temperatures) and the radiator can be integrated with the spacecraft with greater flexibility than for a number of RTGs. The DIPS radiator could also offer significant protection for the spacecraft components from hostile natural and man-made environments. The DOE is conducting a DIPS ground demonstration system program. The objective of this program is to select, develop and demonstrate the performance and reliability of prototypical flight hardware at the 6 kWe power level for use in future military and civilian space missions. The program has completed the selection of the dynamic conversion cycle to be used and is currently in the design phase of the engineering unit for use in an integrated system test of the dynamic power conversion loop scheduled for 1993. The current DIPS concept is shown in Fig. 4. The heat source assembly is composed of six stacks of GPHS modules (17 per stack) for a total heat input of about 25 kWt at time of launch. The power conversion unit includes a completely redundant pair of closed Brayton cycle loops with their turboalternator compressor units and recuperators, each capable of producing the full power output of 6 kWe. The single phase working fluid is a mixture of Helium and Xenon gases. The working fluid is heated in tube coils in the heat source unit; flows to the turbine which turns the alternator and compressor; then flows from the turbine to the recuperator where excess heat is transferred to the fluid entering the heat source unit; then flows to a heat pipe radiator assembly; and then back to the compressor for recirculation through the system. A power conditioning and control unit is included to control the DIPS and to match its power output with the spacecraft. The 6 kWe DIPS is being designed for an operating life of 10 years with a reliability goal of 0.96. Its overall system efficiency is 24% (BOM) and improves slightly over the mission lifetime. The current weight estimate for the DIPS flight system, with redundant power conversion loops, is approximately 905 kg for a specific power of 6.6 We/kg. The DIPs heat source assembly is designed to reject its heat through variable conductance heat pipes backed up with a passive non-reversible melting foil insulation feature in case of total loss of cooling by failure of both working fluid loops. The heat source assembly is also designed to free the GPHS modules in the event of reentry at or near orbital velocities, so the GPHS modules can provide their proven safety attributes. Summary An overview of the DOE program for developing advanced radioisotope space power systems has been presented. This program is based on the experience gained from 28 years of successful use of radioisotope power systems in space. Steady advancements in the technologies involved have been, and will continue to be, made to allow us to provide improved system performance and provide the high degree of safety required for future mission applications of such systems over a wide range of power levels, a wide spectrum of operational environments and for extended mission lifetimes.
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