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Weight did not dominate the design for HEAO-B. Its payload instruments existed before the program developed the spacecraft to carry them. The program was able to establish large weight and power margins. The fairing diameter affected the spacecraft's size, but the instruments fit within this diameter. Power requirements were modest and a planar body-fixed array was used. The spacecraft mission required precision pointing of the payload, which led to 3-axis attitude control using gyroscopes, star sensors, and reaction wheels. Kick-stage propulsion was not required. Hydrazine monopiopellant propulsion was used for attitude control.

Design Budgets

Table 10-34 presents the propellant budget, velocity increments, specific impulse, and spacecraft mass for FLTSATCOM. To derive the velocity required to insert the spacecraft into its final orbit, the designers used the methods in Chap. 7. That chapter also describes injection-error analysis which the designers used to derive the velocity increment that corrected the orbit The ratio of this error to the A KM velocity (0.5%) is typical of current performance for solid motors. The stationkeeping velocity derives from analysis of orbit perturbations over the spacecraft's design life. The stationkeeping increment developed from an operational requirement to move the spacecraft at a rate of 15 deg longitude per day. During orbit injection FLTSATCOM spin-stabilized at 60 ipm and maneuvered through an angle of 65 deg while spinning. Eqs. (10-7) and (10-9) translate these requirements into propellant weight During its lifetime the spacecraft uses propellant for attitude control, mainly to counter solar radiation pressure. HEAO-B needed propellant only to acquire attitude and to cancel disturbance torques (principally aerodynamics and gravity gradient; see Table 10-18). The propellant's mass was 138 kg.

TABUE 10-34. Propellant Budget for FLTSATCOM (See Table 10-4).

Element

Mass (kg)

Design Characteristics

Velocity Correction and Control

0 0

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