US3789608A

US3789608A - Type of colloid propulsion

Info

Publication number: US3789608A
Application number: US00189279A
Authority: US
Inventors: B Free
Original assignee: Comsat Corp
Current assignee: Comsat Corp
Priority date: 1971-10-14
Filing date: 1971-10-14
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05

Abstract

In a colloidal thruster, electrically conductive liquid propellant passes through passages having precisely defined sharp angles at specific points at the exit opening of the passages. Sharp cusps of the liquid propellant are formed at the specific points and due to a stronger electric field gradient near the cusps, caused by an electrode in the vicinity of the exit openings which is charged oppositely to the liquid propellant and by the sharpness of the cusps, the colloidal droplets will be formed at and will be ejected from the cusps. The droplets will be more uniform in size and will be ejected in a more uniform direction than in prior art devices.

Description

nited States Patent [1 1 Free [451 Feb. 5, 1974 [73] Assignee: Communications Satellite Corporation, Washington, DC.

[22] Filed: Oct. 14, 1971 [21] Appl. No.: 189,279

Primary ExaminerCarlton R. Croyle Assistant Examiner-Robert E. Garrett Attorney, Agent, or Firm-Sughrue, Rothwell, Mion, Zinn & Macpeak 5 7 ABSTRACT In a colloidal thruster, electrically conductive liquid propellant passes through passages having precisely defined sharp angles at specific points at the exit opening of the passages. Sharp cusps of the liquid propellant are formed at the specific points and due to a stronger electric field gradient near the cusps, caused by an electrode in the vicinity of the exit openings which is charged oppositely to the liquid propellant and by the sharpness of the cusps, the colloidal droplets will be formed at and will be ejected from the cusps. The droplets will be more uniform in size and will be ejected in a more uniform direction than in prior art devices.

5 Claims, 10 Drawing Figures PATENIEU 5'974 3.789.608

SHEEI 1 0F 2 22 PRESSURE j RESERVOIR Q U 2 ii PROPELLANT i TANK iii i:

VOLTA FIG 3B HG H030 R Q 38 PRIOR ART pRlOR Am A PATENIEU FEB 5 I974 SHEEI 2 [IF 2 TYPE OF COLLOID PROPULSION BACKGROUND OF THE INVENTION The invention is in the field of electric propulsion engines of the type which provide thrust in one direction by ejecting charged particles in an opposite direction.

.There are two general types of electric propulsion engines which eject charged particles. One type, known as an ion thruster, ejects particles of atomic size, i.e. ions. The other type, a colloidal thruster, ejects droplets which, although being of very small size, are much larger than the atomic dimensions of ions. Colloidal thrusters have a potential advantage over ion thrusters because of the increased efficiency which should be caused by the smaller charge to mass ratio involved.

A colloidal thruster operates by passing an electrically conductive liquid through very narrow passageways. In some cases a single passageway suffices (e.g. very low thrust). The liquid is supplied by a tank of liquid from which pressure and/r capillary effects force the liquid into the passageways. Each passageway has an opening at the external end thereof and'colloidal droplets are ejected at the opening due to the electrostatic field set up between the liquid and an oppositely charged electrode in the vicinity of the opening.

In the prior art colloidal thrusters, the principle elements used for passageways are very thin hollow needies. Due to surface tension, the liquid forms a sharp ridge around the inner circumference of the needle near the exit opening. At the ridge the sharpness of the angle formed between the liquid and the inner circumference of the needle results in a compression of the electric field. Since droplets are ejected at regions of greatest field gradient, the colloidal droplets will be ejected from the ridge rather than from any central portion of the exit opening.

There are a number of attendant disadvantages in the prior art of colloidal thrusters. First of all, it is relatively difficult to manufacture uniform needles with the extremely small internal diameters desired. Secondly, the droplets are ejected from anywhere on the circumferential ridge of the liquid. Due to perturbations in the surface, this causes the ejection of non-uniform sized droplets, and the droplet ejection at large angles to the needle axis.

A better understanding of the disadvantages referred to may be had by referring to FIGS. 1,2, and 3 of the drawings, which illustrate the prior art of colloidal thrusters. The propellant liquid is stored in a propellant tank 12 and as a result of pressure provided by a pressure reservoir and hydrostatic forces the propellant is forced into a conductive. manifold 14 which communicates with the entrance opening of hollow passageway 20, which are typically hollow needles. Although only a single column of needles is illustrated it should be understood that there are usually multiple rows and columns of needles. In the vicinity of the exit openings of the needles 20 there is negatively charged electrode 18 having openings therein for passage of the ejected particles. The liquid is charged positively relative to the electrode 18 by connecting the positive and negative terminals of a voltage source 16 to the conductive manifold 14 and electrode 18, respectively. The charged colloidal droplets are ejected in the direction indicated by lines 22 due to the electrostatic field gradient set up between the liquid and the electrode.

In FIG. 2, which is a cross section of the exit opening of one needle 20 and the surrounding electrode 18, it can be seen that due to surface tension the liquid 24 forms a circumferential ridge 26 forming a relatively sharp angle with the needle 20. As can be seen, the lines of constant electrostatic field 28 are compressed around the ridge creating a greater field gradient near the ridge. The droplets are emitted at the points of greatest field gradient and travel in the direction of sharpest field gradient, as indicated by the arrows 34.

Since the sharp ridge is all around the inner circumference of the needle the droplets may be ejected from any point on the ridge and therefor droplets are ejected in many different directions. When a droplet, such as shown at 40 in FIG. 3A, is emitted from point 32 along ridge 26, the momentary absence of liquid from point 32 (before surface tension fills the empty space) creates new and sharper edges 36. The field is further compressed around these points causing emission of smaller size droplets from points 36 as shown in FIGS. 38 and 3C, which represent stages in the formation of droplets 42. These latter droplets travel in different directions from the droplets emitted from the original ridge. Thus the needle arrangement causes variable size droplets to be emitted at various angles. This reduces the efficiency of the colloidal thruster.

SUMMARY OF THE INVENTION In accordance with the present invention, a colloidal thruster is provided which has sharply defined points at the passageway exits thereby creating specific cusps of liquid from which droplets are ejected. The droplets ejected will not travel at as many different angles as in the prior art and will be more uniform in size since they are continuously ejected from the same defined points. Additionally, in the preferred embodiment, the passages are simple to manufacture and the elements which define the passages are sturdier than the prior art needles.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1,2, and 3A, B and C illustrate the prior art colloidal thrusters as has been described in the above section entitled Background of the Invention.

FIG. 4 illustrates the exit opening of a passageway formed by three cylindrical rods, and represents a preferred embodiment of a passageway for the colloidal thruster which is the present invention.

FIG. 5 is a cross sectional view of FIG. 4 taken along lines 5 5 of the latter figure.

FIG. 6 illustrates the relationship of bundles of rods to the manifold and electrode to form the colloidal thruster.

FIG. 7 illustrates an alternate passageway arrangement which also provides sharp points for droplet ejection.

FIG. 8 illustrates the formation of the cusps of the liquid propellant at the exit openings of the passageways shown of the type shown in FIG. 7.

DETAILED DESCRIPTION OF THE EMBODIMENT 56 are closely packed, as shown in FIG. 4. In theory, the contact angle between tangent rods is so that the gap 50 between two rods gets extremely small as the line of contact is approached. Capillary forces will cause a sharp rise in the liquid level near the line of contact, giving rise to three cusps, 58, 60, 62, as shown in FIGS. 4 and 5. In theory, these cusps should be extremely sharp for perfectly cylindrical tangent rods, but in practice the cusps may be only moderately sharp. The sharpeness of the angle of contact between the liquid and the rods at the point where the rods touch may be reduced by using rods which have flat faces in contact. The angle of contact between the rods may be varied in this manner from 0 to 60, which allows considerable variation in the sharpness of the cusps. If desired, the angle of contact may be varied even more by using rods with other cross sections.

With the three cylinder arrangement, surface tension causes the liquid to rise higher at

specific points

58, 60, and 62. The sharpness of the cusps at these points causes the potential gradient, which is AE/Ad (E being potential and d being distance), to be stronger near the cusps than near any other portion of the liquid surface, and thus the droplets will be ejected only from the three defined cusps. The rods should be sealed at their interfaces to prevent leakage and to prevent a ridge from forming at the interface of any two rods.

The technical advantage of this type of device is that the cusps are located precisely, and the potential field gradient in their vicinity is extremely strong. Since the cycle of cusp formation droplet formation droplet ejection will be repeated in the same environment, more uniform droplet size than obtained with needles or linear slits will probably result. Also, since the history of droplet formation and ejection will be repeated very closely, the direction of ejection will be more uniform.

The manufacturing advantages are equally important. Precision uniform wire (rod) drawing down to extremely small diameters is a well established art, as is filament manufacture with glass, plastics, and other materials. Filament and wire drawing dies can be made very accurately in almost any desired cross section. Close packing of the required number of rods and sharp end finishing is much more susceptible to inexpensive manufacturing than the individual fabrication and placement of many precision needles or annular slit devices.

Also, the invention should be much more durable than previous colloidal thrusters, since for a given droplet size the supporting rods will be several times thicker than the propellant passages. This may lead to a lower thrust density for an individual propellant passage, but the possibility of accelerating many close packed individual passages with a single set of electrodes may result in practical thrust densities considerably greater than those possible with needles or linear slits.

An example of the thruster with closely packed rods is shown in FIG. 6. As shown there, four bundles of rods, 70, 72, 74, and 76, comprise seven rods each forming six passages per bundle. The rods are sealed to the manifold in such a manner to allow the liquid in the manifold 80 to flow into-the entrance openings of the passages. Perforations in the negative electrode 78 allow the exit openings of the passages to be in the vicinity of the negative charge and allow the ejected droplets to pass through the plane of the electrode unimpeded. As will be apparent, the number of bundles and the number of rods per bundle shown are only for the purpose of illustration and are not intended to be limiting in any way.

One alternative technique for forming sharply defined cusps would be to stamp holes in the manifold outlet side having sharp edges as shown in FIG. 7. The holes shown in the figure are small squares 82, but it will be apparent that other configurations, such as triangles, would suffice. Due to surface tension, the liquid will form sharp cusps at

points

86, 88, 90, and 92, as shown in FIG. 8, and the droplets will be ejected from these points. The electrode 84 in FIG. 7 could be made with one perforation for each hole or passage 82, or could have one perforation for a group of passages 82, as shown in the drawing.

In general the formation of sharply defined liquid cusps at precise points in the exit openings of the passages of a colloidal thruster will result in more uniform droplet size and more uniform direction of droplet ejection.

What is claimed is:

1. An improved colloidal thruster engine of the type that comprises, a storage means for holding an electrically conductive liquid propellant, means defining at least one exit passageway in communication with said storage means, electrode means positioned adjacent to the outlet of said passageway, means for providing a voltage difference between said electrode means and said liquid whereby charged colloidal particles of said liquid are emitted from said outlet due to the electric field set up by said voltage difference, wherein the improvement comprises said exit passageway having a finite number of internal walls which intersect to form sharp angles creating a finite number of points at said outlet from which said colloidal particles are ejected, said points corresponding to the intersection of the lines of intersection of said internal walls with said outlet.

2. A colloidal thruster as claimed in claim 1 wherein said means for defining at least one passageway comprises at least one group of closely packed cylindrical rods defining passageways between any three mutually touching rods.

3. A colloidal thruster as claimed in claim I wherein said means for defining at least one passageway comprises a plurality of groups of closely packed cylindrical rods, said rods defining multiple passageways communicating with a respective aperature in a wall of said storage means, each passageway being defined by the interspace region between three mutually touching rods.

4. A colloidal thruster as claimed in claim 3 wherein said electrode means comprises an electrically conductive metal plate having aperatures therein and said electrode is positioned parallel to the plane occupied by the outlets of said. passageways, and wherein said plate is positioned so that each group of rods is circumvented near said outlets by an edge of said plate which defines an aperture therein.

5. A colloidal thruster as claimed in claim 1 wherein said passageway has a rectangular cross section.

Claims

3. A colloidal thruster as claimed in claim 1 wherein said means for defining at least one passageway comprises a plurality of groups of closely packed cylindrical rods, said rods defining multiple passageways communicating with a respective aperature in a wall of said storage means, each passageway being defined by the interspace region between three mutually touching rods.

4. A colloidal thruster as claimed in claim 3 wherein said electrode means comprises an electrically conductive metal plate having aperatures therein and said electrode is positioned parallel to the plane occupied by the outlets of said passageways, and wherein said plate is positioned so that each group of rods is circumvented near said outlets by an edge of said plate which defines an aperture therein.