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The Cassini spacecraft, NASA's mission to investigate the Saturn system, has undergone a system-level thermal balance test program to permit verification of the engineering subsystem thermal designs in the simulated worst-case environments. Additionally, other objectives such as functional checkouts, collection of thermal data for analytical model adjustment, vacuum drying of propellant tanks, and flight temperature transducer verification were also completed. In the interest of cost and schedule, transient off-Sunpoint conditions were not tested. The testing demonstrated that the required system resources such as heater power and radiator area were adequate for all engineering subsystems. The only changes required from the results were related to the operation of some of the subsystems. In the instance of the thruster cluster assemblies, allowable flight temperature limits were exceeded for the assumed operational environment.

The Cassini spacecraft, NASA's mission to investigate the Saturn system, has undergone a system-level thermal balance test program to permit verification of the science instrument thermal designs in the simulated worst-case environments. Additionally, other objectives such as functional checkout, collection of thermal data for analytical model adjustment, and flight temperature transducer verification were also attained. In the interest of cost and schedule, transient off-sunpoint conditions were not tested. The test demonstrated that the required system resources such as heater power and radiator area were adequate. In the instance of the Cosmic Dust Analyzer, allowable flight temperature limits were violated, but this problem is being addressed without a significant impact to system resources or thermal design robustness. Finally, the thermal acceptability of a black Kapton “sock” was demonstrated for the magnetometer boom.

Although important efforts are actually devoted to improve Si-Ge materials, their thermoelectric energy conversion efficiency remains relatively low and the adimensional ZT value does not exceed 1. Higher values can be obtained by investigating new materials. A search for new high temperature thermoelectric materials identified a certain number of compounds between transition metals and bismuth, antimony and germanium as potential candidates. Results of the preliminary synthesis of samples by a variety of techniques (Bridgman, mechanical alloying…) are presented as well as some electrical measurements. Some compounds showed interesting properties and need to be investigated in more details.

The effect of changes in the carrier concentration and mobility for heavily doped n-type SiGe on the electrical power factor has been investigated. It has been shown that power factors of 37-40 μV/cm-K2 can be achieved with carrier concentrations of 2.0 - 2.5 × 1020 cm-3 and mobilities of 38-40 cm2/V-sec. Many samples with suitable carrier concentration do not have high mobilities and some rationale for this behavior is presented. Initial results are presented on fabrication of n-type samples from ultra-fine powders. The emphasis in this work is to achieve thermal conductivity reductions by adding inert particles to scatter mid-frequency phonons.

The Mars Pathfinder spacecraft which will be launched in December 1996 features an active cooling system for controlling the temperature of the spacecraft. This will be the first time that such a mechanical pump cooling system is used on an interplanetary or long duration flight (over two weeks) in space. The major element of the cooling system is the Integrated Pump Assembly (IPA). It uses centrifugal pumps to circulate liquid freon to transfer heat from spacecraft electronics to an external radiator. The IPA consists of redundant pumps, motor control electronics, thermal control valves, check valves, and an accumulator. The design and flight implementation of this pump assembly were accomplished in less than two years. This paper describes the design, fabrication, assembly, and testing of the IPA.

The Mars Pathfinder (MPF) Spacecraft, scheduled for a December 1996 launch to Mars, uses a mechanically pumped loop to transfer dissipated heat from the insulated lander electronics to an external radiator. This paper discusses the tradeoffs performed before choosing a mechanical pumped loop as the thermal control system for MPF. It describes the analysis, tradeoffs, design, and predicted performance of this system. The various development tests performed are discussed, along with the current status of this cooling system. Finally, some thoughts on the development of mechanically pumped loops for future spacecraft are presented.

To achieve high thermal-to-electric energy conversion efficiency, it is desirable to operate thermoelectric generator devices over large temperature gradients and also to maximize the thermoelectric performance of the materials used to build the devices. However, no single thermoelectric material is suitable for use over a very wide range of temperatures (~300-1000K). It is therefore necessary to use different materials in each temperature range where they possess optimum performance. This can be achieved in two ways: 1) multistage thermoelectric generators where each stage operates over a fixed temperature difference and is electrically insulated but thermally in contact with the other stages 2) segmented generators where the p- and n-legs are formed of different segments joined in series. The concept of integrating new thermoelectric materials developed at the Jet Propulsion Laboratory into a segmented thermoelectric unicouple has been introduced in earlier publications.

Advanced thermoelectric microdevices integrated into thermal management packages and low power, electrical power source systems are of interest for a variety of space and terrestrial applications. By making use of macroscopic film technology, microgenerators operating across relatively small temperature differences can be conceptualized for a variety of high heat flux or low heat flux heat source configurations. The miniaturization of state-of-the-art thermoelectric module technology based on Bi2Te3 alloys is limited due to mechanical and manufacturing constraints for thermoelement dimensions (100-200μm thick minimum) and number (100-200 legs maximum). We are developing novel thermoelectric microdevices combining high thermal conductivity substrate materials such as diamond or even silicon, thin film metallization and patterning technology, and electrochemical deposition of 10-50μm thick thermoelectric films.