AirForce Tests New Cryogenic Storage System

SPACEDAILY

Kirtland AFB - November 20, 1998 -
John Glenn's experiments on aging weren't the only success stories to come off the recent Space Shuttle Discovery flight. The Air Force Research Laboratory (AFRL) had one of its own on that 9-day-long extraterrestrial excursion, one even "cooler" than the famous astronaut himself -- much cooler.

Early results from an experimental refrigeration device known as the Cryogenic Thermal Storage Unit (CRYOTSU), while perhaps not as glamorous as the septuagenarian's orbital encore, now suggest the Air Force may have found a better way to move, store, and control heat inside satellites. AFRL's CRYOTSU technology, which has no moving parts, will aid existing satellite cryocoolers that already lower internal temperature hundreds of degrees by maintaining the extremely low temperatures for longer periods.

According to project engineer Charlotte Gerhart of AFRL's Space Vehicles Directorate here, CRYOTSU met all AFRL testing goals and promises a new heat management technology for future spacecraft. Unlike current exotic devices such as "heat pipes" and "dewar bottles" that help satellites keep their cool, CRYOTSU used a smaller, lighter configuration of hardware with lower power requirements. CRYOTSU will be used in conjunction with cryocoolers to extend their capacity for applications where the amount of heat to be rejected is not constant.

"Satellite heat build-up from tightly compacted electronics and sunlight degrades the performance of some spacecraft subsystems, especially infrared sensors," explained Gerhart. "We are essentially searching for reliable and affordable supercooling technology that will lower internal satellite temperature around sensors. For sensors to work in space efficiently, they must operate in a frigid environment to properly contrast and accurately identify distant objects through their heat signatures, or 'spectral fingerprints.'"

Looking something like a big, white garbage can bolted against the side of Discovery's payload bay, the CRYOTSU experiment canister held four AFRL-NASA/Goddard Space Flight Center experiments with technical-sounding names.

The primary payload, the "nitrogen thermal storage unit," acted like a block of ice and cooled sensors to -211 degrees Celsius, or 348 degrees Fahrenheit below zero. Second was a "nitrogen-filled capillary pumped loop," which moved heat away from sensors and into space. Another, a hydrogen "gas-gap" thermal switch, directed the heat flow. And finally, another thermal storage unit filled with a type of paraffin wax that melts at 45 degrees Celsius. All the experiments, except for the thermal switch, were developed under Small Business Innovative Research contracts.

Simply, CRYOTSU keeps infrared sensors cold longer so they can continue to function. In the experiment, surplus heat from the cryocoolers was transferred to a radiator-like device, also made of paraffin. Called a "phase-change upper-end plate," the radiator is mounted on the outside of the CRYOTSU canister and dissipated extraneous heat into space. "We hope to see some of this technology used in the next few years on a long-term satellite mission like the planned Space-Based Infrared-Low system," Gerhart added.

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Cryogenic Thermal Storage Unit Flight Experiment (CRYOTSU)

The Cryogenic Thermal Storage Unit Flight Experiment (CRYOTSU ) payload aboard the STS-95 mission will house four thermal control components that will be tested in space.

The four experiments on the CRYOTSU flight experiment are: the Cryogenic Thermal Storage Unit (CTSU), the Cryogenic Capillary Pumped Loop (CCPL), the Cryogenic Thermal Switch (CTSW) and the Phase Change Upper End Plate (PCUEP). The experiments will be mounted in a five-cubic foot Hitchhiker (HH) - Get Away Special canister that mounts to the side wall in the Space Shuttle orbiter bay.

The Cryogenic Thermal Storage Unit Flight Experiment mission is the fifth flight of this HH-GAS joint experiment canister, designated as the Cryogenic Test Bed. The test facility enables experimenters to carry out cryogenic experiments in space. The test bed, which houses five cryogenic refrigerators and associated control electronics, was designed jointly by NASA Goddard Space Flight Center in Greenbelt, Md. and the U.S. Air Force Research Laboratory (AFRL) at Kirtland AFB, N.M. Additional collaboration on the Cryogenic Thermal Storage Unit Flight Experiment payload was provided by Swales Aerospace, located in Beltsville, Md.

The Cryogenic Thermal Storage Unit Flight Experiment mission is designed to demonstrate the functionality of four important spacecraft thermal control devices in the weightless environment of space. Three of the devices operate at very low ("cryogenic") temperatures while the fourth operates at moderately above room temperature. Overall, the payload is a "toolbox" of thermal control elements that aerospace designers can select from to determine ways of solving complex spacecraft thermal design problems. The ultimate goal of any spacecraft thermal designer is to reliably solve these thermal problems with minimum expenditures of power, weight and cost.

For all spacecraft, power is a very scarce resource that must be properly allocated for optimal system performance. The various instruments and electronic components on spacecraft require input power to function and, at the same time, require a means of dissipating this power to maintain their temperatures within allowable limits. The thermal control systems on spacecraft accomplish this goal by combining various low-power or passive thermal control components in an optimal way. In certain types of spacecraft, such as those used in Earth-observing applications, infrared detectors and optics need to be very cold, and these components must co-exist with other much warmer components. So, the thermal control problems in space span a range of temperatures, requiring a range of thermal control components. CRYOTSU will demonstrate four such devices.

The Cryogenic Thermal Storage Unit is a hermetically-sealed, dual-volume, beryllium and stainless steel vessel that contains a cryogenic phase change material. The phase change material used in this experiment is nitrogen. At room temperature, nitrogen is a gas. However, once nitrogen cools sufficiently, it becomes a liquid and, ultimately, a solid. The Cryogenic Thermal Storage Unit functions as a supplement to a cryocooler, which is a small refrigerator designed to cool infrared instruments to low operating temperatures. Although most infrared instruments require quite tight temperature control and dissipate very little heat, some infrared instruments dissipate a moderate amount of heat in a highly variable (non-constant) manner. In some cases, the peak dissipation rate can exceed the average rate by ten times or more.

A Cryogenic Thermal Storage Unit functions by smoothing out the heating variations by periodically melting and refreezing the cryogenic phase change material. Because the heat load seen by a cryocooler (with a CTSU) is now the average load, the engineer can utilize a smaller, less power-consuming cryocooler. For some space systems, the viability of the Cryogenic Thermal Storage Unit will determine whether those systems can be deployed at all, owing to the lack of larger cryocoolers. One very attractive feature of the Cryogenic Thermal Storage Unit is the fact that it operates passively and requires no input power. In addition, this particular Cryogenic Thermal Storage Unit has a hermetically-sealed, seamless beryllium heat exchanger formed by a patent-pending beryllium joining process that Swales Aerospace and its subcontractor partners have developed for this application.

The Cryogenic Capillary Pumped Loop is a lightweight, miniaturized device that provides the thermal link between a cryogenic component and a cryocooler. The Cryogenic Capillary Pumped Loop has no moving parts and operates using a two-phase fluid loop similar to that found in a residential heat pump. It can be constructed using very small diameter tubing that can be routed around mechanisms and components in tight areas. Cryogenic Capillary Pumped Loops are therefore useful in a variety of situations including those where crycooler mounting space is limited, where the cryocooler creates excessive vibration, and where cooling must be transported across a flexible joint. The fluids used in Cryogenic Capillary Pumped Loops are gases at room temperature, but once they have cooled sufficiently, they become liquid. The fluid used in this device is nitrogen. Cryogenic Capillary Pumped Loop benefits include weight savings for highly remote components, the ability to integrate two or more cryocoolers into a single cooling source for a component, and the ability to span joints requiring extreme flexibility. Cryogenic Capillary Pumped Loops will therefore enable certain types of space systems to be deployed and are high performance alternatives to flexible conductive links (FCLs), which are utilized routinely to thermally link cryocoolers to cooled cryogenic components. Besides their substantial weight savings, one important advantage of Cryogenic Capillary Pumped Loops over flexible conductive links is their inherent diode action. That is, a Cryogenic Capillary Pumped Loop-based thermal link can be turned ON or OFF while the flexible conductive links, by definition, is always turned ON.

The Cryogenic Thermal Switch is also a device that enables the thermal link between two components to be turned ON or OFF. For certain cryogenic space applications, the Cryogenic Thermal Switch is an absolute necessity. For example, some very low-temperature infrared sensors need to be cooled by at least two cryocoolers because of reliability concerns; a primary cooler which is normally ON and a back-up cooler which is normally OFF. These very low-temperature cryocoolers require a substantial amount of input power to produce just a small amount of cooling. If Cryogenic Thermal Switches were not available, the unwanted or "parasitic" heat flow from the OFF cryocooler would be overly costly in terms of spacecraft power usage. By using two Cryogenic Thermal Switches in parallel (one for each cryocooler), the flow of heat from the backup (OFF) cryocooler can be minimized and the cooling capability of the primary (ON) cryocooler can be maximized. If the primary cryocooler fails, its Cryogenic Thermal Switch can be turned OFF, and the backup cryocooler, along with its Cryogenic Thermal Switch, can be turned ON.

The Cryogenic Thermal Switch turns ON and OFF by respectively filling or emptying with a very small amount of hydrogen gas (about two millionths of a pound). At room temperature, the hydrogen gas is completely absorbed on porous metal surfaces within a tiny component known as a "hydride pump". The hydride pump, which is mounted in a warmer portion of the spacecraft, is attached to the Cryogenic Thermal Switch by a long, small diameter tube. To activate the Cryogenic Thermal Switch ON, a heater on the hydride pump is turned on. The hydrogen, which is then released, then fills the Cryogenic Thermal Switch and the thermal path is ON. When the hydride pump heater is turned off, the hydrogen is readsorbed and the Cryogenic Thermal Switch turns OFF.

The Phase Change Upper End Plate, like the Cryogenic Thermal Storage Unit, is a thermal storage unit (TSU) and it too provides a thermal load-leveling function that smooths out variable heating loads. The operating temperature of the Phase Change Upper End Plate is 113 degrees Fahrenheit, which is about 77 degrees Fahrenheit above room temperature. The primary use for the Phase Change Upper End Plate is in maintaining the thermal stability of high power components that need to be intermittently turned on and off. The Phase Change Upper End Plate is constructed of an aluminum shell and a carbon fiber core filled with a wax-like PCM known as docosane. When the high power component is turned ON, the docosane melts and the component temperature stays relatively constant. When the high power component is turned off, the docosane freezes and the component temperature, again, stays relatively constant.

On the Cryogenic Thermal Storage Unit Flight Experiment mission, the Phase Change Upper End Plate is an integral part of the overall thermal control system for the flight experiment. With five cryocoolers, the total power dissipation exceeds the capability of the HH-GAS Canister to dissipate the heat to space without overheating. Thus, under normal conditions, the operating time is limited. The Phase Change Upper End Plate allows the cryocoolers to operate longer without overheating, extending the time that the Cryogenic Thermal Storage Unit flight experiments have to gather valuable performance data in space.

Neal Barthelme from Goddard is the overall mission manager for the Cryogenic Thermal Storage Unit Flight Experiment payload. The program manager for the Cryogenic Thermal Storage Unit experiment is Sergeant Scott Stanely of the Air Force Reasearch Laboratory. Mark C. Kobel at Goddard is the program manager for the Cryogenic Capillary Pumped Loop experiment. Lieutenant B.J. Tomlinson of the Air Force Reasearch Laboratory is the program manager for both the Cryogenic Thermal Switch and Phase Change Upper End Plate experiments.

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