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A Small Loop Heat Pipe (SLHP) featuring a wick of only 1.27 cm (0.5 inches) in diameter has been designed for use in spacecraft thermal control. It has several features to accommodate a wide range of environmental conditions in both operating and non-operating states. These include flexible transport lines to facilitate hardware integration, a radiator capable of sustaining over 100 freeze-thaw cycles using ammonia as a working fluid and a structural integrity to sustain acceleration loads up to 30 g. The small LHP has a maximum heat transport capacity of 120 Watts with thermal conductance ranging from 17 to 21 W/°C. The design incorporates heaters on the compensation chamber to modulate the heat transport from full-on to full-stop conditions. A set of start up heaters are attached to the evaporator body using a specially designed fin to assist the LHP in starting up when it is connected to a large thermal mass.

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.

The Earth Observing-1 spacecraft, built by Swales Aerospace for NASA's Goddard Space Flight Center (GSFC), was successfully launched on a Boeing Delta-II ELV on November 21, 2000. The EO-1 spacecraft thermal design is a cold bias design using passive radiators, regulated conductive paths, thermal coatings, louvers, thermostatically controlled heaters and multi-layer insulating (MLI) blankets. Five of the six passive radiators were aluminum honeycomb panels. The sixth panel was a technology demonstration referred to as the Carbon Carbon Radiator (CCR) panel. Carbon-Carbon (C-C) is a special class of composite materials in which both the reinforcing fibers and matrix materials are made of pure carbon. The use of high conductivity fibers in C-C fabrication yields composite materials that have high stiffness and high thermal conductivity.

Using Loop Heat Pipes (LHPs) for controlling the temperature of the source of heat has been considered for many applications. However, traditional LHPs can require significant amounts of power for source temperature control. A number of techniques have been identified and implemented to reduce control power requirements. One of the very first design approaches was to thermally couple the liquid line bringing subcooled liquid from the condenser to the vapor line entering the condenser with a number of “coupling blocks”. In another application, a variable conductance heat pipe (VCHP) was used to couple the liquid line to the LHP evaporator. A third generation approach has been developed that offers even further reductions in control power. The paper discusses earlier generations of control power reduction approaches with their advantages and disadvantages. It also describes the third generation of the approach, which is currently in manufacturing.

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.

The ADRAD deployable radiator is in development at Swales Aerospace to provide additional heat rejection area for spacecraft without envelope impact. The ADRAD design incorporates ALPHA loop heat pipes, an aluminum honeycomb radiator with embedded condenser, OSR optical coating, spherical bearing hinges, pyrotechnic release devices and snubbers. This paper describes the design of ADRAD to a set of “generic” GEO requirements, including a nominal heat rejection capacity of 1250 W. Thermal, structural and mechanism considerations are described along with the comprehensive systems approach necessary to produce an integrated subsystem.

Having been in operation for over 15 years, the Hubble Space Telescope (HST) had experienced significant changes in both hardware upgrades and operational modes. The changes were necessary to improve performance of some equipment and to replace failed electronics in others. Hardware replacements were done in several servicing missions. To accommodate the change in physical condition of HST, alterations in the way the telescope is operated were also required. The final opportunity to make any hardware changes on HST is during Servicing Mission 4 (SM-4) which is currently scheduled for December of 2007. It is important to make the most appropriate changes in order to ensure that HST will be in good operating condition until its planned termination. In order to provide manifest input to the HST project for the final servicing mission, the HST thermal team must conduct careful evaluation of every single piece of hardware on HST.

This paper describes the development and testing status of several novel components and integration tools for space-based cryogenic applications. These advanced devices offer functionality in the areas of cryogenic thermal switching, cryogenic thermal transport, cryogenic thermal storage, and cryogenic integration. As such, they help solve problems associated with cryocooler redundancy, across-gimbal thermal transport, large focal plane array cooling, fluid-based cryogenic transport, and low vibration thermal links. The devices discussed in the paper include a differential thermal expansion cryogenic thermal switch, an across-gimbal thermal transport system, a cryogenic loop heat pipe, a cryogenic capillary pumped loop, a beryllium cryogenic thermal storage unit, a high performance flexible conductive link, a kevlar cable structural support system, and a high conductance make-break cryogenic thermal interface.

This paper describes the development, operation and testing of an across-gimbal ambient thermal transport system (GATTS) for carrying cryocooler waste heat across a 2-axis gimbal. The principal application for the system is space-based remote sensing spacecraft with gimbaled cryogenics optics and/or infrared sensors. GATTS uses loop heat pipe (LHP) technology with ammonia as the working fluid and small diameter stainless steel tubing to transport 100–275 W across a two-axis gimbal. The tubing is coiled around each gimbal axis to provide flexibility (less than 0.68 N-m [6 lbf-in] of tubing-induced torque per axis) and fatigue life. Stepper motors are implemented to conduct life cycling and to assess the impact of motion on thermal performance. An LHP conductance of approximately 7.5 W/C was demonstrated at 200 W, with and without gimbal motion. At the time this paper was written, the gimbal had successfully completed over 500,000 cycles of operation with no performance degradation.

The Spacecraft Radiative Thermal Model Exchange System is a technology developed for the bi-directional exchange of spacecraft radiative thermal models via the TMG thermal software package. It provides a means for quickly and accurately transferring models between TMG and theree of the major thermal radiation codes used in the spacecraft industry, particularly the ESARAD and Thermica packages, which are widely used by contractors to the European Space Agency, and the TSS code which is prevalent in the United States space industry. In order to reconcile element-based and primitives-based modeling approaches, this system includes an interactive primitives-based modeling system, enabling users to construct, import, and manipulate primitives-based radiation models in TMG.