All calculated costs are related to an electrical energy cost of $ 0.07/KWhe with amortization of 30 years by tax on real estate of 4% [4], Assuming the cost for allocation of LOX in dollars is the total cost per year, allocation for propellants is given by: The annual cost for production of propellant is on the order of the money made by selling electricity from one 10 GW SPS, assuming an electrical energy price of $ 0.07 / KWhe. Cost for Transport The lifetime revenue from one 10 GW SPS is on the order of $200 billion. The payload to be transported is 75000 Mg for one 10 GW SPS according to the reference scenario. From an economics perspective the maximum cost for transport from Earth to GEO might be 25% of the lifetime revenue leading to specific transport costs of about $650/kg. Assuming future operation capability and serial production of a standardized vehicle this seems to be attainable. Externally Supplied Vehicle Because of the environmental impact of conventional launch systems the concept of an externally supplied vehicle HDCF (Heavy Duty Cargo Freighter) is being proposed. This launch system is equipped with a laser-thermal propulsion system, the required power is being supplied by laser from the ground or space. For conversion of energy into thrust, an environmentally compatible stabilizing medium is heated up in the combustion chamber of the engine. As the thrust depends no longer on the combustion enthalpy, very great specific impulses are theoretically possible. Generally, large specific impulses yield lower lift-off weights, leading to lower mass flow rates and, consequently, to reduce pollutants emitted to atmosphere. However, the large specific impulses are restricted due to the quadratically increasing combustion chamber temperature (Equation of St. Venant and Wantzel). With magnetic confinement of the jet, temperatures of up to some 10000 Celsius may be handled. This assumption seems to be very optimistic and should be analyzed in more detail. The high combustion temperature is, in addition to the technology of a high power laser, the most significant problem of this configuration. A power estimate resulted in an upper payload limit of approximately 50 Mg. In this case the required laser power is on the order of some 10 GWs, a dimension, the handling of which is presupposed by the SPS concept. By comparison, a conventional launch system of the same magnitude (50 Mg PL/SSTO) and a payload fraction of 5 % produces approximately 30 GW.
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