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The Loop Heat Pipe has become increasingly popular among spacecraft engineers for its operational reliability and robustness. At first glance, the LHP seems like a simple capillary-pumped heat transport device. In reality, the thermodynamics and fluid dynamics in the LHP form a complex system, especially during transient operation. The LHP performance varies with power input and sink temperature, but perhaps more importantly, with transitions from one condition to another. In addition, when multiple LHPs are thermally coupled, a change in one LHP can result in a detrimental effect on another. In this paper, model simulations of a thermal control system having two LHPs will be presented to show the system performance during the expected on-orbit thermal conditions of low Earth orbit.

Loop Heat Pipes have proven as reliable heat transports for spacecraft thermal control systems. NASA Goddard Space Flight Center in collaboration with NASA Jet Propulsion Laboratory recently proposed a miniature dual pump/condenser LHP system for use in future Mars missions. Results of a ground test program indicated that the dual pump/condenser LHP performed very well, but in a complicated manner. No analytical model was available to facilitate the design/analysis of this emerging technology. A generalized LHP theory will be presented in this paper along with the derived governing equations and solution scheme. Model predictions were made and compared with test data for validation.

Passive cooling transport in the cryogenic temperature regime still remains a challenging task since problems regarding parasitic heat gains from the surrounding have not been resolved satisfactorily. A recently-introduced concept of Advanced Loop Heat Pipe (or ALHP) had demonstrated an ability to manage “excessive” vapor generation in the compensation chamber. Nitrogen and Neon were successfully utilized as the working fluids to provide cryocooling transports in the temperature range of 80-120K and 30-40K, respectively. A Hydrogen ALHP in 2004 became the first capillary-pumped system to operate in the 20-30K range. This paper will present the ALHP technology in general and the detailed description of the research program/test results in particular.

Like other spacecraft subsystems, the thermal control system had to comply with the following requirements: (i) robust/reliable in harsh environments, (ii) minimal power consumption, (iii) compact/lightweight, and (iv) long life even without maintenance. For this reason, passive two-phase heat transport technologies such as heat pipes, Loop Heat Pipes, and Capillary Pumped Loops became popular among spacecraft engineers. Future thermal requirements may outgrow the capabilities of the existing devices. The US Naval Research Laboratory is leading a research and development effort to produce advanced technologies for the thermal management of the Navyapos;s next-generation spacecraft and satellites.

Heat transfer characteristics of a Loop Heat Pipe (LHP) are difficult to predict due to the complex nature of thermal interaction between the LHP itself and the environment it operates in. The overall thermal conductance varies not only with the power input, sink temperature and ambient temperature as expected, but also with the system initial condition and/or previous history of its operation. Hence, the analytical modeling of LHPs often yielded inaccurate results when compared with the actual data. The U.S. Naval Research Laboratory (NRL) recently completed the development of a transient LHP model. Accordingly, NRL carried out an extensive LHP test program, in part, to provide test data for the model verification. In this paper, description of the test program and results of the model verification will be presented.

In this paper, results of the NRL LHP experimental studies, conducted by Naval Research Laboratory (NRL) and NASA Goddard Space Flight Center, will be presented. Emphasis in this test program is to examine the “turnkey” startup of the NRL LHP and its operational characteristics. Series of tests were performed, including startup tests, power cycling tests, low power tests, and high power tests. The NRL LHP has demonstrated very robust operations throughout the tests. In addition, hysteresis was found at low power operations. Importance of the two-phase dynamics in the evaporator core is realized, which has shown significant effects on loop operations.

The CPL technology has been under development for the last 15 years. Many technical and manufacturing issues have been worked out. However the deprimes of capillary pumps have remained the most difficult problem to solve. The Advanced CPL (A-CPL) design concept was proposed in an effort to make the capillary pumps more tolerant to both vapor and noncondensible gases. A test loop was constructed to demonstrate the operational principles of the A-CPL at room temperature. Freon 134a was chosen as the working fluid so that Teflon tubing could be used as the loop transport lines for flow visualization purposes.

Capillary Pumped Loops (CPL) are projected to be the primary heat transport device for the next generation spacecrafts and satellites. The major shortcoming of a traditional CPL system has been its inability to tolerate vapor and/or non-condensible gas (NCG) bubbles when they are present in the liquid side of the loop. The vapor/NCG bubbles ultimately caused the capillary pumps to fail. A novel CPL design concept, called Advanced CPL or A-CPL, was introduced to create deprime resistant capillary pumps. The main idea of the A-CPL was to generate a liquid flow through the capillary pump core to flush out vapor/NCG bubbles.