Hybrid Aerogel-MLI Insulation System Performance Studies for Cryogenic Storage in Space Applications By: R. Begag 1, S. White 1, J.E. Fesmire 2, and W.L. Johnson 2 1 2
Aspen Aerogels, Inc., 30 Forbes Rd., Northborough, MA 01532 NASA Kennedy Space Center, Cryogenics Test Laboratory, KSC, FL 32899
ABSTRACT Long duration storage of large quantities of cryogenic fluids for propulsion, power, and lifesupport is an essential requirement for missions into space. Efficient and reliable insulation materials are key to the success of these missions. The required insulation material must outperform the current standard multi-layer insulation (MLI) for thermal insulation and provide additional features such as durability, micrometeoroid orbital debris protection, and flexibility, all in one single-layer material. Ultra-low density and highly hydrophobic fiber reinforced aerogel material integrated with MLI has the potential to offer a great insulation package which will overcome several issues that the current standard MLI alone suffers from such as: 1) damage during installation, 2) high cost, and 3) degradation over time. The hybrid aerogel/MLI solution affords a more reliable alternative because it is robust, and will outperform the MLI in cases of vacuum loss. Low density and highly resilient methylsilicate aerogel will contribute less solid conductivity to the overall heat transfer within the aerogel/MLI system. Sol-gel optimization of low density and low dust methylsilicate aerogels will be presented. Thermal performance of two prototypes of hybrid aerogel/MLI composites and a baseline MLI system (1 inch thick, 90 layers) fabricated by Aerospace Fabrication and Materials (AFM) and tested at cryogenic temperatures under different vacuum level conditions (at Cryogenic Laboratory, NASA KSC) will also be presented. INTRODUCTION The planned Altair/Ares missions to return to the moon include a short-term commute to and from the moon and an extended lunar stay of 210 days. Long duration storage of large quantities of cryogenic fluids for propulsion, power, and life-support is an essential requirement for these missions1,2. Several analytical and experimental studies were conducted over the last several decades which investigated a variety of methods for meeting mission storage requirements2,3. These methods can be divided into two general types: those that use boil-off gasses from the cryogen and those that have zero boiloff. Methods that utilize boil-off for cooling include thermodynamic vent systems and vapor cooled shields, each of which require additional fluid mass to compensate for the boil-off losses. Methods that enable zero boil-off include actively cooled shields that use a cryocooler refrigerator and a simple, well-insulated tank. The required insulation material must outperform the current standard multi-layer insulation (MLI) for thermal 1 2
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insulation and provide additional features such as durability, micrometeoroid protection, and flexibility. Aspen has successfully developed a novel cryogenic aerogel insulation material which when combined with MLI has the potential to revolutionize cryogenic storage fluid in space by providing a durable material with unsurpassed thermal performance, at a reduced cost, when compared to existing vacuum insulation (MLI), and when considering installation and maintenance costs. For NASA applications, highly efficient, readily usable, and low cost cryogenic insulation is a technology immediately required for the cost-effective implementation of the next generation cryogenic storage for space exploration, space orbiters, and space station programs. AEROGEL INSULATION One of the most promising applications of aerogels is their use as thermal insulation. The large thermal resistance of aerogels is attributed to the high porosity of the gel network, which consequently has a very small solid thermal conductivity. Aerogels have extraordinary high thermal insulation values over a wide range of vacuum levels. Flexible aerogel composites are now commercially available from Aspen Aerogels. Methylsilicate aerogels, which are not currently available commercially, exhibit super-hydrophobicity and extraordinary mechanical properties. Unlike other chemically treated aerogels which require two step sol-gel processing (i.e. a gel fabrication step and hydrophobization of the gel during the aging step), methylsilicate aerogels use one step sol-gel processing and are intrinsically superhydrophobic, as shown in Figure 1.
Figure 1. Drops of water on surface of (a) methylsilicate aerogel fiber reinforced (b) Flexibility and resiliency of methylsilicate fiber reinforced aerogel. The tetrafunctional monomer structure of silica aerogels (such as those derived from water glass, tetraethylorthosilicate (TEOS) or tetramethylorthosilicate (TMOS)) readily break down and collapse under small shear stresses due to the rigid siloxane network that makes silica aerogels extremely brittle. Methylsilicate aerogels, typically derived from Me-SiX3 monomer starting materials, afford a highly resilient network structure that does not readily crack, shrink, or irreversibly plastically deform when compressed4.This material characteristic is derived from four structural differences: 1) low surface silanol concentration, 2) repulsive interactions between methyl groups, 3) lower crosslinking density, and 4) a continuous three dimensional gel skeleton. This material resilience to compressive stress is particularly attractive because it facilitates the
aerogel material to be compressed during installation without degradation in thermal performance. EXPERIMENTAL Using Me-SiX3 monomer silica precursors such as methyltriethoxysilane (MTES) or methyltrimethoxysilane (MTMS), ultra-low density aerogels (0.03 – 0.05 g/cc) were prepared by the sol-gel process. The main goal was to preserve the resiliency of the aerogel, which is procured by the methylsilicate structure, and simultaneously try to attain the lowest thermal conductivity value under cryogenic conditions. The aerogel manufacturing process can be divided into four steps: 1) sol-gel process / preparation of the gels, 2) selection of the fiber reinforcement, 3) aging of the gels, and 4) supercritical drying of the gels. During sol preparation the methylsiliconate precursors were hydrolyzed in the presence of alcohol (i.e. ethanol or methanol), and in an acidic medium. Later, the resulting sol was diluted with alcohol to the desired target density and catalyzed with a base to form a gel. Prior to gelation, the sol is cast into a fiber reinforcement and remains undisturbed until the gel forms. Upon gel formation it is often assumed that the hydrolysis and condensation reactions of the silicon precursor reactants are complete. This is far from the case. The gel point simply represents the time when the polymerizing silica species spans the container containing the sol. At this point the silica backbone of the gel still contains a significant number of unreacted alkoxide groups. The gels then have to be aged, at a temperature of 60 ºC in an alcoholic solution for 48 hours, to permit full polycondensation of free alkoxy and silanol groups. Afterward, the fiber reinforced gel samples are dried under supercritical conditions of CO2 (~ 31.06 °C, 1,100 psi) to extract alcohol from the porosity of the gels without causing collapse to the gel network. To quantitatively determine the effect of the sol gel chemistry parameters on the properties and performance of the low-density aerogel, Design of Experiment (DOE) software was used to establish a plan, to analyze data, and to determine the effective weight of each factor on the properties of the aerogel. The optimum aerogel formulation was used to fabricate 8 inch circular and ½ - ¼ inch thick fiber reinforced low density aerogel samples to be used for aerogel-MLI prototype fabrication at Aerospace Fabrication Materials (AFM). THERMAL PERFORMANCE Hybrid aerogel/MLI and MLI baseline insulation prototypes AFM were sent to the Cryogenics Test Laboratory at NASA Kennedy Space Center (KSC) to determine the thermal conductivities at cryogenic temperatures and different levels of vacuum using an insulation test instrument, Cryostat-500. Cryostat-500 uses a steady-state liquid nitrogen boil-off calorimetry method to determine the effective thermal conductivity (k-value) and heat flux. Each test measures the steady-state heat leak (watts) through the specimen for a set of environmental conditions including the warm-boundary temperature (WBT), cold-boundary temperature (CBT), and cold vacuum pressure (CVP). Liquid nitrogen maintains a CBT of approximately 78 K while a WBT of approximately 293 K is maintained by an electric heater with electronic
controller. Vacuum levels for this test covered the full range, from high vacuum (below 10-5 Torr) to no vacuum (760 Torr). The rate of the heat transfer through the insulation system into the cold-mass tank is directly proportional to the flow rate of liquid nitrogen boil off. The kvalue is determined from Fourier’s law for heat conduction through a plane wall. The heat flux is calculated by dividing the total heat transfer rate by the effective area of heat transfer5,6. Table 1. Specifications of aerogel/MLI prototype systems.
(a) (b) Figure 2. (a) Effective thermal conductivity as a function of cold vacuum pressure of two aerogel/MLI prototypes (# 1 and #2) and MLI (#3) insulation systems. (b) Effective thermal conductivity of the prototypes compared to low density aerogel-only blankets. A total of three prototype insulation systems, approximately 200-mm diameter by 25-mm thickness, were supplied to KSC for thermal performance analysis. It should be noted that the
nature of the surface (either aerogel or MLI) in contact with the cold side can have an affect on the overall heat transfer through the system7. The basic material specifications for the prototypes are listed in Table 1. A Mylar stiffener was used around the edge of the prototypes to maintain their height and lofting. This addition most likely had an affect on the thermal properties of the insulation system, but is assumed to be similar for all three test articles. The summary chart of the k-values as a function of the cold vacuum pressure (CVP) of the three prototypes is presented in Figure 2. The thermal testing was performed to compare the thermal performance to the baseline MLI system (Prototype#3) and to investigate the importance of the number of layers of aerogel as well as the order of the MLI and aerogel material layers. As shown in Figure 2a Prototype #2 outperformed Prototype #1 in the vacuum range tested. The thermal performance of Prototype #2 is 23 – 30% better than Prototype #1. Therefore, it is believed that, for the same thickness, a lay-up of thin aerogel combined with MLI should perform better than fewer thick aerogel layers with MLI. More importantly, Prototype #2 outperformed the MLI baseline system, even under high vacuum. Above about 50 microns CVP, the heat transfer is dominated by gas conduction and, as shown in Figure 2 (a) and (b), the shape of the k-value curve (sigmoid) of prototype #3 is mainly due to the macro-size gaps between the layers that allow gas conduction. In comparison, the linear curves for prototypes #1 and #2 include aerogel material with nano-scale spaces that tend to suppress gas conduction. These results do represent approximate absolute values and are not completely calibrated. Factors such as the surface contact resistance under vacuum conditions and the effects of the edge ring (Mylar stiffener) of the samples are unknown. More testing is warranted to quantify these effects. Further measurements in the high vacuum range, including CVP to below 0.01 microns, is also needed for complete comparison with MLI baseline systems. Initially, it was decided to test the prototypes with MLI layers facing the cold side. This configuration has been shown to give the best thermal performance using higher density aerogel blankets7. Mistakenly the prototypes were tested with the aerogel facing the cold side. Nevertheless, the test results for these low density samples have shown a great improvement in thermal performance when compared to test results obtained on aerogel-only insulation blankets8, as shown in Figure 2b. The high vacuum thermal performance of the aerogel material improved by more than two orders of magnitude when combined with MLI (Prototype #1 and Prototype # 2). Other tests are planned by the KSC Cryogenics Test Laboratory to determine the thermal performance of the aerogel/MLI systems, with the MLI materials facing the cold side. Moreover, other tests will also be performed on the same prototypes without the Mylar stiffener, to evaluate its effect on the overall thermal performance of the system. CONCLUSIONS Several experimental studies and considerable research were carried out to optimize the conditions of sol-gel synthesis formulations to produce resilient, dust free and superhydrophobic low density methylsilicate-based aerogels. Multi-layer prototypes of an optimized low density aerogel and MLI materials were fabricated and tested under cryogenic vacuum conditions by NASA’s Cryogenic Test Laboratory at Kennedy Space Center. Test data are of special interest to develop robust MLI systems that can be designed for a minimal drop in thermal performance. This feature is afforded by the presence of low density aerogel blankets. These tests
demonstrated that low density aerogel/MLI lay-ups do not diminish the thermal performance of the aerogel material; in fact, the thermal performance of the system was enhanced by this technique, at high, soft and no vacuum. Because the aerogel layer is also structurally capable, it can help support fragile, easy-to-disturb MLI layers. This type of hybrid aerogel/MLI insulation system will revolutionize cryogenic storage in space applications by providing a durable material with excellent thermal performance compared to existing insulation systems. ACKNOWLEDGEMENT This research has been funded by NASA Marshall Space Flight Center under contract number NNX10CE61P. Aspen would like to thank James E. Fesmire and his team for the valuable thermal tests performed under this contract. This testing was done under a nonreimbursable Space Act Agreement (SAA) between Aspen Aerogels, Inc. and NASA John F. Kennedy Space Center for aerogel composite systems for cryogenic application in high vacuum environments. REFERENCES 1. B.J.Tomlinson, T.M. Davis, J.D. Ledbetter, “Advanced cryogenic integration and cooling technology for space-based long term cryogenic storage”, Cryocoolers, vol. 11. Kluwer Academic/ Plenum Publishers; 2001. 2. P. Kittel, D. Plachta, “Propellant preservation for mars missions”, Advances in cryogenic engineering, vol. 45. Kluwer Academic/ Plenum Publishers; 2000. 3. D. Plachta, P. Kittel, “An updated zero boil-off cryogenic propellant storage analysis applied to upper stages or depots in an LEO environment” NASA Glenn Research Center, NASA TM 2003-211691, June 2003. 4. R. Venkatswara, D. H. Nagaraja, and H. Hiroshima, J. Colloid. Interf. Sci., 2007, 305, 124. 5. J. E. Fesmire, et al.., “Equipment and Methods for Cryogenic Thermal Insulation Testing,” in Advances in Cryogenic Engineering, Vol. 49, American Institute of Physics, New York, 2004, pp. 579-586. 6. J. E. Fesmire, and S. Augustynowicz, “Thermal Insulation Testing Method and Apparatus,” US Patent 6,824,306 November 30, 2004. 7. W. L. Johnson, J. A. Demko, and J. E. Fesmire, “Analysis And Testing of Multilayer and Aerogel Insulation Configurations,” Advances in Cryogenic Engineering, Vol. 55A, American Institute of Physics, Melville, NY, 2010. Pg. 780-787. 8. DOE Phase I, “Flexible Aerogel as a Superior Thermal Insulation for Superconductor Technology”, Contract No DE-FG02-08ER85189.
