US3817320A

US3817320A - Passive cooler

Info

Publication number: US3817320A
Application number: US00120076A
Authority: US
Inventors: R Williams
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1971-03-02
Filing date: 1971-03-02
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18

Abstract

A passive cooler arranged for three stages of cooling including an outer L-shaped member serving as two of the cooling stages and an inner member serving as the third stage of cooling at a preselected equilibrium temperature. Certain surfaces of each leg of the outer member and an exposed surface of the inner member are each given a surface finish having a spectral response in accordance with certain radiation wavelengths such that the inner member is maintained at a preselected temperature which is lower than otherwise possible when the cooler is exposed to thermal radiation, the cooler being located in an environment having a temperature lower than the preselected temperature.

Description

United States Patent [191 Williams [11] 3,817,320 June 18, 1974 PASSIVE COOLER 75 Inventor: Richard Jean Williams, Marlton,

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: 7 Mar. 2, 1971 [21] App]. No.: 120,076

[52] US. Cl 165/47, 165/80, 165/133 [5l] Int. Cl. F24h 3/00 [58] Field of Search 165/47, 80, 133, 44

g [56] References Cited UNITEDSTATES PATENTS 3,550,678 l2/l970 Pfouts....= 165/44 Primary Examiner-Charles Sukato vAttorney, Agent, or Firm Edward J. Norton; William;

Squire [57] ABSIRACT A passive cooler arranged for three stages of cooling including an outer L-shaped member serving as two of the cooling stages and an inner member serving as the third stage of cooling at a preselected equilibrium temperature. Certain surfaces of each leg of the outer member and an exposed surface of the inner member are each given a surface finish having a spectral response in accordance with certain radiation wavelengths such that the inner member is maintained at a preselected temperature which is lower than otherwise possible when the cooler is exposed to thermal radiation, the cooler being located in an environment having a temperature lower than the preselected temperature.

10 Claims, 3 Drawing Figures SPACECRAFT 20 K SPIN AXIS so COOLER Q PAI'ENTEDJun 18 m4 sum 1 0F 2 \WSUN 50 ISPACECRAFT 20 IM'ORBT PLANE 40 SUN ANGLE B INVENTOR. Richard J Williams BY ATTORNEY P'ATENTEDM 18 m4 SHEEI. 2 BF 2 I INVENTOR. v

Richard J Williams ATTORNEY PASSIVE COOLER BACKGROUND OF THE INVENTION This invention relates to passive coolers which are capable of cooling a supported article to a pre-selected temperature by radiation to the surrounding environment in the presence of wide band thermal radiation, which, unless otherwise provided for, would heat the article above the pre-selected temperature.

Coolers have widespread application on spacecrafts for cooling sensitive radiation detectors which are operative at cryogenic temperatures, the detectors being used to scan the earth from an orbit position about the earth. One application for such detectors is in the weather satellites now in use. One such device which may be used with the cooler of the present invention is described in my copending application entitled Anti- Condensation Device for Infra-red Detector, Ser. No. 2

101,328 filed Dec. 24, I970, and assigned to the assignee of the present invention.

Several methods are available for cooling these detectors, the methods including electro-mechanical refrigerant systems, combination of thermal electric and radiation systems, solid cryogenic space coolers which utilize solid cryogenic material which sublimes at a controlled rate, generating cooled gas for cooling the detector, and passive, radiation coolers. The passive cooler is preferred since it has no defined life limitation, requires no external power, and has no moving parts. These operating features lend themselves to make passive coolers smaller, lighter and relatively inexpensive in comparison with the other cooling methods.

Nevertheless, a problem with passive coolers is overheating due to inability to dissipate thermal energy received from thermal sources such as the earth, sun and spacecraft. More specifically, a passive cooler attached to a spacecraft orbiting the earth is subject to thermal radiation of the sun, the earth, and the spacecraft, jointly contributing to heating a radiation detector above its operating temperature. In order to maintain thermal equilibrium in a body at a desired temperature, it is essential that the incident energy be balanced with the radiated energy from the body for that temperature. Prior art coolers operate at equilibrium. temperatures that are unsatisfactory for most present-day passive cooler applications. Coolers should provide an equilibrium temperature at; a preselected temperature in the cryogenic region of radiation detectors in the order of 70K to 100K.

Prior art passive coolers have been constructed which provide staged shielding from thermal sources such as the earth, sun and the spacecraft and black body (entire thermal energy spectrum), radiation surfaces facing that portion of space that serves asa thermal sink. In practice, such coolers cannot maintain. a preselected low temperature due to restricted thermal coupling to dark space. These coolers generally have been formed as hollow pyramids or cones of revolution in which the detector is located in the interior of the apex and the black body radiating base is exposed to dark space. Such cooling devices are not suitable to maintain temperatures in the ranges needed for present day needs.

SUMMARY OF THE INVENTION second wavelength range which tends to raise the temperature of the body above the equilibrium temperature. To maintain the equilibrium temperature, the body is provided with a surface having high reflectivity and low absorptivity of the second wavelength range and low reflectivity and high absorptivity at the first wavelength range. By this arrangement the body is coupled to the environment with respect to radiant energy in the first wavelength range and decoupled to the environment with respect to radiant energy in the second 0 wavelength range, providing significantly improved thermal coupling to the external environment to thereby achieve lower thermal equilibrium than has heretofore been achieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatical illustration of the orientation of the cooler of the present invention in space;

FIG. 2 is a fragmented perspective view of a cooler in accordance with the present invention; and

FIG. 3 is a sectional view of the cooler of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT The cooler of the present invention is adapted to be located in an environment which is estimated to have a temperature of 4K and an absorptivity factor of I. Space can be considered to be a black thermal sink of infinite capacity and therefore is an environment of use of the cooler of the invention. Further, space is assumed in the art to be a vacuum. Thus bodies spatially separated with respect to each other in space are thermally conductively and convectively insulated from one another. Accordingly, the only other means of known heat transfer in a space environment is by radiation. A body in such an environment if exposed to radiated thermal energy from external sources such as the earth and sun will reach an equilibrium temperature higher than the temperature of space. The temperature of equilibrium that is established will depend upon the thermal absorption and radiation equilibrium of that body.

The spacecraft being exposed to radiation from the earth and the sun, has its temperature elevated above the 4K temperature of space, and therefore, is also a radiator of thermal energy relative to certain portions of itself including a cooler which may be attached to it. In practice the radiation from the other celestial bodies including the moon and stars is negligible. Without further provisions, a body such as a cooler attached to an orbiting spacecraft would reach a temperature considerably higher, for example, 200K or higher, than the cryogenic operating range of a radiation detector, for example -80K.

To provide cooling within the cryogenic temperature range of radiation detectors, the present invention provides for a cooler construction that utilizes radiation, radiation shielding, and staging in which each stage is maintained at a progressively lower temperature by means including conductive and radiation thermal isolation of the various stages from each other, noting that no thermal convection occurs in the vacuum of space. In accordance with the invention a selected portion of the cooler is maintained at a preselected cryogenic temperature. For example, a radiation detector in thermal contact with that cooled portion effects conductive cooling of the detector at that preselected temperature.

Instead of using a black body type of surface, the cooler of the present invention is provided with several surfaces each of which differ in their thermal energy response characteristics. In particular, the surfaces are arranged to respond essentially to solar and infrared wavelengths. Solar thermal energy radiation generally includes energy radiated within the entire thermal radiation spectrum. However, most of the thermal energy emitted by the sun falls within the visible region as compared to the portion of the radiation emitted in the infrared region. Thus the essential portion of solar radiation is in a different wavelength region than the infrared radiation region for purposes of passive cooling according to the present invention. To utilize these different wavelengths means are provided which selectively either couples or decouples certain surfaces in each of the stages to radiation in the environment within pre scribed wavelengths. In particular, these wavelengths may be characterized generally as infrared and as solar or visible radiation. Both the radiation of thermal energy to dark space as well as the radiation shielding effected in the cooler is a function of not only the orientation of the various radiating bodies such as the earth, sun and spacecraft with respect to the cooler but also of the wavelength of radiation from these bodies.

A preferred embodiment of the invention will now be described with reference to the drawing. In FIG. 1, cooler 10, to be described in further detail in conjunction with FIGS. 2 and 3, is secured to spacecraft 20 by suitable mounting devices (not shown). Spacecraft 20 orbits about the earth 30 in orbit plane 40. Spacecraft 20 spins about spin axis 60 once per orbit about earth such that cooler is alway in the same position with respect to earth 30. Sun rays 50 are disposed at angle B to spin axis 60, angle [3 remaining constant in a manner to be described.

In describing the preferred embodiment, with respect to FIGS. 1, 2 and 3, like numerals refer to like or identical parts. Member 11 is formed of generally

planar leg portions

13 and 14 and an interconnecting semicylindrical portion 15. Member 12 is located in the cavity formed by portion as shown in FIG. 3.

Leg portions

13 and 14 are located in spaced relationship preferably at right angles with respect to each other, member 12 being located generally in the space between the two

leg portions

13 and 14.

Legs

13 and 14 may be spaced at an angle greater than but not less than 90. Member 11 is further provided with two

web shield portions

21 and 22.

Web portions

21 and 22 are generally triangular and planar in shape, and, together with leg 14 and interconnecting portion 15, form a scooplike cavity.

The structure is arranged for the convenience of description into three portions, each portion serving as one of three stages of thermally independent structures. Each stage is thermally conductive, is isothermal, and is at a different temperature with respect to the other stages. According to the invention, the stages cooperate, in sequence, to reduce to a preselected tem perature the temperature of the article (radiation detector, for example) located in thermal contact with the third stage. This pre-selected temperature is established and maintained at that temperature inherently and without any control means usually considered necessary for temperature control.

Leg portions

13 and 14 and interconnecting portion 15 are preferably constructed such that the legs and interconnecting portion include an outer skin or housing 16. Housing 16 serves as a first stage radiation shield for the portions of the cooler nested therein to be described. Housing 16 is a thin sheet of metal which is structurally rigid and thermally conductive and is preferably a magnesium casting. Housing 16 forms the outer sides of

legs

13 and 14, interconnecting portion 15 and

webs

21 and 22. Edges 41-46, inclusive, of housing 16, are bent inward as shown to form a shallow cavity. The outer side of web 21 is a coplanar extension of

edges

41 and 43 while the outer side of web 22 is a coplanar extension of

edges

42 and 44. Nested within the shallow enclosure cavity formed by housing 16 and adjacent to housing 16 including

webs

21 and 22 is a suitable multilayer thermal insulation blanket 25 formed for example of vapor deposited gold on H- Film, trademark of DuPont Company for a form of polyimide film, separated by alternating layers of nylon net. Insulation blanket 25 serves to thermally insulate the inner portions of the cooler from housing 16.

Disposed adjacent blanket 25 in the cavity formed by housing 16 are

inner panels

17 and 18, frame 19, and

inner web portions

47 and 48 of

webs

22 and 21, respectively, which together serve as a second stage. Panel 17 is nested in leg 13 and panel 18 is nested in leg 14 while frame 19 is nested in the concave cavity of portion 15. Frame 19 serves to mechanically and

thermally interconnect panels

17 and 18. Frame 19 is preferably formed of a magnesium casting while

panels

17 and 18 are preferably formed of aluminum honeycomb sheets. Frame 19,

portions

47 and 48 together with

panels

17 and 18 are an integral rigid structure. The

inner portions

47 and 48 of

webs

21 and 22, re spectively and frame 19 are preferably a magnesium casting. The second stage structure (comprising frame 19,

panels

17 and 18, and inner web portions 47 and 48) is secured in the nested position spaced from the first stage (housing 16) by thermal isolation mounts (not shown). The spaced relationship between the first and second stage is defined by gap 39. Due to the vacuum of space, gap 39 permits the thennal conductive isolation of housing 16 from the first stage. Blanket 25 provides radiation isolation between the first and second stages.

A socalled second surface mirror 38 having a surface 27 of a glass panel 31 and a silver coating 32 is secured in thermal contact with panel 17. Any suitably bonding medium may be used to secure mirror 38 to panel 17. Silver coating 32 of mirror 38 is adjacent to and in thermal contact with panel 17. Coating 32 is vapor deposited on glass panel 31. Panel 17 serves to provide a rigid planar structural support for mirror 38 which is preferably 6-8 mils thick, the silver being 5 microns thick. By second surface mirror is meant in this art a mirror in which a transparent medium such as glass (31) is coated on one side with a reflective medium such as silver (32). The reflective side of the coat- Leg portion 13 thus includes mirror 38, panel 17 and that portion of' blanket 25 and housing 16 within the bracket while leg portion 14 includes panel 18 and that portion of blanket 25 and housing 16 within the bracket as illustrated in FIG. 3.

Member 12 serves as the third stage of cooler and includes a thin metal plate 29 and coating 28 having a surface 33. Member 12 is suspended from frame 19 in interconnecting portion in thermal isolation therefrom by means not shown. An example of the thermal isolation means is described in copending application entitled Suspension System, Ser. No. 79,496, filed Oct. 9, 1970 invented by David Melrose and Derek Binge and assigned to the assignee of the present invention.

The inner surfaces of cooler 10 consist of surface 26 of leg 14, surface 33 of member 12, surface 27 of leg 13 and the inner surfaces, respectively, of

web portions

47 and 48 each of which face each other. Each of these surfaces are adapted to have spectral properties which compliment one another, in a manner to be described, to provide improved overall thermal coupling to dark space (i.e. to the ambient external the spacecraft).

It should be understoodthat the spectral properties of a surface depends in part on the subsurface portions, thus surface 27 includes both glass panel 31 and silver coating 32 which together contribute to the spectral properties of surface 27.

A brief description of the relationships of the various properties relating to this invention will it is believed aid towards a better understanding of the nature of the invention. The spectral properties of materials can be shown to be related by the formulae:

where A, in general, is absorptivity throughout the entire frequency spectrum of radiation, E is emissivity, 'r is transmissivity, and p is reflectivity of radiant thermal energy. For radiation receiving and emitting bodies at the same temperature, emissivity (E) of a body equals absorptivity (A) of that body. Conversely, for radiation receiving and emitting bodies at different temperatures, their respective properties of absorptivity and emissivity are not equal. Equation (1), when used with reference to solar energy usually includes the parameter a of the absorptivity of a surface with respect to solar energy instead of the more general parameter A.

Emissivity (E) is the percent energy radiated by a body relative to that energy which would be emitted by a black body at the same temperature and is a direct function of temperature of the emitting body. Absorptivity (a) of a body is, on the other hand, related to the energy impinging upon the body, and' is a function of the temperature of the other body from which the energy is radiated. Thus, while a particular surface may have low emissivity (E) of its own energy, it may also 6 be provided with a relatively high absorptivity (a) of energy received from a higher temperature body, where the emitting body (sun) temperature is higher than the receiving body (cooler) temperature.

In accordance with the present invention, passive cooling at lower temperatures than heretofore obtained is effected by utilizing the principle that thermal radiation occurs over a wide band of wavelenths, and by providing certain surfaces of the cooler to be selectively responsive to thermal radiation wavelengths within predetermined ranges of the wide band.

Thermal radiation is restricted for this embodiment into two wavelength ranges, essentially within the infrared region and the solar region each of which having been defined above. Particular surfaces of the cooler are each provided with certain properties that have significantly different responses to thermal radiation in accordance with the wavelength of the thermal radiation arranged to impinge upon that surface. These certain properties when arranged according to this invention provide improved thermal radiation coupling to dark space. These properties are arranged to cooperate to provide essentially thermal coupling to dark space in the infrared region and thermal decoupling in the solar (visible) region.

The preselected temperature of member 12 is achieved in accordance with the present invention by providing the inner surface 26 of leg 14 with a layer of gold which is both highly specular, i.e. light reflected therefrom is columnar rather than diffuse, and highly reflective, i.e. energy that is not absorbed, with respect to bodies emitting energy substantially essentially within the infrared region. High reflectivity, as used herein shall be understood to mean reflectivity (p) in the order substantially of 0.95 to 1.00 while low emissivity (E) shall be understood to be in the order substantially of 0.05 to zero for the operating temperature of surface 26. These properties are achieved for example in a surface (26) of gold just described. At the expected low operating temperature of surface 26, the p and E thereof relate to radiation in the infrared region. Therefore, since surface 26 has a low E, very little thermal energy is radiated therefrom. With respect to thermal radiation in the solar region, the gold surface 26 has relatively high absorptivity a. As indicated above, due to the much higher temperature of solar radiation as compared to the temperature of surface 26, emissivity E of surface 26 is low while its absorptivity a is high with respect to E, such that a/E is in the order of 10. Thus, it should be appreciated that while surface 26 absorbs little radiation within the infrared region, surface 26 absorbs by a factor of 10 radiation within the solar region.

Inner surface 27 of leg 13 is arranged by the structural arrangement of the second surface mirror 38 to have a low solar absorptivity a in the order of 0.1 and high infrared emissivity E in the order of 0.85. As previously described, it is to be noted that second surface mirror 38 is thermally coupled to panel 17.

Glass panel 31 has high emissivity in the order of 0.85 and therefore, high absorptivity within the infrared region while silver 32 has high reflectivity within the solar region. Further, the silver coating is specular, that is, there is no scattering of light impinging thereon. Thus, earth reflected solar energy impinging upon surface 27 of mirror 38 is reflected into space predominantly by reflections from the silver coating 32 while earth emitted infrared energy is absorbed by the glass panel 31. The amount of absorbed infrared energy is a function of the thickness of glass panel 31 which is chosen according to design requirements.

Surface 33 of member 12 is the exposed surface of coating 28 and is arranged to be highly emissive in the infrared region suitably in the order of 0.9. Either black or white paint provides such high emissivity. According to the embodiment, coating 28 is conventional space stable, white paint, i.e., white velvet-400 series manufactured by the 3M Company, providing high reflectivity in the solar region in the order of 0.8. The inner surfaces of

web portions

47 and 48 which face each other are coated with vapor deposited gold, suitably polished to high specularity in a manner known in the art to provide high reflectivity (approximately 0.95) in the infrared region and relatively high a in the solar region, a/E being in the order of similar to surface 26 as previously described.

All interior surfaces of housing 16 and

panels

17 and 18, frame 19,

webs

21 and 22, and member 12 are also suitably vapor deposited with gold to minimize radiation coupling between the first and second stages and between the second and third stages.

The outer exposed surfaces of housing 16 are coated with conventional white paint (not shown) which serves as a solar reflector and radiator. Alternatively, the outer surfaces of leg 13,

webs

21 and 22 and section may be covered with a suitable insulation blanket (not shown) while the outer surface of leg 14 may be covered or placed in thermal contact with a second surface mirror (not shown) in a similar manner as the inner surface of leg 13. This latter (alternative) covering of the outer surfaces of housing 16 improves the efficiency of cooler 10 in a manner to be described.

In operation, cooler 10 is oriented as shown in FIG. 1. In this orientation, spacecraft orbits about earth 30 in plane 40 in a sun synchronous, earth oriented orbit. By sun synchronous is meant that the sun angle ,3, the angle of the sun's rays 50 to the spin axis 60 of spacecraft 20, is substantially constant, and is preferably a maximum of 75 to prevent direct radiation by the sun on surface 26. By earth oriented is meant the spacecraft rotates at one orbit per revolution about the earth with cooler 10 always positioned in the same orientation with respect to the earth.

In the position illustrated in FIG. 1, cooler 10 is secured at the bottom of the spacecraft adjacent the end 20a of the spacecraft opposite the sun rays 50 such that the cooler is in the spacecraft shadow 70 with the outer surface of leg 14 facing the earth. When the spacecraft is positioned 180 from the illustrated position such that the spacecraft is at the bottom of the Figure, it is apparent that all of the outer surfaces of member 11 are exposed to sun rays 50, leg 14 still facing the earth. Note also that the orbit plane 40 positions leg 14 to face both the light and the dark sides of the earth. Thus, the outer surfaces of member 11 are exposed to radiation sources of discretely different bands of wavelengths, which nevertheless, do not substantially affect the temperature of final stage member 12, which is always shielded from direct radiation from the sun, earth and spacecraft. in this orientation, the spacecraft appears to cooler 10 as a body having a temperature of about 293K while the earth appears as a body having a temperature of 233K.

The inner surface of leg 13 and the exposed surface of member 12 are positioned to be beyond the field of view of spacecraft 20. Thus, cooler 10 is positioned at the extreme end 20a of spacecraft 20 opposite the sun. The inner surface of leg 13 is either coplanar with end 20a or laterally displaced in the direction of spin axis 60 further from the sun than end 20a. Leg 14 extends generally in the horizontal direction with inner surface 26 facing spacecraft 20 while leg 13 extends generally in the vertical direction. The scale of the drawing is out of proportion for purposes of illustration, but in practice, the scale is such that the inner surface of leg 13 is exposed to dark space and earth radiation, while member 12 is always shielded from earths radiation by leg 14 and

webs

21 and 22.

In the orientation of the spacecraft as described above, the exterior surfaces of cooler 10 are exposed to solar radiation. With an exterior coating of white paint (not shown) on housing 16, there is some absorption of solar energy while most of the solar energy is reflected. Since the cooler is exposed to the sun only during a portion of the orbit, there is a change of temperature in housing 16. A portion of the energy absorbed by housing 16 when exposed to the sun is later radiated to space when the cooler is not exposed to the sun, resulting in expected temperature variations of housing 16 within the range of 220 to 320K.

By thermally isolating the second stage from the first stage, the second stage is maintained at a lower temperature expected to be within the range of l40l60K. Since the first stage is thermally isolated from the second stage, the significant thermal input to the second stage is radiation from the earth received by mirror 38 on leg 13. However, since mirror 38 is a reflector of solar energy and a radiator of infrared energy, the equilibrium temperature in the range of l40-l60K is reached which is significantly lower than the temperature of the first stage.

In the orientation shown, surface 26 is exposed to the spacecraft 20, but due to the highly specular finish of surface 26, the infrared energy radiated from the spacecaraft is substantially totally reflected by surface 26 into space and not to leg 13 or member 12 since there is substantially no scattering of radiation. Similarly, surface 26 is exposed to mirror 38 but, any radiation from mirror 38 is in the infrared region and is also reflected by surface 26 out to space. It is apparent that there is substantially no absorbed radiation on the second stage other than the thermal radiation from earth on mirror 38. Further, any thrrnal radiation absorbed by

panels

17 and 18, and frame 19, by heat leaks due to radiation coupling with the first stage or housing 16 is radiated to space by mirror 38. The undesired thermal coupling (generally, heat leakage) of member 12 (third stage) to the second stage is significantly reduced by minimizing radiation thermal coupling therebetween using insulation such as blanket 25 and gold coating. The coupling can be further reduced by minimizing the conduction thermal coupling therebetween by way of a thermal isolation support (not shown) using, for example, the support described in the copending application Ser. No. 79,496, noted above between member 12 and frame 19. By coating member 12 with emissive (white paint) coating 28, the temperature of this third stage (member 12) can be even further reduced by radiation to dark space. Coating 28 is advantageously made white to prevent bursts of solar radiation during the spacecraft launch from damaging the detector connected to the third stage. Leg 14 together with

webs

21 and 22 shield member 12 from earth radiation.

It should be noted that the net heat flow between the stages varies with the fourth power of absolute temperature, i.e., with T, where T is the absolute temperature of a stage. Thus, as the temperature increases, the heat flow between stages increases at a much greater rate. It can be shown that where the housing 16 reaches a temperature above 300K, the heat flow between the first stage and the second stage is significant enough to cause some change in the second stage temperature owing to the higher housing temperatures. However, due to the thermal attenuation between stages, only minor fluctuations occur in the third stage temperature. For example, with the first stage varying between 220 to 320K the second stage may vary between l40-160K and the third stage will vary only within a few degrees.

In some cases, it is desirable to provide even more efficient cooling and hold the third stage temperature to within a degree. In this latter instance, the white paint coating on housing 16 is substituted by an insulation blanket such as blanket 25 (previously described) over the outer surfaces of leg 13,

webs

21 and 22, and section and a second surface mirror on the outer surface of leg 14 in thermal contact with housing 16. Such an insulation blanket placed on the outer surface of leg 13 and

webs

21 and 22 will absorb thermal energy when the blanket is exposed to direct radiation by the sun, and, by conduction and radiation, this absorbed thermal radiation is transferred to housing 16. During a portion of the orbit when the sun is shielded from the outer surface of leg 13 by spacecraft 20, the thermal energy absorbed by housing 16 tends to be retained thereby due to the insulating effect of the outer blanket. At the same time, the mirror (not shown) on the outer surface of leg 14 radiates the thermal energy present on housing 16 out to space, reduces and stabilizes the temperature of housing 16. Thus, improved temperature stability is provided for housing 16. For example, housing 16 is maintained at a temperature of about 240 K using this latter configuration just described.

A cooler in accordance with the present invention was built and tested to determine its relative cooling capacity in comparison with the theoretical design considerations noted above. The exterior surfaces of housing 16 were completely encased in an insulation blanket similar to blanket 25. Any heat absorbed by the insulation blanket entirely encasing housing 16 when exposed to the sun is absorbed by housing 16 and is prevented from being radiated out to space by this same insulation blanket. Thus, to cool housing 16 the housing is hard-mounted to the spacecarft, that is, by thermally conductive mounting means such as bolts or the like. ln this instance, the cooler housing 16 is maintained substantially at the same temperature as the spacecraft. This temperature has been calculated to be approximately 300K.

A bell jar, serving as a test chamber for the cooler, was provided with an inner liquid nitrogen shroud and a liquid helium shroud in separate test configurations to simulate the thermal sink of space for each of two environmental simulations. The liquid nitrogen shroud provided a thermal sink temperature of approximately 80K, while the liquid helium shroud provided a thermal radiation sink temperature of approximately 35 K. These shrouds have an approximate thermal radiation absorptivity factor of 0.9 as compared to a thermal radiation absorptivity of 1.0 for dark space. These differences between the test environment and the actual space environment can be compensated for mathematically to show that the tested results are essentially equivalent to a cooler in a space environment.

The results of tests performed on the ground-based cooler conform very closely to the calculated predictions.

The thermal inputs to the test cooler at an altitude of 600 nautical miles, are such that housing 16 is expected to have a temperature of 300K while the second stage is expected to have a thermal input of 374 milliwatts due to the energy absorbed by the second surface mirror. The thermal input to the second surface mirror comes from the earths self emitted energy and earth reflected solar energy. The thermal inputs to the third stage are approximately 5 milliwatts due to detector heat dissipation with a radiation detector mounted in thermal contact with the third stage. These thermal inputs were applied directly to the tested cooler. Thus, to maintain the housing at 300K, heater wires were attached to the outer surface of housing 16 adjacent leg 13, and to provide the 374 milliwatt input, heater wires were attached to the second stage at leg 13 adjacent the inner surface 27 thereof. Five milliwatts of thermal energy were applied to the third stage by a small resistor attached thereto. The second stage heater was maintained at a constant temperature while the third stage heater was maintained at a constant heat dissipation level.

With a shroud temperature of 80K, the second stage was maintained at 163K and the detector dissipation thermal input was maintained at approximately 5 milliwatts. With these inputs the third stage measured 99K.

Extrapolating this measured temperature with respect to a space sink at 4K, the third stage has a temperature of 80K. A second test with a shroud temperature at 81K and a second stage temperature at 163K and no detector dissipation thermal input on the third stage, the third stage had a measured temperature of 94K. By extrapolation the third stage temperature was computed to be K relative to a space sink of 4K. With the liquid helium shroud temperature of 35K and a second stage temperature of 163K and no detector dissipation themial input on the third stage, the third stage had a measured temperature of K. This third stage temperature was computed to be 71.5K relative to a space sink of 4K. Using a 5 milliwatt thermal input on the third stage this computed temperature is raised to 8l.5l(.

According to these simulated tests, it has been shown that a cooler according to this invention, can be provided which has a third stage temperature as low as approximately 70K. It will be appreciated by those skilled in this art that these tests are good simulations of operating conditions of a cooler in a space orbit.

It should be noted that the entire cooler assembly size and test configuration is small, requiring an envelope volume of eight inches by seven inches by seven inches. The approximate cooler assembly weight is 1 pound. Further, the size of the radiator surface area 27 of leg 13 of the cooler constructed in accordance with the preferred embodiment of the present invention is 31.5

square inches. The inside area of leg 14 is 39.9 square inches, and the area of member 12 is 9.0 square inches.

In order to provide an even more precise temperature control of the third stage member 12, a heater may be provided in thermal contact therewith to raise the temperature of member 12 to some predetermined temperature above the normal operating temperature of member 12. In this manner, the temperature of member 12 can be maintained within a half of a degree. Thus, with the cooler member 12 having a normal operating temperature in the range of 7080K, a detector normally operating at a temperature of 95K could be employed therewith.

It will be understood that the size of a space cooler made according to this invention will depend on the altitude at which it will orbit as well as the orientation it assumes, particularly with respect to the sun and earth. The preferred embodiment described above illustrates the size and particular dimensions of a cooler at an altitude of 600 miles.

It will now be appreciated that in accordance with the present invention a cooler can be provided which provides very efficient cooling operation. According to the principles of the present invention, a combination of staged radiation shielding and surface finishes having preselected spectral properties to certain wavelengths ranges provide an extremely efficient cooler having a relatively large thermal coupling to space whereby cooler temperatures previously unattainable are achieved.

What is claimed is:

l. A passive device for maintaining an article at a preselected temperature in the presence of a thermal radiation sink, comprising:

an article cooling and supporting member, said member including a surface exposed to the ambient, said surface having high emissivity and low reflectivity to radiation within a first wavelength range when said member is at said preselected temperature, said article and said member being in conductive thermal relationship, and

shielding means adjacent to and in conductive and radiation thermal isolation from said member, said shielding means including a first surface having high absorptivity and low reflectivity of thermal radiation within said first wavelength range and low absorptivity and high reflectivity of thermal radiation within a second wavelength range, said shielding means substantially preventing the impingement of thermal radiation essentially within said first and second wavelength ranges on said member, said member being cooled by radiation to said thermal sink and said shielding means being cooled by radiation and reflection of thermal energy essentially within said first and second wavelengths, respectively, to said thermal sink.

2. The device of claim 1 wherein said shielding means includes a second surface having high reflectivity and low emissivity of thermal radiation within said first wavelength and high absorptivity and low reflectivity of thermal radiation within said second wavelength, said second surface being shielded from impingement by thermal radiation essentially within said second wavelength,

said member being disposed in the field of view of said second surface and beyond the field of view of the other surface of said shielding means, said shielding means surfaces having a temperature at which thermal radiation is emitted essentially within said first wavelength range.

3. The device of claim 2 wherein said second surface is specular.

4. The device of claim 1 wherein said one surface is a second surface mirror in thermal conductive relationship with said shielding means.

5. The device of claim 1 wherein said shielding means includes a first radiation shield nested within a second radiation shield in thermal isolation therefrom, said one surface being disposed on said first shield.

6. A passive device for maintaining an article at a preselected temperature in the presence of a thermal sink, comprising:

an article cooling and supporting member including a surface exposed to the ambient, said surface having high emissivity and low reflectivity to radiation within a first wavelength'range when said member is at said preselected temperature, said article and said member being in thermal conductive relationship, first radiation shield surrounding said supporting member and secured thereto in thermal isolation, said first shield including a first surface having high absorptivity and low reflectivity of thermal radiation within said first wavelength range and high reflectivity and low absorptivity of thermal radiation within said second wavelength range, and further including a second surface having high reflectivity and low absorptivity of thermal radiation within said first wavelength range, and low reflectivity and high absorption of thermal radiation within said second wavelength range, said first surface being disposed beyond the field of view of said supporting member surface, said second surface being disposed within the field of view of said first surface and said supporting member surface, and

a second radiation shield surrounding and receiving said first radiation shield in nested relationship, said first and second shields being secured in thermal isolation with respect to each other, said second shield having the outer surfaces thereof exposed to the ambient,

the temperature of the respective shields and supporting member being progressively lowered and cooled when said supporting member surface faces said thermal sink, said first surface is exposed to radiation within both said first and second wavelengths, and said second surface is exposed to radiation within essentially said first wavelength.

7. The device of claim 6 wherein said first and second radiation shields are L-shaped, and wherein said first surface is disposed on the inner surface of one leg, and said second surface is disposed on the inner surface of the other leg, said supporting member being disposed adjacent the junction of the two legs.

8. The device of claim 6 wherein said first surface is a second surface mirror secured in thermally conductive relationship to said one leg.

9. The device of claim 6 wherein said second surface is specular.

10. The device of claim 6 wherein said second radiation shield has secured thereto on a portion of said outer surfaces a layer of insulation, a second portion of said outer surfaces having high reflectivity and low absorptivity of radiation within said first wavelength range, and low reflectivity and high absorptivity of radiation within said second wavelength range.

Claims

1. A passive device for maintaining an article at a preselected temperature in the presence of a thermal radiation sink, comprising: an article cooling and supporting member, said member including a surface exposed to the ambient, said surface having high emissivity and low reflectivity to radiation within a first wavelength range when said member is at said preselected temperature, said article and said member being in conductive thermal relationship, and shielding means adjacent to and in conductive and radiation thermal isolation from said member, said shielding means including a first surface having high absorptivity and low reflectivity of thermal radiation within said first wavelength range and low absorptivity and high reflectivity of thermal radiation within a second wavelength range, said shielding means substantially preventing the impingement of thermal radiation essentially within said first and second wavelength ranges on said member, said member being cooled by radiation to said thermal sink and said shielding means being cooled by radiation and reflection of thermal energy essentially within said first and second wavelengths, respectively, to said thermal sink.

2. The device of claim 1 wherein said shielding means includes a second surface having high reflectivity and low emissivity of thermal radiation within said first wavelength and high absorptivity and low reflectivity of thermal radiation within said second wavelength, said second surface being shielded from impingement by thermal radiation essentially within said second wavelength, said member being disposed in the field of view of said second surface and beyond the field of view of the other surface of said shielding means, said shielding means surfaces having a temperature at which thermal radiation is emitted essentially within said first wavelength range.

3. The device of claim 2 wherein said second surface is specular.

6. A passive device for maintaining an article at a preselected temperature in the presence of a thermal sink, comprising: an article cooling and supporting member including a surface exposed to the ambient, said surface having high emissivity and low reflectivity to radiation within a first wavelength range when said member is at said preselected temperature, said article and said member being in thermal conductive relationship, a first radiation shield surrounding said supporting member and secured thereto in thermal isolation, said first shield including a first surface having high absorptivity and low reflectivity of thermal radiation within said first wavelength range and high reflectivity and low absorptivity of thermal radiation within said second wavelength range, and further including a second surface having high reflectivity and low absorptivity of thermal radiation within said first wavelength range, and low reflectivity and high absorption of thermal radiation within said second wavelength range, said first surface being disposed beyond the field of view of said supporting member surface, said second surface being disposed within the field of view of said first surface and said supporting member surface, and a second radiation shield surrounding and receiving said first radiation shield in nested relationship, said first and second shields being secured in thermal isolation with respect to each other, said second shield having the outer surfaces thereof exposed to the ambient, the temperature of the respective shields and supporting member being progressively lowered and cooled when said supporting member surface faces said thermal sink, said first surface is exposed to radiation within both said first and second wavelengths, and said second surface is exposed to radiation within essentially said first wavelength.

9. The device of claim 6 wherein said second surface is specular.