taic system and/or increasing the battery life. For simplicity, we will refer to this load control strategy as the "DANMOE" (Day and Night Management of Energy) technique. The DANMOE method is the principal basis for the sizing and orbital life optimization concept presented in this paper. The basic scenario behind this approach is to run the spacecraft in orbit such that electrical loads during the night portion of each orbit are reduced and the daytime energy consumption is correspondingly increased. As will be shown later, the average bus power during an orbit stays at least equal to the case when the day and night loads are equal. To determine the required solar array during the spacecraft design phase, day and night loads are to be defined for the maximum eclipse orbit, but the night load is to be minimized and the day load maximized. The advantages of doing the night to day load shifting are generally known to the power system designers, but no attempts have been made in past spacecraft. The main reasons for this are due to a combination of the following: (1) the use of a single orbit load value (the orbit average power) is easy, and it is a general practice for solar array sizing, (2) the electrical loads for many spacecraft, especially the low-power ones, stay relatively constant once the spacecraft is deployed and all loads are turned on, (3) power switching (on/off) are avoided for reliability reasons, (4) insufficient knowledge of the accuracy of load power and duty cycle, (5) too complicated for on-board or ground implementation, (6) insufficient technical justifications at the spacecraft level, and (7) on-orbit operation and optimization needs, especially for a housekeeping subsystem, usually end up at the bottom of the priority list. The DANMOE scheme, although inherently simple in concept, is not exactly easy to mechanize and perhaps not even practical on low-power spacecraft. Spacecraft system designers as well as the flight controllers and the crew may generally find it difficult to accept the idea of a power subsystem having a maximum capacity for eclipse periods and another for the daylight periods rather than the conventional one orbital average capability. However, the future high power space vehicles, the Space Station in particular, cannot afford the luxury of an unlimited energy source, and should resort to an operational strategy to reduce the weight and cost of the power system. The first manned Space Station, the SKYLAB program, has taught us the following vital lessons [4], for a manned spacecraft: (1) load management is not only desirable but technically mandatory to accomplish certain experiments and to accommodate various emergency and contingency conditions, (2) there was a large amount of reserve power available during the actual orbital operation that was not utilized simply because the subsystem and spacecraft designers did not plan for its use, and (3) mission operations objectives and requirements, specifically the load management area, were not considered and emphasized during the initial design phase. The two basic requirements to effectively implement the DANMOE strategy are as follows: 1. Operate the photovoltaic system in orbit to achieve the following criteria: Maintain a positive bus power margin based on the orbital day and night average power capabilities and the corresponding average power demand for day and night durations. Avoid battery discharge more than the pre-selected depth of discharge limits. Satisfy the basic mission and spacecraft operational constraints. Minimize electrical loads during eclipse periods by reduction of loads or shifting them to the adjacent or succeeding daytime period. 2. Size the solar array and batteries to a pre-defined orbit average bus power level and day to night load ratio, with the night load defined for the maximum eclipse orbit but minimized and the day load maximized.
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