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Multilayer Insulation for Spacecraft Applications
MULTILAYER INSULATION FOR SPACECRAFT APPLICATIONS Che-Shing Kang
Spacecraft Project, National Space Program Office, Hsinchu 300, Taiwan, ROC
ABSTRACT Multilayer Insulation (MLI) blankets provide a lightweight insulation system with a high thermal resistance in vacuum. MLI blankets are utilized to reduce heat loss from a spacecraft to the cold space, or to prevent excessive heating of the surroundings from an internal component with heat dissipation. MLI blankets consist of a number of highly reflecting radiation shields interspaced with a low thermal conductivity spacer material or separated by crinkling the radiation shields themselves. The radiation shields are generally a plastic film metalized on either one side or both sides of the film. The principle of an MLI blanket is to use multiple layers of radiation shields to reflect back, in the opposite direction of heat flow, a large percentage of the radiant heat flux reaching each radiation shield. MLI is therefore very effective if solid conduction through the spacers and gaseous conduction through the gas medium can be minimized. MLI HEAT TRANSFER Heat transfer modes through an MLI system include thermal radiation between the radiation shields, heat conduction and radiation through the gaseous medium in the spacing, and heat conduction through the spacer or the contact points between the crinkled MLI film surfaces. Radiation through the spacing gas medium must be considered if the medium is a participating gas, such as water vapor or carbon dioxide. Otherwise radiation of the gas medium is negligible. For launch vehicle or satellite MLI applications, air is the medium which is almost transparent to thermal radiation. However, gaseous heat conduction can be neglected only if the air pressure is less than 10'^ mm Hg (Torr), such as in space or upper atmosphere for most satellite applications. Radiation Heat Transfer Between the Radiation Shields Radiation heat transfer between two adjacent parallel radiation shields is governed by:
e^=oo^(r/-r/)
(1)
where Tj and T2 are the absolute temperatures of the adjacent radiation shields, A is the open surface area of the radiation shields, a is the Stefan-Boltzmann constant and 3 is the radiative interchange factor (Script-F), In general, the radiative interchange factor between two opposing surfaces is defined in terms
175
of the geometric form factor {F]2)fortwo black surfaces, the surface areas {Aj and A2), and the surface emissivities (a 1 and a 2): ^ ^ •' i\/F,,) + {\/s,-\)
1 + {AJA,)(\/s,-\)
(2)
S^Hd^Cond^ction Jhrough Spacers Solid heat conduction through the spacer may be expressed as: Q.onj=ksA^^
(3)
where k^ is the thermal conductivity of the solid spacer material, A^ is the solid area through the spacer, T] and T2 are the temperatures of the two opposing radiation shields, d is the thickness of the spacer (also spacing between two shields). If contact resistance between the shields and the spacer is also considered, ks represents the combined thermal conductivity. Combined Normal Heat Transfer If heat conduction through a thin metalized film is negligible, the total normal heat transfer between n similar radiation shields closely placed, in a steady state, is expressed by : g,.;=cx4(l-^)^
^^^.
{T:-T:)^k^-^-^(T,-T„)
(4)
where (/f^A/A is the solid fraction of the spacer. In practice, if we consider an MLI system with T^^Tj and Tc=Tyi to be the hot and cold boundary temperatures. Equation 4 can be expressed by : Q,^,^,=GAS{T:-T:)
(5)
where e {E-Star) is the effective emissivity which is a measure of the combined effect of radiative heat transfer through the radiation shields and the solid conduction through spacers. Therefore, e may be calculated from measured data of Qtotal ^h ^ ^ ^c under vacuum conditions for a particularly designed MLI system. MLI BLANKET CONSTRUCTION
Radiation shields are low emissivity metal foils or metal coated plastic films used to attenuate the incoming thermal radiation. Aluminum, gold and silver are the most commonly used metals either for coatings or to form thin metallic foils. Plasticfilmscan be metalized (through a vacuum deposition process) on either one side or both sides. For single-sided metalizedfilms,the metal can be vacuum deposited on either the top side or the back side of 176
the film. The top-sided metalized films (usually called First-Surface Mirrors) and the double-sided metalized films are good solar absorbers (high solar absorption over IR emissivity aje ratio) and infi-ared reflectors. They are commonly used as inner layers (infi*ared heat shield) in MLI blankets and can also be used as the outermost MLI layer (with thicker plastic structure) if the covered area is shaded fi-om the Sun for most portion of the orbit cycle. The back-sided metalized films (usually called SecondSurface Mirrors) are good solar reflectors (low aJe ratio) and are commonly used as radiators to dump the spacecraft internally generated heat to the deep space. Among the available plastic films, Mylar, Teflon and Kapton (trade names of DuPont Company)are commonly selected by space industries as the shield materials for MLI. Mylar, from the family of polyester, is inexpensive, and has been mass produced and widely used in the past. However, it disintegrates under prolonged UV exposure. Therefore Mylar is usually used as inner layers of an MLI system, or test MLI for a spacecraft in a thermal vacuum test. Kapton, fi-om the family of polyimide, is a gold-colored plastic. Its color becomes more darker by increasing the thickness, thereby increasing its emissivity. Kapton is slightly heavier than Mylar, but more expansive (in the past). Kapton is more rugged and high-temperature resistant (non-flammable) than both Mylar and Teflon. Therefore it can be installed as the outermost layer of an MLI blanket to provide protection during installation and handling, and also offer desirable aJe ratios when used as a secondsurface mirror (SSM). The a^ /e value does not change with thickness because both a anda^ increase with increasing thickness. For an aluminized Kapton, theaj 8 value remains at about 0.5 for thickness from 0.5 mil to 5.0 mil. Embossment of Kapton is fairly troublesome that a general rule is to use long cycles to produce the molded shapes from this polymer. Teflon,fi"omthe family of fluoroplastics, is expensive than both Mylar and Kapton, but most stable under UV and charged particle exposure. Teflon, vacuum deposited with silver, offers the best for SSM (with lowest a5 / £ ratio). Its a^ /e ratio also decreases with increasing Teflon thickness because a decreases with increasing thickness while a ^ remains unchanged. Therefore it is usually applied as the outermost surface of an MLI blanket. Its drawback is that it does not adhere easily to spacecraft surfaces. Metal Deposits on Plastic Films Available metals for vacuum deposition are silver, aluminum and gold to offer low solar absorptance, and Inconel and chromium to offer high solar absorptance. The infrared emissivities of silver, aluminum and gold are also lower than those of Inconel and chromium. Aluminum is the most popular metal for vacuum deposition, followed by gold and silver. Aluminum is inexpensive and readily available as a coating on a variety of plastic surfaces. It vaporizes at a lower temperature than gold, making the aluminum deposition process easier to control. As a result, aluminum coated film are of better average quality than gold or silver-coated films. The emissivity of aluminum ( a = 0.03) is only slighfly higher than that of clear silver ( a = 0.02), but, whereas silver tarnishes in air, aluminum forms a very thin layer of oxide which prevents further degradation of the surface. Vapor deposited gold appears very attractive for reusable spacecraft radiation shields because of the lower emissivity ( £ = 0.02) and no degradation effects after exposure to moisture or contaminated air with salt and pollutants which would corrode aluminum metal away.
177 —
Silver has the lowest emissivity value ( a = 0.02) as compared to that of gold. Silverized Teflon, usually used as Second-Surface Mirrors, offers the lowest aje value of less than 0.1 for a layer of 5.0 mil type A Teflon. To protect silver from contamination, it is normally over-coated with silicone oxide or vacuum deposited by an additional layer of Inconel. Thermal Control Tapes and Laminates The various types of thermal control surfaces can be converted into a variety of products, e.g., combining different metal deposits on opposite sides of a substrate may provide one type of surface on one side and another type on the other side. Thermal Control Tapes have pressure-sensitive adhesive coated on the opposite side for easy and positive attachment. Typical thermal control tapes are aluminum or silver Teflons with acrylic of silicone pressure sensitive adhesives. Standard tape widths from manufactures are 1.0 to 4.0 inches and the standard roll length is 108 ft (Shedahl 1989). Thermal Control Laminates are formed by laminating a reinforcement scrim or film to the back side for added toughness as for the covers of an MLI blanket. A typical example is a Dacron scrim sandwiched between two Mylar films (by polyester adhesive) and vacuum deposited by aluminum on the outside of the Mylar films (Shedahl 1989). A Complete MLI System MLI blankets typically consist of three to thirty layers of metalized plastic sheets. The inner layers are typically as thin as practical (0.25 mil) to minimize weight and are usually made of aluminized Mylar or Kapton, separated by spacers. The innermost and outermost layers of the blanket, however, are usually made of thicker 1- or 2- mil aluminized Kapton which provides protection during installation and handling. Thermal control laminates can also be used as the outermost layer for additional toughness. A standard MLI layup of Lockheed-Martin (former GE Astro Div.) is shown in Figure 1 where 8 layers of 0.25 mil, double aluminized Mylar are separated by Dacron polyester nets. A 2-mil aluminized second surface Kapton, coated with Indium Tin Oxide (TTO), is placed on top to face the space UV radiation and atomic oxygen environment. A 1 -mil double aluminized Kapton is located at the bottom to shield the spacecraft.
'I
TSPACE
002 KAPTON ALUn 2N0 SURF flTOCOAT£0)
006 POLYESTER NET (9 LAYERS)
^
^
00025 MYLAR. ALUn BOTH SIDES (8 LAYERS)
\yy^jxj^u^u
001 KAPTON, ALUM BOTH SFDES
5/C
Figure 1. A standard MLI layup.
MLI DESIGN EXAMPLE ROCS AT-L the first Republic of China satellite, has been designated as a scientific satellite to be injected into a 600 km Low Earth Orbit (LEO) to conduct three scientific experiments. Eclipse time of the 178 —
ROCS AT-1 orbit can be as high as 30%, therefore, to protect the electronic components from thermal radiation heat loss to the deep space, MLI is applied to cover most of the exposed areas, except those places designated for radiator ( Kang 1996). The MLI blankets used on ROCS AT-1 include two types of external blankets, one backside and internal blanket, and one spiral wrap tape. Each layer of the ROCS AT-1 MLI blanket is composed of aluminizedKapton, either plain for the inner and outer layers or crinkled for the filled layers. The external ones have either regular outer layer or carbon filled electric conducting outer layer. The backside MLI has a thinner outer layer and less crinkle filler layers. The regular external MLI blankets cover most exposed surfaces while the electric conducting ones cover the front area of the payload enclosure which is facing the ram direction. The thinner MLI blankets cover the backside of some external components and separate the high dissipation internal components from neighboring components. The spiral wrap tapes cover cables, waveguides and struts. Since the ROCS AT-1 600 km orbit typically passes through the Earth's ionosphere, all layers of the MLI blankets shall be grounded between the outer layer's metalized surface to the spacecraft structure with a D.C. resistance less than 1000 ohms. REFERENCES Sheldahl Corp, "Thermal Control Material & Metalized Films," July 1980. Kang, C. S., "An Overview of ROCSAT-1 Thermal Control Design," Proc. of AASROC Annual Meeting, Taipei, Taiwan, Dec 1996.
179
Spacecraft Project, National Space Program Office, Hsinchu 300, Taiwan, ROC
ABSTRACT Multilayer Insulation (MLI) blankets provide a lightweight insulation system with a high thermal resistance in vacuum. MLI blankets are utilized to reduce heat loss from a spacecraft to the cold space, or to prevent excessive heating of the surroundings from an internal component with heat dissipation. MLI blankets consist of a number of highly reflecting radiation shields interspaced with a low thermal conductivity spacer material or separated by crinkling the radiation shields themselves. The radiation shields are generally a plastic film metalized on either one side or both sides of the film. The principle of an MLI blanket is to use multiple layers of radiation shields to reflect back, in the opposite direction of heat flow, a large percentage of the radiant heat flux reaching each radiation shield. MLI is therefore very effective if solid conduction through the spacers and gaseous conduction through the gas medium can be minimized. MLI HEAT TRANSFER Heat transfer modes through an MLI system include thermal radiation between the radiation shields, heat conduction and radiation through the gaseous medium in the spacing, and heat conduction through the spacer or the contact points between the crinkled MLI film surfaces. Radiation through the spacing gas medium must be considered if the medium is a participating gas, such as water vapor or carbon dioxide. Otherwise radiation of the gas medium is negligible. For launch vehicle or satellite MLI applications, air is the medium which is almost transparent to thermal radiation. However, gaseous heat conduction can be neglected only if the air pressure is less than 10'^ mm Hg (Torr), such as in space or upper atmosphere for most satellite applications. Radiation Heat Transfer Between the Radiation Shields Radiation heat transfer between two adjacent parallel radiation shields is governed by:
e^=oo^(r/-r/)
(1)
where Tj and T2 are the absolute temperatures of the adjacent radiation shields, A is the open surface area of the radiation shields, a is the Stefan-Boltzmann constant and 3 is the radiative interchange factor (Script-F), In general, the radiative interchange factor between two opposing surfaces is defined in terms
175
of the geometric form factor {F]2)fortwo black surfaces, the surface areas {Aj and A2), and the surface emissivities (a 1 and a 2): ^ ^ •' i\/F,,) + {\/s,-\)
1 + {AJA,)(\/s,-\)
(2)
S^Hd^Cond^ction Jhrough Spacers Solid heat conduction through the spacer may be expressed as: Q.onj=ksA^^
(3)
where k^ is the thermal conductivity of the solid spacer material, A^ is the solid area through the spacer, T] and T2 are the temperatures of the two opposing radiation shields, d is the thickness of the spacer (also spacing between two shields). If contact resistance between the shields and the spacer is also considered, ks represents the combined thermal conductivity. Combined Normal Heat Transfer If heat conduction through a thin metalized film is negligible, the total normal heat transfer between n similar radiation shields closely placed, in a steady state, is expressed by : g,.;=cx4(l-^)^
^^^.
{T:-T:)^k^-^-^(T,-T„)
(4)
where (/f^A/A is the solid fraction of the spacer. In practice, if we consider an MLI system with T^^Tj and Tc=Tyi to be the hot and cold boundary temperatures. Equation 4 can be expressed by : Q,^,^,=GAS{T:-T:)
(5)
where e {E-Star) is the effective emissivity which is a measure of the combined effect of radiative heat transfer through the radiation shields and the solid conduction through spacers. Therefore, e may be calculated from measured data of Qtotal ^h ^ ^ ^c under vacuum conditions for a particularly designed MLI system. MLI BLANKET CONSTRUCTION
Radiation shields are low emissivity metal foils or metal coated plastic films used to attenuate the incoming thermal radiation. Aluminum, gold and silver are the most commonly used metals either for coatings or to form thin metallic foils. Plasticfilmscan be metalized (through a vacuum deposition process) on either one side or both sides. For single-sided metalizedfilms,the metal can be vacuum deposited on either the top side or the back side of 176
the film. The top-sided metalized films (usually called First-Surface Mirrors) and the double-sided metalized films are good solar absorbers (high solar absorption over IR emissivity aje ratio) and infi-ared reflectors. They are commonly used as inner layers (infi*ared heat shield) in MLI blankets and can also be used as the outermost MLI layer (with thicker plastic structure) if the covered area is shaded fi-om the Sun for most portion of the orbit cycle. The back-sided metalized films (usually called SecondSurface Mirrors) are good solar reflectors (low aJe ratio) and are commonly used as radiators to dump the spacecraft internally generated heat to the deep space. Among the available plastic films, Mylar, Teflon and Kapton (trade names of DuPont Company)are commonly selected by space industries as the shield materials for MLI. Mylar, from the family of polyester, is inexpensive, and has been mass produced and widely used in the past. However, it disintegrates under prolonged UV exposure. Therefore Mylar is usually used as inner layers of an MLI system, or test MLI for a spacecraft in a thermal vacuum test. Kapton, fi-om the family of polyimide, is a gold-colored plastic. Its color becomes more darker by increasing the thickness, thereby increasing its emissivity. Kapton is slightly heavier than Mylar, but more expansive (in the past). Kapton is more rugged and high-temperature resistant (non-flammable) than both Mylar and Teflon. Therefore it can be installed as the outermost layer of an MLI blanket to provide protection during installation and handling, and also offer desirable aJe ratios when used as a secondsurface mirror (SSM). The a^ /e value does not change with thickness because both a anda^ increase with increasing thickness. For an aluminized Kapton, theaj 8 value remains at about 0.5 for thickness from 0.5 mil to 5.0 mil. Embossment of Kapton is fairly troublesome that a general rule is to use long cycles to produce the molded shapes from this polymer. Teflon,fi"omthe family of fluoroplastics, is expensive than both Mylar and Kapton, but most stable under UV and charged particle exposure. Teflon, vacuum deposited with silver, offers the best for SSM (with lowest a5 / £ ratio). Its a^ /e ratio also decreases with increasing Teflon thickness because a decreases with increasing thickness while a ^ remains unchanged. Therefore it is usually applied as the outermost surface of an MLI blanket. Its drawback is that it does not adhere easily to spacecraft surfaces. Metal Deposits on Plastic Films Available metals for vacuum deposition are silver, aluminum and gold to offer low solar absorptance, and Inconel and chromium to offer high solar absorptance. The infrared emissivities of silver, aluminum and gold are also lower than those of Inconel and chromium. Aluminum is the most popular metal for vacuum deposition, followed by gold and silver. Aluminum is inexpensive and readily available as a coating on a variety of plastic surfaces. It vaporizes at a lower temperature than gold, making the aluminum deposition process easier to control. As a result, aluminum coated film are of better average quality than gold or silver-coated films. The emissivity of aluminum ( a = 0.03) is only slighfly higher than that of clear silver ( a = 0.02), but, whereas silver tarnishes in air, aluminum forms a very thin layer of oxide which prevents further degradation of the surface. Vapor deposited gold appears very attractive for reusable spacecraft radiation shields because of the lower emissivity ( £ = 0.02) and no degradation effects after exposure to moisture or contaminated air with salt and pollutants which would corrode aluminum metal away.
177 —
Silver has the lowest emissivity value ( a = 0.02) as compared to that of gold. Silverized Teflon, usually used as Second-Surface Mirrors, offers the lowest aje value of less than 0.1 for a layer of 5.0 mil type A Teflon. To protect silver from contamination, it is normally over-coated with silicone oxide or vacuum deposited by an additional layer of Inconel. Thermal Control Tapes and Laminates The various types of thermal control surfaces can be converted into a variety of products, e.g., combining different metal deposits on opposite sides of a substrate may provide one type of surface on one side and another type on the other side. Thermal Control Tapes have pressure-sensitive adhesive coated on the opposite side for easy and positive attachment. Typical thermal control tapes are aluminum or silver Teflons with acrylic of silicone pressure sensitive adhesives. Standard tape widths from manufactures are 1.0 to 4.0 inches and the standard roll length is 108 ft (Shedahl 1989). Thermal Control Laminates are formed by laminating a reinforcement scrim or film to the back side for added toughness as for the covers of an MLI blanket. A typical example is a Dacron scrim sandwiched between two Mylar films (by polyester adhesive) and vacuum deposited by aluminum on the outside of the Mylar films (Shedahl 1989). A Complete MLI System MLI blankets typically consist of three to thirty layers of metalized plastic sheets. The inner layers are typically as thin as practical (0.25 mil) to minimize weight and are usually made of aluminized Mylar or Kapton, separated by spacers. The innermost and outermost layers of the blanket, however, are usually made of thicker 1- or 2- mil aluminized Kapton which provides protection during installation and handling. Thermal control laminates can also be used as the outermost layer for additional toughness. A standard MLI layup of Lockheed-Martin (former GE Astro Div.) is shown in Figure 1 where 8 layers of 0.25 mil, double aluminized Mylar are separated by Dacron polyester nets. A 2-mil aluminized second surface Kapton, coated with Indium Tin Oxide (TTO), is placed on top to face the space UV radiation and atomic oxygen environment. A 1 -mil double aluminized Kapton is located at the bottom to shield the spacecraft.
'I
TSPACE
002 KAPTON ALUn 2N0 SURF flTOCOAT£0)
006 POLYESTER NET (9 LAYERS)
^
^
00025 MYLAR. ALUn BOTH SIDES (8 LAYERS)
\yy^jxj^u^u
001 KAPTON, ALUM BOTH SFDES
5/C
Figure 1. A standard MLI layup.
MLI DESIGN EXAMPLE ROCS AT-L the first Republic of China satellite, has been designated as a scientific satellite to be injected into a 600 km Low Earth Orbit (LEO) to conduct three scientific experiments. Eclipse time of the 178 —
ROCS AT-1 orbit can be as high as 30%, therefore, to protect the electronic components from thermal radiation heat loss to the deep space, MLI is applied to cover most of the exposed areas, except those places designated for radiator ( Kang 1996). The MLI blankets used on ROCS AT-1 include two types of external blankets, one backside and internal blanket, and one spiral wrap tape. Each layer of the ROCS AT-1 MLI blanket is composed of aluminizedKapton, either plain for the inner and outer layers or crinkled for the filled layers. The external ones have either regular outer layer or carbon filled electric conducting outer layer. The backside MLI has a thinner outer layer and less crinkle filler layers. The regular external MLI blankets cover most exposed surfaces while the electric conducting ones cover the front area of the payload enclosure which is facing the ram direction. The thinner MLI blankets cover the backside of some external components and separate the high dissipation internal components from neighboring components. The spiral wrap tapes cover cables, waveguides and struts. Since the ROCS AT-1 600 km orbit typically passes through the Earth's ionosphere, all layers of the MLI blankets shall be grounded between the outer layer's metalized surface to the spacecraft structure with a D.C. resistance less than 1000 ohms. REFERENCES Sheldahl Corp, "Thermal Control Material & Metalized Films," July 1980. Kang, C. S., "An Overview of ROCSAT-1 Thermal Control Design," Proc. of AASROC Annual Meeting, Taipei, Taiwan, Dec 1996.
179