In addition to the technological problems another critical aspect for an externally supplied vehicle is the selection of the stabilizing medium. On the one hand the stabilizing medium is the emitted pollutant (in type and amount), on the other hand the stabilizing medium determines configuration and engine-type of the vehicle. The specific impulse is inversely proportional to the square root of the molecular mass, and the density of the (liquid) stabilizing medium determines size of vehicles. With respect to type, amount of pollutant, technical suitability and availability, hydrogen (H2), helium (He), nitrogen (Nj), oxygen (O2) and argon (Ar) are being selected for analysis. As the molecular mass should be as small as possible, heavier elements or chemical compounds do not come into consideration. Table 1 shows the characteristic data of the selected materials. Assuming the prescribed scenario with a launch vehicle of 50 Mg payload capacity (SSTO) the following approach is used: The launch mass is given by the basic equation of rockets (Ziolkovski):
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