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Loop Heat Pipe (LHP) technology has advanced to the point that LHPs are baselined for thermal control systems in many spacecraft applications. These applications typically utilize a loop heat pipe with a single evaporator. However, many emerging applications involve heat sources with large thermal footprints, or multiple heat sources that would be better served by LHPs with multiple evaporators. Dual evaporator LHPs with separate reservoirs for each evaporator have been successfully developed, but the volume and weight of such systems become impractical as the number of the evaporators increase to more than three or four. Other investigators have proposed systems containing several evaporators that are coupled to a common reservoir with a conduit to contain a capillary link (secondary wick). This approach places several restrictions on the relative location of the evaporators due to the limitation of the capillary link.

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.

In typical loop heat pipe (LHP) applications, the LHP design calls for a dedicated evaporator and a dedicated condenser. Applications exist for reversible loop heat pipes (LHPs), which can transport heat in either direction. In the reversible LHP design, two evaporator pumps are plumbed together, one which acts as an evaporator while the other acts as a condenser. The two pumps can reverse roles, simply by reversing the temperature gradient across the loop. Thus, either pump can be used as an evaporator or a condenser, depending upon the environment. Reversible LHPs can be used to share heat between components, or to cross-strap opposing spacecraft radiators. A reversible LHP was built and tested to demonstrate feasibility and to characterize its performance capabilities and attributes. The device was tested by either alternately heating each evaporator electrically or by inducing a temperature difference between the two ends of the device.

Loop Heat Pipe (LHP) technology has advanced to the point that LHPs are baselined for thermal control systems in spacecraft applications. Many of the applications also require redundant systems to address reliability concerns. In the redundant design, two LHPs are plumbed in parallel to the same heat source and sink. The LHPs are totally separate, and each is designed to fully accommodate the total heat load at the source if the other LHP should fail. Due to the self-regulating nature of an LHP, questions have been raised regarding the expected behavior of two LHPs operating in parallel between the same source and sink, particularly their ability to self-start and equally share the heat load. To demonstrate the application of LHPs in a redundant system, two totally independent LHPs, each with the same condenser plate and heat source, were fabricated and tested.

Loop Heat Pipe (LHP) technology has advanced to the point that LHPs are baselined for thermal control systems in many spacecraft applications. These applications typically utilize a loop heat pipe with a single evaporator. However, many emerging applications involve heat sources with large thermal footprints, or multiple heat sources that would be better served by LHPs with multiple evaporators. Other investigators have proposed systems containing several evaporators that are coupled to a common reservoir; however, this approach places severe restrictions on the relative locations of the evaporators. This paper describes a multiple evaporator loop heat pipe, with a separate reservoir for each evaporator, which can accommodate payloads with large or spatially separated heat sources.

Capillary Pumped Loops (CPLs) have been under development for almost two decades and are emerging as a design solution for many spacecraft thermal control systems. Three Capillary Pumped Heat Transport Systems (CPHTS) using CPL technology have been selected for accommodating the two Earth Observing System (EOS-AM) instruments that require advanced waste heat dissipation. The Capillary Pumped Loop Flight Experiment (CAPL-2) [Ref 7], which was carried on board STS-69, successfully demonstrated an EOS-like capillary system utilizing a prototyped starter pump cold plate (SCP). The CAPL-2 SCP is almost identical to the EOS-AM configuration and was designed, built, and demonstrated to overcome the start-up difficulty under fully flooded conditions. The SCP was rigorous ground tested as part of a simulated EOS-AM / CAPL-2 capillary loop, and the SCP successfully met or surpassed all of its performance requirements.

NASA's Earth Observing System (EOS) Program will place a series of unmanned polar-orbiting spacecraft in low-earth orbit to support a variety of scientific and Earth-observing missions. The EOS AM spacecraft has a requirement for an advanced heat transportation system for thermal energy. A Capillary Pumped Heat Transportation System (CPHTS) using Capillary Pumped Loop (CPL) technology was selected to accommodate the instruments requiring advanced heat transport.

This paper presents the COMmercial Experiment Transporter (COMET) Service Module thermal control system design using a Capillary Pumped Loop (CPL). The COMET satellite is scheduled for launch in early 1993 aboard the Conestoga rocket. COMET provides the United States commercial research and development community with a dependable and economical means to access space. The COMET program is defined and funded by the NASA Centers for the Commercial Development of Space (CCDS). The Center for Space Transportation and Applied Research (CSTAR) was given the authority to establish and implement the COMET Program. COMET is designed to carry experiments to the micro-gravity of space and return one of two modules back to Earth. COMET provides basic utilities such as electric power, a tightly controlled thermal environment, attitude control, data management and communications while in orbit. COMET is a two part Free Flyer, which will carry experiments into a low Earth orbit.