Abstract:
A thruster for providing thrust for spacecraft positioning, which has a propellant reservoir for storing propellant, a reaction chamber for discharging a vapor for providing thrust, a pump module comprising one or more micropumps for drawing propellant from the reservoir and for systematically metering propellant to the reaction chamber in a controlled manner, and a controller for actuating the pump module.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to microthrusters, and more particularly relates to microthrusters utilizing micropumps.  
         BACKGROUND OF THE INVENTION  
         [0002]    Spacecraft or satellite attitude control requires periodic use of thrusters to maintain or change spatial positioning. Because of severe restrictions in weight and size in spacecraft and satellites, it is important to provide very small thrusters capable of delivering small, precisely metered, controllable amounts of propellant in order to allow exact position control of satellites, especially small satellites. Many existing thrusters are cold gas thrusters, the gas propellants of which are stored under extremely high pressure in order to provide an adequate quantity of propellant, especially when the mission of the satellite is expected to be lengthy. The propellant is stored in relatively large and heavy tanks because of the high pressure. Additionally, the valves used to precisely meter the gases to the thruster nozzle are necessarily quite large and heavy as well, and usually comprise solenoid-operated on-off valves having carefully machined, and therefore expensive, components.  
           [0003]    For smaller spacecraft and satellites, relatively low thrust may be required for, for example, three-axis positioning systems. Recently there has been an interest and significant activity in the design and use of microthrusters. Since the spacecraft or satellites that the microthrusters control are quite small, and space and weight are at a premium even with respect to larger craft and satellites, the thrusters must be exceptionally small and light as well, although capable of delivering thrust on the order of 1*10 −6  lb. to 0.005 pounds or so. Larger spacecraft and satellites require larger thrusters.  
           [0004]    Accordingly, it is desirable to provide a thruster that is both lightweight and small. In addition, it is desirable to provide a thruster having a valve and metering system that is small, lightweight, and relatively inexpensive to manufacture. Also it is desirable to provide a thruster having a valve that can be scaled to accommodate a number of different thrust requirements. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    An apparatus is provided for use in satellites and other spacecraft. The apparatus includes a thruster for providing thrust for spacecraft positioning, which has a propellant reservoir for storing propellant, a reaction chamber for discharging a vapor for providing thrust, a pump module comprising one or more micropumps for drawing propellant from the reservoir and for systematically metering propellant to the reaction chamber in a controlled manner, and a controller for actuating the pump module. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0007]    [0007]FIG. 1 is a schematic illustration of a microthruster according to the instant invention.  
         [0008]    [0008]FIG. 2 is a schematic illustration of a micropump that is capable of use in the microthruster of the instant invention;  
         [0009]    [0009]FIG. 3 is a schematic illustration of a plurality of the micropumps of FIG. 2 coupled together.  
         [0010]    [0010]FIG. 4 is a schematic illustration of a reaction chamber usable in the instant invention;  
         [0011]    [0011]FIG. 5 is a schematic illustration of an alternative reaction chamber usable in the instant invention; and  
         [0012]    [0012]FIG. 6 is a schematic illustration of another alternative reaction chamber usable in the instant invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the drawings.  
         [0014]    [0014]FIG. 1 is a schematic illustration of a microthruster according to the instant invention. As previously noted, microthrusters are relatively small thrusters used for positioning and maintaining the position of a spacecraft, such as a satellite, in space. As spacecraft become smaller, the thrusters must also become smaller because of space limitations. Also weight is always important in spacecraft design and construction because the cost to launch a spacecraft increases dramatically with weight. Older thrusters relied primarily on gas under pressure as a propellant. As the need for smaller and lighter thrusters became more important, different propellant technologies were considered, which do not require storage under pressure since the thrust from these propellants is developed by chemical or electromagnetic means. Included among these technologies are monopropellants such as hydrazine or HAN (HydroxylAmine Nitrate) used with TEAN (TriEthanolAmmonium Nitrate), which require a catalyst to change from a liquid to a gas, ionic liquids such as glycerol, NaI (sodium iodide), and liquid metals, heated liquids such as water over a hot surface, and hydrides such as Lithium borohydride and sodium borohydride.  
         [0015]    The thruster  10  of FIG. 1 has a propellant reservoir  12  that is preferably, but not exclusively, a non-pressurized propellant reservoir. As noted above pressurized reservoirs or tanks tend to be large and heavy, so a non-pressurized solution is preferred. As will be explained below, however, a pressurized reservoir, at least of moderate pressurization, could also be used in the instant invention.  
         [0016]    Thruster  10  also has a pump module  14  which will be described in greater detail below, and which communicates with the reservoir  12 . While the term pump module is used herein to describe module  14 , the module could also be considered a valve or metering module since, as will be seen, the pump module provides all of those functions. A pump controller  16  is coupled to the pump module  14  to actuate the pump on command from an attitude controller  18  to which the pump controller is coupled. The attitude controller  18  is a device which sends signals to the pump controller  16  to meter a precise amount of propellant from the reservoir.  
         [0017]    Pump module  14  also communicates with a reaction chamber  20  in which chemical, electrothermal, electrostatic, or electromagnetic reactions are produced to provide propulsion for the thruster  10 . Reaction chamber  20  may be one of several types of reaction chamber depending upon the type of propellant used for a particular thruster and the type of reaction necessary to provide a propellant plume which is directed from a nozzle to the exterior of the spacecraft to provide thrust. For example, if the propellant is a monopropellant, the reaction chamber will have some provision for directing the mixture at the catalyst (usually a solid, chemically-active surface) and turbulently mixing the propellant on the catalyst surface in order to produce the propellant plume that may be directed such as to provide thrust for the thruster. If the propellant is an ionic liquid, the reaction chamber may comprise an electrostatic screen in front of the nozzle to draw the propellant plume to and through the screen and if the reaction chamber is electrothermal, a heated element may be provided to transform a liquid propellant into a gaseous form or to transform a gaseous propellant into a gas of higher volume. Each of these and others will be described in greater detail below in conjunction with FIGS. 4, 5, and  6 .  
         [0018]    [0018]FIG. 2 is a schematic illustration of a micropump cell that is capable of use in the microthruster of the present invention. As may be seen in FIG. 2, the cell has a molded pump body  22  with an upper actuation electrode  24  and a lower actuation electrode  26 . Body  22  also mounts an electrically grounded diaphragm  28  such that diaphragm  28  is capable of movement inside chamber  30  between upper electrode curved surface  32  and lower electrode curved surface  34 . Body  22  also includes an inlet lateral conduit  36  and an outlet or inter-chamber conduit  38  in curved surface  34  which permits material in chamber  30  between diaphragm  28  and the lower electrode  26  to be discharged when voltage is applied to move diaphragm into substantial contact with surface  34 . Body  22  also includes a back pressure control conduit  40  in the upper electrode curved surface  32 .  
         [0019]    Diaphragm  28  conforms to curved surfaces  32  and  34  when it is electrostatically driven to one or the other surfaces through application of a voltage to the particular electrode via voltage source  42  for upper electrode  24  and voltage source  44  for lower electrode  26 . Diaphragm  28  and the curves surfaces  32  and  34  are coated with thin dielectric layers (not shown) for electrical insulation and protection.  
         [0020]    [0020]FIG. 3 illustrates a micropump that has been fabricated in a configuration which uses a plurality of cells of FIG. 2 in series. The micropump,  50  generally, consists of a plurality of cells  52  that efficiently and effectively transfer fluid from an inlet  54  to an outlet  56 . This specific micropump  50  has an upper channel  58  and a lower channel  60 , arranged in parallel relationship, with both channels functioning in the same manner, in accordance with the invention.  
         [0021]    The body  62  is constructed by molding a high temperature plastic such as ULTEM (registered trademark of General Electric Company, Pittsfield, Mass.), CELAZOLE (registered trademark of Hoechst-Celanese Corporation, Summit, N.J.), or KETRON (registered trademark of Polymer Corporation, Reading, Pa.). The electrodes themselves can be formed by printing, plating or EB deposition of metal followed by patterning by using dry film resist, as is known in the art. Low temperature organic and inorganic dielectric is used as an insulator between the actuating electrodes.  
         [0022]    In operation of the cell of FIG. 2, the input conduit  36  is coupled to a reservoir ( 12  in FIG. 1) of the material to be pumped, and the diaphragm  28  is drawn to electrode  26  by application of an actuating voltage on terminal  44 . When it is desired to pump material from the reservoir  12  the voltage on terminal  44  is released and an actuating voltage is applied to terminal  42 . The diaphragm  28  is attracted to the upper electrode  24  and the vacuum thus created draws material from the reservoir  12  into chamber  30 . The voltage on terminal  42  is then released and a voltage is applied to terminal  44  to draw diaphragm  28  toward electrode  26  and force the material in cavity  30  out through outlet conduit  38 .  
         [0023]    The operation of micropump  50  is similar, except that pump  50  has (in each of its two sections  58  and  60 ) six cells placed in a series configuration to increase the pumping pressure of the fluid to be pumped. The two sections  58  and  60  operate in parallel to increase the volume of fluid that may be pumped in a period of time. Material from a reservoir coupled to inlet  54  is drawn into the cavity of cell  62  by the process described above with respect to FIG. 2. The material is then pumped out of the cavity of cell  62  to an inlet of cell  64 . Cell  64 , in turn pumps its contents into cell  66 , thence to cells  68 ,  70  and  72  from which the material is output through conduit  56 . Of course on each cycle new material is drawn from the reservoir into cell  62 , so the process is continuous.  
         [0024]    A more detailed operation of the micropump  50  can be found in U.S. Pat. No. 6,106,245, Cabuz, which is assigned to the assignee of the present invention.  
         [0025]    Micropump  50  is particularly suited for use in microthrusters used in spacecraft or satellites. As spacecraft become smaller, the thrusters must also become smaller because of space limitations. Also weight is always important in spacecraft design and construction because the cost to launch a spacecraft increases dramatically with weight. Older thrusters relied primarily on gas under pressure as a propellant. As the need for smaller and lighter thrusters became more important, different propellant technologies were considered, which do not require storage under pressure. The present invention provides such a microthruster.  
         [0026]    [0026]FIG. 4 is schematic diagram of one type of reaction chamber  20  usable in the present invention. This reaction chamber is particularly suited for use in electrothermal or thermochemical propulsion systems. Electrothermal systems generally use cryogenically compressed gases or monopropellant liquid fuels (fuels which in their chemical structure contain both an oxidant decomposition species and a reductant decomposition species), or easily gasified liquids. Suitable cryogenically compressed gases include hydrogen, helium, xenon, and nitrogen. Some usable monopropellants are hydrazine, ammonia, N2O4, and monomethylhydrazine. Another suitable mono-propellant is an appropriate mixture of HAN and TEAN. Some easily gasified liquids include alcohols, ketones, alkanes, water, etc. If the propellant is a monopropellant or easily gasified liquid, the reaction chamber should have some provision for mixing the propellant. If the propellant is a monopropellant, the chamber could include a catalyst surface in order to assist ignition and produce the propellant plume that may be directed such as to provide thrust for the thruster For oxidizable propellants, when an oxidant is provided (either as part of a monopropellant fuel, or as a separate reactant stream, the oxidizable propellant can be ignited by a suitable heat source or in the presence of a suitable catalyst surface.  
         [0027]    Another suitable propellant for the chamber shown in FIG. 4 would be water reacted with a hydride. For example, if liquid water is pumped onto solid lithium borohydride (or a powder compact thereof), the result is solid lithium boroxide plus gaseous hydrogen. The gaseous hydrogen so produced is then used as a propellant plume. Sodium borohydride can also be reacted in this manner.  
         [0028]    [0028]FIG. 4 shows the reaction chamber  20  having a mixing chamber  80  that receives a propellant through conduit  82  from the pump module ( 14  in FIG. 1). A catalyst surface(s)  84  or a heated surface(s) as described above is added to the mixing chamber The said surface(s) may be roughened or geometrically arranged to create a suitable degree of mixing, heat transfer, turbulence, and surface contact between the propellant reactants and the chamber. As the volume of the propellant and any gaseous reaction products increase, that gas is expelled as a propellant plume  86  which provides thrust for the thruster. For HAN/TEAN mixtures, the catalytic surface may be composed of iron, copper, or other metals which are known to catalyze the ignition of the monopropellant.  
         [0029]    For cases in which water is pumped onto a hydride, the reaction chamber ( 20 ) and the hydride bed are arranged in such a manner as to facilitate reaction of the hydride with water and the production and evolution of gaseous hydrogen.  
         [0030]    [0030]FIG. 5 is a schematic illustration of an alternative reaction chamber usable in the instant invention. In particular, FIG. 5 shows a reaction chamber suitable for use with an electrostatic or electromagnetic propellant. Electrostatic or electromagnetic propellants are ionizable liquefied gases, or ionic liquids that are drawn to a cathode by either an electrical voltage or a magnetic flux, or both. One suitable liquefied gas is xenon, Some suitable ionic liquids usable in the reaction chamber of FIG. 5 are doped glycerol, doped formamide, substituted imidazolium solutions (for example, solutions containing EMI (1-ethyl-3-methylimidizolium)), solutions containing [BF4]-ions, solutions containing sodium iodide, solutions containing mercury chloride, solutions containing aluminum chloride, solutions containing tributyl phosphate, solutions containing tetrabutylammonium tetraphenylborate, cesium, rubidium, indium, and gallium.  
         [0031]    The reaction chamber  20  of FIG. 5 has a conduit  82  coupled to the pump module ( 14  in FIG. 1) through which the propellant is pumped into nozzle  88 , which is fabricated from a conductive material. A source of electrical energy, shown here as power supply  90  but which may be any source of electrical energy, is connected with one terminal (usually the positive terminal) coupled to the nozzle  88  and the other terminal coupled to a conductive screen  92  (or multiple conductive screens) that is located at the exit port  94  of the reaction chamber  20 . Alternatively, depending upon the propellant used, a flux generator may generate a magnetic field in front of the exit port  94  of the reaction chamber  20 . In certain embodiments, and dependant upon the propellant used, both an electrical field and a magnetic field may be employed.  
         [0032]    In operation, a propellant fluid is pumped into the reaction chamber  20  from the pump module ( 14  in FIG. 1). The nozzle breaks up the propellant liquid into droplets  86  of ionic material (or ionized gas) and the droplets (or ionized gas) are attracted to the cathode screen  92 , or the magnetic field, or both. The droplets  86  are accelerated by the electrical or magnetic fields and exit through the exit port  94  of the reaction chamber  20  as a propellant plume to provide thrust for the thruster.  
         [0033]    [0033]FIG. 6 is a schematic illustration of another alternative reaction chamber  20  usable in the instant invention, showing additional design configurations over FIG. 4. In particular, FIG. 6 shows a reaction chamber  20  suitable for use with easily gasified liquid propellants, such as water, alcohols, alkanes, ketones, gasoline, kerosene, etc.  
         [0034]    [0034]FIG. 6 shows a reaction chamber  20  having a conduit  82  from the pump module ( 14  in FIG. 1) for receiving an easily gasified liquid. The liquid passes onto or through a heated element  100 , which is heated to a temperature sufficient to turn the liquid into its gaseous phase. A power source  102 , which may preferably be a source of electrical power, causes the element  100  to be heated and maintained at an appropriate temperature. In operation, a liquid is pumped through conduit  82  and passes onto or through a segment heated by heating element  100  where it is turned into a gas. Pressure forces the gas through a nozzle  104  positioned at the exit port  94  of reaction chamber  20 , producing the propulsion plume  86  to provide thrust.  
         [0035]    While an exemplary embodiment(s) has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that these exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing a preferred embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary preferred embodiment without departing from the spirit and scope of the invention as set forth in the appended claims.