section. If initial results are not favorable for today's environment, factors effecting energy cost have been identified which may change this outcome in the not so distant future. Figure 2.8 Comparative Fuel Costs for Cumulative U.S. Energy Demand 2.3 Space Energy To fully characterize the status of space power today, we must know what the power is used for, the places where the power is used, and the types of power available to satisfy space energy needs. This includes not only electrical power, but also chemical, thermal and mechanical power. Major uses for space power are for transportation and satellite station keeping. Using typical examples, energy use rates are established for communication and other-purpose satellites. The current populations of satellites are estimated and multiplied by these rates to establish total energy demand in space. And finally, a general discussion of types of space power available to satisfy these needs is presented. Uses of Energy in Space In space as on Earth, transportation, or propulsion systems account for the greatest part of energy usage. The vast majority of the space propulsion energy is used in ascending from the Earth's surface to Low Earth Orbit. After this, the propulsion energy used for orbital transfer from LEO to GEO and beyond is the next greatest consumer, and is still an order of magnitude greater than the remaining propulsion needs of present-day spacecraft. For example, the AV required for transfer from 200 km LEO at an inclination of 28° is 5.42 km/sec, which means that even when using high-ISn LOX-LH2 engines 70% of the spacecraft's LEO mass must be devoted to propellant for the transfer. By contrast, a typical satellite might use about 400 m/sec of AV over the rest of its lifetime. [Chetty, 1991] Even using a lower ISp monopropellant to provide this energy, only about 5% of the final spacecraft mass need be devoted to propellant for non-launch, non-transfer purposes. This final portion of propellant usage serves several purposes. It is used to make fine corrections to the orbit injection provided by the launch vehicle or transfer stage, for station keeping, for attitude control, and for stability. It also includes extra propellant for contingencies and an additional margin, usually on the order of 10%. Figure 2.9 below shows the propellant usage breakdown for a sample satellite in a 900 km orbit. The values in the pie chart representing the velocity budget (ft/sec) required to maintain a 3000 pound satellite in an 850 km orbit.
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