by Alan Kopelove, Quest Thermal, alan.kopelove@questthermal.com

Part 1: Use of Discrete Spacers for Advanced Thermal Insulation

Cryogenic thermal engineers are familiar with multilayer insulation (MLI), developed in the late 1950s as lightweight insulation for cryogenic propellants. MLI is used as high performance thermal insulation for launch vehicles, spacecraft, cryogenic tanks, dewars and spaceborne instruments. MLI operates in high vacuum, where its performance exceeds other insulations by a factor of ten. However, the heat leak through acreage MLI is still a major one for cryotanks, making it difficult to achieve the NASA goal of zero boiloff needed for long duration missions, and real world application of MLI also have some well-known challenges.

Conventional MLI technology is over 60 years old and is currently based on gold or aluminized metalized polymer films separated by polyester or silk netting. The metalized films act as radiation barriers and reduce radiated heat flux, while the netting separates the barrier layers and reduces solid heat conduction. MLI soft blankets involve a lot of touch labor and it is hard to control MLI density and compression. MLI also can be difficult to support and control on large cryogenic tanks. In the 1970s, a study by Lockheed Corporation on MLI resulted in the Lockheed Equation, formulated as a semi-empirical fit to data on thermal performance.

Heat leak through conventional as-received netting MLI is dependent on layer density to the 3.56 power and is therefore sensitive to compression, which is not well controlled. MLI thermal performance is also dependent on design, workmanship and installation. MLI is neither robust nor structural, i.e., it can’t support itself well on large tanks, nor any external loads such as lightweight vacuum shells or broad area cooled thermal shields. Due to layer compression and other factors, MLI performance is difficult to predict, leading to “degradation factors” from the Lockheed Equation that can range from 1.5 into the 20s (heat leak 1.5 to 20 times larger than predicted).

These factors led to the creation and development of a next generation MLI technology, called integrated MLI (IMLI), by Gary Mills of Ball Aerospace and Scott Dye and Alan Kopelove at Quest Thermal Group. IMLI replaces the netting separator with discrete, low thermal conductance, micro-molded polymer spacers. Discrete spacers offer numerous advantages over netting MLI, including precise control over layer spacing and density, a robust bonded structural MLI system and an engineered geometry that can be thermally modeled accurately and performs close to predicted behavior with repeatable thermal performance.

Discrete spacers offer elegant engineered solutions that can be designed for specific properties, such as low heat flux or structural strength. This is accomplished by careful control over the geometry of the spacer, including the cross sectional area to length ratio (that controls solid heat conduction from layer to layer) and static or dynamic response properties. The IMLI spacer is a micromolded polymer that provides about 1/1000 the contact area that netting spacing has, is fabricated from a low thermal conductance engineering polymer and has an area/length ratio of about 0.0001 m. IMLI performance has been measured via boiloff calorimetry on test tanks ranging from 10L to 500L at Quest, Ball Aerospace and the NASA KSC Cryogenics Test Lab (with help from Wesley Johnson and James Fesmire), and had a measured heat flux of 0.41 W/m2 for a 20 layer IMLI structure (78K, 292K, 3.7 cm). IMLI typically has 30-50 percent less heat flux per layer than conventional MLI, so fewer layers are needed for a specific heat flux. IMLI will reach TRL (Technology Readiness Level) 9 with its first spaceflight in 2016 on the NASA/Ball Aerospace Green Propellant Infusion Mission. Ball and Quest began work on IMLI in 2007, so you can see how long the technology development and infusion cycle is for aerospace! This work was made possible because cryogenic fluid management groups at multiple NASA centers saw the possibilities to improve on traditional MLI, and supported this work via NASA Small Business Innovation Research (SBIR) contracts.

With the success of Discrete Spacer Technology in IMLI, the team began looking at other applications in need of advanced thermal insulation. Load responsive MLI (LRMLI) was designed to provide both ultrahigh performance in space and high performance in air, as a possible Spray On Foam Insulation (SOFI) replacement. LRMLI uses a unique dynamic spacer with a central support rib, which in the unloaded condition (in-space) has a 0.005″ gap between the support rib and the underlying layer and no heat leak through the rib. When loaded, for example with external air pressure, the LRMLI spacer dynamically responds and supports the load (but with additional heat leak).

The 30 mg load responsive dynamic spacer blends both low thermal conductivity and structural strength, and has supported 90 lbf. A 0.25″ three-layer LRMLI system has a measured heat flux of 4.8 W/m2 in vacuum and 29.1 W/m2 in air (77K, 295K, 0.63 cm), which is a 24x advantage in air over SOFI per thickness and a 144x lower heat leak per thickness than SOFI on-orbit. LRMLI self-supports loads; for example, operation in air requires only a thin, lightweight metal vacuum shell. First generation LRMLI had a mass of 2.5 kg/m2, equal heat leak through SOFI would require 91 cm and 33.7 kg/m2. Later work developed a lower mass polymer laminate vacuum shell LRMLI with an areal mass of 1.35 kg/m2.

Another application considered is MLI for cryogenic propellant feedlines. Heat leak through spiral wrapped MLI on pipes is 3 to 10 times higher than tank MLI. The poor performance of traditional MLI wrapped on feed lines is due in part to compression of the MLI layers, with increased interlayer contact and heat conduction. Quest Wrapped MLI uses a novel discrete spacer to maintain layer spacing and reduce heat leak. A Triple Orthogonal Disk spacer was engineered to minimize contact area/length for use in concentric MLI configurations. Wrapped MLI prototypes were fabricated and tested, and offered superior performance, 2.2 W/m2 heat flux compared to 26.6 W/m2 for traditional spiral-wrapped MLI (five layers, 77K to 295K). Wrapped MLI as inner insulation in vacuum jacketed pipe had a heat flux as low as 0.09 W/m, compared to industry standard products with 0.3 W/m, and could enable improved spacecraft cryogenic feedlines and industrial hot/cold transfer lines.

The structural strength of discrete spacers can be used to support a variety of loads, including vacuum shells or thermal shields. An interesting project that Quest Thermal worked on with NASA focused on large tank MLI for reduced boiloff. Load bearing MLI was engineered so that the insulation itself supported the external load of a broad area cooled thermal shield as part of an actively cooled system on the large NASA Glenn SMiRF LH2 tank. Tank standoffs were not needed since the discrete spacers in LBMLI easily supported the thermal shield and external MLI. LBMLI provided a 51 percent reduction in heat leak per layer over traditional MLI and thermal shield supports, with a 38 percent reduction in mass. These advances in MLI may help achieve the zero boiloff goals required for long duration space exploration missions.

Part 2: New applications and thermal insulation designs

Thermal insulation is used all around us and opportunities for advanced insulation for aerospace and terrestrial applications are quite interesting. Integrated MLI (IMLI) and variants are not necessarily the best solution for all cryogenic insulation applications but they offer the opportunity for improved performance, structural strength and the unique capability to precisely engineer insulation with novel properties for new mission requirements.

The Quest Thermal and Ball Aerospace team has developed advanced launch vehicle specific insulation systems to improve upon the spray on foam insulation (SOFI) used on launch vehicle cryopropellant tanks. Launch Vehicle MLI (LVMLI) uses the robustness and strength of discrete spacers to form a ruggedized IMLI structure potentially strong enough to survive launch loads on exposed cryotank sidewalls. Current Centaur sidewall insulation is SOFI, with a heat flux of approximately 230 W/m2. The team’s 2.5 layer LVMLI prototype survived simulated aerodynamic ascent forces. It has 3.5 W/m2 heat leak, 68 times lower than SOFI, and 33 percent of the mass of SOFI. LVMLI requires vacuum for good performance and as such is designed for in-space operation.

Load Responsive MLI (LRMLI, described in Part 1) can operate both in-air and in-space, but requires the insulation internal space to be pumped down to hard vacuum. With this limitation in mind, the team began designing two new launch vehicle insulation systems that would not require mechanical pumping.

Cellular Load Responsive MLI (CLRMLI) is a novel technology with a cryopumping cellular core containing Load Responsive MLI layers. CLRMLI self-evacuates via cryocondensation or cryosorption (depending on cryogen and temperature) when in contact with cryogenic propellant tanks, allowing high thermal performance both in-air and in-space. The cellular structure provides damage tolerance, and internal LRMLI layers support atmospheric pressure with a lightweight laminate vacuum shell layer.

CLRMLI dramatically outperforms SOFI both in-air and in-space. Compared to SOFI’s 230 W/m2 heat flux, CLRMLI first generation prototypes have a measured heat flux of 11 W/m2 in vacuum and 46 W/m2 in-air. Quest demonstrated the feasibility of CLRMLI in SBIR Phase I, reaching TRL 4, and is now in Phase II R&D. SINDA-like thermal modeling predicts second generation CLRMLI should have 8 W/m2 in-space and 27 W/m2 in-air (77 K, 295 K, 1.5 cm).

Another new launch vehicle system, Vacuum Cellular MLI (VCMLI), uses a thin self-evacuating dual layer vacuum cell honeycomb for thermal insulation in-air, with exterior LVMLI layers for excellent performance in-space. VCMLI can replace SOFI with a high performance, robust system that provides 76 percent lower heat leak in-air and 98 percent lower heat leak in-space than SOFI.

An interesting (and fun) aspect of these new insulation technologies is that we can engineer these systems to modify their properties. Four-layer VCMLI provides a different operational trade-off than CLRMLI, with 4.7 W/m2 in-space and 42 W/m2 in-air, offering better performance in-space and still preventing icing pre-launch. Heat leak through the cellular structure, heat leak through the internal or external radiation barriers and strength of outer skins are each adjustable for specific applications. Ball Aerospace and Quest have a current VCMLI development program funded by the US Department of Defense to begin further testing and integration into a launch vehicle for increased payloads and coast time capabilities. The benefits are clear, but as these technologies are currently at TRL 3 to 5, more development and testing are required.

Quest Thermal is also working on two new insulation systems with potential to help meet NASA cryogenic fluid management goals. The 2015 NASA thermal management roadmap has goals for passive and active thermal control, including load responsive insulation that can support broad area coolers or vapor-cooled shields and low thermal conductivity structural supports, for reduced or zero boiloff. As acreage tank MLI improves, conductive heat leak through tank supports such as skirts and struts has become more important, and reducing heat load through these support elements is a productive area for innovation.

Quest has developed and tested novel Vapor Cooled Structure MLI (VCSMLI) with custom shaped discrete spacers that create a lightweight vapor transport layer for vapor cooling. VCSMLI uses discrete spacers and IMLI to provide robust insulation that intercepts heat load conducted through tank supports. VCSMLI models predicted a 45 percent reduction in total tank and support heat leak for a skirt mounted tank, and 57 percent reduction in skirt alone heat leak. A VCSMLI prototype achieved a 41 percent reduction in heat leak by using vapor cooling through a lightweight vapor layer applied directly to tank supports and integrated with IMLI insulation. Application of VCSMLI to the Advanced Common Evolved Stage (a cryo upper stage) could reduce heat flux into the LH2 tank from 10,900W to 3,400W, and VCSMLI integrated with Quest LVMLI is modeled to further reduce heat flux to 755W.

VCSMLI Phase II development is beginning and will increase technical maturity by optimizing the design and testing in more relevant environments to include larger tanks, tank strut supports and LH2 tanks. VCSMLI could prove very useful for reducing heat flux into skirt mounted cryotanks such as those on the Delta Cryogenic Second Stage, Vulcan or SLS cryogenic upper stages.

Another interesting insulation system developed for NASA was to provide an effective Mars thermal control strategy, insulating LCH4 storage tanks during the cruise phase to Mars and for LCH4 ISRU storage tanks on Mars surface. NASA provided challenging thermal goals, including <1 W/m2 heat flux on-Mars surface, <0.5 W/m2 in-space, better than SOFI thermal performance in-air prelaunch and low mass.

A hybrid system (LV-LRMLI) using Launch Vehicle MLI (very good in space) with inner layers of a modified LRMLI (very good in-air and on-Mars) enclosed in a ventable and sealable lightweight vacuum shell was developed. Various spacers and geometries—with different structural strength and solid heat conductance—offer unique capabilities to custom engineer thermal solutions. For on-Mars operation with a low atmospheric pressure (600 Pa), the team designed spacers with adequate structural strength to support a polymer laminate vacuum shell with low external on-Mars load while minimizing heat flux. LV-LRMLI prototypes were fabricated and thermal performance measured with boundary temperatures of 77 K and 295 K, with the heat flux for LCH4 tanks calculated to be 127 W/m2 in-air, 1.0 W/m2 on-Mars surface condition (4.5torr CO2) and 0.25 W/m2 in-space. This system provides unique thermal properties, with very good thermal performance in Mars atmosphere for future Mars missions.

Noting the long adoption cycle for aerospace applications, Quest Thermal is currently developing commercial grade superinsulation, based on our discrete spacer technology, initially focusing on home and commercial appliances like refrigerator/freezers (RF). Quest High Performance MLI (HPMLI) uses a new spacer designed for low cost and full time in-air use; a 0.22" thick panel is expected to have a heat flux of 2.4 W/m2. State-of-the-art RF insulation is polyurethane foam, which for 1" thick insulation has about 22 W/m2. HPMLI could lead to highly energy efficient appliances. Prototypes are currently in development and discussions have begun with appliance manufacturers.

I want to give credit for this R&D work to Dye and Phillip Tyler of Quest and Mills of Ball Aerospace. Quest has received great support from our NASA technical monitors, including Johnson and Dave Plachta at Glenn, Shuvo Mustafi at Goddard and Brian Banker at Johnson Space Center, among many others interested in advancing passive thermal insulation. This work was initially supported by the NASA SBIR program, which took the concepts from the back of a napkin to TRL 4, then by NASA Game Changing Development, and finally by a NASA Technology Demonstration Mission that will fly IMLI next year and reach TRL 9. This is a great success story of NASA investing in and helping new technologies (and small businesses) mature.