Title:
LIGHTWEIGHT COMPOSITE REFLECTOR DISH
United States Patent 3694059


Abstract:
A lightweight precision dish structure for reflectors, particularly large parabolic microwave reflectors for space applications. The dish structure has a laminated reflector dish with a central core and bonded facing sheets of differing coefficients of thermal expansion, and a rigid reinforcing ring secured by adjustable fastening means to the rear convex side of the reflector dish and having a coefficient of thermal expansion closely approximating the resultant coefficient of the dish. The reinforcing ring and its adjustable fastening means permit initial adjustment of the dish to a precise parabolic or other configuration and prevent later thermal distortion of the dish as a consequence of the differing coefficients of its core and facing sheets in an adverse, asymmetric thermal environment.



Inventors:
SHAKESPEARE WILLIAM B J
Application Number:
05/076741
Publication Date:
09/26/1972
Filing Date:
09/30/1970
Assignee:
TRW INC.
Primary Class:
Other Classes:
343/912, 359/883, 428/64.1, 428/115, 428/119, 428/152
International Classes:
F24S23/71; G02B5/08; G02B7/18; (IPC1-7): G02B7/18
Field of Search:
343/840,912 350
View Patent Images:
US Patent References:



Primary Examiner:
Schonberg, David
Assistant Examiner:
Leonard, John W.
Claims:
What is claimed as new in support of Letters Patent is

1. A lightweight precision reflector dish structure comprising a laminated reflector dish including a central core having a front concave face and a rear convex face, and facing sheets conforming and bonded to said core faces:

2. A reflector dish structure according to claim 1 wherein:

Description:
The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Air Force.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to reflectors, especially large parabolic microwave reflectors for space applications. More particularly, the invention relates to a lightweight precision dish structure for such reflectors.

2. Prior Art

As will become evident from the ensuing description, the invention may be utilized in a variety of reflectors for both terrestrial and space applications. However, the principal application of the invention is a large parabolic microwave reflector for space use. The invention will be disclosed in connection with such use.

A reflector intended for space applications is subject to conflicting design constraints which heretofore have been difficult, if not impossible, to satisfy. Thus, in order to minimize structural deformation of the reflector due to temperature cycles in space, such as are experienced in certain orbital flights, the reflector must be constructed of materials having low coefficients of thermal expansion. However, even when constructed of such materials, the reflector is subject to significant thermal expansion and contraction when exposed to extreme temperature cycles and must possess sufficient stiffness to maintain its close tolerance profile under such temperature cycling. On the other hand, normal launch dynamic conditions impose on the reflector the constraint of lightweight construction which is inherently characterized by relatively low finite stiffness.

At the present state of the art, the above design constraints can best be satisfied by utilizing a laminated reflector dish consisting of a central core of aluminum honeycomb or other similar material and resin impregnated facing sheets bonded to the front concave and rear convex sides of the core. However, the resulting reflector is a non-homogeneous structure which displays inherent unpredictable creep characteristics. Because of these creep characteristics, the reflector dish, when exposed to extreme temperature cycles, tends to deform, primarily within a concentric annular band or region adjacent the rim of the dish in a manner which produces within such region wavelike distortions. These distortions are unacceptable within the constraints of the close tolerances limits of the reflector profile.

SUMMARY OF THE INVENTION

The present invention avoids the distortion problems and satisfies the design constraints discussed above and thus provides a lightweight precision reflector dish structure which is ideally suited for space applications involving extreme temperature cycles. According to the invention, the laminated reflector dish is stiffened or reinforced by a relatively rigid ring concentrically surrounding the rear side of the dish adjacent its rim. This ring is attached to the reflector dish at positions spaced uniformly about the ring by adjustable fastening means. In the disclosed inventive embodiment, these fastening means comprise screws which pass through the ring and are threaded in the reflector dish and shims which may be placed between the ring and dish. The ring is constructed of a material which has a coefficient of thermal expansion closely approximating the overall or resultant coefficient of thermal expansion of the laminated reflector dish.

During initial assembly of the dish structure, the reflector dish is adjusted to the desired profile or shape by the adjustable ring-dish fastening means. In the disclosed embodiment, for example, this adjustment involves adjustment of the ring screws and selective placement of shims between the ring and dish in such a way as to draw the dish to the desired profile. Subsequently, in operational use of the reflector, the ring serves to reinforce or stiffen the reflector dish against distortion when exposed to extreme temperature cycles and thereby preserves the close tolerance profile of the dish.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a rear view of a parabolic reflector dish structure according to the invention; and

FIG. 2 is an enlarged section taken on line 2--2 in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings illustrate a lightweight precision reflector dish structure 10 according to the invention. This dish structure includes a reflector dish 12 proper, a reinforcing or stiffening ring 14 concentrically surrounding the rear convex side of the dish adjacent its rim, and adjustable fastening means 16 joining the ring and dish at positions spaced uniformly about the ring. In this instance, the reflector dish 12 is a parabolic dish whose front concave surface is coated with a metallic film to form a parabolic microwave reflector for installation on an orbiting space satellite.

The reflector dish 12 has a laminated construction and includes a central core 18 and facing sheets 20, 22 bonded and conforming to the front and rear faces of the core. For the space application referred to above, the core comprises a high strength lightweight material having a low coefficient of thermal expansion, such as aluminum honeycomb. The facing sheets comprise high strength resin impregnated Fiberglas bonded to the core.

Dish 12 is fabricated by applying the facing sheets to the honeycomb core to form a laminated structure and then forming this laminated structure into a dish between a pair of forming dies having the desired profile of the finished dish. After forming, the dish is cured to relieve the stresses produced during the forming operation. As noted earlier, a laminated dish of this kind is a non-homogenous structure which possesses unpredictable creep characteristics. It has been found that owing to these creep characteristics, the dish tends to creep and deform when exposed to the temperature changes or cycles which occur during the stress relieving process and later during operational use of the reflector in space, particularly in some orbital flights which produce extreme temperature changes or cycles. However, this deformation of the dish occurs only within the annular region R of the dish extending radially in from its rim, a distance approximating one-third the radius of the dish. The remaining central region of the dish remains undeformed and conforms to the desired dish profile.

Deformation of the dish 12 within the region R takes the form of wavelike distortions. These distortions are unacceptable within the close tolerance limits of a dish for a parabolic reflector.

The stiffening ring 14 constitutes a major improvement feature of the invention. The ring and its fastening means 16 serve two purposes, namely initial adjustment of the reflector dish to its proper profile or contour, and retention of this profile during operational use of the reflector. To this end, the ring surrounds the rear side of the reflector dish 12 within the region R of dish distortion. The ring is constructed of a material whose coefficient of thermal expansion closely approximates the overall or resultant coefficient of thermal expansion of the laminated dish. For a composite dish having an aluminum honeycomb core and resin impregnated facing sheets for example, the stiffening ring may comprise a stainless steel tube. The ring fastening means comprise screws 24 which pass through holes in the wall of the ring adjacent the dish and are threaded in inserts 26 bonded within the dish. The fastening means also include shims 28 which are adapted to be placed on selected screws between the dish and the ring. Screws 24 are arranged at uniform intervals about the dish.

Stiffening ring 14 is diametrically sized to closely approximate diameter of the reflector dish at the plane of attachment of the ring to the dish, such that this dish region may be forced or adjusted to the proper profile by tightening the ring attachment screws 24 to firmly clamp the dish to the ring. Shims 28 are placed between the dish and the ring as required to provide the dish with the proper profile. During subsequent operational use of the reflector, when the latter is subjected to extreme temperature cycles, the dish 12 and ring 14 expand and contract in unison so that the ring retains the outer region R of the dish in the proper profile.

The number of ring fastening screws 24 depends on the diameter of the dish. For a 44-inch diameter dish, for example, 32 fastening means have been found to be suitable. While the illustrated dish has only a single stiffening ring 14, it will be understood that additional rings may be employed, if necessary. One ring has been found to be sufficient for a 44-inch parabolic dish, for example. Larger reflector dishes may require additional rings.