thrusters are also accounted for as required. Component specifications on a per unit basis derive from expert opinion obtained at the Technical Interchange Meeting, and represent 20-year technology projections as well as low-cost manufacturing techniques in place at that time. This basic data includes unit mass estimates as well as development (non-recurring) cost and production (recurring) cost. The calculation procedure employed in this spreadsheet model is designed to customize an overall GN&C subsystem appropriate to the specific functional requirements of each different SSP system concept under consideration. For example, the LEO SunTower utilizes gravity-gradient stabilization and fixed pointing of the solar power collectors in a sun-synchronous orbit, which greatly simplifies the attitude control problem. In contrast, the MEO SunTower requires variable forced Sun-tracking implemented by a combined program of SunTower roll-axis control and individual collector pointing control about a single axis; this requirement imposes a significant mass and cost burden on the attitude control subsystem The model calculations begin with quantitative estimates of the moments-of-inertia for any given SSP concept as determined from the system mass and size characteristics data passed to the GN&C spreadsheet. The inertia information together with specified angular limits or rates then determines the momentum and torque authority required by the control effectors. Orbit trim propellant is also calculated based on estimates of the annual AV budget and electric thruster specific impulse performance. The overall GN&C subsystem mass and cost is thus synthesized in this manner. 4.3.5. Thermal Control Like the Energy Storage subsystem, the contribution of the Thermal Control subsystem to the overall mass and cost of the SSP flight segment is contained within the Power Transmission portion of the model It is within that subsystem’s worksheet that the waste heat generated by the power transmission chain is computed, and that value is used to specify the requirement for thermal control Although thermal control is distributed throughout the other subsystems as well, the removal of waste heat from the transmitting devices and the Power Transmission subsystem as a whole, represents the most significant requirement for thermal control, and is the major driver of mass and cost for that subsystem A lookup table is again employed as the means of relating values of specific mass and cost to user- selected technologies. The available technologies include device-integrated radiators, large dedicated radiators, and advanced systems such as belt radiators and droplet radiators. For the most part this study has assumed the use of integrated radiators with a specific mass of 0.07 kg/kW and recurring cost of SlOO/kg. These values are passed to the Power Transmission worksheet and used in the computation of overall mass and cost of that subsystem 4.3.6. Telecommunications/Command & Data Handling Modeling of the TCM/C&DH subsystems for both power and relay satellites is fairly trivial compared to the other elements of the SSM. The reason for this is that SSP concepts appear to levy no special requirements upon these functions beyond that found in conventional Earth-orbiting spacecraft, and suggest lesser requirements for these subsystems than their counterparts in planetary spacecraft. Inputs to the TCM/C&DH worksheet are therefore limited to the number of modules per spacecraft and the estimated mass of each module (in the latter case, the user can default to values based on historical data). A built-in value of $100,000/kg is used to estimate the cost of these modules. The estimated mass and cost of the complete subsystem is subsequently combined with values for Structure and Harness and passed on to the Summary worksheet to be reported as a separate category in the context of platform support.
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