Aeronautics and astronautics – Spacecraft – Spacecraft formation – orbit – or interplanetary path
Reexamination Certificate
1998-12-11
2001-01-23
Carone, Michael J. (Department: 3641)
Aeronautics and astronautics
Spacecraft
Spacecraft formation, orbit, or interplanetary path
C244S158700
Reexamination Certificate
active
06176453
ABSTRACT:
This invention relates to radiators, and, more particularly, to radiators used in spacecraft.
A radiator is designed to radiate heat to its surroundings. Spacecraft whose occupants, electronics, or power sources generate large amounts of heat employ one or more radiators to transfer the generated heat from the interior of the spacecraft to free space and to reflect incident heat from solar radiation exposure. The radiators also aid in dissipating static electricity on the surface of the spacecraft. The radiators are necessary to prevent heating of the interior of the spacecraft to unacceptably high levels. For some spacecraft such as large communications satellites that generate and utilize large amounts of power, removal of excess heat is a significant factor in the design of the spacecraft, and large amounts of radiator surface are required.
In a commonly used construction of a spacecraft radiator, the radiating surface is formed of a large number of individual mirror-like radiators. A single larger mirror-like radiator is not used because of the likelihood that it would crack due to thermal strains. Each mirror-like radiator is 1-2 inches on a side. Each mirror-like radiator is formed of a ceramic-glass substrate about 0.002-0.010 inches thick that is coated on the inwardly facing surface with a metallic silver coating. The metal-coated mirror has a relatively low solar absorptance and a relatively high infrared emittance, so that heat is effectively radiated away without absorbing excessive energy from incident sunlight. The opposite, outwardly facing surface of the ceramic-glass substrate is coated with a layer of transparent indium-tin-oxide that serves to dissipate static charges. The metallic silver-coated inwardly facing surface is bonded to the underlying structure with a silicone adhesive.
This radiator construction is operable and widely used on communications satellites. However, the fabrication of the spacecraft using the individual mirror-like radiators is a time-consuming, expensive process. Hundreds or thousands of individual mirror-like elements are fabricated by deposition processes and then individually attached to the underlying support surface. Because the glass ceramic mirror substrates are thin and large in lateral extent relative to their thickness, they are fragile and easily broken during fabrication, assembly, or service.
There is accordingly a need for an improved approach to the construction of radiators used in spacecraft and for other applications. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a radiator and a spacecraft which utilizes such a radiator. The radiator has excellent performance in the space environment. It is fabricated and installed in the spacecraft much less expensively and more quickly than a radiator using conventional mirror-like radiating elements. The weight of the radiator is reduced as compared with the mirror-like radiator, an important consideration for launching the spacecraft from earth. The radiator of the invention is more robust than a mirror-like radiator.
In accordance with the invention, a radiator comprises a structure which generates heat, and a radiator element in thermal communication with the structure. The radiator element comprises a radiating surface, with a coating on the radiating surface comprising a white thermal control paint. The paint has an initial solar absorptance of not greater than 0.16, preferably not greater than about 0.14 and more preferably not greater than about 0.10, and an initial infrared emittance of not less than about 0.80. (The “initial” properties are those measured after the paint is applied and dried, but before any substantial exposure to a space environment.) The structure which generates heat is preferably the internal structure of a spacecraft such as a satellite.
The paint used in the coating on the radiating surface of the radiator is formed of white pigment particles in a binder. The preferred pigment particles have a composition of Zn[xAl(1−x)Ga]
2
O
4
(&dgr;In), termed a zinc aluminate gallate, where the value of x is from 0 to 1 and the value of &dgr; is from 0 to about 0.2. The pigment particles may also be a zinc-containing pigment such as the doped or undoped ZnO pigment disclosed in U.S. Pat. No. 5,094,693, whose disclosure is incorporated by reference. The binder is preferably an inorganic binder such as a silicate, and preferably potassium silicate, but an organic binder may also be used for less-demanding applications. The weight ratio of pigment to binder is preferably from about 3:1 to about 5:1, but may be less than about 3:1. The paint thickness is preferably from about 0.003 to about 0.006 inches, after drying. The zinc aluminate gallate paint with a potassium silicate binder has an initial solar absorptance &agr; of less than 0.10, and aluminum-doped zinc oxide paint with a potassium silicate binder has an initial solar absorptance of about 0.13-0.18.
The paint is applied to the radiating surface of the radiator using conventional painting techniques such as brushing or spraying, or by non-vehicle painting techniques such as plasma spray.
Many white coatings have been used for the exterior portions of spacecraft other than the radiators. During extended exposure to the space environment of ultraviolet radiation, gamma radiation, electrons, and protons, the initially white coatings become yellow and then gray as their solar absorptance rises. As the solar absorptance rises, the coatings become less efficient thermal coatings, because they absorb increasing amounts of solar energy. This loss of efficiency typically takes at least several years and is not a concern for short-lived spacecraft or satellites.
The gradually increasing solar absorptance with increasing exposure is of relatively little significance for the non-radiator portions of the exterior of the spacecraft, even those spacecraft such as communications satellites that spend many years in space. These non-radiator regions are not designed to dissipate larger amounts of heat than they absorb, and in many cases do not face the sun. If the solar absorptance for these portions of the spacecraft rises, there is relatively little effect on the total heat balance of the spacecraft. The radiators, on the other hand, must dissipate large amounts of interiorly generated heat. Due to the required orientation of the antennas and solar cells of the spacecraft, the radiators often must face toward the sun. If the solar absorptance of the radiators increases significantly over the operating life of the spacecraft, the radiators begin to absorb large amounts of heat and consequently radiate the interiorly generated heat less efficiently. The thermal balance of the spacecraft is degraded. The temperature of the interior of the spacecraft begins to rise, and eventually exceeds the acceptable operating temperatures.
Thus, the radiators of the spacecraft have different operating conditions and requirements than the exterior non-radiator walls of the spacecraft. The coatings that have long been used on the non-radiator exterior walls of the spacecraft could not be used for the radiators because of their inadequate long-term stability in the space environment. The development of the white coatings with low initial values of solar absorptance and high initial values of solar emittance has made possible the present approach to an improved radiator, because the initial values of solar absorptance are sufficiently far removed from the maximum permissible value after extended exposure that the gradual increase in solar absorptance does not result in the inoperability of the radiator.
At the present time, there is no known approach for preventing the gradual increase in the solar absorptance that occurs over time. Instead, the initial solar absorptance must be selected to be sufficiently low such that, after the gradual increase in solar absorptance over time and exposure, the paint still has an acceptable solar a
Denman James R.
Long Lynn E.
Carone Michael J.
French III Fredrick T.
Gudmestad Terje
Hughes Electronics Corporation
Sales M. W.
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