of these capabilities are considered adequate for the Powersat demonstrator mission. The total mass of the Service Module is approximately 10-12 kg, including the launch vehicle interface adaptor. The total height above the ASAP would be 242 mm. If the camera was deleted, this would be reduced to 192 mm. These Microbus modules are both proven and reliable and would offer the lowest cost option for the Powersat mission. Microwave Source Discussion It became evident during the previous Eurospace Powersat study that the use of space-qualified TWTs was by far the simplest, most efficient, available and least expensive method of generating the required microwave energy. This was reaffirmed by the detailed trade-off analysis performed by Thomson Tubes and presented in the Appendix. Even though these proposed TWTs are as much as 55-60% efficient, radiating the waste heat may still present a problem within the small area available on top of the Service Module. However, because of the high mass margin available, the TWT collectors could be clustered on a heat sink - several kilograms of aluminium, for example. Thus, as the experiment proceeds, the heat-sink temperature will slowly rise to the maximum operating limits of the TWTs, at which point the experiment would be terminated to allow the heatsink to cool. It should also be noted that these TWTs can be pushed to higher levels of power (e.g. beyond qualification limits) during the final experiment runs. According to Thomson Tubes, the basic characteristics of the TWTs baselined for this reference concept are as follows:
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