Abstract:
A spacecraft having a primary structural frame and a propellant tank, in which the spacecraft may include a tank mount adopted to engage a portion of the propellant tank, the tank mount being configured to transfer launch loads directly from the propellant tank to a lunch vehicle interface ring.

Description:
BACKGROUND 
     The present disclosure is directed spacecraft and, more particularly, to spacecraft propellant tank mount systems. 
     It is desirable to minimize the mass or weight (i.e., the force exerted on a mass by Earth&#39;s gravity) of a spacecraft. Each kilogram of spacecraft mass that is to be launched into Earth orbit may require as much as 10 kilograms of fuel. Accordingly, it is desirable to minimize spacecraft mass in order to reduce the overall cost and fuel requirements of a launch vehicle. Further, by reducing the mass of the spacecraft propulsion structure, more available spacecraft mass may be devoted to instrumentation and other spacecraft payload. One mechanism for reducing spacecraft mass is to eliminate any unnecessary structure from the spacecraft. 
     These considerations apply to spacecraft powered by electric propulsion systems, as well as to spacecraft that may employ chemical, or hybrid propulsion systems. Currently, spacecraft of the type having a central thrust tube design require internal propellant tanks that match the tube diameter to transfer launch loads to the cylindrical wall of the central thrust tube with minimal bending. A disadvantage with such a design is that it may require a custom tank design to match the inside diameter of the central thrust tube. Further, such a design may not work well with electric propulsion spacecraft because the size of the propellant tank required by a mission may be less than what would be required to fill the interior of the central thrust tube. Accordingly, there is a need for a spacecraft having a propellant tank mount that may be used in conjunction with a central thrust tube, but does not need to match the interior diameter of the central thrust tube. 
     SUMMARY 
     The disclosure is directed to a spacecraft having a primary structural frame and a propellant tank, the spacecraft including a tank mount adapted to engage a portion of the propellant tank, the tank mount being configured to transfer launch loads directly from the propellant tank to the launch vehicle interface ring. In one aspect, the propellant tank mount includes a conical shell having a first end adapted to engage an end of a propellant tank, and a second end adapted to engage a launch vehicle interface ring. In another aspect, a method of mounting a propellant tank to a spacecraft having a primary structural frame includes providing a tank support adapted to engage a portion of the propellant tank, the tank support being configured to transfer launch loads directly from a propellant tank to a launch vehicle interface ring. 
     An advantage of embodiments of the disclosed spacecraft, propellant tank mount and method is that the propellant tank may be supported independently of the central thrust tube of the spacecraft. Consequently, the spacecraft and mount may accommodate propellant tanks of a variety of shapes and diameters. The shape and diameter of the propellant tank need not be dictated by the inside diameter of the central thrust tube. Another advantage of embodiments of the disclosed spacecraft, propellant tank mount and method is that the launch load of the propellant tank may be transferred directly from the propellant tank to the launch vehicle interface ring, and not borne by the central thrust tube of the spacecraft. 
     The disclosed design may provide a mass-efficient solution because the propellant tank load (i.e., the force exerted by the mass of the propellant tank during launch, and when the launch vehicle is accelerating, as a result of acceleration of the launch vehicle and spacecraft) may bypass the spacecraft&#39;s primary structure. This may enable use of a simplified and relatively lighter primary structure, so that a larger portion of the available mass of the spacecraft may be allotted to instrumentation and other spacecraft payload. 
     Other objects and advantages of the disclosed structure will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation in section of an embodiment of a spacecraft incorporating the disclosed tank mount, shown mounted in a launch vehicle fairing; 
         FIG. 2  is a side elevation in section of a second embodiment of a spacecraft incorporating the disclosed tank mount, shown incorporated in a larger launch vehicle fairing; 
         FIG. 3  is a perspective view of the disclosed tank support of  FIG. 1 ; 
         FIG. 4  is a side elevation in section of disclosed tank support structure of  FIG. 1 ; and 
         FIGS. 4A ,  4 B,  4 C, and  4 D are enlarged detail views, in section, of portions of the tank support structure shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIG. 1 , the disclosed spacecraft, generally designated  10 , may include a primary structural frame that may be in the form of a cylindrical central thrust tube  12  that extends substantially the entire length of the spacecraft. The thrust tube  12  also may support stiffener panels  14 , solar wing drives  16  and thrusters  18 . Thrusters  18  may be used for attitude control and/or moving the spacecraft  10  to a different orbit. The solar wing drives  16  may support solar panels  20 . 
     The thrusters  18  may be include an electric propulsion units connected to a propellant tank  22 . In embodiments, the thrusters  18  may be gridded electrostatic ion thrusters, or Hall effect thrusters. The propellant tank  22  may contain xenon gas propellant under pressure and may be dimensioned to be spaced from the inner surface  24  of the central thrust tube  12 . Although shown in  FIG. 1  as having a cylindrical shape, in embodiments the propellant tank  22  may be spherical, elliptical or oval in shape. In an embodiment, the propellant tank  22  may be a metallic pressure vessel with a composite overwrap reinforcement. In embodiments, the propellant tank  22  may be made of aluminum or titanium, and may or may not have overwrap reinforcement. 
     As shown in  FIGS. 1 and 3 , the spacecraft  10  may include a tank mount, generally designated  26 . The tank mount  26  may serve to attach the spacecraft  10  to the base  28  of a launch vehicle  30 , so that the spacecraft  10  may be positioned within the payload region  32  of the launch vehicle. The tank mount  26  may include a conical shell  34  that may be made of a lightweight, strong composite material. In an embodiment, the material may include graphite or carbon fiber and may have a honeycomb structure. In other embodiments, the conical shell  34  may be made of metal, such as titanium, steel or aluminum alloy. The conical shell  34  may be attached to a launch vehicle interface ring  36 , which may be part of the base  28  of a launch vehicle  30 . 
     At an opposite end of the propellant tank  22 , the tank mount  26  may include a forward tank support panel  38 . The forward tank support panel  38  may be disk-shaped and sized to engage the inner periphery  24  of the central thrust tube  12 . The forward tank support panel  38  may be a solid disk, as shown, or may have voids to reduce weight. The forward tank support panel  38  may be attached to the propellant tank  22  by a pivotal mount  40 , such as the monoball bearing axial slip joint shown. Other types of pivotal mounts may be employed. The forward tank support panel  38  may be attached to the inner periphery  24  of the central thrust tube  12  by welding, brazing, adhesives or other means. 
     As shown in  FIGS. 4 and 4A , the propellant tank  22  may include an axially extending forward tank boss  42  that may extend through and is captured by the monoball bearing joint  40 . The monoball bearing joint  40  may be attached to the forward tank support panel  38  by fasteners such as bolts  44 . In other embodiments, the monoball bearing joint  40  may be attached to the forward tank support panel  38  by a suitable adhesive, by welding, by rivets, or a combination of the foregoing. The monoball bearing joint  40  may be made of metal, such as an aluminum alloy or titanium. 
     As shown in  FIGS. 4 and 4B , the upper end  44  of the conical shell  34  may be attached to a cap  46  that may be made of a hardened material such as titanium or other metal. The attaching mechanism may be by adhesives, or bolts  48  as shown in  FIG. 4B . As shown in  FIGS. 4 and 4C , the cap  46  may include a pivotal mount  50 , such as the moment-free monoball bearing mount shown. Monoball bearing mount  50  may receive an aft tank boss  52  of the propellant tank  22 . The aft tank boss  52  may be attached to the propellant tank by screws  54  and may include an adapter tube  56  that extends through and is captured by the monoball bearing mount  50 . The tube  56  may be hollow and shaped to receive an outlet tube  57  of the propellant tank  22 . In one embodiment, the tube  56  may be slidable relative to the monoball bearing mount to allow for expansion and contraction of the propellant tank  22 , and accommodate any out-of-tolerance conditions. Similarly, the forward tank boss  42  ( FIG. 4A ) may be slidably retained by the monoball bearing slip joint  40 . In embodiments, both joints  40  and  50  may allow axial (i.e., in the direction of the longitudinal axis of the spacecraft  10 ) and pivotal movement of the propellant tank  22  relative to the spacecraft  10 , central thrust tube  12  and conical support  34 . 
     As shown in  FIGS. 4 and 4D , the lower end  58  of the conical shell  34  may be attached to the launch vehicle interface ring  36  by bolts  60  that extend through the lower end and through tabs  62  formed on the interface ring  36 . As shown in  FIG. 4D , the interface ring  36  also may include an angular slot  64  shaped to receive the bottom of the central thrust tube  12  ( FIG. 1 ), and the joint may be secured by means such as an adhesive, welding or brazing, mechanical fasteners such as screws (not shown), or a combination of the foregoing. 
     As shown in  FIG. 2 , a spacecraft  10 ′ may include a central thrust tube  12 ′ that is flared outwardly at the bottom of  66  to accommodate a greater thrust load, in the event that the central thrust tube  12 ′ may support the thrust tube  68  of a second spacecraft (not shown) to be launched in tandem with the spacecraft  10 . In this embodiment, the propellant tank  22  may be supported in a similar fashion as that described with reference to  FIG. 1 , except that the conical shell  34 ′ may be shaped to flare outwardly at a greater angle than shell  34  engage a larger interface ring  70 . 
     The disclosed spacecraft  10 ,  10 ′ and tank mount  26 ,  26 ′ provide a low-cost mounting system that may transfer launch loads from the lower propellant tank nozzle  52  through the conical shell  34  and to the launch vehicle interface ring  36 . Therefore, the launch load of the propellant tank  22  may be conveyed directly to the interface ring  36  without transferring a load to the central thrust tube  12 . Because the connection between the propellant tank  22  and the forward tank support panel  38  is by way of a slip joint  40 , there is not thrust load transmitted to the central thrust tube  12  at that location. Thus, the entire thrust load of the propellant tank may be borne by the interface ring  36  and not the structural frame of the spacecraft  12 ,  12 ′. Further, because the propellant tank is attached to the spacecraft  12 ,  12 ′ at its upper and lower ends by boss  42  and nozzle  56 , the support system will accommodate a variety of propellant tank dimensions and diameters. 
     While the forms of apparatus and method described herein may constitute preferred embodiments of the spacecraft and propellant tank mount system, it is to be understood the invention is not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention.