relative to the ship) and its propellant mass flow. Thus one can achieve the same thrust by either ejecting more material at low velocity or less at high velocity. Clearly, since the material must be carried on board, the latter approach is preferred. Thus, the fundamental goal of rocket propulsion is to achieve the highest possible exhaust velocity. Generally, in thermal rockets (chemical, nuclear or otherwise) this leads to high exhaust temperatures. Thus, these motors must be quite sturdy, yet lightweight enough for flight. This implies that a careful choice of materials must be made. In non-thermal rockets, other forms of materials issues are still present, having to do with the bombardment of walls and components with very fast (albeit cold) moving particles. Witnessing a rocket launch makes one aware of the extreme conditions under which they operate. Indeed, their design is a tribute to human ingenuity. These devices operate at the limits of known materials, in an exquisite and unforgiving balance. For example, a space shuttle main engine has an exhaust velocity of about 4500 m/sec, at an exhaust temperature of about 1400° C. Its high-strength alloy engine nozzle must be actively cooled by liquid hydrogen, in order to prevent melting. The power level of such an engine is about 5 GigaWatt, enough to provide electrical power to a large city. Yet, all of this power comes in a package no bigger than a small automobile. Our society has colloquially recognized the remarkable achievement of our early rocket pioneers by the popular term "rocket science." |
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