Evaluation of Thermal Insulation Performance of a New ...

45th International Conference on Environmental Systems ICES-2015-220 12-16 July 2015, Bellevue, Washington

Evaluation of Thermal Insulation Performance of a New Multi-Layer Insulation with Non-Interlayer-Contact Spacer

Takeshi Miyakita1, Ryuta Hatakenaka2 and Hiroyuki Sugita3 Japan Aerospace Exploration Agency, Tsukuba city, Ibaraki prefecture, 305-8505, Japan

Masanori Saitoh4 Orbital Engineering Inc., Yokohama city, Kanagawa prefecture, 221-0822, Japan

Tomoyuki Hirai5 Toska-Bano’k Co., Ltd., Bunkyo ward, Tokyo, 112-0014, Japan

Efficient insulation for tanks of the cryogenic propulsion stage or for infrared astronomical satellites is one of the crucial key requirements. Multi-layer insulation (MLI) blankets are used for spacecraft as excellent insulations. For conventional MLI blankets using net spacers or embossed films however, it is difficult to control the layer density and the thermal insulation performance degrades due to the increase in conductive heat leak through interlayer contacts. At low temperatures, the proportion of conductive heat transfer through MLI blankets is large compared to that of radiative heat transfer, hence the decline in thermal insulation performance is significant. A new type of MLI blanket using new spacers; the Non-Interlayer-Contact Spacer MLI (NICS MLI) has been developed, which uses small discrete spacers and can exclude uncertain interlayer contact between films. The insulation performance is measured with a boil-off calorimeter. Because the NICS MLI blanket can exclude uncertain interlayer contact, the test results correlated well to estimations. Furthermore, the NICS MLI blanket shows significantly good insulation performance (effective emissivity is 0.0008 between 300K and 77K boundary temperature), particularly at low temperatures, due to the high thermal resistance of this spacer.

Nomenclature

= the Stefan-Boltzmann constant eff = effective emissivity s = layer surface emissivity qtotal = total heat flux through MLI qrad = radiative heat flux qcond = conductive heat flux qconv = convective heat flux Thot = hot boundary temperature Tcold = cold boundary temperature N = number of layers R = thermal resistance of a spacer lp = pitch between spacers m = evaporation rate of liquid nitrogen hlg = latent heat of evaporation SBT = heat transfer surface area of the boil-off tank

1 Engineer, Thermal Systems Group, Aerospace Research and Development Directorate, [email protected] 2 Engineer, Thermal Systems Group, Aerospace Research and Development Directorate, [email protected] 3 Head, Thermal Systems Group, Aerospace Research and Development Directorate, [email protected] 4 Engineer, Engineering Group, [email protected] 5 General Manager, Product Development Division, [email protected]

International Conference on Environmental Systems

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I. Introduction ulti-Layer Insulation (MLI) blankets are the most efficient thermal insulation element. Conventional MLI blankets comprise multiple layers of low-emissivity films and netting spacers, the latter of which prevent

direct contact between films to reduce conductive heat leaks (Fig. 1 (a)). Instead of netting spacers, embossed films are sometimes used to decrease the contact area between films. However, neither netting spacers nor embossed films can exclude interlayer contact between film and the netting spacer or films, which has a major impact on conductive heat leak through the MLI blankets, depending on the degree of contact. The thermal insulation performance of MLI blankets is thus strongly dependent on the mounting arrangements. One of the parameters of thermal insulation performance is the effective emissivity of an MLI blanket, as defined by the following equation:

44coldhot

totaleff TT

q

(1)

Effective emissivity of 0.0050 or lower for a conventional MLI is achievable in well-controlled laboratory tests. However, experience has shown that when a blanket is configured for spacecraft applications, effective emissivity closer to the range 0.0150 to 0.0300 is representative of the current design, manufacturing and installation methods for medium-area applications3. Extremely high thermal insulation performance is particularly important for exploration spacecraft such as lunar surface rovers, cryogenic science platforms, and long duration, in-space, cryogenic propulsion systems. The Japan Aerospace Exploration Agency (JAXA) is planning one such mission: the Space Infrared Telescope for Cosmology and Astrophysics (SPICA). The SPICA satellite has a cryogenically-cooled (< 6 K), large telescope (in the 3 m class). Most heat radiation from the Sun to the spacecraft is blocked by sun shields and thermal radiation shields covered with MLI blankets. It is important to maintain the insulation performance of MLI to retain the extremely low temperature of its sensor.

For long-term propulsion systems, including cryogenic propellant tanks, insulation performance is also important as a key technology to reduce heat entering the propellant storage tank and minimize boil-off losses of propellant during long-term missions such as Mars exploration. The performance of conventional MLI blankets declines significantly in cryogenic systems due to the low proportion of radiative heat transfer through MLI relative to that of conductive heat transfer at low temperatures. The temperature of SPICA’s thermal radiation shields is 55 K to 121 K13, while that of the hydrogen tank is 20 K, which makes it challenging to maintain the thermal insulation performance of MLI blankets installed in such cryogenic space missions. In this study, a new type of MLI, Non-Inter layer-Contact Spacer MLIs (NICS MLIs) are experimentally designed and assembled and the insulation performance is measured by a boil-off calorimeter.

(a) Conventional MLI with netting spacers (b) MLI with discrete spacers

Fig. 1 Schematics of a conventional MLI (a) and MLI with discrete spacers (b).

II. Non-Interlayer-Contact Spacer MLI development Outside a well-controlled laboratory, e.g. for an installation configuration on a cuboid body like a satellite or a cylindrical side structure like the rocket propellant tank, the insulation performance of conventional MLIs declines. Our team studied the proportion of heat leak through conventional MLIs10 (Fig. 2). The edges of conventional MLIs are seamed to combine films, but the degradation of conventional MLI is primarily attributable to machine sewing and secondarily to the contact between films and netting spacers. Conductive heat leak through MLIs increases with the contact area of films and spacer or with contact pressure. For ideal MLI the layer density is well controlled in the laboratory, the performance is high and easier to predict. However, for conventional MLI when actually used on

M


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spacecraft, the degree of interlayer contact is difficult to control or predict which makes it difficult to quantify the performance before thermal balance testing of spacecraft. Many equations are proposed to estimate the thermal performance of conventional MLI blankets4-7. However, since these must take the kind of spacer, sewing condition and so on into account, it is difficult to apply them to each MLI blanket. A new type of spacer, non-interlayer-contact spacers (NICS) has been developed14. While conventional spacers such as netting spacers are inserted in the whole surface layer, these new spacers are intermittently arranged and hold up the film to exclude any incidental interlayer contact (Fig. 1 (b)). This section describes the NICS design in detail as well as the assembly method for NICS MLI. The NICS is designed to meet the SPICA satellite requirement for thermal insulation14 and the schematics of an NICS and NICS MLI are shown in Figs. 3 and 4 respectively. NICS controls the gaps between films to be intermittently arranged on the film and prop the upper film. The contact points of the films are only the top face or subsurface of the discrete spacers and the film surfaces do not come into contact. Consequently, conductive heat leaks can be easily estimated because only conduction through the spacer need be considered, rather than interlayer contacts of films and thermal performance can be more precisely quantified. Such spacers have also been developed by S. Dye et al.2 Their MLI has also been tested using a discrete type spacer, revealing a lower heat leak than conventional MLI, while obtaining high thermal resistance improves insulation performance. The detailed thermal resistance design of NICS is described in Ref. 14. As shown in Fig. 3, a NICS comprises three stages. The role of the second stage is to ensure high thermal resistance with an elongated path. The diameter is 10 mm and the height between the first and third stages is 4 mm. The thermal conductive path is 40.9 mm long, with a cross-sectional area ranging from 0.36 mm2 to 0.51 mm2. If the thermal conductivity of the spacer material is 0.30 W/m K, its thermal resistance is calculated at 3.32 × 105 K/W, which is extremely high in a small body. The NICS specifications are shown in Table 1. Films are held between the upper and lower spacers and interconnected, while the first NICS stage includes a locking pin to join the joint hole at the third stage. The film has small holes at equivalent intervals to the spacer pitch, through which the locking pin passes and joins the joint hole of another spacer. Accordingly, the NICS MLI eliminates the need for sewing to integrate films. NICSs are made by injection molding, which allows them to be produced in bulk, economically and with high dimensional accuracy. Polyetheretherketone (PEEK) was selected as the spacer material with heat tolerance, injection moldability and the tolerance of electron beam radiation in mind.

Table 1 Specifications of a non-interlayer-contact spacer.

Thermal resistance Cross-sectional area to

length ratio Diameter Height Mass Material

3.32 × 105 K/W 1.0 × 10-5 m 10 mm 6.9 mm 0.101 g PEEK

Fig. 2 The proportion of heat leak through conventional MLI10

(The layer number is 12, layer density is 2.3 layer/mm and the outermost and innermost layer temperatures are 300 K and 77 K respectively)

Fig. 3 Schematics of

the non-interlayer-contact spacer Fig. 4 Schematics of the non-interlayer-contact spacer MLI

(assembling)


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III. Materials and Method

3.1. Measurement equipment The thermal insulation performance of MLI blanket samples was measured by a double-guarded calorimeter with liquid nitrogen. A boil-off calorimeter is one of the most famous methods used to measure insulation performance of MLI. In particular, the vertical-type boil-off calorimeter has an advantage whereby gravity does not apply in the direction to press the layers. Two types of tank shape are used, one of which is rectangular, to simulate a spacecraft body and evaluate the degree of degradation due to the corner. The other is cylindrical, to simulate a curved surface shape like the sun shield of an astronomy satellite or the propellant tank of a rocket. As shown in Fig. 5, the inner tanks comprise the main boil-off tank and the upper and lower guard tanks, all of which are filled with liquid nitrogen. This configuration ensures uniform temperature distribution around the measurement tank along the axial direction. The size of the rectangular boil-off tank is 300 mm (height) × 236 mm (width) × 236 mm (depth) while the cylindrical tank is 300 mm (height) × 300 mm (diameter). These two boil-off tanks have equivalent heat-receiving areas. The top flange suspends these inner tanks and is equipped with a cryogen exhaust port from each tank, an evacuation port, a safety valve, a vacuum gauge installation port, a feed-through terminal for thermocouples and a water tube for thermostat circulation. An aluminum shroud surrounds the test piece, while the temperature of its inner surface is maintained by thermostat circulation, from 276 to 353 K. The temperature distribution in the inner surface of the shroud is within 1 K. The inner surface of the shroud and the outer surface of the liquid nitrogen tanks are finished in black anodized aluminum as high-emissivity surfaces to minimize radiative thermal resistance between MLI surfaces to the shroud or tank and minimize the impact of uncertain contact heat transfer between the innermost MLI surface and the tank surface. The heat flux through the MLI blanket is measured by the evaporation rate of liquid nitrogen from the main tank. Equation (2) is used to calculate the heat flux q.

BTS

hmq lg

(2)

The flow rate of nitrogen gas having evaporated from the measurement tank is measured using three types of flow meter as follows:

(a) Wet-type volume flow meter W-NK-0.5 (Shinagawa Corp.) (b) Mass flow meter CMS-9500 (Azbil Corp.) (c) Soap-film volume flow meter (50 ml type, time measured manually, for cross-checking)

The wet-type flow meter allows us to measure wide-ranging flow rate (from 0.016 to 5000 mL) with a single flow meter, but has the disadvantage of yielding a fluctuating flow rate due to the difficulty in rotating the measurement wheel when the flow rate is small, whereas the averaged flow rate calculated from the integrated flow rate is considered reliable owing to its measurement principle. Conversely, the mass flow meter is suitable for this experiment since the final desired value is not a volume flow rate but a mass flow rate, despite the relatively narrow measurement range. In this paper, some of the data is taken with (a) and the rest with (b). It was confirmed in another experiment that the averaged flow rates of these two flow meters match.

Fig. 5 Schematics of the double-guarded calorimeter


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The temperatures of the tanks and MLI surface are measured using Type-T thermocouples and the measurement points are individually chosen per test piece. The diameter of the copper and constantan wire is 100 m or 50 m and the wires of the thermocouple for the MLI surface are routed on the surface for a certain length. The tip of the thermocouple is electrically insulated with Kapton tape and covered with a piece of vapor-deposited polyimide tape (UTC-025R-NANA, Ube Industries, Ltd.) to form the same surface as samples.

3.2. Test piece A list of the test pieces for thermal performance tests is shown in Table 2. Two configurations of NICS MLIs

were evaluated, one of which is installed on a rectangular tank and the other on a cylindrical tank. For comparison, ‘reference MLI’ and ‘conventional MLI’ were also evaluated. As the detail of the each test pieces is described later, average areal density of each test pieces is calculated and shown in Fig.6. NICS MLI is heavier than conventional MLI and reference MLI with same number of layers, because NICS MLI uses thicker film.

Table 2 List of test pieces

No. Name Calorimeter Layers Connection Perforation Inner layer film Outer layer film

1 Reference MLI (1) Cylindrical

tank Film ×24 /Net ×23 Interleaved

lapping ×1

(a)Dia.=2mm Pitch=50×50mm Random pattern

Vapor-deposited polyester 6μm film

KF-6B

(Both sides aluminized)

Polyimide surface side of aluminized

25μm film

UTC-025R-NANN (Single side aluminized)

2 Reference MLI (2) Rectangular

tank

3 Reference MLI (3) Cylindrical

tank Film ×12 /Net ×11

4 Conventional MLI (1) Rectangular

tank Film ×12 /Net ×11

Butting joint ×4

(edges of tank)

5 Conventional MLI (2) Simple overlap

×4 (edges of tank)

6-8 Non-Interlayer-Contact

Spacer MLI (1) Rectangular

tank Film 6/5/4

Interleaved lapping

with spacer ×4 (edges of tank)

(c)Via Spacer: Dia.=3mm on film Min Dia. of spacer

pin=0.6mm Pitch≒50 mm×50mm

Vapor-deposited polyimide 50μm film KC-50B (Both sides aluminized)

Only outermost layer is KC-50S (Single side aluminized)

Spacer pitch = 50 mm × 50 mm

9-13 Non-Interlayer-Contact

Spacer MLI (2) Cylindrical

tank Film

12/10/8/6/4

Interleaved lapping

with spacer ×1

(b)Via Spacer: Dia.=3mm on film Min Dia. of spacer

pin=0.6mm Pitch≒90 mm×100mm

Vapor-deposited polyimide 125μm film UTC-125S-AANN (Both sides aluminized)

Spacer pitch ≒ 90 mm × 100 mm

Fig. 6 Calculated average areal density


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3.2.1. Reference MLI The reference MLIs were prepared as samples which were ideally mounted on a spacecraft. Concerning the connection method, interleaved lapping is performed for one layer after another when test pieces are attached to the boil-off tanks, as shown in Fig. 7. Vapor-deposited polyimide tape (UTC-025R-NANA, Ube Industries, Ltd.) is used to fix the outermost layer, while another vapor-deposited polyimide tape (UTC-025R-ANNA, Ube Industries, Ltd.) is used to fix the inner layer and maintain the emissivity of each surface equivalent to the original surface. This connection method reduces the heat leak on lapping but is almost impossible in practice because of the immense time and effort involved. Vapor-deposited polyimide film (UTC-025R-NANN, Ube Industries, Ltd.) is used for the outermost and innermost layers and the polyimide sides are directed toward the outside of the blanket. Thin vapor-deposited polyester film (KF-6B, Kaneka Corp.) is used for the inner reflective shields while the net spacer (KN-20, Kaneka Corp.) is also used.

3.2.2. Conventional MLI The conventional MLIs mimic samples which are

ordinarily mounted on spacecraft and the material of each layer is the same as the reference MLI. All edges are sewn with a sewing machine and taped with vapor-deposited polyimide tape (UTC-025R-NANA, Ube Industries, Ltd.), which has optical properties equivalent to the outermost layer of the blanket, for reinforcement. A hook and loop fastener (A8693Y-71/B2790Y-00, 16 mm wide, Kuraray Fastening Co., Ltd.) is sewn to blankets with a polyester string, also using a sewing machine. The stitches for the fastener are also taped with vapor-deposited polyimide tape (UTC-025R-NANA, Ube Industries, Ltd.) on the outer side of the blanket. At the corner, sample No. 4 is connected by a butting joint, while sample No. 5 is connected by simple overlapping, as shown in Fig. 8.

3.2.3. Non-Interlayer-Contact Spacer MLI Two types of non-interlayer-contact spacer MLI were made. Sample Nos. 6 to 8 include NICS MLI (1), which are installed on the rectangular tank to demonstrate the ease of installation on the cubed spacecraft body. (Fig. 9). NICS MLI (1) was made for each of the tank sides and a tuck was made on the film, which was bent at the corner. The spacer on the corner fixes the right and left films together as shown in Fig. 10. The spacer works for tapes, meaning NICS MLI needs no seams or tapes. Spacers are arranged with a 50 mm grid and 50 µm films are used to maintain the film flatness. There are a total of six film layers. Vapor-deposited polyimide film (KB-50S, Kaneka Corp.) is used for the outermost layers, while thin vapor-deposited polyimide film (KB-50B, Kaneka Corp.) is used for the inner reflective shields. Sample No. 7 is formed by removing the outermost layer from sample No. 6, while sample No. 8 is made by removing the outermost layer from sample No. 7, meaning the outer layer surface of sample No. 6 is the polyimide side and those of sample Nos. 7 and 8 are aluminized surfaces. Samples Nos. 9 to 13 are NICS MLIs (2), which are installed on a cylindrical tank to determine the ease of installation on the rocket propellant tank or astronomy satellite shield (Fig. 11). NICS MLIs (2) have one joint line on the side and the spacer on the corner fixes the films together just like the corner of NICS MLI (1). Spacers are arranged by 90 mm in an axial direction by (about) 100 mm in a circumferential direction and vapor-deposited polyimide 125 µm films (UTC-125S-AANN, Ube Industries, Ltd.) are used. Using thick films for NICS MLI (2), the spacer pitch is extended twice beyond NICS MLI (1). Sample No. 9 includes 12 film layers, while sample Nos. 10 to 13 are formed by removing the outermost layer from sample No. 9.

Fig. 7 Interleaved lapping

Fig. 8 Simple overlap (left) and butting joint (right)


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3.3. Test Procedure To prevent MLI blankets from becoming contaminated with dust, the test MLI blanket is prepared in a clean

room. Using thermocouples, the temperatures of the outermost layer of the MLI film and inner layers of NICS MLI, the surface of the inner tanks and the surface of the shroud were all measured. The evaporation rate and pressure of the vacuum tank were also measured using a flow meter and ionization vacuum gauge. The vacuum chamber was evacuated by a turbo-molecular pump while the vacuum chamber was warmed to 353 K over two days for moderate baking. After baking, liquid nitrogen is transferred into the inner tanks. The pressure of the vacuum tank after the liquid nitrogen is transferred is less than 10-4 Pa. To investigate the temperature dependency of the thermal insulation performance of MLI blankets, the shroud temperature was controlled to 276, 300 and 353 K.

IV. Results and Discussion

4.1. Results of the thermal performance test and data correction method. The heat fluxes measured by the boil-off calorimeter are shown in Fig. 12. Reference MLI and NICS MLI use the left axis and conventional MLI uses the right. The values of the mass flow rate to assign equation (2) are corrected for the influence of atmospheric pressure. Liquid nitrogen in the measurement tank is under saturated vapor pressure and affected by ever-changing atmospheric pressure. If the measurement is performed when the atmospheric pressure declines by 1 Pa, the temperature declines by 8.6×10-5 K. To compensate for the removed heat capacity of liquid nitrogen and the decreased measurement tank temperature, the mass flow of evaporated nitrogen is increased and overestimated and vice versa15. The pressure at the mass flow meter outlet and the volume of liquid nitrogen in the tank are measured by a pressure manometer and capacitance-type level gauge, respectively. The error bar sums up the accuracy of the flow meter and level gauge. The measurement heat fluxes are also corrected by deducting heat which is not through the center MLI sample.

Fig. 9 Picture of the non-interlayer-contact

spacer MLI (1) installed on the rectangular tankFig. 10 Schematic of the overlapping of

the non-interlayer-contact spacer MLI

Fig. 11 Picture of the non-interlayer-contact spacer MLI (2) installed on the cylindrical tank

(Left: overhead view, Right: bottom view)


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Some experimental equations to predict the temperature dependency of MLI thermal performance are proposed in literature [4-7]. The coefficients of the equation depend on many parameters such as the number of layers, spacer material, film material, pre-treatment method, layer density and so on. Although the general form of the equation should resemble those of literature, the coefficients should be evaluated individually per MLI system. Basically, the equation proposed by Cunnington et al.6 is adopted in this study, while the coefficient of each term is simplified as follows using three free-parameters A, B and C:

CHCH

CHMLI TTCTT

BTTAq

)2

()( 67.467.4 (3)

where TH and TC denote the temperatures

of the hot- and cold-side boundaries respectively. Fig 12 shows both the raw test data and fitting curves generated using equation (3) and Fig.13 shows effective emissivity.

4.2. Reference MLI and conventional MLI.

No. 1 reference MLI (1) and No. 3 reference MLI (3) are installed on a cylindrical tank with interleaved lapping and these two samples differ in terms of the number of layers, No. 1 has 24 layers, while No. 3 has 12. Comparing the two, Reference MLI (1), with many layers, shows superior performance at high temperatures, but the performance of reference MLI (1) and reference MLI (3) rarely differs at low temperature. At a high temperature, when radiation dominates, increasing the number of layers to improve thermal performance is effective. At a low temperature however, when conduction dominates, increasing the number of layers has little effect on thermal performance.

No. 2 reference MLI (2) comprises 24 layer films as with No. 1 reference MLI (1), but reference MLI (2) is installed on a rectangular tank and reference MLI (1) on a cylindrical tank. Both comprise the same materials and are applied to the same connection method, interleaved lapping. However, since it is difficult to install one blanket of reference MLI on a rectangular tank with a corner, the average layer density of reference MLI (2) on a rectangular tank may become smaller than reference MLI (1) on a cylindrical tank. The thermal performance of MLI samples using conventional netting spacers like reference and conventional MLIs are more severely affected by the installation methods due to the change in layer density or contact pressure between films.

Despite the fact conventional MLIs comprise the same materials as reference

Fig. 12 Experimental results of boil-off calorimeter and fitting curves

(Temperature of cold boundary is 77 K.)

Fig. 13 Effective emissivity calculated from experimental results


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MLIs except for the installation method and edge sewing, the thermal performance of conventional MLIs is much lower than that of reference MLIs. As discussed in chapter II, about 70 % of the heat leaks through conventional MLIs are attributable to conductive heat through stitching and overlap or joint lines. In this test, the thermal performance on edge stitching and the joint line of conventional MLIs was severely degraded.

4.3. Non-interlayer-contact spacer MLI As shown in Fig. 11, the thermal performance of NICS MLIs is far superior to conventional MLIs. The thermal performance of No. 9 NICS MLI (2) is 39 times than that of No. 4 conventional MLI (1), despite both having 12 layers. Furthermore, the thermal performance of No. 13 NICS MLI (2), which has only 4 layers, is 14 times that of No. 4 conventional MLI (1).

The differences between Nos. 6-8 NICS MLI (1) and Nos. 9-13 NICS MLI (2) emerge in the spacer pitch and boil-off tank configuration. When NICS MLI (1) is compared to NICS MLI (2), the latter shows superior thermal performance with an equivalent number of layers. When the thermal performances of NICS MLIs were measured, the temperature distributions of the outermost films were also measured and the temperature difference between the center and corner of the NICS MLI (1) sample was less than 2 °C. Therefore, the difference in thermal performance between NICS MLI (1) and NICS MLI (2) is primarily attributable to the spacer pitch rather than the installation configuration. As will hereinafter be described in detail, when the spacer pitch increases, not only does the conductive heat leak through the spacer decrease but also radiative heat exchange between the spacer and film, meaning improved thermal insulation performance.

The temperature dependency of thermal insulation performance is noted. The gradient of heat flux over the temperature of conventional and reference MLIs is small and the relationship seems linear. Conversely, the gradient of heat flux over the temperature of NICS MLIs is large at high temperature and small at low temperature. Because NICS MLI only comprises a few layers, radiative heat leak increases at high temperature, but the thermal resistance of the spacer is very high, which, in turn, means minimal conductive heat leak through the spacer and total heat leak at low temperature, where conduction dominates.

4.4. Analysis of the thermal insulation performance and proportion of heat leak

As already mentioned in section II, the thermal performance of NICS MLI can be easily estimated because the conductive heat of NICS MLI only passes through the spacer and is very simple. To estimate the thermal performance of the NICS MLI precisely, heat transfer through the NICS MLI is modeled using a Thermal Desktop as shown in Figs. 15 and 16. The spacer is modeled in three parts, namely the first, second and third stages. The area and figure of the horizontal surface of the spacer model is the same as the real thing and radiative heat flux between the spacer and film is taken into consideration. The temperature dependency of film emissivity is considered using Eq. (4) 9,

3241069.6 ii T (4)

The three parts of the spacer are connected by

thermal conductors and the total thermal resistance of the spacer corresponds to 3.32 × 105

K/W, which is the design value. As boundary conditions, the temperatures of the shroud are specified from 77 K to 400 K and that of the boil-off tank at 77 K. The horizontal dimension of the model is the spacer pitch and the spacer is arranged in the center. To eliminate the edge effect, all sides are covered by surfaces that act as reflectors/blockers in the radiation calculations.

The calculated heat fluxes of NICS MLIs are shown in Fig. 14 with the measured heat flux. When the calculated and measured heat fluxes are compared, the calculations agree very well with the experiment, with an error rate of less than 17 %. Reducing the uncertainty of conductive heat leak due to interlayer contact and reducing conductive

Fig. 14 Comparison of experimental and calculated results of heat flux through NICS MLI.

(Cold boundary temperature is 77 K)


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heat leak due to the high thermal resistance of the spacer allows the thermal performance of NICS MLI to be precisely estimated.

Fig. 15 Schematic of the heat transfer model through NICS MLI

Subsequently, the proportion of heat leak for each sample is

analyzed, whereas for NICS MLIs, the radiative heat leak between films and the film and spacer and the conductive heat leak through the spacer are analyzed using the thermal mathematical model as referred to above. With regard to conventional and reference MLIs, the following thermal mathematical model is conceivable:

The total heat flux qtotal is expressed as the sum of radiative heat flux qrad, conductive heat flux qcond and convective heat flux qconv as follows:

convcondradtotal qqqq (5)

Here, it is assumed that radiation and conduction are not

coupled and the principle of superposition can be applied4. Under the condition of applicability to spacecraft, convective heat transfer via rarefied gas is negligible and radiative heat flux is expressed in the form of radiation exchange between large parallel planes as follows:

44

1)1()1(

1ji

jirad TTq

(6)

i is the emissivity of the hotter layer surface, j is the emissivity of the colder layer surface and both follow by equ

ation (4). The conductive heat flux is expressed as follows:

jicond TThq (7)

Here, h is the total conductive heat transfer coefficient between two layers, which means the sum of the spacer

thermal conductance and contact thermal conductance between the film and netting spacer. The total conductive heat transfer coefficients between each layer are assumed to be constant. The temperatures of the outermost and innermost layers are given as boundary conditions and the heat flux between each of the films is repeatedly calculated until the calculated heat flux matches the measured value by changing the total conductive heat transfer coefficient and the temperature of each layer.

The proportion of heat leak of No. 6 NICS MLI (1) and No. 12 NICS MLI (2), which comprise 6 layers when the outermost layer temperature is 300 K, are shown in Fig. 17. Moreover, the proportion of heat leak of No. 3 reference

Fig. 16 Schematic of the thermal mathematical model of NICS MLI


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MLI (3) and No. 4 conventional MLI (1), which comprise 12 layers, are shown in Fig. 18 when the outermost layer temperature is 300 K. The calculated temperature of each layer is shown in Fig. 19.

Shown as Figs. 17 and 18, the conductive ratio of heat leak through conventional MLI (1) exceeds 89 %, while the conductive ratio of heat leak through NICS MLIs is less than 8 %, less than a tenth of that of conventional MLIs. Alternatively the conductive ratio of heat leak through reference MLI (3) is 28 % at the hot boundary layer and 79 % at the cold boundary layer respectively. As seen above, the ratio of conduction to radiation is changing in each interlayer of the same sample. As the temperature declines, the radiation ratio does the same. Due to the varying proportion of heat leaks, the temperature dependency of thermal insulation performance shows different features for each of the samples. The temperature dependency of the thermal insulation performance of conventional MLIs, which is conduction dominated, seem linear, with a significant degradation of performance at low temperatures. Conversely, the conductive heat leak of NICS MLI is quite small, meaning thermal performance can be maintained, even at low temperatures. The calculated conductive heat transfer coefficient in each layer is 3.03×10-

6 W/K for NICS MLI, 3.45×10-2 W/K for reference MLI (3) and 7.10×10-1 W/K for conventional MLI.

Fig. 18 The calculated proportion of heat leak of No. 3 reference MLI (3) and No. 4 conventional MLI (1)

with a hot boundary temperature of 300 K. As shown in Fig. 17, the radiation via spacer in NICS MLI is considerable. This is because the emissivity of the

spacer surface is relatively higher than that of the film surface, so despite the small spacer area ratio, the contribution of the spacer radiation is relatively high for the heat exchange in each film. Accordingly, increasing the spacer pitch improves the thermal insulation performance of NICS MLI.

Finally, the effective emissivity is calculated. The effective emissivity between 77 K and 300 K is 0.0326 for No. 4 conventional MLI (1), 0.0027 for No. 3 reference MLI (3) and 0.0008 for No. 9 NICS MLI (2), respectively.

Fig. 17 The calculated proportion of heat leak of No. 6 NICS MLI (1) and No. 12 NICS MLI (2) at the hot

boundary temperature is 300 K.


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Fig. 19 The calculated layer temperature of No. 3 reference MLI (3), No. 4 conventional MLI (1), No. 6

NICS MLI (1) and No. 12 NICS MLI (2) at a hot boundary temperature of 300 K.

V. Conclusion and future work A new type of MLI using new spacers, the non-interlayer-contact spacer MLI, was developed and thermal

performance tests using a rectangular and cylindrical boil-off calorimeter were performed. The rectangular boil-off tank simulates a cubic spacecraft body, while the cylindrical tank has a curved surface resembling the sun shield of an astronomy satellite or a rocket propellant tank, and the degree of degradation due to the mounting method is evaluated using two types of boil-off tank. Accordingly, the thermal insulation performance of the NICS MLI is independent of the mounting procedure on the tanks. Conversely, reference MLI, which is mounted idealistically and conventional MLI, both of which use conventional netting spacers, are significantly influenced by the mounting procedure given the difficulty of controlling the layer density or interlayer-contact. Because the NICS MLI excludes any uncertain interlayer contact between films due to use of discrete spacers, heat passes of NICS MLI are very simple. Accordingly, the estimations of thermal insulation performance of NICS MLI using a simple thermal mathematical model effectively matches the experimental results with an error rate of less than 17 %.

According to the experimental results, the thermal insulation performance of NICS MLIs is far superior to conventional MLIs, particularly at low temperature. Moreover, when the spacer pitch increases, thermal insulation performance improves, because not only does the conductive heat leak through the spacer decline, the radiative heat exchange between the spacer and film is also reduced. Thermal analysis shows that the conductive ratio of heat leak through conventional MLIs exceeds 89 %, conversely, the conductive ratio of heat leak through NICS MLIs is less than 8 %; less than a tenth of that of conventional MLIs. For the above reasons, NICS MLI performs well, particularly at low temperatures, where conduction dominates. The effective emissivity of the NICS MLI, with a spacer pitch of about 100 mm and a film layer of 12, achieved 0.0008 at Thot = 300 K.

Because of the features of the previously predictable and quite high insulation performance at low temperature, the benefit of the NICS MLI in space missions like cryogenic astronomy satellites or long-term propulsion systems has been shown. This unique feature should be confirmed by low-temperature testing in the future. The cryogenic calorimeter to measure insulation performance using Gifford-McMahon coolers have been designed, and we have the plan to manufacture the new calorimeter during the current year and measure thermal performance of the NICS MLI at lower than 20 K.

Variety of issues for the NICS MLI should be examined like perforation hole specification, fastener to a spacecraft and durability for launch load and so on. The thickness of the NICS MLI is thicker than conventional MLI so the NICS MLI must evacuate more air inside the layers. Vent hole configuration identification and depressurization testing have been initiated and will be reported on in a future report. For rocket propellant tank, insulation is subjected to force at low temperature during launch. The stand-alone tensile and compaction tests at ambient temperature have already completed. We should also look at tensile and compaction property of the spacer at low temperature and evaluate the NICS MLI blanket under vibration and acoustic loading. For long-term


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propulsion systems, radiation tolerance will be an important aspect of the spacer. The above considerations for the proposed MLI will be addressed prior to its application to specific missions.

References 1 Leslie Buchanan and Steve Buerger, “MLI Layup Optimization Test,” Proceedings of Space Thermal Control Workshop,

2010. 2 S. Dye, A. Kopelove and G. L. Mills, “Integrated and Load Responsive Multilayer Insulation,” Cryogenic Engineering

Conference, Vol. 55, 2010, pp. 946-953. 3 David. G. Gilmore, ‘Spacecraft Thermal Control Handbook Volume 1: Fundamental Technologies,’ The Aerospace

Corporation, 2002, pp. 161-168. 4 Moshfegh, B. et al., “Thermal Analysis of Multilayer Insulation at Room Temperature,” Advanced Computational Methods

in Heat Transfer, Vol. 3, 1990, pp. 357-368. 5 Jochen, Doeneche, “Survey and Evaluation of Multilayer Insulation Heat Transfer Measurement,” SAE technical paper series, No. 932117, 1993.

6 C. W. Keller, G. R. Cunnington and A. P. Glassford, ‘Thermal Performance of Multilayer Insulations,’ NASA CR-134477, 1974. 7 L. D. Stimpson and W. Jaworskis, “Effect of Overlaps, Stiches, and Patches on MLI,” AIAA paper, No. 72-285, 1972.

8 C. L. Tien, G. R. Cunnington, “Radiation Heat Transfer in Multilayer Insulation having perforated Shields,” AIAA Paper, pp. 73-718, 1973.

9 N. Inai, Japan Society of Mechanical Engineers, Vol. 43, No. 365(1977-1), 217, 1977. 10 S. Okazaki, M. Murakami, H. Kawasaki, T. Yabe, H. Sugita, and Y. Kanamori, “Experimental Study of the influence of

processing on MLI performance for Space Use,” AIAA-2008-01-2067, 2012. 11 H. Kawasaki, S. Okazaki, H. Sugita and M. Murakami, “Temperature dependence of Thermal Performance in Space using

Multilayer Insulation,” AIAA-2012-3407, 2012. 12 Ryuta Hatakenaka, Takeshi Miyakita, Hiroyuki Sugita, Masanori Saitoh, Tomoyuki Hirai, “Thermal Performance and Practical Utility of a MLI Blanket using Plastic Pins for Space Use”, American Institute of Aeronautics and Astronautics, 2013, pp. 1-11 13 Y. Sato, H. Sugita, et al., “Conceptual design of cryogenic system for the next-generation infrared space telescope SPICA”, Proc. of SPIE Vol. 7731, 2012. 14 Takeshi. Miyakita, et al., “Development of a New Multi-Layer Insulation Blanket with Non-Interlayer-Contact Spacer for Space Cryogenic Mission”, Cryogenics 64 (2014) pp. 112-120. 15 Ryuta Hatakenaka, Takeshi Miyakita, et al., “Development and testing of a zero stitch MLI blanket using plastic pins for space use”, Cryogenics 64 (2014) pp. 121-134.

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