GB2241481A

GB2241481A - Apparatus for damping rotation and/or oscillation of a spacecraft orbiting earth or other celestial body

Info

Publication number: GB2241481A
Application number: GB9004829A
Authority: GB
Inventors: Peter Richard Scott
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1990-03-03
Filing date: 1990-03-03
Publication date: 1991-09-04
Also published as: GB9004829D0

Abstract

Apparatus for damping rotation and/or oscillation of a spacecraft orbiting Earth or other Celestial body includes casing means (1) fixable to the spacecraft and having a substantially spherical cavity (2) therein, magnet means (6) containable in the cavity (2) for motion relative to the spacecraft under the influence of the magnetic field of Earth or other Celestial body and viscous fluid means locatable in the spherical cavity (2) and operable to dissipate energy. The magnet means (6) is shaped to interact hydrodynamically with the viscous fluid means on relative rotation between the magnet means (6) and casing means (1) to maintain a separation between the magnet means and the casing means, sufficient to prevent excessive torques from acting between the spacecraft and the magnetic means. As shown, the magnet is at least part-spherical to leave a gap (8) between the casing ( ) and magnet (6) to contain the viscous fluid e.g. polydimethyl siloxane. <IMAGE>

Description

APPARATUS FOR DAMPING ROTATION AND) OR OSCILLATION OF A SPACECRAFT ORBITING BARTH OR OTHER CELESTIAL BODY This invention relates to an apparatus for damping rotation andl or oscillation of a spacecraft orbiting Earth or other Celestial body and is particularly, but not exclusively, suitable for use with a satellite operating within the Earths magnetosphere.

There arises for many spacecraft the problem of removing any rotation which may be imparted to the spacecraft on release from a launch vehicle. There also arises for many spacecraft the problem of minimising the oscillatory motions which may be permitted by the spacecrafts attitude control system, especially in cases wherein the attitude is controlled by passive means such as gravity gradient means.

It is known that a potentially very efficient means of damping such rotations consists of a magnet means mountable on the spacecraft for motion relative thereto under the influence of the magnetic field of Earth, and energy dissipating means locatable on the spacecraft and operable to reduce the motion of the spacecraft relative to that of the magnet means. However, attempts to use this principle have sometime led to failures of attitude control which are attributable to excessive viscous drag or friction between the magnet means and the spacecraft body.

Other forms of damper are also known, for example based on the use of gravity gradients and energy dissipation means, or else for example utilising attitude sensors and active actuators such as thrusters, rotating wheels or electromagnetic torquing devices. Apparently all such alternatives suffer from at least one of two disadvantages, the first disadvantage being a slow rate of damping, and the second disadvantage being the combination of high cost, high mass and relatively low reliability which is inherent in a system having many interacting mechanical and/or electronic and/or chemical parts.

There is thus a need for a generally improved apparatus for damping rotation andlor oscillation of a spacecraft orbiting Earth or other Celestial body which avoids loss of attitude control, which at least minimises the other above noted difficulties and problems and which preferably does so in a simple manner.

According to the present invention, there is provided apparatus for damping rotation andlor oscillation of a spacecraft orbiting Earth or other Celestial body, including casing means fixable to the spacecraft and having a substantially spherical cavity therein, magnet means containable in the cavity for motion relative to the spacecraft under the influence of the magnetic field of Earth or other Celestial body, and viscous fluid means locatable in the spherical cavity and operable to dissipate energy, so as to reduce the relative motion of the magnet means and casing means and thereby damp down rotation andlor oscillation of the spacecraft, the shape of the magnet means being such as to interact hydrodynamically with the viscous fluid means on relative rotation between the magnet means and casing means to maintain a separation between the magnet means and the casing means sufficient to prevent excessive torques from acting between the spacecraft and the magnet means.

Preferably the magnet means is shaped, relative to a rectangular co-ordinate system having axes X, Y, Z with the Z-axis being directed along the magnetic moment vector of the magnet means, to provide a first portion operable to rotate the magnet means into a desired orientation and a second portion operable to tend to centre the magnet means relative to the cavity of the casing means.

Conveniently the shape of the first portion of the magnet means is such that first parts of the first portion of the magnet means which lie in the region where the co-ordinates (X, Y, Z) satisfy the condition that the product (YZ) is greater than zero, are relatively large so as to cause greater resistance to rotation of the magnet means relative to the casing means, and second parts of the first portion of the magnet means which lie in a region where the co-ordinates (X, Y, Z) satisfy the condition that the product (YZ) is less than zero, are smaller than said first parts so as to cause lesser resistance to rotation of the magnet means relative to the casing means, thereby to generate in response to any relative rotation component of the magnet means about the Y-axis, a hydrodynamic torque about the Z-axis, which torque tends to rotate the magnet means into the orientation where the X-axis coincides with the axis of rotation.

The first portion of the magnet means may have the shape of at least part of a sphere concentric with the magnetic centre of the magnet means , with the radius of the sphere being less than that of the said cavity by an amount sufficient to define a clearance between the magnet means and casing means.

Advantageously the first parts of the magnet means take the form of protrusions on the at least part of the sphere, arranged in a pattern which extends around the X-axis.

Conveniently the second parts of the magnet means take the form of indentations on the at least part of the sphere, arranged in a pattern which extends around the X-axis.

Preferably the first portion of the magnet means is substantially rod-like in shape provided with at least two substantially planar fins projecting therefrom so as to lie in the YZ plane.

Conveniently the fins lie within the regions where Y and Z are both greater than zero or where Y and Z are both less than zero.

Advantageously the second portion of the magnet means is in the form of at least one array of grooves each of which is substantially V-shaped in cross section, which extend around the at least part of the sphere between the protrusions and/or indentations, between and across the protrusions and/or indentations, or in one or more rings laterally spaced from and adjacent the protrusions andjor indentations, and which are oriented such that the point of the V-shape is directed substantially oppositely to the direction of motion of the protrusion or indentation which would result from any positive rotation of the magnet means about the X-axis in a conventional right handed sense.

Preferably the second portion of the magnet means has the shape of at least part of a sphere concentric with the magnetic centre of the magnet means, with the radius of the sphere being less than that of said cavity by an amount sufficient to define a clearance between the magnet means and the casing means, with at least one array of grooves, each of which is substantially V-shaped in cross section, being provided around the exterior of the at least part of the sphere which forms a ring connecting the ends of the rod-like first portion, and with the grooves being oriented such that the point of the V-shape is directed substantially oppositely to the direction of motion of the fins which would result from any positive rotation of the magnet means about the X-axis in a conventional right handed sense.

Conveniently the second portion of the magnet means is in the form of at least one array of tooth-like projections and hollows which extend around all or part of the at least part of a sphere in the manner of a stepped ratchet-like ring around the X-axis between the protrusions andior indentations, between or across the protrusions andlor indentations or in one or more rings laterally spaced from and adjacent the protrusions and/or indentations.

The second portion of the magnet means may be in the form of at least one array of tooth-like projections and hollows which are arranged to form a stepped ratchet-like ring extending around the X-axis and interconnecting the ends of the rod-like first portion.

Preferably the ring is so oriented in the casing means that the clearance between an adjacent point on the casing means and each projection and hollow is subject alternately to a relatively slow rate of decrease and then to a relatively rapid rate of increase upon a positive rotation of the magnet means about the X-axis relative to the casing means, so that on rotation of the magnet means about the X-axis relative to the casing means, a force is generated tending to centre the magnet means relative to the cavity.

The second portion of the magnet means may be a combination of grooves and tooth-like projections and hollows.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which Figure 1 is diagrammatic illustration of a rectangular co-ordinate system appertaining to apparatus of the present invention, Figure 2 is a diagrammatic illustration of varying viscous drag as a function of a spatial direction for apparatus according to the present invention, Figure 3 is a diagrammatic illustration of a first form of surface modification to a substantially spherical magnet means of apparatus of the invention, Figure 4 is a second form of magnet means for use in the apparatus according to the present invention, Figure 5 is a third form of surface modification of a substantially spherical magnet means for use in the apparatus of the invention, and Figure 6 is a fourth form of magnet means for use in the apparatus according to the present invention.

Apparatus according to the present invention, as shown in the accompanying drawings, is intended for damping rotation andlor oscillation of a spacecraft orbiting Earth or other Celestial body. All embodiments of the apparatus include casing means, of which a short section is shown by way of example in Figure 3, fixable to a spacecraft (not shown). The casing means 1 has a substantially spherical cavity 2 therein and all embodiments of the apparatus include magnet means containable in the cavity 2 for motion relative to the spacecraft under the influence of the magnetic field of Earth or other Celestial body.

Contained in the cavity 2 between the magnet means and the casing means 1 is viscous fluid means which conveniently takes the form of a polydimethyl siloxane having a viscosity value of about 70 poise. The fluid means is operable to dissipate energy so as to reduce the relative motion of the magnet means and casing means 1 and thereby damp down rotation andior oscillation of the spacecraft.

In all embodiments of the apparatus of the invention the shape of the magnet means is such as to interact hydrodynamically with the viscous fluid means, on relative rotation between the magnet means and casing means 1, to maintain a separation between the magnet means and the casing means 1 sufficient to prevent excessive torques from acting between the spacecraft and the magnet means. The shaping of the magnet means is defined with reference to a rectangular co-ordinate system, as illustrated in Figure 1, which is fixed in relation to the magnet means. The co-ordinate system has axes X, Y and Z with the Z axis being directed along the magnetic moment vector of the magnet means. The X axis in operation of the apparatus of the invention, is caused to coincide with the axis of rotation by hydrodynamic forces.In the illustration of Figure 1 the magnet means 3 is substantially rod-like in shape with the North Pole being shown at N and the South Pole being shown at S.

The magnet means is shaped, relative to the rectangular co-ordinate system, to provide a first portion operable to rotate the magnet means into a desired orientation and a second portion operable to tend to centre the magnet means relative to the cavity 2 of the casing means 1.

The shape of the first portion of the magnet means is such that first parts of the first portion of the magnet means, which lie in a region where the co-ordinates (X, Y, Z) satisfy the condition that the product (YZ) is greater than zero, are relatively large so as to cause greater resistance to rotation of the magnet means relative to the casing means 1, and second parts of the first portion of the magnet means, which lie in a region where the co-ordinates (X, Y, Z) satisfy the condition that the product (YZ) is less than zero, are smaller than the first parts so as to cause lesser resistance to rotation of the magnet means relative to the casing means 1, thereby to generate in response to any relative rotation component of the magnet means about the Y-axis, a hydrodynamic torque about the Z-axis, which torque tends to rotate the magnet means into the orientation where the X-axis coincides with the axis of rotation. Thus, as illustrated in Figure 2, the shape of the magnet means is such as to produce varying viscous drag, as a function of spatial direction. There is a greater drag in the regions 4 where YZ is greater than zero than in the regions 5 where YZ is less than zero.

In the preferred embodiment of the invention as shown in Figure 3 the first portion of the magnet means 6 has the shape of a least part of a sphere concentric with the magnetic centre 7 of the magnet means 6. The radius of the sphere is less than that of the cavity 2 in the casing means 1 by an amount sufficient to define a clearance 8 between the magnet means 6 and the casing means 1.

In the Figure 3 embodiment the first parts of the magnet means 6 take the form of protrusions 9 on the at least part of the sphere arranged in a pattern which extends around the X-axis and second parts of the magnet means take the form of indentations 10 on the at least part of the sphere arranged in a pattern which extends around the X-axis. In the most basic form of the embodiment of Figure 3 the protrusions 9 may be omitted. The indentations 10 may be of any convenient shape such as concave or merely a flat area on the sphere surface.

In the presence of the indentations 10 only, relative rotation between the magnet means 6 and the casing 1 about the X-axis results in the indentations 10 producing no torque other than an irrelevant change of drag. However, if there is relative rotation between the magnet means 6 and casing means 1 about the Y-axis, there is reduced drag at the indentations 10 which creates a net torque, making the X-axis swing towards the rotation axis. This swing is relatively slow, so that the rotation rate is dominated by the external magnetic field and its rotation relative to the spacecraft.

The system is linear with respect to the rotation of the magnet means 6, because the torques are dominated by the action of viscosity. A preferred maximum Reynolds number is about 0.00007. Thus all rotations consisting of a combination of rotation about the X and Y-axes lead to a similar result, namely that the magnet means 6 swings round until its rotation acts about the X-axis in the conventional positive sense as shown by the arrow 11 about the X-axis in Figure 3.

In the embodiment of Figure 3 the second portion of the magnet means 6 is in the form of at least one array of grooves 12 each of which is substantially V-shaped in cross section.

The grooves 12 which extend around the at least part of the sphere between the protrusions 9 andlor indentations 10, between and across the protrusions andlor indentations, or in one or more rings 13 laterally spaced from and adjacent the protrusions 9 andlor indentations 10. The grooves 12 are oriented such that the point of the V-shaped is directed substantially oppositely to the direction of motion of the protrusion 9 or indentation 10 which would result from any positive rotation of the magnet means 6 about the X-axis in a conventional right handed sense. When the magnet means 6 rotates about the X-axis the grooves 12 provide a centring force which is proportional to the distance of the magnet means 6 from its centred position.

In the embodiment illustrated in Figure 4 of the accompanying drawings the magnet means has a substantially rod-like shaped first portion 14 provided with at least two substantially planar fins 15 projecting therefrom so as to lie in the YZ plane. The fins 15 lie within the regions where Y and Z are both greater than zero or Y and Z are both less than zero.

In the Figure 4 embodiment the second portion of the magnet means has the shape of at least part of a sphere concentric with the magnetic centre 7 of the magnet means, with the radius of the sphere being less than that of the cavity 2 by an amount sufficient to define the clearance 8 between the magnet means and the casing means 1, with at least one array of grooves 16, each of which is substantially V-shaped in cross section, being provided around the exterior of the at least part of the sphere which forms a ring 17 connecting the ends of the rod-like first portion 14. The grooves 16 are oriented such that the point 16a of the V-shape is directed substantially oppositely to the direction of motion of the fins 15 which would result from any positive rotation of the magnet means about the X-axis in a conventional right handed sense.

Preferably, as illustrated in Figures 3 & 4 the rings 13 or rows 17 of grooves 12 or 16 are centred on the great circle normal to the preferred rotational axis which is the X-axis.

Preferably the ring 13 or row 17 extends fully around the circumference of the great circle and the width of the ring 13 or row 17 preferably is about half the radius of the magnet means sphere.

The depth of each grove 12, 16 preferably is O. 7 x the clearance or spacing 8 between the magnet means sphere and the casing means 1. As illustrated each grove 12, 16 preferably intersects the great circle at about 45 degrees. The effect of the ring 13 or row 17 of V-shaped groves 12, 16 is to produce a centring force on the magnet means 14 in the casing means 1 when rotation is about the X-axis. This centring action results from the grooves 15 creating an increase of pressure at points on the great circle which pressure increases at points where spacing 8 is reduced.

The embodiment of Figure 5 is basically imilar to that of Figure 3, and like parts will not be described again in detail but in this embodiment the second portion of the magnet means 18 is in the form of at least one array of tooth-like projections 19 and hollows 20 which extend around all or part of the at least part of a sphere in the manner of a stepped ratchet-like ring 21 around the X-axis between the protrusions 9 andlor indentations 10, between and across the protrusions 9 and/or indentations 10 or in one or more rings laterally spaced from and adjacent the protrusions 9 andlor indentations 10.

The embodiment of Figure 6 is basically similar to that of Figure 4 and like parts will not be described again in detail, but in this embodiment the second portion of the magnet means 22 may be in the form of at least one array of tooth-like projections 23 and hollows 24 which are arranged to form a stepped ratchet-like ring 25 extending around the X-axis and interconnecting the ends of the rod-like finned first portion 14.

In Figures 5 and 6 the rings 17, 25 extend around the X-axis and are orientated in the casing means 1 so that the clearance 8 between an adjacent point on the casing means 1 and each projection 19, 23 and hollow 20, 24 is subject alternately to a relatively slow rate of decrease and then a relatively rapid rate of increase upon a positive rotation of the magnet means about the X-axis relative to the casing means 1. In this way on rotation of the magnet means about the X-axis relative to the casing means 1, a force is generated tending to centre the magnet means relative to the cavity 2 of the casing means 1.

Alternatively, although not illustrated, the second portion of the magnet means oan be a combination of grooves 12, 16 and tooth-like projections 19, 23 and hollows 20, 24 in the embodiments of Figures 3, 4, 5, and 6.

Claims

1. Apparatus for damping rotation andlor oscillation of a spacecraft orbiting Earth or other Celestial body, including casing means fixable to the spacecraft and having a substantially spherical cavity therein, magnet means containable in the cavity for motion relative to the spacecraft under the influence of the magnetic field of Earth or other Celestial body, and viscous fluid means locatable in the spherical cavity and operable to dissipate energy, so as to reduce the relative motion of the magnet means and casing means and thereby damp down rotation and or oscillation of the spacecraft, the shape of the magnet means being such as to interact hydrodynamically with the viscous fluid means, on relative rotation between the magnet means and casing means, to maintain a separation between the magnet means and the casing means sufficient to prevent excessive torques from acting between the spacecraft and the magnet means.

2. Apparatus according to Claim 1, wherein the magnet means is shaped, relative to a rectangular co-ordinate system having axes X, Y, Z with the Z-axis being directed along the magnetic moment vector of the magnet means, to provide a first portion operable to rotate the magnet means into a desired orientation and a second portion operable to tend to centre the magnet means relative to the cavity of the casing means.

3. Apparatus according to Claim 2, wherein the shape of the first portion of the magnet means is such that first parts of the first portion of the magnet means, which lie in a region where the co-ordinates (X, Y, Z) satisfy the condition that the product (YZ) is greater than zero, are relatively large so as to cause greater resistance to rotation of the magnet means relative to the casing means, and second parts of the first portion of the magnet means, which lie in a region where the co-ordinates (X, Y, Z) satisfy the condition that the product (YZ) is less than zero, are smaller than the said first parts so as to cause lesser resistance to rotation of the magnet means relative to the casing means, thereby to generate in response to any relative rotation component of the magnet means about the Y-axis, a hydrodynamic torque about the Z-axis, which torque tends to rotate the magnet means into the orientation where the X-axis coincides with the axis of rotation.

4. Apparatus according to Claim 3, wherein the first portion of the magnet means has the shape of at least part of a sphere concentric with the magnetic centre of the magnet means, with the radius of the sphere being less than that of the said cavity by an amount sufficient to define a clearance between the magnet means and casing means.

5. Apparatus according to Claim 4, wherein the first parts of the magnet means take the form of protrusions on the at least part of the sphere, arranged in a pattern which extends around the X-axis.

6. Apparatus according to Claim 4 or Claim 5, wherein the second parts of the magnet means take the form of indentations on the at least part of the sphere, arranged in a pattern which extends around the X-axis.

7. Apparatus according to Claim 2, wherein the first portion of the magnet means is substantially rod-like in shape provided with at least two substantially planar fins projecting therefrom so as to lie in the YZ plane.

8. Apparatus according to Claim 7, wherein the fins lie within the regions where Y and Z are both greater than zero or where Y and Z are both less than zero.

9. Apparatus according to Claim 6, wherein the second portion of the magnet means is in the form of at least one array of grooves each of which is substantially V-shaped in cross section, which extend around the at least part of the sphere between the protrusions andlor indentations, between and across the protrusions andlor indentations, or in one or more rings laterally spaced from and adjacent the protrusions and/or indentations, and which are oriented such that the point of the V-shaped is directed substantially oppositely to the direction of motion of the protrusion or indentation which would result from any positive rotation of the magnet means about the X-axis in a conventional right handed sense.

10. Apparatus according to Claim 8, wherein the second portion of the magnet means has the shape of at least part of a sphere concentric with the magnetic centre of the magnet means, with the radius of the sphere being less than that of said cavity by an amount sufficient to define a clearance between the magnet means and the casing means, with at least one array of grooves, each of which is substantially V-shaped in cross section, being provided around the exterior of the at least part of the sphere which forms a ring connecting the ends of the rod-like first portion, and with the grooves being oriented such that the point of the V-shape is directed substantially oppositely to the direction of motion of the fins which would result from any positive rotation of the magnet means about the X-axis in a conventional right handed sense.

11. Apparatus according to Claim 6, wherein the second portion of the magnet means is in the form of at least one array of tooth-like projections and hollows which extend around all or part of the at least part of a sphere in the manner of a stepped ratchet-like ring around the X-axis between the protrusions andlor indentations, between or across the protrusions and/or indentations or in one or more rings laterally spaced from and adjacent the protrusions andlor indentations.

12. Apparatus according to Claim 8, wherein the second portion of the magnet means is in in the form of at least one array of tooth-like projections and hollows which are arranged to form a stepped ratchet-like ring extending around the X-axis and interconnecting the ends of the rod-like first portion.

13. Apparatus according to Claim 11 or Claim 12, wherein the ring is so oriented in the casing means that the clearance between an adjacent point on the casing means and each projection and hollow is subject alternately to a relatively slow rate of decrease and then to a relatively rapid rate of increase upon a positive rotation of the magnet means about the X-axis relative to the casing means, so that on rotation of the magnet means about the X-axis relative to the casing means, a force is generated tending to centre the magnet means relative to the cavity.

14. Apparatus according to Claims 9 and 11, wherein the second portion of the magnet means is a combination of grooves and tooth-like projections and hollows.

15. Apparatus according to Claims 10 and 12, wherein the second portion of the magnet means is a combination of grooves and tooth-like projections and hollows.

16. Apparatus for damping rotation andlor oscillation of a spacecraft orbiting Earth or other Celestial body, substantially as hereinbefore described and as illustrated in Figures 1 and 2, Figures 3 and 5, Figures 3 and 6, Figures 4 and 5 or Figures 4 and 6 of the accompanying drawings.

17. A spacecraft incorporating apparatus according to any one of Claims 1 to 16.