For unmanned spacecraft in general, power requirements for subsystems of all types come to about twice the power needs of the payload. Locations of Energy Demand in Space For the geographical energy demand in space, the number of satellites or other space vehicles at a particular location are counted and then a multiplier for typical energy demand is applied. For the estimation of energy demand of satellites or other unmanned space vehicles, roughly 1.5 kW per communication satellite and 1 kW per each other-purpose satellite is assumed. So daily energy demand becomes 36 kWh per 1 communication satellite and 24 kWh per 1 other-purpose satellite. In the case of manned vehicles, we have to add 1 kW per person for his or her life support. But currently, very few persons are on satellites or vehicles, so we may neglect them. T rans-atmospheric In this area, the main energy demands are propulsion. For the estimation of propulsion energy, we calculate the energy of launching a two ton satellite to 1000 km altitude orbit. The initial vehicle has fuel and the total mass is about 100 tons. The velocity of exhaust gas is about 4500 m/s. We can calculate the kinetic energy of the exhaust gas. The kinetic energy of the vehicle is negligible. From this rough estimation, the energy is about 250 MWh. Considering the 350 satellites in operation and the typical mission time (10 years), the energy demand becomes 25 MWh/day. As energies are necessarily concentrated on propulsion, a more meaningful estimate might be based on a typical first stage burn rather than on average energy. Earth Orbit Defining low altitude orbit as less than about 5000 km, over 100 satellites are on missions in this orbit. So energy demand is about 2500 kWh/day. Higher altitude orbits currently contain about 200 satellites (140 communications satellites and 60 other purpose satellites) generating an energy demand of about 6500 kWh/day. Interplanetary Three satellites are on interplanetary missions. One is Magellan, another is Galileo. Each satellite uses about 600 W so in a 24 hour day the total energy demand is roughly 30 kWh/day. Interstellar At this time, 3 spacecraft are on interstellar missions: Voyager 1, Voyager 2, and Pioneer 10. These spacecraft use only about 300 W with a cumulative energy demands of about 22 kWh/day. Providing Space Power There are many sources of electrical power available that have flown in space. Photovoltaic arrays, batteries, fuel cells, radioisotope thermoelectric generators (RTG) and nuclear reactors represent the systems used in space. Each of these technologies have been used on many space missions. A photovoltaic array intercepts light from the Sun and converts it to electricity. This array typically consists of many individual solar cells that are wired together to produce the total power needed for the satellite or spacecraft. In Earth Orbit, a battery system is often needed for storage of energy during the period when the vehicle is in shadow. The solar array will charge the battery while in sunlight and the battery will be used during the shadowed part of the orbit. On each successive orbit, the battery is recharged. Batteries are typically not used exclusively on satellites unless their lifetime is very, very short: several hours or days. The total mass of batteries is very, very high for missions that last longer than this short period. Fuel cells are also an option for energy storage. These cells would use solar array power to electrolyze water (H2O) into O2 and H2. The O2 and H2 would then be recombined and release energy during the shadow period of the orbit. Water is produced again as the O2 and H2 recombine. On the next orbit, the cycle of electrolysis and recombination would repeat. For missions that require flight far from the Sun or are in shadow for long periods, nuclear power may be preferred. Examples of these missions would be flights beyond Mars or powering a lunar base during the long 354 hour lunar night. Both radioisotope thermoelectric generators (RTG) and nuclear reactors have been used on Earth orbiting and planetary spacecraft. However, current international
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