David C. Bugby

Cryogenic Thermal Storage Unit (CRYOTSU) flight experiment

This paper presents an overview of the Cryogenic Thermal Storage Unit (CRYOTSU) flight experiment. CRYOTSU is developed from a series of reflown Cryogenic Test Bed (CTB) flight experiments and incorporates design, integration, and testing techniques for cryogenic systems. Some of the applications described in this paper include: reducing vibration induced by cryogenic refrigerators; mounting the experiments without introducing parasitic heat leaks; selecting thermally conductive and isolation interface materials that would meet the requirements and perform over the temperature range of 55 to 300 K; supporting experiments structurally via kevlar cable suspension straps; developing a beryllium thermal storage unit with dissimilar metals; and testing in 1g in preparation of the flight experiment.

Experimental investigation of a neon cryogenic capillary pumped loop

This paper describes the first known demonstration of a cryogenic capillary pumped loop (CCPL) using neon as the working fluid. One obvious application for a neon CCPL is in thermally coupling redundant 35 K cryocoolers to a long-wave TR sensor operating at 40 K without the need for additional flexible links and with negligible (off-cooler) parasitic penalties. The test hardware utilized in this investigation is the 3rd-generation CCPL which was developed by Cullimore and Ring Technologies and Swales Aerospace under NASA/GSFC SBIR funding. Since neon melts at 24.5 K, boils at 27 K, and is a gas above 44.5 K, the practical operating range for a neon CCPL is 30-40 K. The specific demonstrations described herein include several start-ups from a supercritical state without the aid of a cooled shroud, power cycling from 0.1-3.5 W, long-term operation, and start-up/operation under 1.0 cm of adverse evaporator elevation. This paper describes the test set-up, analysis, and results associated with this first neon CCPL.

Thermal Performance of Multi-Evaporator Hybrid Loop Heat Pipe (ME-HLHP) with a Liquid Cooled Shield (LCS)

A multi-evaporator hybrid loop heat pipe (ME-HLHP), with a unique design modification enabling sub-ambient operation in a warm laboratory setting, was developed and successfully tested at heat loads up to 600W and saturation temperatures up to 30K below local ambient. The ME-HLHP architecture, which utilizes a remote, cold-biased, flow-through reservoir (linked to a secondary ”sweepage” evaporator) to provide the operational robustness of a loop heat pipe (LHP) and the temperature controllability of a capillary pumped loop (CPL), has the same limitations that LHPs and CPLs do when trying to cool a component significantly below the temperature of its surroundings. In general, when a two-phase loop is operated in a warm environment, liquid returning to each evaporator must be protected from conductive, convective, and radiative heat leaks. If the liquid line is not sufficiently protected, a loss of subcooling may ensue that could lead to suboptimal loop performance, such as higher operating temperatures (due to autoregulation) in LHPs, higher than necessary sweepage power in HLHPs, or possible deprime in CPLs. To maximize loop robustness, a unique design feature known as the liquid cooled shield (LCS) was incorporated into the ME-HLHP architecture to protect the liquid return line from parasitic heat gains. This paper describes the background, design and test results of ME-HLHP for compact Laser applications.

Experimental verification of a 60 K thermal storage unit

To reduce the cooling power requirements and weight of space-based IR sensor systems with time varying focal plane loads, cryogenic thermal storage units (TSUs) with embedded phase change materials (PCMs) represent a viable solution. This paper describes the design and verification of a dual-volume 60 K TSU, which uses nitrogen trifluoride (NF/sub 3/) as the working fluid. The dual-volume design divides the TSU into a cryogenic temperature heat exchanger volume and an ambient temperature storage volume. This approach increases TSU energy storage capacity and thermal stability while reducing total weight and volume. The TSU heat exchanger has a beryllium shell to ensure CTE compatibility with beryllium focal planes. The heat exchanger core is constructed of a 40 ppi, 6101 aluminum foam, compressed to 35% relative density. Developmental testing has shown that the aluminum foam core, which is vacuum furnace brazed to the beryllium end-caps, tends to suppress NF/sub 3/, single-phase supercooling. Also, by virtue of its slight capillary wicking capability, the foam core eliminates concerns over heat exchanger filling in 0-g. The TSU will be tested on-orbit in a Hitchhiker GAS Canister flight experiment (designated as CRYOTSU) on the STS is late-1998. In this flight experiment, the TSU storage tank will be made of thin-walled stainless steel and the system will be pressurized to less than 100 psi to minimize safety concerns.

Development and Testing of a Variable Conductance Thermal Acquisition, Transport, and Switching System

This paper describes the development and testing of a scalable thermal control architecture for instruments, subsystems, or systems that must operate in severe space environments with wide variations in sink temperature. The architecture is comprised by linking one or more hot-side variable conductance heat pipes (VCHPs) in series with one or more cold-side loop heat pipes (LHPs). The VCHPs provide wide area heat acquisition, limited distance thermal transport, modest against gravity pumping, concentrated LHP startup heating, and high switching ratio variable conductance operation. The LHPs provide localized heat acquisition, long distance thermal transport, significant against gravity pumping, and high switching ratio variable conductance operation. Combining two variable conductance devices in series ensures very high switching ratio isolation from severe environments like the Earths moon, where each lunar day spans 15 Earth days (270 K sink, with a surface-shielded/space viewing radiator) and each lunar night spans 15 Earth days (80-100 K radiative sink, depending on location). The single VCHP-single LHP system described herein was developed to maintain thermal control of International Lunar Network (ILN) anchor node lander electronics, but it is also applicable to other variable heat rejection space missions in severe environments. The LHPVCHP system utilizes a stainless steel wire mesh wick ammonia VCHP, a Teflon wick propylene LHP, a pair of one-third square meter high radiators (one capillary-pumped horizontal radiator and a second gravity-fed vertical radiator), a half-meter of transport distance, and a wick-bearing co-located flow regulator (CLFR) to allow operation with a hot (deactivated) radiator. The VCHP was designed with a small reservoir formed by extending the length of its stainless steel heat pipe tubing. The system was able to provide end-to-end switching ratios of 300-500 during thermal vacuum testing at ATK, including 3-5 W/K ON conductance and 0.01 W/K OFF conductance. The test results described herein also include an in-depth analysis of VCHP condenser performance to explain VCHP switching operation in detail. Future multi-VCHP/multi-LHP thermal management system concepts that provide scalability to higher powers/longer transport lengths are also discussed in the paper.

Flight Testing of a Cryogenic Capillary Pumped Loop

Future space-based cryogenic systems will require enhanced integration flexibility, lower weight reduced parasitic penalties, better vibration isolation, and a variety of other improvements to meet performance goals. Additionally, there is an increasing need to locate cooling sources remotely from cooled components. In the past flexible conductive links were used and worked well in most cases. However, as the transport lengths increase, conductive couplings become heavier and less effective, and must be replaced by higher performance systems. One available option, which can meet many of these future requirements, is the cryogenic capillary pumped loop (CCPL). The development of the CCPL technology started in 1992, following the success of the room temperature CPLS. The extrapolation of CCPL technology to cryogenic temperatures offers many performance benefits, which are not currently within the reach of traditional heat pipes or conductive links. Specific advantages of the CCPL technology pertaining to cryocooler integration include: (1) greater capillary pumping pressure for improved ground testability; (2) improved mechanical isolation; (3) faster diode shutdown and lower reverse heat leaks; (4) tighter control of detector temperature; (5) variable or fixed conductance operation; and (6) ease of integration due to their flexibility. The applications of CCPL technology are numerous. Military and commercial applications include surveillance satellites, earth observing satellites, deep space observation systems, medical devices, and many other cryogenic systems. Over the past few years, several breadboard and prototype CCPLs have been built and ground tested. A prototype CCPL has demonstrated successful operation between 80K and 110K with heat loads between O.5W and 12W using nitrogen as the working fluid, and 35K and 40K with head loads of 0.25W to 3.5W using neon. In order to verify CCPL performance in a microgravity environment, a flight unit, CCPL-5, was tested onboard 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. The CCPL-5 utilized nitrogen as the working fluid and operated between 75K and 110K. Flight results indicated excellent performance of the CCPL-5 under zero-G environment The CCPL could start from a supercritical condition in all tests, and the loop operating temperature could be tightly controlled regardless of changes in the heat load and/or the sink temperature. In addition, the loop demonstrated successful operation with a heat load of 0.5W as well as with parasitic heat loads alone. There were no noticeable differences between zero-G and one-G operation.

Ceramic Flat Plate Evaporator for Loop Heat Pipe Cooling of Electronics

Two-phase loops are extremely efficient devices for passively transporting heat over long distances with low temperature drop. The heat acquisition component of a two-phase loop, the evaporator, is commonly made from conventional metal materials (aluminum, copper, etc.) and has cylindrical geometry. Neither characteristic is optimally suited for close integration to common electronic or photonic heat sources, which generally have flat interfaces and are constructed from low thermal expansion coefficient (CTE) semiconductor materials. This paper describes the development of a ceramic flat plate evaporator for cooling processor chips in network computers used onboard Navy submarines. The unique requirements of submarines give added motivation for the advantages offered by two-phase loops. The ceramic flat plate evaporator is constructed of low CTE, high thermal conductivity material and thus enables a low thermal resistance interface between the heat source and the working fluid of the loop heat pipe. Alumina and aluminum nitride flat plate evaporators were integrated into a water-based two-phase loop and thermally tested to a heat flux of 30 W/cm2 .Copyright