Requirements for Spacecraft Materials

Abstract

The Space Age began in 1957, with an 83 kg Russian Sputnik satellite bleeping greetings to a surprised world. Since that spectacular beginning, intensive effort has gone into the scientific exploration of space, exploration of the Moon and distant planets, manufacturing of materials in space laboratories, and exploiting orbiting satellites for communication, navigation and observation of the Earth. The early steps have passed into history, and most equip-ment and instrumentation has been and will continue to be replaced by lighter and more complex substitutes. The remarkable achievements of the Apollo Lunar Exploration Programme two decades ago still tend to overshadow the unmanned automated satellite flights, and it is not always realized that spacecraft orbiting above all continents of the world have already revolutionized global communications, maritime navigation, and worldwide weather forecasting. These satellites are now vital links in a global network. They would not have been economically or technically feasible before the advent of near-Earth space explorations. Satellite communications started on a commercial basis with the launch of Early Bird in 1965, less than eight years after the launch of the first Sputnik. This was the first satellite to remain stationary over the Earth, and it was able to provide a continuous connection between any two Earth stations. Until comparatively recently these so-called 'ap-plications' satellites were merely assemblies of separately designed components rather than thoughtfully integrated systems. Often component interfaces failed to match, reducing the overall system performance. These satellites, and to a more limited extent the 'scien-tific' satellites, are now incorporating standardized subsys-tems in an attempt to optimize performance factors including weight, reliability, and cost. It seems likely that the spacecraft designer has placed greatest emphasis on mass, as this is usually set by the capabilities of the assigned launch vehicle which will take the satellite from the Earth's surface and inject it into the desired orbit. The lighter the satellite, the cheaper will be the launch costs. Another major performance factor, reliability, can also be purchased if money is preferentially funneled into reliability and test programmes rather than launch vehicles. The important point is that performance factors of weight, reliability, and cost are all interrelated. The designer of an applications satellite will be more willing to pay for the reliability level that would give him 10 years of operation than the designer of a scientific satellite designed to shut off transmission after only one year when the mission objectives are attained. One of the major aims of the European satellite manu-facturer has been to set up a European communications programme which will develop and launch long-life satel-lites. A supporting technology programme has been under-taken to develop and qualify most of the critical subsystems that will enter the design of future operational satellites. An experimental satellite (Orbital Test Satellite—OTS) was launched in 1978 to evaluate and test the performance of the various subsystems of future European communication satellite systems. OTS and its launcher are illustrated in Figs. 2.1a and 2.2. Its major subsystems under evaluation included: • communications—to relay information (data and