4.25 K (at 1013 mbar). In addition, the resources of helium are strongly limited (helium is 0.0005 % of the atmosphere, by volume ). Due to the economics and scarcity of resources, helium seems unsuitable for a stabilizing medium. Nitrogen (N 2) For nitrogen, the desired specific impulse of 1000s probably cannot be maintained. Assuming I sp= 500s, a combustion chamber temperature of 15000 K and a required power of about 21 GW are expected. However, for an optimistic assessment, the utilization of nitrogen might be feasible. The production of liquid nitrogen is technically simple and inexpensive, the resources are inexhaustible. After expansion there are still jet temperatures of several 1000 K, thus nitrogen will react with oxygen/ozone to form nitroxide (NOX). The impact to atmosphere of nytroxides in this amount is to be analyzed more detailed. However, a noticeable negative impact is certain. Oxygen (O2) Enormous technological problems are expected if oxygen is used as a stabilizing medium. However, in comparison to the other mediums the technical complexity is in the medium range. As neither hydrogen nor helium nor nitrogen are suitable, oxygen is - on the condition of the concept's principle feasibility - the most likely handle-able stabilizing medium. Magnetic confinement of the jet is indispensable, because dissociated oxygen is extremely aggressive. Thus, it would destroy the hardware of the engine. An impact to atmosphere cannot be identified, but a detailed analysis is requisite for every case. Due to emission of oxygen into the ozone cycle even a "curing effect" is conceivable (hypothetically!). The resources are inexhaustible (21% of the atmosphere by volume, 89% in water), liquefaction is technically simple and inexpensive. Argon (Ar) With argon, the desired specific impulse of 1000s cannot be maintained. Reducing the Isp = 500s leads to a combustion chamber temperature of about 25,000 K and power to be supplied of 26 GW. It is questionable, whether the technology can be provided at all. If necessary, smaller vehicles or booster support up to 10 km altitude must be analyzed. The impact to the environment is non-critical, similar to helium. Production and liquefaction of argon is technically simple and inexpensive, the resources are sufficient (argon: 0.93% Vol. of atmosphere). Appraisal The concept of an externally supplied vehicle is based on reduced environmental load and increased payload fraction of up to 25%. Thus, analyzing only the technical feasibility would not be sensible because conventional systems are remarkably easier to develop and operate. Nevertheless systems also have to be cost effective. Environmental compatibility and technology are strongly connected. If high specific impulses are not achievable the mass flow rate and. consequently the amount of pollutant, increases. As analysis has shown, only oxygen or argon is suitable for an externally supplied vehicle. However, these concepts will lead to nearly insurmountable technical problems.
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