Thus, it can be concluded that either chemical or electric propulsion systems can be chosen to fulfill the task of drag compensation, since both can easily supply a countering force of this magnitude. Now, from Figure 10.4.8, the worst case scenario provides a delta V of about 180 m/s per year. Using this value, a comparison of chemical and electric propulsion for this task can be performed. This analysis uses the ‘Rocket equation', as shown below: In this case, we assume an initial mass of 70000 kg. The resulting mass data is tabulated below (note: Mp = mass of propellant used): ____________________ Table 10.4.2 Propellant Masses____________________ From the table, it can be seen that the electric propulsion system offers a significant propellant mass saving. It also has the property that it is compatible with the propulsion system for orbit raising although the thrust requirements for each operation are different. The main disadvantage of using electric propulsion at this phase of the mission is the power requirement. The specific power for an electric propulsion system is typically 35 kW/N for the magnitude of specific impulse given. Hence, to supply 0.65 N to counteract the drag, the electrical power required is 22.75 kW. This may be a problem in the initial stages of construction of the spacecraft, when the solar arrays are not operational (although the drag will be lower when the solar arrays are not in place). However, it has been decided that this disadvantage is outweighed by the advantages of fuel mass saving, system compatibility and avoidance of attitude disturbances (as mentioned at the beginning of this section). Orbit Raising Here, the maneuver considered is that of raising the spacecraft's orbit from LEO (350 km) to GEO coupled with a 28.5° plane change. There are two basic options: • a Hohmann transfer using high thrust, chemical propulsion, coupled with a plane change at apogee (minimum energy case) • a spiral transfer using electrical propulsion, with the plane change integrated optimally in the maneuver. These two options are illustrated schematically in Figure 10.4.9 and are analyzed below.
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