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The Capillary Pumped Loop Flight Experiment (CAPL 2) employs a passive two-phase thermal control system that uses the latent heat of vaporization of ammonia to transfer heat over long distances. CAPL was designed as a prototype of the Earth Observing System (EOS) instrument thermal control systems. The purpose of the mission was to provide validation of the system performance in microgravity, prior to implementation on EOS. CAPL 1 was flown on STS-60 in February, 1994, with some unexpected results related to gravitational effects on two-phase systems. Start-up difficulties on CAPL 1 led to a redesign of the experiment (CAPL 2) and a reflight on STS-69 in September of 1995. The CAPL 2 flight was extremely successful and the new “starter pump” design is now baselined for the EOS application. This paper emphasizes the design history, the CAPL 2 design, and lessons learned from the CAPL program.

This paper describes the flight test results of the fifth generation cryogenic capillary pumped loop (CCPL-5) which flew on the Space Shuttle STS-95 in October of 1998 as part of the CRYOTSU Flight Experiment. This flight was the first in-space demonstration of the CCPL, a lightweight heat transport and thermal switching device for future integrated cryogenic bus systems. The CCPL-5 utilized nitrogen as the working fluid and operated between 75K and 110K. Flight results indicated excellent performance of the CCPL-5 in a micro-gravity environment. The CCPL could start from a supercritical condition in all tests, and the reservoir set point temperature controlled the loop operating temperature regardless of changes in the heat load and/or the sink temperature. In addition, the loop demonstrated successful operation with heat loads ranging from 0.5W to 3W, as well as with parasitic heat loads alone.

A Capillary Pumped Loop (CPL) is an advanced two-phase heat transport device which utilizes capillary forces developed within porous wicks to move a working fluid. The advantage this system has over conventional thermal management systems is its ability to transfer large heat loads over long distances at a controlled temperature. Extensive ground testing and two flight experiments have been performed over the past decade which have demonstrated the potential of the CPL as a reliable and versatile thermal control system for space applications. While the performance of CPL's as “black boxes” is now well understood, the internal thermo-fluid dynamics in a CPL are poorly known due to the difficulty of taking internal measurements. In order to visualize transient thermohydraulic processes occurring inside an evaporator, a see-through capillary evaporator was built and tested at NASA's Goddard Space Flight Center.

The loop heat pipe (LHP) was invented in Russia in the early 1980’s. It is a two-phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat, and the capillary forces developed in fine porous wicks to circulate the fluid. The LHP is known for its high pumping capability and robust operation because it uses fine-pored metal wicks and the integral evaporator/hydro-accumulator design. It has gained rapid acceptance in recent years as a thermal control device in space applications. This paper presents an overview of the LHP operation. The physical processes and the thermal-hydraulic behaviors of the LHP are first described. Operating characteristics as functions of various parameters including the heat load, sink temperature, ambient temperature, and elevation are presented. Peculiar behaviors in LHP operation such as temperature hysteresis and temperature overshoot during start-up are explained.

Loop Heat Pipes (LHPs) are being considered for cooling of military combat vehicles and spinning spacecraft. In these applications, it is important to understand the effect of an accelerating force on the performance of LHPs. In order to investigate such an effect, a miniature LHP was installed on a spin table and subjected to variable accelerating forces by spinning the table at different angular speeds. Several patterns of accelerating forces were applied, i.e. continuous spin at different speeds and periodic spin at different speeds and frequencies. The resulting centrifugal accelerations ranged from 1.2 g's to 4.8 g's. This paper presents the first part of the experimental study, i.e. the effects of an accelerating force on the LHP start-up. Tests were conducted by varying the heat load to the evaporator, condenser sink temperature, and LHP orientation relative to the direction of the accelerating force.

Loop Heat Pipes (LHPs) are being considered for cooling of military combat vehicles and spinning spacecraft. In these applications, it is important to understand the effect of an accelerating force on the performance of LHPs. In order to investigate such an effect, a miniature LHP was installed on a spin table and subjected to variable accelerating forces by spinning the table at different angular speeds. Several patterns of accelerating forces were applied, i.e. continuous spin at different speeds and periodic spin at different speeds and frequencies. The resulting centrifugal accelerations ranged from 1.2 g's to 4.8 g's. This paper presents the second part of the experimental study, i.e. the effect of an accelerating force on the LHP operating temperature. It has been known that the LHP operating temperature under a stationary condition is a function of the evaporator power and the condenser sink temperature when the compensation temperature is not actively controlled.

Most existing loop heat pipes (LHPs) consist of one single evaporator and one single condenser. LHPs with multiple evaporators are very desirable for cooling multiple heat sources or a heat source with large thermal footprints. Extending the current LHP technology to include multiple evaporators and multiple condensers faces some challenges, including operating temperature stability, adaptability of loop operation to rapid power and sink temperature transients, and sizing of the compensation chambers (CCs). This paper describes an overview of an extensive testing program for an LHP with two evaporators and two condensers. Tests performed include start-up, power cycle, sink temperature cycle, CC temperature cycle, and capillary limit. Test results showed that the loop could be started successfully in most cases, and the operating temperature was a function of the total heat load, heat load distribution between the two evaporators, condenser sink temperature and ambient temperature.