configuration was dictated by minimum mass shield requirements, especially the requirement to eliminate neutron scattering around the steel-reinforced lithium hydride shadow shield. The base diameter was established by the Agena vehicle payload and the upper diameter was determined by the effective area of the reactor. The length was determined by the total radiator area requirement. The total system mass of the final flight unit (known as FS-4) was 435 kg including the shield [22]. The reactor is shown in Fig. 22. The power conversion system basically consisted of 2880 SiGe thermoelectric elements mounted in groups of 72 along 40 stainless steel tubes through which the NaK coolant flowed. Figure 23 shows the overall thermodynamic cycle including a thermoelectric module. Despite its lower figure of merit at the SNAP-10A operating temperatures SiGe was chosen over PbTe because of (1) its stability to higher temperatures; (2) its potential for future performance growth; (3) its ease of manufacture; and (4) its mechanical properties. The converter hot side operating temperature was about 765 K and the mean radiator temperature was about 590 K [22]. On 3 April 1965, SNAP-10A was placed into a 1288 km by 1307 km orbit by an Atlas/Agena launch vehicle. Once it was confirmed that SNAP-10A was in a very long-lived orbit, the AEC authorized startup of the reactor [22]. Figure 24 is an artist's concept of SNAP-10A in space with the Agena. The automatic startup of SNAP-10A was accomplished flawlessly. The response of the FS-4 flight system was in excellent agreement with predictions based on analog computer studies and ground test results obtained from the FS-3 reactor. Net power
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