Spacecraft protective blanket

Abstract: A spacecraft multilayer insulation blanket is made from multiple sheets of polyamide e.g. KAPTON. A preferred six sheet embodiment employs three carbon loaded sheets (26, 32, 34) separated by two dielectric sheets (28, 30). A third dielectric sheet (36) insulates the carbon loaded sheets from the spacecraft body. The outermost carbon loaded sheet (32) has a high resistivity and is primarily transmissive while the other two carbon loaded sheets (26, 34) have a low resistivity and are primarily reflective.

Description: The present invention relates to the use of blankets for protecting spacecraft componentry from a passive intermodulation. Communications satellites and other spacecraft typically employ a protective blanket to shield electrical and other componentry from the space environment through which the spacecraft travels. This environment includes high and low temperature extremes, solar radiation, electron bombardment and radio frequency interference from a great variety of sources. A conventional protective blanket shown, for example, in U.S. Patent No. 4,489,906 to Fellas is constructed from a polyimide sheet upon which aluminum has been vapor deposited to reflect radiation. The sheet is fastened to the spacecraft using rivets and adhesives. The conventional blanket typically generates passive intermodulation because of small cracks and short circuits through the vapor deposited aluminum coating and because of contact of the aluminum coating with itself, the connecting rivets and other parts of the spacecraft. In "Journal of Spacecraft and Rockets", volume 28, No. 2, March 1991, Washington, D.C., pages 130 to 138, there is disclosed in an Article with the title "Thermal Redesign of the Galileo Spacecraft for a VEEGA Trajectory" that the Galileo spacecraft is provided with a blanket for thermal protection. This blanket is (re)designed with a carbon-polyester-coated Kapton outer layer backed up with multiple alternating layers of Mylar and Dacron net as separators and an additional indium-tin-oxide coating being applied to the exterior surface to ensure that surface electrical resistivity requirements are adequately met. However, this blanket cannot withstand a passive intermodulation (PIM). Therefore, it is an objective of the present invention to facilitate the use of a blanket as indicated above which always reliably protects a spacecraft from a passive intermodulation. According to the present invention, this objective is solved by the use of a blanket including at least one carbon loaded plastic sheet formed substantially of a flexible polyimide attached to at least a portion of a spacecraft exterior for reducing passive intermodulation about the spacecraft exterior in a space environment. The present invention provides the use of a lightweight, durable protective blanket that not only offers equal or better shielding from the external environment compared to conventional blankets in many cases but also generates far less passive intermodulation. The invention encompasses the use of a protective blanket comprising a carbon loaded flexible polyimide sheet. It is preferred that several sheets be combined to form a six layer blanket with alternating layers of dielectric polyimide sheets and carbon loaded polyimide sheets, the sheet contacting the spacecraft being dielectric and the sheet exposed to the space environment being carbon loaded. These and other aspects of the invention will be more clearly understood with reference to the following drawings wherein like numerals are used to refer to like features throughout and, in which: FIG. 1 shows a communications satellite incorporating the present invention; FIG. 2 shows a blanket constructed according to the present invention; FIG. 3 shows an alternate blanket constructed according to the present invention; FIG. 4. shows a second alternate blanket constructed according to the present invention; and FIG. 5 shows a third alternate blanket constructed according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention is preferably used as a spacecraft multilayer insulation (MLI) blanket with three types of polyimide film. The three types of film create an effective passive intermodulation (PIM) shield and suppressor. The blanket also protects against thermal effects, electrostatic discharge, contamination and other space environment hazards. A typical communications satellite spacecraft suitable for use with the present invention is shown in FIG. 1. The spacecraft has a pair of solar panels (10) on either side of a central body (12). The body has a pair of stabilizing booms (14) and a variety of antennas. As shown in FIG. 1 these include telemetry tracking and control antennas (16), an L-Band receive antenna (18), an L-Band transmit antenna (20) a C-Band receive antenna (21) and a C-Band transmit antenna (22). As is conventional in the art, all of the antennas are covered with a MLI blanket (24) to protect these components from the space environment. The radiating and absorbing portions of the antennas are not covered with a MLI blanket, but may be covered with some other material. The satellite of FIG. 1 is shown for general background. The present invention may be used with a great variety of different spacecraft. A blanket (24) for use according to the present invention is preferably constructed of at least one carbon loaded polyimide sheet (26) as shown in FIG. 2. The sheet has an external side (26a) for facing toward the space environment and an internal side (26b) for facing toward the spacecraft. The two sides are structurally the same because the sheet preferably has a uniform consistency through its thickness. A thickness of approximately .002 to .0025 inches (.050 to .064mm) is presently preferred. Such a sheet is available from the DuPont Corporation under the trademark "KAPTON", typically referred to as Black KAPTON and available in a variety of resistivities and thicknesses. Resistivities within the range of approximately 100 to 10,000 ohms per square are believed to be the most useful. The resistivity of free space is on the order of 377 ohms so that a sheet with a resistivity greater than 377 ohms per square will be primarily transmissive or adsorptive whereas a sheet of less than 377 ohms per square will be primarily reflective. Accordingly, for a single sheet blanket, it is presently preferred that the carbon loaded sheet have a resistivity in the range of 100 to 350 ohms per square preferably about 200 ohms per square. The sheet avoids the passive intermodulation problems of the prior art in part because the carbon loading is substantially consistent through the sheet material. The sheet has no surface coating which could be subject to cracking or chipping. FIG. 3 shows an enhanced blanket which uses two coextensive sheets. A first sheet (26) of carbon loaded polyimide like that of FIG. 2 having a resistivity of approximately 200 ohms per square and a second sheet (28) of polyimide for use between the first sheet and the spacecraft that is substantially dielectric. The fabric of the dielectric sheet is not carbon loaded and is also available for example from DuPont under the trademark KAPTON. The second sheet is in contact with the internal side of the first sheet and has its own internal side (28b) for facing the spacecraft. The dielectric sheet (28) is preferably from .0005 to .001 inches (.013mm to .026mm) thick. The second dielectric sheet (28) provides an insulating barrier to electrostatic discharge and passive intermodulation between the first sheet (26) and the spacecraft. FIG. 4 shows a further enhanced blanket constructed according to the present invention which incorporates a third sheet (30) which is dielectric and a fourth sheet (32) which is carbon loaded. The third sheet (30) is substantially identical to the other dielectric sheet (28) and has an internal side (30a) which faces the external side of the first sheet (26). The fourth sheet faces the third sheet and its external side faces the space environment. All four sheets are substantially the same size so that they are coextensive. It is preferred however that the resistivity for the two carbon loaded sheets (26, 32) vary, one sheet having a relatively high resistivity preferably about 1,200 ohms per square and the other having a relatively low resistivity preferably about 200 ohms per square, the high resistivity sheet preferably being the outermost or external sheet relative to the spacecraft. The high resistivity outer sheet provides substantial electrostatic discharge protection and isolation and generates essentially no passive intermodulation of its own. It has been measured to provide approximately a 3 decibel reduction in radio frequency passthrough. The low resistivity sheet is provided primarily for radio frequency shielding purposes and has been measured to provide a 7 decibel radio frequency passthrough reduction. While it does generate a small amount of passive intermodulation, at the same time it shields other parts from passive intermodulation from other sources. In antenna applications, the sheet shields passive intermodulation sources from antenna transmit fields preventing still higher levels of passive intermodulation generation. As shown in FIG. 5 the blanket can be enhanced still further by providing a fifth and sixth polyimide sheet. To enhance radio frequency interference shielding, the fifth sheet is a high resistivity carbon loaded KAPTON sheet (34) facing the internal side of the second sheet and the sixth sheet is an additional dielectric KAPTON sheet (36) facing the internal side of the fifth carbon loaded sheet. Such a blanket has three coextensive carbon loaded KAPTON sheets (26, 32, 34). The outer and inner sheets (32, 34) have a higher resistivity, preferably about 1200 ohms per square, and the middle sheet (26) has a relatively lower resistivity, preferably about 200 ohms per square. The sheets are separated by substantially identical coextensive dielectric KAPTON sheets (28, 30, 36) placed in between the carbon loaded sheets for insulation purposes. In other words, each sheet of each embodiment has an external side for facing toward the space environment and an internal side for facing toward the spacecraft. These sides are placed adjacent to each other to form the blanket. As shown in FIG. 5, the internal side (36a) of the sixth sheet faces and may contact the spacecraft in use. The sixth sheet external side (36b) is adjacent the fifth sheet internal side (34a). The fifth sheet external side (34b) is adjacent to the second sheet internal side (28a). The second sheet external side (28b) is adjacent to the first sheet internal side (26a). The first sheet external side (26b) is adjacent the third sheet internal side (30a). The third sheet external side is adjacent the fourth sheet internal side and the fourth sheet external side is directly exposed to the space environment. Such a blanket using the sheets described above has been measured to provide a 12 1/2 decibel reduction in radio frequency passthrough and a 40 decibel reduction in passive intermodulation in addition to its other thermal isolation and other characteristics. If desired, the outermost surface of any of the blankets in FIGS. 2-5 can be coated with a reflective coating, such as white paint as is well known in the art. The precise number and order of sheets may be varied further to suit different applications. The blanket is preferably attached to the spacecraft conventionally using rivets. Grounding wires as are well known in the art are preferably used to ground all layers of the blanket to the spacecraft body. The layers of the blanket are preferably stitched together using conventional thread as is known in the art. Applicant first tested a blanket embodying the present invention in a spacecraft launched October 30, 1990. The spacecraft first entered a true space environment sometime after the launch date. It was in this space environment that the blanket's true effectiveness could first be accurately measured. While the blanket has been described only in a few different embodiments it will be apparent to those skilled in the art that a wide variety of modifications and adaptations may be made. The blanket may be applied to a great variety of different spacecraft to shield different types of spacecraft componentry including but not limited to antennas and electronics. The blanket provides protection for spacecraft not only in earth orbit but also on deep space missions. The invention is not to be limited to the embodiments described above but only by the claims which follow.

Claims: Use of a blanket including at least one carbon loaded plastic sheet (26) formed substantially of a flexible polyimide attached to at least a portion of a spacecraft exterior for reducing passive intermodulation about the spacecraft exterior in a space environment. Use of a blanket according to claim 1, characterized in that said plastic sheet (26) has a resistivity of between 100 and 10,000 ohms per square. Use of a blanket according to claim 1 or 2, characterized in that the carbon loaded polyimide sheet is KAPTON (trademark). Use of a blanket according to claim 2 or 3, characterized in that the plastic sheet (26) has a resistivity of between 100 and 350 ohms per square. Use of a blanket according to at least one of claims 1 to 4, characterized in that said blanket further comprises a dielectric plastic sheet (28) substantially coextensive with the carbon loaded plastic sheet (26) adjacent to the internal side of said carbon loaded plastic sheet (26). Use of a blanket according to at least one of claims 1 to 5, characterized in that said blanket further comprises an electromagnetic radiation reflective layer on the external side (26a) of the carbon loaded plastic sheet (26).

Inventors: REYNOLDS ROBERT L, ROSS BARRY S, VANBEBBER GLEN A, WHITE DANIEL A, REYNOLDS, ROBERT L., ROSS, BARRY S., VANBEBBER, GLEN A., WHITE, DANIEL A.
Applicants: HUGHES ELECTRONICS CORP, HUGHES ELECTRONICS CORPORATION
Application year: 1992
Main classification: B32B5
IPC codes: B32B27