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| Cryogenic Thermal Storage Unit (CRYOTSU) |
| Payload Bay |
| Total with adapter beam: 900 lbs; without: 710 |
| Prime: Stephen Robinson |  |
Principal Investigator: Overall mission manager: Neal Barthelme, NASA GSFC |
| Backup: Scott Parazynski | |
Project Scientist: CRYOTSU experiment prog. mgr., Lt. M. Rich, USAF Research Lab; Cryogenic Capillary Pumped Loop experiment prog. mgr., Theodore Swanson, NASA GSFC; Cryogenic Thermal Switch/Phase Change Upper End Plate experiments, Lt. B.J. Tomlinson, USAF Research Lab |
| Overview |
| 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. The Cryogenic Thermal Storage Unit Flight Experiment (CRYOTSU) payload is designed to demonstrate the functionality of four important spacecraft thermal control devices in microgravity: the 60K Thermal Storage Unit (TSU), the Cryogenic Capillary Pumped Loop (CCPL), the Cryogenic Thermal Switch (CTSW) and the Phase Change Upper End Plate (PCUEP). Three of the devices operate at very low ("cryogenic") temperatures, while the fourth operates at just above room temperature. Overall, the payload is a "toolbox" of thermal control elements that aerospace designers can select from to determine ways of reliably solving complex spacecraft thermal design problems with minimum expenditures of power, weight and cost. 60k Thermal Storage Unit (TSU) The 60K TSU is a hermetically-sealed, dual-volume, beryllium and stainless steel vessel that contains a cryogenic phase change material--in this case, nitrogen. At room temperature, nitrogen is a gas. However, once nitrogen cools sufficiently, it becomes a liquid and, ultimately, a solid. The 60K TSU 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 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. The 60K TSU smoothes out the heating variations by periodically melting and refreezing the cryogenic phase change material. By stabilizing the heat load seen by the cryocooler into an average load, engineers can use a smaller, less power-consuming cryocooler. For some space systems, the viability of the 60K TSU will determine whether those systems can be deployed at all, due to a lack of larger cryocoolers. One very attractive feature of the 60K TSU is the fact that it operates passively and requires no input power, which also lessens power consumption of the entire payload system. In addition, this particular 60K TSU 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. Cryogenic Capillary Pumped Loop (CCPL) The CCPL is a lightweight, miniaturized device that provides the thermal link between an infrared or electrical cryogenic component and a cryocooler. The CCPL 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. CCPLs are therefore lightweight and 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 CCPLs are gases at room temperature, but once they have cooled sufficiently, they become liquid. The fluid used in this device is nitrogen. CCPL 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. CCPLs will therefore enable certain types of space systems to be deployed and are high-performance alternatives to flexible conductive links (FCLs), which are used routinely to thermally link cryocoolers to cooled cryogenic components. Besides their substantial weight savings, one important advantage of CCPLs over flexible conductive links is their inherent diode action--that is, a CCPL-based thermal link can be turned on or off, while flexible conductive links, by definition, are always turned on. Cryogenic Thermal Switch (CTS) The CTS is also a device that enables the thermal link between two components to be turned on or off. For certain cryogenic space applications, the CTS 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 that is normally on and a backup cooler that is normally off. These very low-temperature cryocoolers require a substantial amount of input power to produce just a small amount of cooling. If CTSs 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 CTSs 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 CTS can be turned off, and the backup cryocooler, along with its CTS, can be turned on. The CTS 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 CTS by a long, small diameter tube. To activate the CTS on, a heater on the hydride pump is turned on. The hydrogen, which is then released, then fills the CTS and the thermal path is on. When the hydride pump heater is turned off, the hydrogen is readsorbed and the CTS turns off. Phase Change Upper End Plate (PCUEP) The PCUEP, like the 60K TSU, stores energy and also provides a thermal load-leveling function that smooths out variable heating loads. The operating temperature of the PCUEP is 113 degrees Fahrenheit, which is about 77 degrees Fahrenheit above room temperature. The primary use for the PCUEP is in maintaining the thermal stability of high-power components that need to be intermittently turned on and off. The PCUEP is constructed of an aluminum shell and a carbon fiber core filled with a wax-like phase change material 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 CRYOTSU mission, the PCUEP 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 Hitchhiker-Getaway Special Canister to dissipate the heat to space without overheating. Thus, under normal conditions, the operating time is limited. The PCUEP allows the cryocoolers to operate longer without overheating, extending the time that the CRYOTSU flight experiments have to gather valuable performance data in space. Flight Hardware & Configuration CRYOTSU will be mounted in a five-cubic-foot Hitchhiker-Getaway Special canister that mounts to the side wall in Discovery's payload bay. Total length of the payload is 71.5 inches. The CRYOTSU electronics, using the Hitchhiker avionics, provide power and commands to the experiment as well as data acquisition, signal conditioning, and telemetry transmission. CRYOTSU desires a thermal environment cooler than or equal to +ZLV (bay-to-Earth) during operations. A thermal environment warmer than this may result in longer test cycles. The payload has no sun pointing constraints. Crew Operations The STS-95 crew will activate and deactivate the CRYOTSU Hitchhiker carrier using a standard switch panel. Crew participation is also required to position Discovery to the required attitudes for payload data acquisition. They will also be required to shoot general photographs of CRYOTSU during the mission. Payload Operations Control Center (POCC) The POCC at NASA's Goddard Space Flight Center will monitor and control the payload from the ground. CRYOTSU requires two 24-hour and two 12-hour test cycles. Two additional 24-hour and 12-hour test cycles are highly desired. |
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| History/Background |
| This will be the first flight of CRYOTSU, and the fifth in a series of cryogenic test bed flights managed by the United States Air Force Phillips Laboratory. The other flights were: . Cryogenic Heat Pipe Experiment (CRYOHP)--STS-53 . Cryogenic Two-Phase Experiment (CRYOTP)--STS-62 . Cryogenic Flexible Diode Experiment (CRYOFD)--STS-83 and -94 CRYOTSU was designed jointly by NASA's Goddard Space Flight Center, Greenbelt, Md., and the U.S. Air Force Research Laboratory (AFRL), Kirtland AFB, N.M. Additional collaboration on CRYOTSU was provided by Swales Aerospace, Beltsville, Md. CRYOTSU is managed by the Department of Defense Space Test Program and the United States Air Force Phillips Laboratory. The GSFC Hitchhiker program, which is managed by the Shuttle Small Payloads Project Office, provided mission support. For more info on Hitchhiker, see http://sspp.gsfc.nasa.gov/hh/hh.html. |
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| Benefits |
| CRYOTSU will provide aerospace designers with a "toolbox" of thermal control elements that they can select from to determine ways of reliably solving complex spacecraft thermal design problems with minimum expenditures of power, weight and cost. The 60K TSU, CCPL, and CTSW experiments benefit future integrated cryogenic bus systems. The PCUEP experiment will benefit future cooling test bed missions and spacecraft requiring load-leveling for power dissipating components. |
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