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 example, the SM-RCS propellant valves are located adjacent to the SM skin, and their temperature is relatively sensitive to spacecraft orientation (even though they are insulated). Heaters are provided for the valves to accommodate the unavoidable thermal cycling experienced in lunar orbit, but the use of passive thermal control during the translunar and transearth coast phases, allows conservation of electrical energy which would otherwise be required for heater operation. More important, however, the use of passive thermal control negates the necessity for an active coolant loop to cool these same sensitive components.

Passive thermal control is usually referred to by the less technical, but more descriptive, term of "barbequing". The orientation maneuver is made such that the spacecraft longitudinal axis is placed perpendicular to the vehicle-sun line (figure 32). The orientation of the longitudinal axis about

the sun vector is chosen to minimize the interference of the subsequent roll maneuver with high gain antenna coverage. After stabilizing the spacecraft in this orientation, a slow rotational rate about the X-axis is established to achieve the desired thermal cycling.

As a result of small residual rates about all these axes when

the spacecraft was "stabilized", and as a result of other sources of disturbances such as fuel slosh and steam venting, the vehicle will begin slowly to precess about its angular momentum vector. Current analysis indicate that a roll rate of about 2.5 revolutions per hour will be required in order to maintain the spacecraft YZ plans within 20° of the sun line, which is the tolerance required to maintain effective thermal control.

It is emphasized that the thermal cycling operation can be interrupted for periods of up to three hours, provided these attitude hold periods are followed by an appropriate period of barbequing (5 to 7 times the hold period). In addition, the thermal design is such that a three hour period prior to lunar orbit insertion and prior to entry can be accommodated without the necessity of subsequent thermal cycling.

Following the establishment of the barbeque mode, operations onboard the spacecraft will settle down to a routine for the next 2½ days. Periodic systems status checks will be performed by the crew, the spacecraft's position and velocity will continue to be monitored by the ground, and data will be transmitted continuously by the spacecraft. For the sun's position such as to give us a 7° to 20° elevation angle at lunar landing, the passive thermal control maneuver can usually be set up such as