second spacelab flight under German management (D2-mission) is scheduled for December 1991. As an alternative the D3-mission is possible. Comparison and Summary Alternatives The characteristics of the three types of microgravity experiments are summarized in Table II. As an alternative to the drop tower experiment a parabolic aircraft flight experiment should be mentioned. It offers a microgravity period of about 30 seconds but the worse microgravity level of 10-2 g and higher security requirements are disadvantageous. There are a lot of alternatives for the spacelab experiment if no human interaction is necessary, e.g. MAUS (Materialwissentschaftliche autonome Experimente unter Schwerelosigkeit; autonomous material science experiments in weightlessness), GAS (Get Away Special) and TOPAS (Transport Operation of microgravity Payloads Assembled on Scout; not yet available). The Institute of Space and Aeronautical Science (ISAS) in cooperation with the National Space Development Agency (NASDA) and the Ministry of International Trade and Industry (MITI), Japan, is preparing a sounding rocket experiment and an experiment on the Space Flyer Unit (SFU) to test a complete small-scale solar dynamic space power system (SDSPS) [5,6], A Technology Development Mission with the US Space Shuttle was proposed by NASA-Lewis in 1986 to test a complete SDSPS with about 7 kWe [7]. Microgravity experiments in general are necessary for the development of a SDSPS with heat of fusion storage. Cost and Profit The main problems to be solved to carry out the microgravity experiments have to be solved anyway for TES development. All of the described experimental PCM configurations are possible TES configurations, e.g. the drop tower experiment configuration resembles the packed bed tube design in fabrication at AiResearch, USA [8]. Therefore the additional costs caused by the microgravity experiments are very small compared to the TES development costs. This coincidence of microgravity experiments and TES development will minimize costs and risks of the whole project. For each microgravity
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