by Dr. Peter Shirron, NASA/Goddard Space Flight Center, peter.shirron@nasa.gov

This column presents a topical summary of the 2015 Space Cryogenics Workshop that was held June 24-26 in Phoenix AZ. The workshop was organized by David Plachta and Jason Hartwig of NASA/Glenn Research Center, and continued the tradition of bringing together specialists in the field of space cryogenics to discuss upcoming and potential space missions, and the development of technologies that support, or more often enable, the science and exploration goals of the world’s space agencies. The workshop consisted of two days of talks and poster sessions, and provided ample opportunity for more informal discussions that foster collaborations and cooperation in the space cryogenics community. Selected papers from the workshop will be published in a special issue of the journal Cryogenics, which is expected to be available by the end of 2015.

Near-term missions

As has become traditional, the workshop opened with presentations on the nearest-term missions. In this case, the first series of talks by Shirron and DiPirro (both of NASA/Goddard Space Flight Center), Ezoe (Tokyo Metropolitan University), and Narasaki (Sumitomo Heavy Industries, Japan) described the Soft X-ray Spectrometer instrument on the Japanese Astro-H mission, which is currently slated for launch in late January 2016. The instrument is a successor to the X-ray spectrometer instrument launched on Astro-E2 in July 2005, but involves a more complex cryogenic system intended to achieve full operational redundancy. The heart of the instrument is a 6×6 array of X-ray microcalorimeters cooled to 50 millikelvin by a three-stage adiabatic demagnetization refrigerator (ADR). The cryogenic system consists of a 30+ liter superfluid helium tank and a 4.5K Joule-Thomson (JT) cryocooler, and two pairs of two-stage Stirling cryocoolers that pre-cool the JT cooler and vapor cooled shields within the dewar. The unique instrument configuration allows it to meet its science goals if either the superfluid helium tank or the 4.5K JT cooler remain operational, and it has some tolerance for degradation or failures of the Stirling cryocoolers.

The talks discussed the operation and thermodynamic performance of the ADR, the porous plug and helium dewar, and the cryocoolers. The system has been shown to meet all of its requirements (most importantly 50mK detector operation and >90% observing efficiency) in both cryogen and cryogen-free modes. The Astro-H satellite is shown in the figure just prior to thermal vacuum testing at the Tsukuba Space Center, Japan. The testing was successful, and now Astro-H moves on to final vibration and acoustic testing before being shipped to the launch site at Tanagashima Space Center, Japan.

The second mission discussed was the Robotic Refueling Mission 3, by Boyle (NASA/GSFC), which will demonstrate the transfer and solidification of methane and xenon, as well as the technologies needed to achieve zero-boiloff (ZBO) storage and a no-vent fill. The methane source tank will use conic vanes to position the liquid for transfer. Both it and the receiver tank have cryocoolers to support the ZBO and no vent fill operations. The system features two liquid transfer lines and one gas transfer line used for condensing and freezing the methane. The demonstration will be conducted on the International Space Station, currently expected to launch in April 2017.
Narasaki (Sumitomo Heavy Industries, Japan) also presented on a possible collaboration between the Russian Space Agency and the European Space Agency (ESA) on the development of the Millimetron Space Telescope that will use a 4.5K mirror and detectors cooled to 100mK. The baseline concept is to use passive cooling of shields surrounding the telescope to 30-40K, and active cryocoolers from 20K down to 1.7K. The instrument’s detectors will then be cooled to 100mK using the hybrid He-3 sorption cooler and ADR stage developed by Duband (CEA).

Cryocoolers and sub-Kelvin cooling

Although other missions currently in formulation, such as Athena (the next major x-ray observatory being studied by ESA), were not presented directly, there were numerous presentations on the technology developments being undertaken to support them. In particular, recent progress in the development of a closed-cycle dilution refrigerator capable of cooling detectors to 50mK was described by Butterworth (Air Liquide, CSA CSM) and Vermeulen (Neel Institute). The work extends the operation of the open-cycle DR used on Planck to achieve surface tension confinement of the helium-3/helium-4 mixture and phase separation of the two isotopes needed to achieve the necessary circulation rates in a closed system. Recently, a full-scale demonstration unit was tested in -1 g. The helium-3 was circulated using the compressor developed for SHI’s 1.7K JT cryocooler, and a base temperature of 80mK was achieved, believed to be limited by the suction pressure (8 mbar actual vs 5 mbar required).

Other innovative cooling system developments were presented, including an active magnetic refrigerator capable of cooling into the sub-kelvin regime (Miller, Univ of Wisconsin-Madison). The concept involves oscillating flow of cold liquid helium created by demagnetizing a packed bed of GGG, driven by the fountain effect caused by magnetizing a second bed. The concept is capable of cooling powers of ~1 mW at 0.75K. Also pushing into the sub-kelvin regime, Ullom (NIST, Boulder) has extended the concept of cooling based on superconducting tunnel junctions to be able to cool macroscopic objects. The technique has been shown capable of producing ~60mK from a 290mK base temperature. Sullivan (NASA/GSFC) also discussed on-going work on continuous ADRs and a new piezo-electrically actuated mechanical heat switch.

Cryocooler development talks were not as prominent at this workshop as in the past, but did include the following: Dave Frank discussed work at Lockheed-Martin on their Coax microcooler (whose compressor is 0.3 W at 15K. Butterworth gave a separate talk on a concept for using the gas from a PT cooler, rectified by check valves and buffers, to transfer cooling to the load. The advantage is an ability to couple redundant coolers, and minimize the parasitic load that a failed cooler imposes, without the use of auxiliary heat switches. In a related presentation, Prouve (CEA) investigated redundant PT coolers (with the 30K and 100K stages directly linked to each other) and directly measured the parasitic heat leaks from an inactive cooler. Sato (JAXA) reported on continuing efforts to develop and qualify the 1K-class JT cooler in support of future astronomy satellites, and Narasaki (SHI) also presented a thermodynamic assessment of a JT cooler using 2-stage JT valves, showing the feasibility of increasing the cooling power of SHI’s 1.7K JT cooler from 10 mW to 20 mW.

Cryogenic fluid management

Since 2005, NASA’s exploration focus has given rise to renewed efforts to develop and demonstrate the technologies needed for managing cryogenic fluids in space. Consequently, a large segment of the workshop was devoted to technologies and methods associated with liquid acquisition, pressure and temperature control, zero-boiloff systems, thermal insulation and chilldown of cryogenic tanks, as well as zero-g experimentation with both cryogenic and simulant fluids.

As mentioned, RRM-3 will use conic vanes for fluid control, but screen channel systems can support higher flow rates based on the higher surface tension forces acting on the fluid at the screen surface. Several papers presented studies of pressure drop (Darr, Univ. of FL) and bubble point testing (Hartwig, NASA/GRC) of screen channel systems, as well as the effectiveness of these devices in liquid H2 outflow tests (Zimmerli, NASA/GRC) in the engineering demonstration unit built for the Cryogenic Propellant Storage and Transfer (CPST) project. Modeling of the chilldown and filling process for the same tank was presented by Hedayat (NASA/Marshall Space Flight Center), and modeling of the effect of a thermodynamic vent system to control ullage pressure was presented by Majumdar (also of NASA/MSFC).

In large scale systems, the testing by Plachta (of NASA/GRC) validated model predictions that show that for storage of liquid H2 and O2 propellants, adding a cryocooler (and its power system) to achieve zero boiloff is the lower mass option when storage times exceed a few weeks. Using broad area cooling to distribute the cooling along the tank walls, a demonstration of ZBO was conducted using a Creare (CSA CSM) turbo-Brayton cooler (like that used on Hubble’s NICMOS instrument) that resulted in temperature gradients on the liquid nitrogen tank of less than 4K and ZBO tank pressure control demonstrated to within ±0.1 psi. From a related paper by Guzik (NASA/GRC), while ZBO for a liquid H2 tank would require cooling powers of kilowatts at 20K, a direct analytical method for reduced boil-off strategies require cooling only to 90K was presented, which distributes cooling along the shield surrounding the tank. Another ZBO development was presented by Chato (NASA/GRC) on the progress and development of the ZBOT (Zero Boiloff Technology) micro-gravity science experiment, which is slated to fly on Space X 10 and scheduled to launch in 2016.

In related work, Stephens (NASA/GRC) showed that the effect of introducing pressurant gas, in order to transfer liquid H2, into bulk liquid as opposed to the ullage space significantly increases the amount of gas needed to reach desired pressures, as the gas thermalizes far more rapidly. The side effect is a ~2x times larger heat input from the pressurant gas.

Zero-G Fluid Behavior

Although large-scale investigations of fluids in zero-g, such as CPST (Cryogenic Propellant Storage and Transfer), have not gotten off the ground, so to speak, a number of smaller-scale experiments have made use of drop-towers, sounding rocket launches and zero-g airplane flights to make shorter duration observations. The talks on Cryofenix by Mathey (Air Liquide) and Legrand (CNES) report the first observations of liquid H2 from sounding rocket platforms since the 1960s. With six minutes of microgravity, the experiment used a cold gas system to impose accelerations and an array of sensors to monitor pressures, thermal stratification and settling times (including de-spin). Good agreement between CFD model predictions and observations was reported.

Schmitt (Univ. of Bremen) described drop-tower tests looking at the behavior of liquid H2 in response to step changes in gravity. In particular, the experiment used visualization of the fluid to see the effect of heat inputs to the walls. Fluid flow along the walls was not obviously affected, but the pressure in the vapor bubble that forms was significantly increased.

Other technology reports

A number of other noteworthy presentations were made whose topics don’t neatly fit into the broad groupings outlined above. In particular, Schlachter (Kahrlsruhe Institute of Technology, CSA CSM) summarized testing of an HTS magnet that was designed to divert the plasma produced during re-entry in an effort to mitigate the radio blackout normally experienced. Tests of a 10 cm diameter bore REBCO magnet subjected to 450K plasma showed an ability to maintain 2 T external fields. Modeling of the plasma distribution shows the ion density is low enough to prevent loss of communication.

Vanapalli (Univ. of Twente) described a heat switch made entirely by additive machining techniques for connecting electronics boxes to radiators for thermal control. Fesmire (NASA/Kennedy Space Center) presented data on the effectiveness of layered composite thermal insulation using multi-layer insulation and aerogel blankets, and Johnson (NASA/GRC) described a test platform for evaluating the performance of large-scale thermal insulation and structural supports as part of the Evolvable Cryogenics (eCryo) Project. Tuttle (NASA/GSFC) reported on the thermal absorptance of gold-coated stainless steel tubing, used on the James Webb Space Telescope, in which the absorptance was determined to be higher than that expected from pure gold. Courts (Lake Shore Cryotronics, CSA CSM) described the effort at Lake Shore to produce a standard line of flight-qualified thermometers, including diodes, Cernox and germanium sensors. The goal is to reduce delivery time and cost.

Pamidi (Florida State University) described a test system for direct caloric measurements of ac losses in superconducting motors for use in aerospace planes. Allen (Michigan Technological University) described a measurement technique for determining evaporation and condensation coefficients for cryogenic propellants, and Kassemi (NASA/GRC) presented on the effect of interfacial turbulence and accommodation coefficients in CFD modeling of pressure control in cryogenic storage tanks.

Titan mission studies

To conclude this summary, it seems fitting to come back to the central theme of space exploration, and specifically to two talks that presented the most far-reaching and creative thinking: how to explore Saturn’s moon Titan. Titan’s atmosphere is 95% nitrogen at 1.5 bar, and has a methane cycle that is similar to the water cycle on Earth. DeLee (NASA/GSFC) presented a study of a cryogenic propulsion system for the Titan Orbiter Polar Surveyor (TOPS). The mission duration would be 10+ years, and any cryogens used in propulsion would have to be preserved for at least eight years. Power is limited, so no active cooling is possible. The conclusion was that liquid H2/O2, using the most advanced load responsive MLI and launching subcooled, represented the most mass advantageous solution.

But nothing at the workshop compared to the eye-catching subject of Hartwig’s (NASA/GRC) presentation “Exploring the depths of the Kraken Mare.” Kraken Mare, we are to understand, is one of three stable hydrocarbon seas on Titan (the others being Punga Mare and Ligeia Mare, as seen in the above picture taken by the Cassini spacecraft), and the goal was to design a submersible vehicle that could navigate in and study these 90-96K oceans, in part to see if hydrocarbon-based life is possible on Titan. The task is complicated by the fact that the seas vary in ethane/methane concentration, so buoyancy and thermal control requirements are different. The concept presented was based on maintaining the internal environment near 300K, while managing heat exchange and power generation for the instruments and submarine communication and propulsion systems. There is too much to the study to describe here, but it’s definitely a mission that offers an exciting challenge to the cryogenics community.