Patent Application: US-43686903-A

Abstract:
propellant liquid is supplied to a colloidal thruster for micro satellite vehicles in space by capillarity induced flow through a wick element comprising a permeable porous aggregate of fibers or particles of material that is wetted by the propellant liquid . an intense electric field at the tip of the wick element dispersed the arriving liquid into a fine spray of charged droplets . electrodes having appropriate design , location and potentials accelerate the charge droplets to high velocity , thereby providing reactive thrust to the vehicle . in this method of propellant liquid introduction the flow rate and exhaust velocity , and therefore the thrust level , are determined by the applied potential difference , thereby eliminating the need for pumps or pressurized gas and flow controllers to provide the desired flowrate for the propellant liquid .

Description:
we have found that very stable electrosprays are readily produced when capillarity driven flow through a wick structure introduces a relatively nonvolatile conducting liquid into a high field region at very low pressure or in vacuum . fig1 shows schematically the essential features of an arrangement by which such an electrospray is readily produced . reservoir 10 contains a supply of propellant liquid 20 having a low volatility . wick 30 , of a porous material wettable by liquid 20 , extends from immersion in liquid 20 through conduit 40 , terminating at or near its exit plane . it may sometimes be desirable to let the wick extend for a short distance beyond that exit plane . a seal 50 placed at the exit end of the tube limits leakage of liquid by flow around the wick . such a seal may comprise a drop of cement that is allowed to set while the wick is dry and before liquid is added to reservoir 10 . the cement should be such that it is not soluble in liquid 20 after it has set . alternatively , a suitable plug may comprise a plug of soft plastic material , also insoluble in the liquid , through which a needle whose eye is “ threaded ” by the wick may be pushed . the plug may then be compressed into the end of tube 40 thus providing a seal that will allow liquid to flow through the wick but not around it , i . e . between its outside surface and the inner wall of tube 40 . we have also found that a sewing needle can be used to pull the wick through a thin film or membrane of rubber or elastic polymer while it is stretched . when the stretching force is released the return of the membrane to its original dimensions can provide a tight enough fit around the wick to minimize leakage . the membrane may then be wrapped around the tube and bound to its outside surface by a winding of wire or string . indeed , one can also provide an effective seal with one of the several varieties of compression packing glands , tees and couplings widely used for in chromatography “ plumbing .” such glands must not be so “ tight ” that they compress the wick substance to the point where the capillarity driven flow is reduced to the extent that the desired flow rate cannot be achieved . other ways to provide seal 50 between the wick and the tube will be apparent to those reasonably skilled in the art of plumbing on a miniature scale . in general the end of the wick should be approximately flush with the exit plane but it may be desirable in some cases that it extends slightly beyond or before that exit plane . the main point is to avoid a wettable surface by which liquid 20 can be lost to the surroundings by creeping flow from the wick along that surface . also to be remembered is that capillarity driven flow through a wick does not depend upon body forces on the liquid due , for example , to hydrostatic pressure , gravity or centrifugal acceleration . indeed , such forces can accelerate , inhibit , stop or even reverse capillarity driven flow . for this reason the propellant liquid in a system like the one illustrated in fig1 must be confined or constrained during periods in which the vehicle may be subject to such forces , e . g . during launch . how such problems can be addressed are beyond the scope of the subject invention which deals primarily with the problem of supplying propellant liquid for electrospray dispersion into charged droplets that can be electrostatically accelerated to provide thrust at the low levels required for positioning or slow maneuvering of a small satellite . opposite the end of the wick is an open mesh electrode 90 maintained at an opposite potential relative to the wick sufficient to provide at the wick tip the field necessary to disperse the liquid into an electrospray of fine droplets . power supply 100 provides a potential difference between the wick and electrode 90 that determines the intensity of the field at the wick tip . the intensity of that field determines the rate at which the liquid flows through taylor cone 60 into a thin jet of liquid from the cone tip 70 which breaks up into the charged droplets that form electrospray 80 . the kinetic energies and therefore the velocities of the charged droplets that arrive at the plane of electrode 90 are determined primarily by the potential difference between that electrode and wick 30 but are somewhat less than that potential difference because of the electrical resistance of the liquid in the taylor cone and especially the thin jet of liquid that issues from its tip . the mesh grid of electrode 90 should be as open , i . e . “ transparent ”, as possible so as to pass as large a fraction as possible of the arriving droplets . additional mesh electrodes ( not shown ) may be located beyond electrode 90 to provide the desired translational energy and therefore velocity of the droplets leaving the vehicle . that departing velocity is what determines the impulse or thrust provided to the vehicle per unit mass of droplets . it may be desirable to provide additional electrodes “ downstream ” from electrode 90 so that the effective exhaust velocity of the droplets can be specified somewhat independently of the field at the wick tip that governs the size of the droplets and the rate at which they are formed .” [ 0015 ] fig1 illustrates the essential features of the invention as they might be embodied in a single thrust - producing element . however , depending upon the size of the vehicle , there may be a need for providing more thrust than one such thrust - producing element can provide . moreover , it seems likely that in many applications there may also be a need for thrust vectors of different directions and magnitudes from more than one location on the vehicle . [ 0016 ] fig2 shows schematically how such a plurality of thrust - producing elements might be provided . with reference to fig2 reservoir 10 contains a supply of propellant 20 that flows continuously through the loop of conduit 30 back to reservoir 10 . flow of propellant liquid through that loop is maintained by pump 40 . it is to be understood that the actual path of conduit 30 en route from reservoir 10 and back , passes near all locations on the vehicle at which a thruster is to be located . at each such location there is a tee 50 in the conduit . examples of such “ tees ” are represented by 50 and along conduit loop 30 shown schematically in fig2 . a wick 60 is inserted in the arm of each tee 50 so that its interior end is bathed by the liquid 20 circulating in the loop . the wick extends to the exit plane of the tee arm , or slightly beyond , and is provided with a seal 70 as explained in the description of fig1 . opposite the tip of each wick is a mesh electrode 130 as in fig1 that can be maintained at a desired potential relative to their opposing wicks 60 by power supplies 100 respectively . when a sufficiently high potential is applied to electrodes 130 , a taylor cone of liquid 80 will be formed at the end of each wick . from the tip of each cone emerges a filament or jet of liquid 110 which breaks up or disperses into an electrospray of charged droplets 120 . clearly , this method of providing separate wick injectors from a single source of liquid can be readily expanded to produce a plurality of such wick injectors , each with its own counter electrode comprising a highly open or transparent grid electrode 130 . in addition , each such separate grid electrode 130 can be followed by subsequent similar electrodes whose potentials can be maintained any desired level by an appropriately adjustable power supply , or plurality of power supplies such that the potential of each electrode on each thruster can be maintained at a desired level by methods well known to those skilled in the art of designing electrical power supplies and their controls . in sum , by procedures such as those just described , a plurality of electrospray thrusters can be easily distributed on a particular vehicle in such a way that each one provides thrust vector in its particular direction . a desired magnitude for that thrust vector can be obtained by an appropriate choice of voltages on each electrode associated with that thruster . an appropriate combination of thrust magnitudes for each of the said plurality of thrusters can provide a resultant thrust vector for the vehicle as a whole in any desired direction over a range of magnitudes . thus , the position and velocity of the vehicle can be varied over a wide range thereby providing readily controlled maneuverability . it is noteworthy that the only variables that need to be controlled are voltages . this wick injection takes advantage of the self - balancing feature of capillarity driven flow , namely the fact that such flow will occur only at a rate sufficient to replace liquid that is removed , for example by combustion in a candle or by field dispersion in electrospray . in the latter case the field is produced by potential difference between the wick and the counter electrode . the required potential difference is created by connecting the wick to one pole of an appropriate power supply , the other pole of which is connected to the counter electrode . a wick wet with conducting liquid is itself a good electrical conductor so that the wick connection to the power supply can be made anywhere along the wick or to the reservoir of liquid in which one end of the wick is immersed . capillarity drives the liquid to the tip of the wick where it forms the same kind of cone - jet configuration that occurs at the end of a small diameter tube maintained at high potential in conventional electrospray systems . in the case of a tube , the diameter of the cone base generally equals the effective flow diameter at the tube exit , i . e . its bore . in the case of a relatively porous wick that flow diameter very close to the diameter of the wick . with wicks of very small dimensions it is sometimes not possible to see the cone - jet configuration of liquid at the wick tip . even so when one can detect a measurable spray current between the wick and the counter electrode and a slow but measurable flow of liquid through the wick , one can be reasonably be sure that the cone - jet configuration obtains . indeed in every case when there has been a detectable current , a cone has become visible when viewed with sufficient magnification . numerous experiments have clearly shown that with wick injectors both total current in the spray and selected ion currents at the detector of the mass analyzer are remarkably steady even with liquids having high conductivities and / or high surface tensions . when gas pressure or a pump is used to supply liquid at very low flow rates it can be very difficult to obtain and maintain the flow rate required for a stable spray . thus , wick injection of liquid into an electrospray provides convenient and effective flow control and stability . also noteworthy is the fact that in the case of the wick the magnitude of the flow rate is determined by the strength of field , i . e . the applied voltage . thus , in the case of a colloidal thruster one can control and vary the thrust over a substantial range , simply by adjusting the voltage . indeed , though it has not yet been investigated , one can contemplate the possibility of providing impulse thrust in very short bursts , a procedure that might be very useful in achieving accurate control of the position and orientation of a small satellite . because of the inherent simplicity and ruggedness of the hardware required for producing a wick spray , it should be fairly easy to provide wick thrusters at various positions on the satellite . all such thrusters might obtain the propellant liquid from a single common source or they might be fed from a plurality of sources at strategic locations in the satellite . such an arrangement could provide net thrust vectors of readily variable magnitudes in almost any direction . a systematic study to determine the optimum structure and composition for useful wicks has not been carried out but successful operation has been obtained with wicks comprising bundles of small fibers made of glass , graphite , paper , cotton and linen that have ranged in diameter from 8 to perhaps 200 microns . nor is the cross sectional shape important . thin flat strips of cloth or paper work just as well as threads or fibers of circular or oval cross section . tubes packed with granular or porous material can also be used . an effective wick can comprise a single monofilament fiber in a tube whose bore has a diameter only slightly larger than that of the wick . if the thickness of the annular gap between wick and tube is sufficiently small , and if the attractive forces between molecules of liquid and the surfaces of the fiber and filament are sufficiently larger or smaller than the attractive forces of the molecules for each other , capillarity can either lift the level of liquid in the tube to a substantial height above the surface level of liquid in which this filament cylinder wick is immersed , or lower the level liquid in tube below the level of that outside surface level . of course , if liquid is to be electrosprayed from this or any other type of wick , acceleration or gravitational forces must not be so strong that capillarity is unable to pull the liquid from the supply container and raise it to the tip of the tube where the applied electric field can pull it into the spray . unwaxed dental floss seems to work very well so a short length of this material has comprised the workhorse in the use of wick injectors for esims . however , successful operation has also been achieved with wicks comprising fibers of glass , carbon and a wide range of natural and synthetic polymers . the necessary and sufficient property of the fiber substance is that it be wettable by the liquid . in bench top experiments with electrometer measurements of total spray current we have readily obtained apparently stable “ sprays ” with a wide variety of liquids . gradually increasing the applied voltage results in a smooth very gradual transition to a corona discharge that seems to be readily reversible without the usual hysteresis loop . this use of capillarity driven flow through a wick to supply an electrospray has been found effective and useful in electrospray ionization mass spectrometry ( esims ) as taught in u . s . pat . no . 6 , 297 , 499 b1 [ 17 ]. in that application the objective is to disperse a solution of analyte species as charged droplets into an inert bath gas , typically at or near atmospheric pressure . the bath gas then provides the enthalpy that evaporates solvent from the droplets , thereby transforming solute species in the droplets into gaseous ions that can then be analyzed by mass spectrometry . thus , the liquid must be volatile and the dispersion must be into a gas , which provides the enthalpy required for vaporization . in this space propulsion application the electrospray dispersion must be carried out in vacuum and liquid must have as low a vapor pressure as possible to avoid evaporative losses of mass from the droplets before they have been accelerated to provide reactive thrust as well as to minimize evaporative losses of the propellant liquid during extended periods in space . the vitalizing feature of present invention is the discovery that wick injection does work beautifully in vacuum , thus providing a simplicity and flexibility which are always at a premium in space propulsion applications . the requirement in space propulsion for liquids with ultra low vapor pressures also raised questions to which answers were needed . it turned out that such liquids did work with wick injection and we have found a number of liquids with very low vapor pressures that seem to work very well . they include amides , alcohols , glycols , esters , ketones and mixtures of one or more of these compounds . of particular interest are so called “ ionic liquids ” that comprise organic mixtures of cations and anions with polyatomic “ super structures ” surrounding the charge bearing groups that prevent charges of opposite sign from coming so close together that they become in effect a neutral particle . in a sense the substance of the molecule outside the charge plays a role similar to that of water in an electrolyte solution by forming a cage that keeps the cations and anions separated from each other . there are a great variety of these materials , which are effective solvents for many species properties and very low vapor pressures . these characteristics have made them increasingly attractive candidates as media in which to carry out chemical synthesis on an industrial scale . stable electrosprays have been obtained with representative candidates from a variety of most of this class of liquids . in sum , the key feature of the invention is the use of capillarity driven flow through a wick element to provide self - stabilizing electrosprays of tiny charged droplets in vacuum . various other applications of this discovery will no doubt suggest themselves to other investigators who understand the nature of the invention . 1 . j . zeleny , proc . phil . soc . 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