cargo transport from Earth to LEO, two principal approaches for the transportation system are being analyzed. One concept consists of SSTO vehicles with fully reusable engines and avionics modules. These vehicles are equipped with conventional engines and the propellant consists of LH2/LOX. The structure (tanks etc.) is left in orbit and used as part of the structure of the SPS. As an alternative concept an externally supplied vehicle is being considered. The required power is being supplied by laser from the ground, by which the stabilizing medium (working gas or liquid) in the combustion chamber of the engine is heated up. The structure of this vehicle is also used as part of the structure of SPS. Conventional Launch System In order to minimize the expenditure for operations, the HLLVs must be optimized with respect to size and construction, propellants and propulsion system, reusability and reliability. One approach consists of a conventional vehicle, the structure of which is left in orbit and used as part of the structure of SPS. Sizing these vehicles leads to a payload capacity of about 50 Mg; much larger vehicles are not suitable for using their structure as part of the SPS [1]. In addition, problems regarding flight stability can be identified, and the costs for development would probably be much higher than for a 50 Mg vehicle. On the other hand smaller vehicles are not efficient for the high transportation requirements Assuming a vehicle with a pay load capacity of 20 Mg, the launch rate increases to about 10000 launches per year, equivalent to 30 launches per day! Whether the structure of these vehicles can be used for build-up of an SPS seems to be doubtful at least. In principle, the highest efficiency will be obtained by consequent design to cost The payload is to be standardized and the vehicle has to be designed to the payload. As the structure is used as "secondary payload" in orbit, an SSTO-vehicle is preferred. The engine/avionics module can be constructed as a fully reusable, ballistic reentry system. The propellant consists of LH2/LOX. All other propellant combinations lead, due to lower specific impulses, to considerable higher pollution. In order to maximize the efficiency, a rocket engine is to be developed, that provides the (staged) combustion in the ratio 1:8 (LH2: LOX). Assuming a specific impulse of 450 s and a structure ratio of 5% (engine/avionics module) leads to a payload fraction of about 6% for launch near the equator. This results in a total propellant mass per year of about 2.375 E106 Mg, subdivided into 264 E10^ Mg for LH2 and 2.11 ElO^ Mg for LOX. Energy Demand and Cost The energy required for production of LH2 is divided into energy for electrolysis and energy for liquefaction.
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