Abstract:
An active satellite dispenser is preferably attachable to a reusable launch vehicle for deployment of one or more satellites into one or more desired orbits. The active satellite dispenser includes a center mast that releasably receives the satellite(s), a liquid propellant rocket, and an orbital control system on an avionics pallet. In the preferred embodiment, a pressurized gas selectively pressurizes the propellant tanks (which may include fuel and oxidizer tanks), to provide propellant to the rocket. In operation, the launch vehicle releases the satellite dispenser in a first deployment orbit. The active dispenser rocket and orbital control system then transport the active dispenser and satellite(s) into the final deployment orbit. In the preferred embodiment the active dispenser can operate multiple times to place individual satellites in different orbits.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/285,908, filed Apr. 23, 2001, the benefit of which is hereby claimed under 35 U.S.C. §119. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to hardware for the launch, transportation, transfer, and release of satellites, cargo, and other payloads in earth orbit. More particularly, the present invention relates to a satellite dispenser providing propulsion and avionics systems for the controlled release of multiple satellites  
         BACKGROUND OF THE INVENTION  
         [0003]    The transportation of cargo to space is expensive. The high cost is partially attributable to the aggregation of multiple, costly individual operations that are needed to transport and deploy satellites and other cargo to orbit. The latter part of the orbital transportation operation, i.e., transporting the payload from low earth orbit to the desired final higher orbit, is typically accomplished by an upper, or final, stage. This stage is ignited and transports the payload to the final orbit. Sometimes this stage remains attached to the payload, satellite, or cargo; but it is usually discarded in orbit. Upper stages vary in their capability of independent actions from a passive, simple, small, solid rocket motor just providing additional propulsion capability, which goes in the direction it is pointed when ignited, to very capable, restartable, liquid rocket engines capable of multiple restarts and significant maneuvering using thruster(s) and related avionics. Some active upper stages are able to dispense multiple satellites.  
           [0004]    Launch vehicles used to deploy satellites in a predetermined orbit are evolving from expendable, and partially expendable rockets, toward fully reusable launch vehicles (RLVs) capable of multiple reuse. The primary advantage of RLVs is the potential for significant cost savings. RLVs frequently utilize active satellite dispensers and upper stages. Satellite launch systems still include a device that couples the satellite to the launch vehicle; but in today&#39;s world, this dispensing hardware takes on a more active role than the simple release of the satellite in response to a control signal. Recent reusable vehicle advances, technological improvements, and commercial market forces are moving the satellite industry toward smaller, more sophisticated commercial networks of communications satellites requiring multiple satellites in various orbital planes around earth.  
           [0005]    The increased demand for communication satellite networks and higher orbits has given rise to active, more capable, customer-sensitive satellite dispensers that satisfy the new privately-financed communication network markets. The new commercial networks require innovative satellites of different size, weight, and transportation requirements. Existing satellite dispensers lack the flexibility and cost effectiveness of reusable vehicles. Moreover, existing satellite dispensers lack subsystem commonality, propellant compatibility, and/or the ability to adapt to the emerging reusable launch vehicles. Existing satellite dispensers fail to adequately take advantage of the reusable vehicles&#39; reusable shrouds and orbital satellite release techniques.  
           [0006]    Previous dispensers stack multiple satellites vertically, and therefore typically employ discardable shrouds that can clutter the orbital environment with spent hardware, which can endanger other space operations. Shortly, international agreements are expected to limit the placement of satellites to those systems that minimize the debris impact on others.  
           [0007]    U.S. Pat. No. 5,568,901 to Stiennon, entitled “Two Stage Launch Vehicle and Launch Trajectory Method,” is purported to be a fully reusable launch vehicle, but fails to mention anything of a satellite dispenser.  
           [0008]    The space shuttle is a partly-reusable vehicle and has several expendable satellite assist modules, or third stages, but at significant cost and requiring extensive hardware dispensers specific to the shuttle payload bay and, in some cases, requiring the changing of the axial orientation of the satellite by a significant amount or to exit the payload bay via a Frisbee®-throwing method.  
           [0009]    U.S. Pat. No. 5,242,135 to Scott et al., entitled “space Transfer Vehicle and Integrated Guidance System,” is a space transfer system capable of continuing the propulsive cycle of earlier stages, but lacks the full service structure system to handle the new generation of communications satellites. No provision for both forward and sideways satellite ejection appears to be available from this patent. No shroud reuse or any reuse of any hardware, except the space transfer vehicle, is implied. No provision in the hardware is evident for disposal at end of life.  
           [0010]    U.S. Pat. No. 5,884,866 to Steinmeyer et al., entitled “Satellite Dispenser,” is a dispenser for the 1990s, but has no propulsion or other active subsystems. The post can accommodate the newer network communication satellites, but it appears the entire upper stage of the poorly defined launch vehicle must be taken to each individual orbit for satellite dispensing.  
           [0011]    U.S. Pat. No. 5,337,980 to Homer et al., entitled “Spacecraft-to-Launch-Vehicle Transition,” is more a structural connection between a vehicle and a satellite, rather than a dispenser. The drawings seem to depict a rocket engine, but it appears to be a part of the satellite. The adapter and/or structural hardware appears to have no active dispensing function except a release capability.  
           [0012]    U.S. Pat. No. 5,199,672 to King et al., entitled “Method and Apparatus for Deploying a Satellite,” is a solid rocket version of a dispenser designed to place many small satellites into a specific orbital plane and focuses on the specific design to accommodate the Pegasus. The four separate pallets are fired into different orbital planes by waiting to fire the solid rocket motors at different times as the earth turns below.  
           [0013]    U.S. Pat. No. 5,816,539 to Stotelmeyer et al., entitled “Integrated Storage and Transfer System and Method for Spacecraft Propulsion Systems,” is an orbital assist module (OAM) built into the stack of an expendable launch vehicle (ELV). The orbital assist module propellant comprises hydrazine and is difficult to handle.  
           [0014]    In contrast, the present invention uses propellants, preferably liquid propellants, used in a single, pressure-feed engine that preferably utilizes components having commonality with the reusable launch vehicle. This hardware commonality in engine, main propellants, pressurants, radiation-hardened avionics, and attitude control systems (ACS), including the propellant used in the ACS tanks, reduces the testing, procurement, ground handling, support systems, and-most importantly-the cost. The propellants used also lend themselves to increased ground safety and cost-effective operations within the present invention.  
           [0015]    Although the present invention will be described with reference to a preferred embodiment that is designed to be used with the K-1 reusable launch vehicle system, which includes a launch assist platform and an orbital vehicle (hereinafter, the K-1 orbital vehicle, or the like) currently under development by Kistler Aerospace Corporation, it will be appreciated by one of ordinary skill in the art that the invention can be readily adapted for use with other launch systems without departing from the spirit and scope of the disclosed invention.  
         SUMMARY OF THE INVENTION  
         [0016]    A primary object of the active dispenser is to transfer the satellite to a higher orbit and position it in the proper orbital location in a cost-effective manner.  
           [0017]    An advantage of the present invention is the cost-effective combination of a reusable launch vehicle subsystem, including efficient propulsion subsystems, pre-engineered common computer avionics, propellant, pressurant, and attitude control thruster hardware subsystems, and a dispenser for multiple satellites.  
           [0018]    An advantage of the active dispenser on a reusable launch vehicle is the ability to integrate commonality with subsystems used on the launch vehicle and/or satellite hardware providing cost-effective common subsystems through commonality in design, procurement, testing, and propellant loading.  
           [0019]    Another advantage of the invention is the ability to integrate common ground handling techniques, technical maintenance, financing, and ownership of the active dispenser, launch vehicle, and satellites.  
           [0020]    Another advantage of the invention is an integrated commercial, satellite delivery operation that all works together in a cohesive manner to accomplish the ground processing, launch, transfer to the proper orbit, release of the satellite, and potential follow-on support.  
           [0021]    Another advantage of the active dispenser is an integrated design, flexible enough to be capable of accommodating on a general active dispenser, a number of different satellites from numerous organizations with varying requirements, different weights, various final orbits, different ground processing requirements, and varying financial needs.  
           [0022]    Another advantage of the invention is the increased energy efficiency in the use of liquid propellants, including the liquid storable propellants, the decreased cost in simplifying the ground handling by using the storable propellants, increased flexibility from the restartable orbital propulsion, and the increased reliability and simplified handling from the common, pre-tested hardware and propellant systems.  
           [0023]    Another advantage of the invention is a single, pressure-fed, restartable, liquid bipropellant main engine located at the base of the active dispenser, which fires to propel the active dispenser into its proper orbit.  
           [0024]    Another advantage of the active dispenser is a reusable, conical payload adapter at the base of the dispenser and a clampband that attaches the K-1 vehicle or other vehicles to the active dispenser.  
           [0025]    Another advantage of the invention is the K-1 orbiter or other launch vehicle, which opens its payload module dome in orbit and deploys either vertically or horizontally everything above the clampband into space.  
           [0026]    Another advantage of the invention includes the various satellites with different shapes that are able to be attached to the support mast using adaptable structural interfaces.  
           [0027]    Another advantage of the invention is the small thrusters mounted to the active dispenser used for attitude control, a pallet for avionics hardware, and gas storage bottles for pressurization and attitude control.  
           [0028]    Another advantage of the invention includes two similar tanks (one or more for fuel and one or more for oxidizer) mounted on either side of the support mast, which connects the engine to the propellant via feed lines.  
           [0029]    Another advantage of the invention is the propulsion system design using hardware common to the K-1.  
           [0030]    Another advantage of the invention is that the propellant tanks are pressurized by gas storage bottles filled with pressurant gas.  
           [0031]    Another advantage of the invention is an active dispenser avionics system, which uses the radiation-hardened hardware for control as the K-1 orbital vehicle.  
           [0032]    Another advantage of the invention is the avionics system, which uses a triplex computing architecture with three cross-strapped 1553 avionics databases and a hardware voter.  
           [0033]    Another advantage of the invention is the electronics power, which is triply redundant, and the utility power, which is dual redundant.  
           [0034]    Another advantage of the invention is the flight software, which is a modified version of the software used on the K-1 orbital vehicle.  
           [0035]    Another advantage of the active dispenser, in a nominal mission mated with one or more customer satellites, is its being processed, handled, fueled, and mated to the K-1 payload module in the K-1 payload processing facility, using the same hardware, consumables, and staff already on hand for other similar hardware on the existing reusable launch vehicles.  
           [0036]    Another advantage of the invention is that the orbital vehicle deploys the active dispenser and satellites after it reaches a nominal 200 km circular orbit at the desired inclination, depending on the mission.  
           [0037]    Another advantage of the invention is achieved after adequate clearance between the active dispenser and orbital vehicle is achieved and the active dispenser performs a series of burns used to place the satellites into their desired final orbits, without further assistance form the launch vehicle.  
           [0038]    The active dispenser places satellites into higher orbits than the host launch vehicle. This third stage is cost-effective because it combines the advantages of a reusable launch vehicle with the flexibility and common subsystems.  
           [0039]    In a nominal mission, the active dispenser is mated with the customer&#39;s satellite in an off-line vehicle processing operation, including propellant loading, and mated to the K-1 payload module in the K-1 payload processing facility. The launch vehicle deploys the active dispenser and satellites after it reaches a higher circular orbit at the desired orbital inclination. The active dispenser initiates a series of burns to place the satellites into their desired final orbits.  
           [0040]    Other objects, advantages, and novel features, and further scope of applicability will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, FIGS.  1 - 5 , and, in part, will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the inventions particularly pointed out in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0042]    [0042]FIG. 1 is the active dispenser;  
         [0043]    [0043]FIG. 2 is the active dispenser propulsion system;  
         [0044]    [0044]FIG. 3 is the active dispenser attached to the Kistler K-1 reusable launch vehicle;  
         [0045]    [0045]FIG. 4 is the mission profile for the active dispenser; and  
         [0046]    [0046]FIGS. 5A and 5B are second configurations of the active dispenser designed to launch a different satellite. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0047]    The active dispenser relates to enhance existing and emerging reusable launch vehicle (RLV) technology and its use in the satellite transportation industry. The present invention provides a more cost-effective transportation cycle capable of placing cargo into orbits beyond the capability of typical existing RLVs or expendable launch vehicle. Reference is now made to FIG. 1, which illustrates a preferred embodiment of the invention. FIG. 1 is a side view of an active satellite dispenser  18  for transporting satellites, payloads, or cargo to space. A launch vehicle—for example the K-1 reusable launch vehicle  20 —is attached at the bottom of the active dispenser  18 . It is contemplated that the launch vehicle may be a single-stage rocket vehicle, a two-stage launch vehicle, or multi-stage space vehicle, and may be either expendable after launch or reusable on future launches. A conical payload adapter  22  is used to attach the active dispenser  18  to the K-1 orbital vehicle  20 . A clampband  26 —a device capable of structurally connecting active dispenser  18  to conical payload adapter  22  and capable of being released upon command—is shown.  
         [0048]    The active dispenser  18  includes a structural frame or support mast  30 , at least one engine  32 , a plurality of attitude control system thrusters  34 , a pallet of avionics hardware  36 , pressurized gas storage bottles  38 , one or more tanks for fuel  40 , one or more tanks for liquid propellant  42  (i.e., oxidant), and feed lines  44 . The structural frame or support mast  30  of the preferred embodiment is a box-shaped, metal or composite support member for the attachment of one or more satellites  28 . In the disclosed configuration, the pallet of avionics hardware  36  and the gas storage bottles  38  are located inside the box of support mast  30 . FIG. 1 depicts a support mast  30  for two satellites  28  of a long, narrow configuration and requiring an extended payload module  47  (shown in fragmentary view in FIG. 1 and more completely in other figures). The extended payload module  47  preferably provides approximately 17 feet inside height. Other customer satellites may require other support structure configurations and may, for example, permit the use of only a shorter payload module  23 , for example, providing approximately 9 feet inside height.  
         [0049]    The active dispenser  18  may use either the shorter payload module  23  alone or with the extended payload module  47  (both shown in more detail in FIGS. 3 and 5). The payload modules  23 ,  47  provide ascent and re-entry protection. The shorter payload module  23  and/or the extended payload module  47  are covered by a payload module dome  24 , that cooperatively with the other components defines a protected enclosed volume that offers protection during flight and on the ground after the installation of the satellite  28  or other payloads. This complete payload module with satellite  28 , or other payload, is attached and detached from the launch vehicle or K-1 orbital vehicle  20  by payload module attachment bolts  49  between the payload module  18  and the orbital vehicle  20 . This “quick bolt-on” of the closed volume unit permits the user to decouple the entire customer payload preparation operation from the transportation space launch K-1 orbital vehicle  20 . This compartmentalizing function provides the ability to store the payload and/or satellite  28  in a payload module for extended periods of time apart from the launch vehicle  20  increasing the flexibility of the launch system. This is a departure from the traditional time-consuming method of stacking the payload on the launch vehicle while the assembly is on the launch pad and the integration procedures used in the space transportation system industry. The payload module  23  or extended payload module  47 , with the payload module dome  24 , is mated or integrated with the active dispenser  18  in a separate ground processing location and separate timeline from the ground processing of K-1 orbital vehicle  20 .  
         [0050]    It is contemplated that the active dispenser  18  will use, as much as possible, the pre-tested and cost effective subsystems of K-1 orbital vehicle  20 , or other host vehicle subsystems or, in some cases, subsystems from the satellite  28 , given satellite  28  requirements-including structural frame or support mast  30 , engine  32 , attitude control system thrusters  34 , pallet of avionics hardware  36 , gas storage bottles  38 , tank for fuel  40 , tank for liquid propellants  42 , and feed lines  44 .  
         [0051]    The main propulsion of the active dispenser  18  is preferably a single, restartable, liquid bipropellant rocket engine  32  located at the base of active dispenser  18 . The rocket engine  32  propels the active dispenser  18  using liquid propellants from tank for fuel  40 , tank for liquid propellants  42 , and feed lines  44  from each tank  40 ,  42  to the engine  32 , in order to place the active dispenser  18  with satellites  28  into the desired deployment orbit (see FIG. 4). The avionics pallet  36  and gas storage bottles  38 , and some other items, are mounted inside support mast  30  for additional protection. The avionics pallet  36  controls the smaller attitude controls system thrusters  34  of the active dispenser  18 . One or more tanks for fuel  40  and one or more tanks for liquid propellants  42  are located on either side of the support mast  30 , and feed lines  44  from each tank to the engine  32  are provided to connect propellant to the engine  32 . Liquid propellant is forced or pressure-fed through the feed lines  44  using pressurized gas from gas storage bottles  38 . The satellite(s)  28  are connected to support mast  30  via satellite interfaces  46 .  
         [0052]    It should be appreciated that upon deployment the active dispenser  18  and satellite(s)  28  are released from the payload module  23  and/or extended payload module  47 . It is contemplated, as shown in FIG. 4 and discussed below, that the payload module  23 ,  47  in the preferred embodiment, will then return to earth for reuse. This is an improvement over prior art systems wherein a shroud or similar payload enclosure typically is released in orbit to burn up on reentry.  
         [0053]    [0053]FIG. 2 depicts a close-up of a preferred subsystem layout within the active dispenser  18  with the support mast  30  removed for clarity, and some other support items shown. The avionics pallet  36  is derived from the K-1 orbital vehicle  20  avionics and controls the other components of the active dispenser  18 . It will be appreciated that the active dispenser  18  can be used on vehicles other than the K-1 orbital vehicle  20 .  
         [0054]    The avionics pallet  36  controls the gas storage bottles  38 , preferably by controlling a valve to selectively release the high pressure gas into the tank for fuel  40  and the tank for liquid propellants  42 , forcing liquid propellant through feed lines  44  to the engine  32 . The avionics pallet  36  also controls other subsystems, such as attitude control system thrusters  34 . It should be apparent that many of the components, such as pallet of avionics hardware  36 , gas storage bottles  38 , and the feed lines  44 , are protected by support mast  30 .  
         [0055]    [0055]FIG. 3 depicts the active dispenser  18  in the payload module  23 , with the extended payload module  47 . In orbit, the payload module dome  24  is released and moved or positioned out of the satellite  28  deployment path, which may be either sideways or forward. In the preferred embodiment, the payload module dome  24  opening operation occurs at active dispenser  18  deployment orbit  61  (see FIG. 4). This provides advantages over the convention exposure of the satellite during ejection of the shroud or nose covering altitude on the ascent of prior art launch vehicles. This later payload module dome  24  opening operation affords more protection for the satellite(s)  28 .  
         [0056]    In the preferred embodiment, the active dispenser  18  is connected to a conical payload adapter  22  with a clampband  26 , which is adapted to release the active dispenser  18  at a specified time in orbit, and with one or more (one shown) releasable attachment bolts  49 . At the desired time, the clampband  26  and the payload module attachment bolts  49  that attach the payload module  23  to the K-1 orbital vehicle  20  release the active dispenser with the satellite. It will be appreciated that, before launch, the high-pressure gases and liquid propellants are loaded into the tank for fuel  40 , tank for liquid propellants  42 , and gas storage bottles  38 . The feed lines  44  provide fluid flow paths generally to and/or from the tank for fuel  40 , tank for liquid propellants  42 , and gas storage bottles  38  to supply the rocket engine  32  (hidden, but shown in FIG. 2) with propellant. The attitude control system thrusters  34  are used to stabilize the active dispenser  18  after the separation from the conical payload adapter  22  until separation of the satellite  28 . The satellite(s)  28  are preferably connected to the support mast  30  through any conventional interface  46 , as are well known in the art.  
         [0057]    [0057]FIG. 4 depicts the typical mission profile for the active dispenser  18  inside K-1 orbital vehicle  20 , showing the orbital vehicle  20  and the active dispenser  18  at various sequential stages of deployment. The typical mission profile for the active dispenser  18  inside an alternative launch vehicle may vary slightly from that shown in  
         [0058]    [0058]FIG. 4. The transportation cycle typically starts with a satellite owner contracting with a launch vehicle operator or transportation provider. The satellite  28  and orbital vehicle  20  requirements are exchanged and agreed to by the parties. The satellite  28  then arrives at spaceport  50 , and is put into storage, if required. The satellite  28  is then integrated with the active dispenser and then attached to the K-1 orbital vehicle  20 . Alternatively, the satellite owner may provide the satellite pre-integrated with the active dispenser  18  and ready for attachment to the K-1 orbital vehicle  20 .  
         [0059]    In the preferred embodiment, the K-1 launch assist platform  48  launches with K-1 orbital vehicle  20  attached, including the active dispenser  18  and satellite(s)  28  from spaceport  50 .  
         [0060]    The two-stage, K-1 launch assist platform  48  achieves a predetermined trajectory and the entire vehicle moves to stage separation altitude  52 , where the K-1 orbital vehicle  20  separates from the K-1 launch assist platform  48 . The K-1 orbital vehicle  20  with active dispenser  18  and satellite(s)  28  proceeds to the main engine cut-off (MECO) altitude  54 . The K-1 orbital vehicle  20  then proceeds to the payload deployment altitude  61 . The payload module dome  24  is opened and the active dispenser  18  and satellite  28  are separated from K-1 orbital vehicle  20 .  
         [0061]    The K-1 launch assist platform  48  cuts its main engines, separates, reverses direction, and restarts its center engine for the return journey back to the original launch site in spaceport  50 . The K-1 launch assist platform  48  re-enters the atmosphere on flyback trajectory  56 , deploys parachutes at parachute deployment altitude  60 , and lands on air bags near the original launch site, ready for several weeks of refurbishment work to permit its reuse for a later launch.  
         [0062]    The K-1 orbital vehicle  20  with the active dispenser  18  and the satellite(s)  28  proceeds to a typically circular orbit at a desired inclination. The K-1 orbital vehicle  20  deploys the active dispenser  18  and satellite(s)  28  after attaining a desired nominal orbit, e.g., 200 km circular orbit, at the desired inclination, which is the active dispenser deployment orbit  62 . After adequate clearance is achieved between active dispenser  18  and K-1 orbital vehicle  20 , the active dispenser  18  initiates a series of burns using the rocket engine  32  to place the customer&#39;s satellite  28  into its desired final orbit. The attitude of the active dispenser  18  may be adjusted by the attitude control system thrusters  34 , whereby the satellite is placed in the desired orbit and orientation for deployment of the satellite(s)  28 . If multiple satellites are on-board, then the rocket engine  32  and attitude control system thrusters  34  may be used to place the additional satellites  28  in somewhat different orbits. In the preferred embodiment, the K-1 orbital vehicle  20  proceeds to an orbiter-phasing altitude and waits until the earth rotates back to a location permitting the K-1 orbital vehicle  20  to enter a desired re-entry  68  path and return to the original spaceport  50  or another spaceport.  
         [0063]    [0063]FIGS. 5A and 5B depict another configuration of the active dispenser  18 ′ using less host vehicle subsystems and more of the satellite  28 ′ subsystems, and requiring more payload weight launch. The active dispenser  18 ′ preferably still uses the K-1 orbital vehicle  20  as the host vehicle, but the active dispenser  18 ′ configuration changes structural, propellants, and weight capability to accommodate different satellite(s)  28 ′.  
         [0064]    With a differently-shaped satellite  28 ′, active dispenser  18 ′ evolves into a different structure for support mast  30 ′ and a different solution for conical payload adapter  22 ′ by changing the structure to a square structure frame with stabilization struts  74  and thrust cone and spacecraft adapter interface ring  76  due to the requirements of a different shape and type of satellite  28 ′. The square structure frame with stabilization struts supports a large communication satellite within extended payload module  47  from the bottom structural interfaces or payload module interface at the bottom of extended payload module  47  to thrust cone and spacecraft adapter interface ring  76 , which supports satellite  28 . Also supported on the square structure frame with stabilization struts  74 , are ACS thruster mounts  78 , helium pressure bottle  82 , main propellant tank  84 , and an engine  86 . The active dispenser  18 ′ is protected by multiple layer aluminized Mylar heat shield  88 , partly shown in FIG. 5, and opening and closing payload module dome  24 .  
         [0065]    The preceding examples are repeated with similar success by substituting the generically- or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.  
         [0066]    While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.