Abstract:
A spacecraft comprised of a plurality of functionally organized, fully independent modules. Selected ones of the modules are moveable foldably in a “clamshell”-like manner between assembled positions, wherein various components of the spacecraft are generally not readily accessible, to a service position in which virtually all of the components of the spacecraft are substantially readily accessible for service or testing. Logically organized and routed harnesses comprised of numerous electrical and mechanical elements enable the clamshell-like folding movement of selected modules. Conventional electrical and mechanical connectors associated with the harnesses permit quick detachment and complete removal of selected modules from the spacecraft. The fully modular design significantly reduces the time required to gain access to the components on each of the modules and eliminates time consuming and extensive disassembly procedures that would ordinarily be required when performing service or test procedures on the spacecraft. The fully modular configuration also permits parallel manufacturing and testing of the independent modules, thus significantly reducing the design-to-market cycle time for the spacecraft. The modular design also facilitates replacement of selected modules with modified modules to produce derivative spacecraft or to tailor the performance of the spacecraft to better meet the needs of a particular mission.

Description:
TECHNICAL FIELD 
     This invention relates to spacecraft, and more particularly to a spacecraft formed by a number of independently removable, testable and serviceable modules which simplify and expedite the manufacture, testing and service of the spacecraft, and which enable modifications and/or future variations of various modules of the spacecraft to be easily integrated and implemented. 
     BACKGROUND OF THE INVENTION 
     Background Art 
     Present day approaches to spacecraft design are often single-focused on the goal of achieving compliance with technical parameters governing mission performance, such as the power required by the spacecraft, its mass or volume of payload. This single-minded design approach often produces spacecraft which are unique for each new program or application, and which have little or no components or subassemblies in common with one another. Thus, since each spacecraft must essentially be designed as a unique vehicle, the design-to-market cycle time of producing a spacecraft is quite long. Presently, a spacecraft designed for a specific application may take as long as 36-40 months or more to design, construct and test before the spacecraft is ready to be launched. These drawbacks lead to significant non-recurring labor costs, lengthy production cycle time, increased scrap materials, and repeated redesign of various subsystems of the spacecraft. 
     Another significant drawback with many present day spacecraft designs is the lack of serviceability. For example, the various subsystems of the spacecraft are often interconnected by a highly complex plurality of electrical, mechanical, fluid carrying and thermal elements. These elements are routed in a complex arrangement to the various components of the spacecraft and are typically not capable of being quickly and easily disconnected, should one or more subsystems of the spacecraft need to be accessed for service and/or testing. Disassembly and uncoupling of one major subsystem often requires extensive disassembly of cabling, mechanical coupling elements, fluid transfer lines and thermal elements, many of which are not capable of being quickly separated from their associated components. Often, fluid lines need to be physically cut and then re-welded or re-brazed, and then pressure tested, during re-assembly. Thus, uncoupling and reassembly of even one major subsystem often takes several days, or even weeks, to complete. 
     With present day spacecraft, the organization of its various subsystems are further not arranged in logical, fully functional, modular sub-component form to facilitate access to, service and testing of functionally related sub-components. This further contributes to extremely time consuming disassembly procedures being required before access can be gained to the specific subsystems of the spacecraft being serviced or tested. Often, a significant number of other unrelated subassemblies of the spacecraft may need to be removed before being able to access and test the desired subassembly/component. These drawbacks with present day spacecraft designs contribute significantly to their lengthy design-to-market cycle times. 
     As will be appreciated, the inability to quickly and easily access and/or uncouple and physically remove a particular subsystem of the spacecraft introduces significant additional cost, in terms of labor, into the overall cost of manufacturing the spacecraft. This is in large part because of the significant testing that must be performed on the numerous components of the spacecraft before the spacecraft is ready for launch. It also can cause significant time delays which could jeopardize delaying a planned launch of the spacecraft if a portion of the spacecraft has to be disassembled, re-tested and then reassembled shortly before a scheduled launch. 
     Since most present day spacecraft are designed with the goal of meeting particular customer requirements, often the subassemblies of the spacecraft are not usable in subsequent spacecraft, thus requiring new design efforts when designing and manufacturing a subsequent spacecraft to different customer specifications. This also contributes significantly to the high cost of present day spacecraft and the long design-to-market cycle times needed to produce such spacecraft. The re-designing of the various subassemblies of the spacecraft also prevents the cost savings that would otherwise come with the manufacture of standardized modules or subassemblies which could be used on more than one spacecraft. 
     Still another disadvantage of the present day spacecraft design is the inability to easily integrate improvements into various subsystems or subassemblies of the spacecraft without requiring re-design of other subsystems. Thus, if one spacecraft required a more powerful propulsion system, it would be much more cost effective and require much less design time if merely the propulsion system of the spacecraft could be re-designed and then either re-introduced into the same spacecraft or used in a different spacecraft without interfering with the design or operation of other subsystems, and without requiring disassembly of a major portion of the spacecraft. 
     It is therefore a principal object of the present invention to provide a spacecraft comprised of a plurality of functionally independent major modules which may be independently accessed and/or removed from the spacecraft without requiring significant and time consuming disassembly of numerous electrical, mechanical, fluid carrying and thermal interconnecting components securing the affected module to other modules of the spacecraft. In this manner specific modules may be accessed, serviced and tested without requiring significant additional disassembly of other modules of the spacecraft. This would significantly reduce the man hours required for testing and servicing the various modules of the spacecraft as well as the cost associated with such procedures. 
     It is another object of the present invention to provide a spacecraft having a highly modular design such that the various modules thereof may be used in various derivative and future spacecraft applications. 
     It is still another object of the present invention to provide a spacecraft comprised of a plurality of functionally independent modules each having its electrical, mechanical, fluid carrying and thermal interconnecting elements configured such that each module may be selectively moved, relative to one another, to enable access to portions of the spacecraft which would otherwise only be accessible through hours, or possibly days or weeks, of time consuming disassembly efforts. 
     It is yet another object of the present invention to provide a method of manufacturing a spacecraft which involves using functionally independent modules that are secured to one another via fastener assemblies including quick disconnect fasteners and conventional electrical connectors, to thereby enable parallel manufacturing, design, testing and/or removal of selected modular subsystems, to thus significantly reduce the overall time required to construct, test and service the various major functional subsystems of the spacecraft. 
     SUMMARY OF THE INVENTION 
     The above and other objects are provided by a fully modular spacecraft in accordance with preferred embodiments of the present invention. In one preferred embodiment the spacecraft comprises a plurality of functionally independent modules which may be quickly and easily released from fixedly held positions by actuating a selected plurality of quick release fastener assemblies associated with the module requiring access thereto. Each of the modules are further constructed such that all of the electrical, mechanical, fluid carrying and thermal elements are arranged and routed in such a manner to permit each of the modules to be moved foldably in a “clamshell”-like fashion between a compact assembled position, wherein its various components are not generally readily accessible, to a service or test position, in which all, or substantially all of the components on the selected module, as well as many of the components on adjacent modules, are readily accessible. This fully modular approach and clamshell-like folding movement of the modules of the spacecraft drastically reduces the time and disassembly effort required to gain access to the numerous sub-components disposed on each of the modules when performing service and/or testing on the numerous components of the spacecraft. This feature also eliminates the need to disassemble/disconnect various other electrical cabling, thermal elements or fluid transfer or support lines, or other components, before access can be gained to the desired component, which is a significant drawback of present day spacecraft designs. 
     The primary modules of the spacecraft of the present invention comprise a bus electronics module, a payload module, an electrical power module and a propulsion module. Each of these modules essentially forms a fully functional, line replaceable unit (LRU) that functions as a substantially separate and discrete subsystem having a complex plurality of logically related subcomponents. Each of the modules can be quickly and easily coupled to and uncoupled from the other modules of the spacecraft such that each module may be separately fabricated, tested, delivered and installed to facilitate rapid construction and testing of the spacecraft. 
     The fully modular design of the spacecraft of the present invention also enables the various modules to be designed such that they may be used with other derivative spacecraft, thus significantly shortening the time-to-market cycle time for design and manufacture of such derivative spacecraft. This modular approach further enables one module of the spacecraft to be replaced with a module having different performance characteristics without requiring modification or redesign of other modules of the spacecraft. 
     A principal advantage of the modular spacecraft of the present invention is the ability to quickly and easily access, and remove if necessary, only those modules of the spacecraft that may require service or testing. This is accomplished through standardized electrical, mechanical and thermal connectors and couplings which permit rapid manual disconnection, if needed, to completely separate one module from the remainder of the spacecraft. Thus, if the need arises to gain access to or completely remove a module shortly before a scheduled launch of the spacecraft, the fully modular design of the spacecraft permits service and testing of one or more modules within a greatly reduced time frame, and with significantly reduced labor expense in terms of man hours. 
     Still another important advantage of the modular spacecraft of the present invention is the ability to rapidly modify the performance characteristics of the spacecraft to accommodate new power, payload or thermal requirements. For example, if the power requirements of the spacecraft are reduced, the design of the electrical power module can be quickly and efficiently scaled down to meet the lesser power requirements. This can be accomplished simply by removing and modifying the electrical power module with little or no modification to the other major modules of the spacecraft. If thermal loads are reduced, the number of other deployable components such as radiators and/or thermal heat pipes may also be significantly reduced. In production, these changes can be easily managed. With conventional spacecraft designs, even minor design changes can take many months to implement and test, while such design modifications could be implemented within days or weeks with the spacecraft of the present invention. Thus, the modular spacecraft of the present invention lends itself well to retrofitting procedures when one or more of the modules thereof require modifications to suit the requirements of a specific mission. 
     The modular design of the spacecraft of the present invention also enables fast, efficient and consistently high quality manufacturing of the spacecraft. The various modules may be delivered pre-tested for assembly to a central location, assembled and then fully tested to ensure that electrical and mechanical connections are securely made. Beyond thermal cycle testing to assess the interconnections, no other environmental tests need be performed. This approach allows very high volume integration and testing of a much greater number of spacecraft, thus significantly reducing the manufacturing and test time required for each spacecraft. 
     In one preferred embodiment of the present invention the payload module comprises an assembly having a central payload panel and a pair of outer payload panels. At least one of the outer payload panels is movable in a folding manner relative to the central payload panel to enable access to the interior area of the payload module if needed. The bus electronics module is also movably secured to the payload module so that it can be folded outwardly therefrom to enable even greater unimpeded access to the payload module. Quick disconnect fasteners allow the payload panels, as well as the bus electronics module, to not only be uncoupled from one another to permit folding movement, but also to be quickly removed from the remainder of the spacecraft for service and/or testing if needed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and subjoined claims and by referencing the following drawings in which: 
     FIG. 1 is an exploded perspective view of the various modules of a spacecraft in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a perspective view of the spacecraft of FIG. 1 shown in a fully assembled arrangement; 
     FIG. 3 is a perspective view of the spacecraft of FIG. 1 showing one of the payload module panels folded into a service position enabling access to the components mounted thereon as well as to components mounted on other panels of the payload module, and further illustrating the routing of a harness that permits folding movement of the outer payload panel without interference from various electrical cables, mechanical elements, fluid lines and thermal elements; 
     FIG. 4 is a perspective view of an alternative preferred embodiment of the present invention wherein the two outer panels of the payload module are both capable of folding movement, and illustrating both payload module outer panels folded outwardly into their service positions; 
     FIG. 5 is an exploded perspective view of the various modules of the spacecraft illustrating where fasteners may be used to releasably secure the modules to one another; 
     FIG. 6 is a perspective view of a supporting tool used to support the spacecraft of the present invention when performing testing or service on the spacecraft; 
     FIG. 7 is an enlarged perspective view of the propulsion module of the present invention; and 
     FIG. 8 is a perspective view of the spacecraft of the present invention within its solar array panels and heat radiators fully deployed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a fully modular spacecraft  10  in accordance with a preferred embodiment of the present invention. The spacecraft  10  generally comprises a payload assembly module  12 , a functionally independent propulsion module  14 , a functionally independent bus electronics module  16  and a functionally independent electrical power module  18 . The payload module  12  is comprised of preferably three distinct, fully independent payload panels  12   a ,  12   b  and  12   c , although it will be appreciated that a larger or smaller plurality of payload panels could be incorporated depending on the specific needs of the mission which the spacecraft  10  will carry out. Payload panel  12   a  forms a central payload subassembly while panels  12   b  and  12   c  form outer payload subassemblies which are disposed adjacent opposite longitudinal edges of the central payload panel  12   a . Outer payload panel  12   b  typically includes a pair of deployable heat radiators  12   b   1 , and  12   b   2  secured thereto for assisting in dissipating heat from the electronic components mounted on the outer payload panel  12   b . It will be appreciated that the other payload panels  12   a  and  12   c  may include between zero and two such similar heat radiators although only one such other radiator  12   c   1  is visible in FIG.  1 . Electrical power module  18  provides electrical power to all of the modules  12 - 16  of the spacecraft  10 . The spacecraft  10  is shown in its fully assembled, pre-deployed condition in FIG.  2 . 
     With further reference to FIG. 1, the electrical power module  18  comprises a battery assembly  20  and a solar array assembly  22 . The solar array assembly  22  is comprised of solar panels  22   a  and  22   b , and a solar array deployment mast  22   c  for deployably extending the solar array panels  22   a  and  22   b . In one preferred form the battery assembly  20  comprises a lithium ion battery assembly and the solar array panels  22   a  and  22   b  comprise silicon solar array panels. The electrical power module  18  is secured to the bus electronics module  16  and movable therewith, as will be described further momentarily. 
     It is a principal advantage of the present invention that each of the modules  12 ,  14 ,  16  and  18  described above is constructed as an independent, fully functional module capable of being readily interfaced with one or more of the other modules in a manner that permits selected ones of the modules  12 ,  14 ,  16  and  18  to be moved in a folding manner relative to one another between an “assembled” position, such as shown in FIG. 2, and a “service” or “test” position, such as shown in FIG. 3, to permit easy access to the numerous components disposed on each of the modules  12 - 18 . This folding movement is accomplished in part by logical routing of a large plurality of electrical cables, fluid lines, thermal elements, etc., in a manner such that these elements do not interfere with folding movement of the modules  12 ,  14 ,  16  and  18  when access to the various components on the modules is required. Thus, there is no risk of breakage or need to disconnect or cut the various fluid transfer or support lines associated with each module  12 - 18 . Heat transfer across the modules  12 - 18  further occurs without breaking any fluid lines or heat pipes. Such movement would not be possible with prior designs of spacecraft without extensive disconnection and cutting of various fluid transfer lines and other mechanical, electrical and thermal elements. 
     With further reference to FIG. 3, the spacecraft  10  can be seen with the bus electronics module  16  and electrical power module  18  extended pivotably away from payload panel  12   b , and with panel  12   b  folded outwardly into the service position. In this orientation, a large plurality of the components on several of the modules  12 - 18  are accessible. More specifically, access to numerous components mounted on the central payload panel  12   a , the outer payload panel  12   c , the bus electronics module  16 , as well as to components  24 - 38  mounted on outer payload panel  12   b , are readily accessible. It will be appreciated that components  24 - 38 , as well as the other components mounted on the payload panels  12   a ,  12   b ,  12   c  and on the bus electronics module  16 , represent highly complex electrical, electromechanical and thermal components, and have been shown in a highly simplified fashion. 
     Referring further to FIG. 3, outer payload panel  12   b  is intended to be moved foldably about a longitudinal axis extending parallel to its longitudinal edge  40  and also to longitudinal edge  42  of the central payload panel  12   a . The bus electronics module  16  and the electrical power module  18  are also intended to be moved as a single unit pivotably along a longitudinal axis extending along an upper corner of the propulsion module  14 , at the point indicated by reference numeral  44 . The highly complex collection of electrical wiring, fluid lines and thermal elements leading from each of the components  24 - 38  on the outer payload panel  12   b  is collected and routed to form a harness  46  that extends along the longitudinal edge  40  of the outer payload panel  12   b . Thermal coupling is by means of bolted interfaces and flexible heat pipes to enable folding movement, thus precluding the need to cut the heat pipes to allow movement of the modules  12 - 18 . The harness  46  electrically couples the components  24 - 38  to various other components on the central payload panel  12   a , as well as to other ones of the modules  12 - 18 . Importantly, the routing of the wiring harness  46  permits unimpeded folding movement of the outer payload panel  12   b  between its assembled and service positions. 
     With brief reference again to FIG. 1, it can be seen that a second harness  47  is preferably included for facilitating pivoting movement of the bus electronics module  16  while allowing this module to remain operatively coupled to other ones of the modules  12 - 18 . While only two major harnesses  46  and  47  are shown, it will be appreciated that additional harnesses could be employed if needed to help facilitate removal of other modules  12 - 18  from the spacecraft  10 . 
     It will also be appreciated that the harnesses  46  and  47  could be replaced by a larger plurality of smaller harnesses. For example, harness  46  in FIG. 3 could be replaced by a plurality of smaller harnesses  46 ′ which extend directly to suitable connectors associated with the central payload panel  12   a . This arrangement would eliminate the need to route all of the electrical conductors to one end of the outer payload panel  12   b . It will be appreciated that any arrangement of harnesses that permits folding movement of the modules  12 - 18  is contemplated as being within the scope of the present invention. 
     When folding movement of one or more of the modules  12 - 18  is required to obtain access to its components, it will be appreciated that conventional frame handling tooling, such as components  12   a ′ and  12   b ′, shown in FIG. 3, will preferably be employed. These components are rigid, frame-like members made from aluminum or another suitably strong material and attach securely at suitable attachment points on the payload panels  12   a  and  12   b  with bolts or other conventional fastening elements. The two frame handling tools  12   a ′ and  12   b ′ are coupled together by hinging mechanisms  13  which permit and control movement of the outer payload panel  12   b  to allow the panel  12   b  to be pivoted into the position shown in FIG.  3 . It will be appreciated that other such frame handling tooling will need to be attached to the other modules  12 - 18  to facilitate convenient pivoting movement. While only two hinges  13  have been shown in simplified form, it will be appreciated that a greater number of hinges may need to be employed to adequately support the weight of the outer payload panel  12   b  during pivotal movement thereof. 
     With further reference to FIG. 3, the entire removal of outer payload panel  12   b  from the remainder of the spacecraft  10  may be accomplished by providing a suitable plurality of connectors, indicated by rectangular box  48 , preferably at an intermediate point along the harness  46 . Connectors  48  are comprised of simple, commercially available electrical and mechanical connectors which can be quickly and easily manually separated to effect removal of the entire payload panel  12   b  from the remainder of the spacecraft  10  if needed. It will be appreciated, however, that other points of attachment could be employed for the harness  46 . For example, harness  46  could be secured by suitable connectors to mating connectors mounted on the central payload panel  12   a , rather than at an intermediate point along the length of harness  46 . Other attachment points will be apparent to those of ordinary skill in the art, the only limitation being that the harness  46  still permits full, unimpeded folding movement and quick separation of its associated panel  12   b . This feature permits the entire outer payload panel  12   b  to be removed from the spacecraft  10  for service and/or testing, if needed, without requiring significant disassembly of any other module  12 - 18  or wiring, fluid carrying elements or thermal elements associated with other modules  12 - 18  of the spacecraft  10 . 
     It will be appreciated that the above-described feature provides a number of significant advantages over present day spacecraft designs which do not permit for rapid access to and/or removal of various subsystems without a significant degree of disassembly being required of the spacecraft  10 . The ability to quickly access and remove, if necessary, an entire functional module  12 - 18  without undertaking massive disassembly efforts of numerous other subsystems of the spacecraft  10  provides for a significant savings in the man hours needed to service, test and repair the various and complex components associated with each functional module  12 - 18  of the spacecraft  10 . Accordingly, disassembly procedures that previously took days or months to perform on previous spacecraft designs can be performed often in several hours on the spacecraft  10 . 
     Referring further to FIG. 3, to assist in providing structural rigidity to the spacecraft  10 , a plurality of bulkheads  52  and  54  may be provided. If used, bulkheads  52  may be attached fixedly to portions of the central payload panel  12   a  while bulkheads  54  may be attached fixedly to portions of the bus electronics module  16 . The bulkheads  52  and  54  engage one another when the bus electronics module  16  is in its assembled position and form a means for providing additional structural support to the various modules  12 - 18  when the spacecraft  10  is in its fully assembled position. 
     It will also be appreciated that outer payload panel  12   c  could also be configured for folding movement relative to the central payload panel  12   a  if desired. This is illustrated by the alternative embodiment  10 ′ of the spacecraft shown in FIG.  4 . This folding movement of outer payload panel  12   c  could further be accomplished with little or no modification to the spacecraft  10 . Outer payload panel  12   c  could also be formed to be completely removable from the spacecraft in a manner similar to that illustrated for outer payload panel  12   b . The principal requirements would be a harness, such as harness  46 , with suitable connectors enabling the outer payload panel  12   c  to be quickly separated from the remainder of the spacecraft  10 . It will be appreciated that with this embodiment additional tooling may be required to further enhance the structural integrity of the spacecraft  10  during movement of the payload panels  12   b  and  12   c , and further to better handle the weight of the outer payload panels  12   b  and  12   c  when these panels are in their service (i.e., extended) positions. 
     While the central payload panel  12   a  and the bus electronics module  16  have not been described as being completely readily removable from the spacecraft  10 , it will be appreciated that such could be easily accomplished with suitable harnesses and connectors such as harness  46  and connectors  48 . 
     Referring now to FIG. 5, the payload panels  12   a ,  12   b  and  12   c , the propulsion module  14 , the bus electronics module  16  and the bulkheads  52  are shown in highly simplified form without the various components that would ordinarily be mounted thereon to illustrate the points where the modules  12 - 16  may be attached to one another through suitable quick disconnect mechanical fastener assemblies. Outer payload panel  12   b  is preferably secured along longitudinal edge  40 , along ends  60 , along longitudinal edge  62  and further along longitudinal sections  64  coinciding with the vertical edges  66  of the bulkheads  52  by a plurality of quick disconnect mechanical fasteners  68 . Fasteners  68  may comprise any suitable form of quick disconnect type of connector forming a first element which is adapted to releasably engage with an associated second fastening element. For simplicity, fasteners  68  have been illustrated as circular dots. 
     With further reference to FIG. 5, the fastening elements  68  are secured to fastening elements  70 , which are represented in highly simplified form by small squares. Fastening elements  70  represent second fastener components that can be quickly and easily secured to the fasteners  68 . The propulsion module  14  may also be secured about substantially its entire periphery by the interconnection of fastening elements  68  disposed along edge  72  of the central payload panel  12   a  and fastening elements  70  disposed on the periphery of the propulsion module  14 . Outer payload panel  12   c  may be similarly secured to vertical edges  74  of the bulkheads  52  by the connection of fastening elements  68  and  70 . The bus electronics module  16  is preferably secured along its longitudinal edge  76  by the connection of fastening elements  70  disposed along edge  76  and fasteners  68  disposed along edge  62  of outer payload panel  12   b . The bus electronics module  16  may also be secured by fasteners  68  connecting with fasteners  70  placed along an upper edge  80  of an endmost one of the bulkheads  52 , and possibly also along an upper edge  82  of the propulsion module  14 . Edge  78  of the bus electronics module  16  may also be secured to edge  69  of outer payload panel  12   c  via an additional plurality of fasteners  68  and  70 . 
     It will be appreciated that the placement of the fasteners  68  and  70  shown in FIG. 5 is merely representative of one arrangement by which the modules  12 - 16  can be mechanically interconnected with one another to form a highly rigid structure that can still be disassembled quickly for service and/or testing. Accordingly, the preferred embodiments of the present invention are not limited to any specific layout or arrangement of fastener elements  68  and  70 . Rather, any arrangement which holds the various modules  12 - 18  securely to one another, when fully assembled, and which still permits quick, convenient access to the fasteners  68  and  70  is contemplated as being within the scope of the present invention. 
     It will be appreciated that one or more suitable mechanisms could be employed for simultaneously releasing the fasteners  68  from fasteners  70  disposed along vertical edges  66  of the bulkheads  52  so that removal of the heat radiators  12   b   1  and  12   b   2  is not needed to gain access to fasteners  68 . Such a structure could be formed by, for example, a plurality of elongated rods each extending underneath the radiator  12   b   2  and parallel to an associated one of the bulkheads  52 , which can each be rotated about its longitudinal axis to engage each of the fasteners  68  along one vertical edge  66  simultaneously when it is turned. Alternatively, a rod could be disposed along and parallel to longitudinal edge  40  of payload panel  12   b  which cooperates with a plurality of rod-like elements disposed adjacent each vertical edge  66  to simultaneously release fasteners  68  from the bulkheads  52 . This would enable the payload panel  12   b  to be released from the bulkheads  52  without first requiring the heat radiators  12   b   1  and  12   b   2  to be moved a predetermined amount for fastener access. 
     It is anticipated that an alternative preferred embodiment of the present invention could comprise a frame structure to which each of the modules  12 - 16  could be attached with a lesser plurality of mechanical fastener components. Thus, the drawing of FIG. 5 should be understood as being merely one preferred example of how the various modules  12 - 16  may be releasably secured to one another to permit quick and easy assembly and disassembly of the spacecraft  10 . 
     Referring now to FIG. 6, a supporting tool  86  is shown for supporting the spacecraft  10  to facilitate convenient service and testing thereof. The supporting tool  86  is adapted to be secured directly to portions of opposing frame handling end panels  88  (only one being shown in FIG. 6) via an inverted “U”-shaped upper frame member  90 . Upper frame member  90  is in turn supported on lower frame elements  92 , which are in turn supported on base assemblies  94 . The base assemblies  94  have casters  96  or other mechanisms for permitting the entire supporting tool  86  to be moved as needed. In this manner, the entire spacecraft  10  can be supported in a convenient manner to permit ready access, testing and servicing on all areas of modules  12 - 18 . 
     Referring now to FIG. 7, the propulsion module  14  will be described in greater detail. As explained previously herein, the propulsion module  14  comprises a functionally independent component that can be readily removed from the spacecraft  10  by uncoupling electrical and mechanical connectors. This permits the propulsion module  14  to be accessed much more conveniently for service or testing. It also permits the propulsion module  14  to be completely removed from the spacecraft  10  with minimal disassembly/disconnection of connecting elements, and modified if needed to suit the needs of a particular application. 
     The propulsion module  14  includes a frame member  102  upon which is mounted a plurality of axial control thrusters  104 , a first plurality of attitude control thrusters  106  and a second plurality of attitude control thrusters  108  (also visible in FIG.  2 ). A tank  110  containing propellant, which may comprise Xenon propellant, is also mounted to the frame  102  for supplying propellant to the thrusters  104 ,  106  and  108 . In a preferred embodiment of the propulsion module  14 , the frame member  102  comprises a pair of aluminum frame sheets between which is disposed a honeycomb panel to provide a relatively lightweight yet very structurally rigid and strong frame member  102 . Locating the tank  110  on the inboard side of the frame member  102  also provides the advantage of protecting the tank  110  from damage caused by debris floating in orbit. The axial control thrusters  104  may comprise well known Hall thrusters. These thrusters are principally responsible for propelling the spacecraft  10  from drop off orbit to the operational orbit, but can just as readily be used for orbit maintenance purposes or plane changes. 
     Attitude control thrusters  106  and  108  are used principally for controlling the attitude (i.e., pitch, yaw and roll) of the spacecraft  10 . Thrusters  106 , in one preferred embodiment, comprise well known Resistojet thrusters. Thrusters  108 , in one preferred embodiment, may comprise cold gas thrusters. 
     A principal advantage of providing all of the thrusters  104 ,  106  and  108  on one propulsion module  14  disposed at one end of the spacecraft  10  is that the routing of lengthy and complex fluid carrying tubing all over the spacecraft  10  is eliminated. This significantly simplifies and expedites movement and removal of the various other modules of the spacecraft  10 . Just as importantly, the propulsive gasses from the thrusters  104 ,  106  and  108  are all directed such that they are not blown on any portion of the spacecraft  10 . 
     Referring to FIG. 8, the spacecraft  10  is illustrated in its fully deployed orientation. Outer payload panel  12   c  can be seen to include a heat radiator  12   c   1 . Deployment mast  22   c  assists in erecting and supporting solar array panels  22   a  and  22   b  in their fully deployed positions. 
     The spacecraft  10  of the present invention thus forms a fully modular spacecraft vehicle which allows rapid and convenient access to the numerous components on each of modules  12 - 18  thereof. The ability to move various modules in a folding manner between assembled and service positions to gain access to the components on each module, and completely remove a module if necessary, without requiring days or even weeks of complicated disassembly procedures, allows testing and service operations to be performed quickly and with significantly less cost to the spacecraft manufacturer. The fully modular design of the spacecraft  10  further permits integration and retrofitting of modified modules should the specifications and/or performance requirements of the spacecraft  10  need to be changed to suit the demands of a specific mission. 
     The fully modular design further permits parallel manufacturing and testing of the various modules  12 - 18  of the spacecraft  10 . By “parallel” manufacturing it is meant that the independent modules  12 - 18  could be constructed and tested at different locations and then brought to one or more common assembly locations to be assembled, as a final phase of the assembly process, to form the spacecraft  10 . This feature significantly increases efficiency in manufacturing the spacecraft  10 , in addition to significantly reducing the design-to-market cycle time for derivative spacecraft. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.