Abstract:
Reaction wheels are placed around an enclosure so that the reaction wheel rotors provide a radiation shield for electronic components inside the enclosure.

Description:
BACKGROUND 
     This invention relates to satellites, in particular, using reaction wheel assemblies (RWA) to provide a radiation shield for electronics on a satellite and simplify the integration of RWA&#39;s and electronics in a satellite system. 
     An RWA comprises a dense, metalic rotor and electronics to change the speed of its rotation. A plurality of RWAs are deployed on a satellite along different rotor axis producing an aggregate angular momentum the direction and magnitude of which is varied, by changing the speed of the RWAs, to change satellite attitude spherically (rotate the satellite). 
     Typically, the RWAs are located at different locations in the satellite payload area and connected by cables to associate electronics. The electronics needs to be shielded from ambient radiation found in space, and the common technique is to place the electronics in a metal container. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is provide a modular reaction wheel assembly with the electronics inside and protected from radiation. 
     According to the invention, the rotors are used to shield the electronics. In particular, the RWAs are arranged around an electronics enclosure on their respective, required off-normal axis orientations to provide full attitude control, the electronics, on circuit boards are located with the enclosure and the RWA rotators, which are thick compared to the housing, provide a barrier between the circuit boards and radiation outside the enclosure. Connectors for the electronics are located on one surface between the RWAs and provide a barrier for one end of the circuit boards. The circuit boards are oriented inside the enclosure so that RWAs shield the surface (maximum exposure area of the electronics) of the circuits boards. 
     A feature of the present invention is that the RWA assembly can be installed as a singe unit on a satellite reducing integration costs. Another feature is that the total weight of RWA package (electronics and reaction wheels) is reduced compared to traditional approaches where each reaction wheel is separately mounted from the electronics. The invention takes advantage of the radiation shielding qualities of the rotors. 
     Other objects, benefits and features of the invention will apparent to one of ordinary skill in the art from the drawing and following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective of a modular reaction wheel assembly according to the present invention. 
     FIG. 2 is a section along line  2 — 2  in FIG.  1 . 
    
    
     DESCRIPTION 
     In FIG. 1, four reaction wheels (RW)  10  are arranged around an enclosure  12 , each on an inclined or sloped metal wall  14 , giving the enclosure something like a igloo shape in perspective. In section, FIG. 2, the enclosure looks like a pyramid with a flat top. Each RW contains the rotator, not shown but mentioned above, and the rotational axis, e.g. axis  10   a  of each rotor (its associated angular momentum vector) points in a direction (normal to the wall  14 ) between horizon H and the zenith Z. Electronic connectors  16  are located on top metal surface  18 . Brackets  20  are attached to walls  14  to attach the enclosure  12  to a surface on a satellite or spacecraft, not shown. The enclosure and the RWs comprise the RWA or reaction wheel assembly. 
     The four surfaces or walls  14 , to which the RWAs  10  are attached, may be stamped from a single metal piece along with the op surface  16 , giving the enclosure a shape resembling an igloo and roof-like in section, as shown in FIG.  2 . The surface area, diameter shape of the wall  14  and the RWA  10  are substantially the same. The wall  14  surface area, in other words, is only large enough to support the reaction wheel. 
     Referring to FIG. 2, a plurality of circuit boards  22 , each containing components  24 , such as multichip module on the top surface  22   a , is vertically oriented inside the enclosure  12 . As a practical matter, sometimes components are located on the top and bottom surfaces. Each circuit board  22  is plugged into a bus or card slot  26  and connected by a cable  28  to a connector  16 . 
     The top  22   a  and bottom  22   b  surfaces of each board  22  are within the shadow of the RWAs  10 . To be more precise, radiation R outside the enclosure  12  is deflected or absorbed by RWA  30  or RWA  32 . Similarly, RWA  34  and  36 , shield the vertical edges of the boards. However, it can be appreciated that the area of maximum possible exposure and damage is on those bottom and top surfaces, where components are typically connected. It is possible to locate additional boards, orthogonal to boards  22 , in which case the RWAs  34  and  36  would shield the bottom and top surfaces (containing components, such as components  24 ) of those boards and RWAs  30  and  32  would shield the vertical edges. 
     The surface  18 , as well as the connectors  16 , shield the top (horizontal) edges  22   c  of the boards, but as explained before, the surface area from that exposure angle is small. Likewise, the bottom edges  22   d , also a small area exposed to radiation R, are shielded by a removable metal floor  38 , by which the interior is accessed. 
     One skilled in the art may make modifications, in whole or in part, to a described embodiment of the invention and its various functions and components without departing from the true scope and spirit of the invention.