SE1650555A1 - A thruster assembly for a spacecraft - Google Patents

Abstract

A thruster assembly for a spacecraft (100) is described. The spacecraft (100) has a coordinate system fixed in the spacecraft (100) having an X axis, a Y axis, and a Z axis. The thruster assembly comprising a first set (101) of at least three thrusters (102a-c,202a-d), configured to provide thrust forces (F102a-c, F202a-d) in the positive Z direction of the spacecraft (100), wherein the at least three thrusters are arranged such that a two-axis adjustable thrust torque can be created along the X axis and along the Y axis. The thruster assembly further comprises a second set. (103) of at least four thrusters (104a-d) being configured to provide thrust force (F104a-d) along the Y axis and along the X axis, and thrust torque along the Z axis of the spacecraft. The thruster assembly further comprises a third set (105) of at least one thruster (109,509a-b,609a-c,709a-d) being configured to provide thrust force (F109,F509a-b,F609a-c,F709a-d) in the negative Z direction of the spacecraft.(Publication Figure: FIG. 1)

Description

A THRUSTER ASSEMBLY FOR A SPACECRAFT TECHNICAL FIELD The present invention relates to a thruster assembly for a spacecraft, and inparticular the invention relates to a thruster assembly that provides control of sixdegrees of freedom to a spacecraft.

BACKGROUND The interest in Active Debris Removal (ADR) missions in space has recentlyincreased significantly due to the ever increasing amount of debris in orbit around theearth. This debris constitutes a real danger for other missions and spacecraft (S/C).The ADR missions are complex and require more powerful attitude and orbit controlcompared to regular missions. A solution to this problem is to utilize a set of ReactionControl Thrusters (RCTs), providing the following functions: Three-axis orbit control capability, the S/C needs to be able to modify its orbitalong any axis for large orbit transfers (from the launch vehicle insertion orbit to thetarget orbit, and later on from the target orbit to the final disposal orbit) or for small orbit maneuvers (rendezvous phases).

Three-axis attitude control capability when the desired torque cannot behandled by Reaction Wheels (RWs). This situation occurs in particular during largeorbit transfers, during Angular Momentum Management (after Launch Vehicleseparation, for RW off-loading, for Target Attitude Stabilization, or in Safe Mode), andin case no RW are used at all (RCTs are then the only actuators used for attitudecontrol).

A complete set of thrusters needs to be defined and accommodated on thespacecraft in order to provide the desired 6-degrees-of-freedom (6-dof) capability.

A known thruster configuration for ADR missions comprises 24 RCTs. ThreeRCTs are provided in the “corners” of the spacecraft. This configuration poses anumber ofdisadvantages. First and foremost this configuration utilizes a large number of RCTs, resulting in high mass and cost penalties. Secondly, the RCTs arenot configured in cold-redundant pairs. lf any thruster fails, then the force and torquecapabilities are divided by a factor of two. Compared to the baseline solution, thismeans that the thrust level required of each thruster must be doubled. Once againthis adds costs and mass. Finally, such a thruster configuration is fully symmetric anddoes not take into account the specific force and torque demands in the different directions.

These problems have been resolved by the technical solution given in a patentapplication SE1450438-5. However, this configuration provides limited torquecapability along one direction: any torque command and momentum off-loading inthat direction require a significantly higher amount of propellant and lasts muchlonger than on the other axes. This can be an issue if the target for deorbit spins at ahigh rate, which means that a large amount of angular momentum needs to beunloaded.

Therefore, there exists a need for an improved thruster configuration that at least obviates some of the above mentioned problems.

SUMMARY lt is an objective of exemplary embodiments of the invention to address theissue outlined above. This objective and others are achieved by the thrusterassembly according to the appended independent claims, and by the embodiments according to the dependent claims.

An exemplary embodiment provides a thruster assembly for a spacecraft with acoordinate system fixed in the spacecraft having an X axis, a Y axis, and a Z axis.The thruster assembly comprising a first set of at least three thrusters, configured toprovide thrust forces in the positive Z direction of the spacecraft, wherein the at leastthree thrusters are arranged such that a two-axis adjustable thrust torque can becreated along the X axis and along the Y axis. The thruster assembly furthercomprises a second set of at least four thrusters being configured to provide thrustforce along the Y axis and along the X axis, and thrust torque along the Z axis of thespacecraft. The thruster assembly further comprises a third set of at least one thruster being configured to provide thrust force in the negative Z direction of the spacecraft.

An advantage of exemplary embodiments is that an improved thruster assembly is provided.

An advantage of certain embodiments is that the torque capability in the +/-Z directions is significantly increased.

BRIEF DESCRIPTION OF THE DRAWINGS ln the following description of embodiments of the invention, reference will be made to the accompanying drawings of which: Fig. 1 is a schematic perspective drawing of a spacecraft illustrating an embodiment of the invention; Fig. 2 is a schematic drawing of the spacecraft projected into the XY plane,illustrating force directions and application points from the first set and the second set of thrusters; Fig. 3 a) and b) are schematic drawings of an embodiment of the first set ofthrusters in a perspective view and in the XY plane; Fig. 4 a) and b) are schematic drawings of an embodiment of the first set of thrusters in a perspective view and in the XY plane; Fig. 5 a)-c) are schematic drawings of an embodiment of the third set ofthrusters in different views; Fig. 6 a)-c) are schematic drawings of an embodiment of the third set of thrusters in different views;; Fig. 7 a)-d) are schematic drawings of an embodiment of the third set of thrusters in different views.

DETAILED DESCRIPTION OF EMBODIMENTS The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodiments of theinvention are shown. This invention may, however be embodied in many differentforms and should not be construed as limited to the embodiments set forth herein;rather, these embodiments are provided so that this disclosure will be thorough andcomplete, and fully convey the scope of the invention to those skilled in the art. ln thedrawings, like reference signs refer to like elements. ln Fig. 1 an embodiment of the invention is disclosed. ln Fig. 1 a spacecraft(S/C), generally designated 100, is schematically illustrated as a transparent box withline edges. The S/C has a center of mass, designated CoM within the S/C 100. Thecenter of mass CoM is also an origin of a coordinate system fixed to the S/C 100,with a Z axis in the main velocity direction of the S/C 100, the X axis pointing downwards, and the Y axis pointing into the drawing.

The S/C 100 comprises a first set of thrusters 101, comprising at least threethrusters 102a-c. The first set of thrusters 101 are arranged in a plane parallel withthe XY plane, and the first set of thrusters are mainly aligned with the Z direction sothat it provides large torque components along the X- and the Y-axis. The first set ofthrusters 101 can be configured to be tiltable relative the Z-axis, such that a smalltorque component along the Z axis can be generated (see Fig. 3). The S/C 100further comprises a second set of thrusters, generally designated 103. This secondset 103 comprises four thrusters 104a-d. This second set 103 is mainly provided forgenerating ing thrust force in the X direction and the Y direction as well as torque inthe Z direction. The second set 102 is preferably arranged in the XY plane andadjusted such that no thrust force component is provided in the Z direction, whichmeans that the second set 103 does not provide any torque with X or Y componentsupon activation. This means that the S/C 100 may be induced to rotate about the Zaxis by means of activation of some thrusters in the second set 103 which may bebeneficial if the target rotates.

Furthermore, the S/C 100 comprises a third set of thrusters, generallydesignated 105. This third set 105 comprises at least one thruster 109 with a thrustforce vector F109 directed in the -Z direction.

FIG. 2 is a view of the S/C 100 as seen from a negative Z position along the Zaxis. The thrusters 102a-c of the first set 101 are in this embodiment evenly arrangedaround the CoM of the S/C in the Z direction, which is shown in FIG. 2 as a dottedcircle. This way, a tvvo-axis adjustable thrust torque can be created along the X axisand along the Y axis. ln this embodiment the first set 101 of thruster 102a-c with force vectors F102a-c provide thrust force in the positive Z direction. ln FIG. 2 the second set 103 ofthruster 104a-d provide thrust force (F104a-d) along the Y axis and along the X axis,and thrust torque along the Z axis of the spacecraft. The second set 103 is preferablyarranged with a small distance (most preferably 0) in the Z direction to the CoM of theS/C 100, which minimizes the torque. ln this embodiment each of the thruster 104a-d in the second set of thrusters103 is arranged with a tilt angle oi relative the Y axis. The thrusters are arrangedevenly around the CoM with an intermediate angle Gab, Gbg Bad, and God which in thisembodiment are equal to 90°. The tilt angle oi relative the Y axis can be optimizeddepending on the mission, in order to increase the torque capability around Z (i.e.thrust direction normal to the direction to CoM) or to increase the thrust capability inthe X or Y direction. ln another embodiment, the angles Gab, 00,, are equal to a first angle, and Bad,and Gbc are equal to a second angle. ln one embodiment the first angle is equal to80°, and the second angle is equal to 100°.

FIG. 3 discloses an alternative embodiment of the first set 101 of thrusters,which comprises four thrusters 302a-d with corresponding thrust force vectorsF302a-d. The thrust force vectors are mainly directed to provide thrust force in thepositive Z direction, but in this embodiment the thrust force vectors also includes Xand Y components, which means that the thrust force vectors are not parallel with the Z axis. This is shown in FIG. 3b which is a view of the XY plane from a position withnegative Z coordinates. ln this way small torques can be generated about the Z axis while using only the first set of thrusters.

FIG. 4 discloses yet another embodiment of the first set 101, in this embodimentthe first set 101 comprises four thrusters 402a-d with corresponding thrust forcevectors F402a-d. In this embodiment the thrust force vectors only provide thrust forcecomponents in the positive Z direction and in the X direction, which is shown in FIG.4b. ln some cases it may be suitable not to have any thrust component along X oralong Y, and the projection in the XY plane may look like FIG. 4b with no forcecomponent in the Y direction.

FIG. 5 discloses an alternative embodiment of the third set 105 comprising twothrusters 609a,b with corresponding thrust force vectors F509a,b. The thrust forcevectors are directed in such a way that the plume from the thrusters 609a,b are notdirected in a direction parallel with the Z axis, which means that the plume is notdirectly directed against the target to deorbit if the target is approached from thepositive Z direction. As illustrated in FIG. 5b and 5c the thrust force vectors has no Ycomponent, and the thrust force vectors can be directed to intersect each other at aposition at first distance from the CoM, as illustrated with the dash-dotted line in FIG.5b and 5c. The first distance can in different embodiments be in the interval from 0 toinfinity. The vector sum of these two thrust force vectors is preferably a vector with a major Z component and minor X and Y components.

FIG. 6 discloses yet another embodiment of the third set 105, which comprisesthree thrusters 609a-c with thrust force vectors F609a-c. Preferably, these threethrusters are arranged symmetrically in the XY plane, as shown in FIG. 6b, aroundthe CoM with their impingement plumes not directed parallel to the Z direction, asshown in FIG. 6c. This way contact between the impingement plume and the target todeorbit is avoided which is preferable for several different reasons, and one of themore important ones is that it is undesirable to transfer impulse from the S/C 100 tothe target to deorbit.

Finally in FIG. 7 another embodiment of the third set 105 is disclosed. ln thisembodiment the third set 105 comprises four thrusters 709a-d with correspondingthrust force vectors F709a-d as can be seen in FIG. 7b the thrusters 709a-d arearranged symmetrically around the center of mass (CoM). Similarly to the previouslydisclosed embodiments the thrusters are directed such that the impingement plumeis directed such that contact with the target to deorbit is avoided.

The embodiments disclosed with reference made to FIG 6 and 7 has theadvantage of decoupling the orbit control on the X and Y axes (in the particular casewhen the thrust forces are in the direction of the Centre of Mass). Thrust in -Zdirection can be obtained by actuating 2 thrusters only.

Some embodiment provides a solution that has the advantage of not thrustingdirectly toward the Target in case of close operations with an approach along +Z(reduced plume impingement effects).

Even if some embodiments of the present invention are perfectly adapted toActive Debris Removal missions, it is not the only case when it could be suitable. lt isalso adapted to the following kinds of missions: Missions which includes a rendezvous with an uncooperative (rotating) targets.

The target can be another satellite, an asteroid or any other type of body.

Missions which include both large orbital manoeuvres (orbit transfers) using alarge/powerful orbit control thruster, and precise rendezvous / formation flying orbit control. ln the drawings and the specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being set forth in the following claims.

Claims (1)

1. _ A thruster assembly for a spacecraft (100) with a coordinate system fixed in the spacecraft (100) having a X axis, a Y axis, and a Z axis, comprising: a first set (101) of at least three thrusters (102a-c,302a-d,402a-d),configured to provide thrust forces (F102a-c,F302a-d,F402a-d) in thepositive Z direction of the spacecraft (100), wherein the at least threethrusters are arranged such that a two-axis adjustable thrust torque canbe created along the X axis and along the Y axis; a second set (103) of at least four thrusters (104a-d) being configuredto provide thrust force (F104a-d) along the Y axis and along the X axis,and thrust torque along the Z axis of the spacecraft; and a third set (105) of at least one thruster (109,509a-b,609a-c) beingconfigured to provide thrust force (F109,F509a-b,F609a-c) in thenegative Z direction of the spacecraft. _ A thruster assembly for a spacecraft (100) according to claim 1, wherein the at least three thrusters of the first set are primarily directed to provide thrustforces in the positive Z direction of the spacecraft. _ A thruster assembly for a spacecraft (100) according to claim 1, wherein the second set (103) of at least four thrusters are arranged symmetrically aboutthe Z axis (101). _ A thruster assembly for a spacecraft (100) according to claim 3, wherein the second set (103) of thrusters are arranged in a plane parallel with the XY-plane, which is adjacent or in contact with the center of mass (CoM) of thespacecraft (100). _ A thruster assembly according to any preceding claim, wherein the thrust force components of the second set (103) is angularly separated in the XY plane. _ A thruster assembly according to claim 1, wherein the third set (105) comprises at least two thrusters (501 a-b, 601 a-c, 701 a-d) with thrust forces (F501a-b, F601a-c, F701a-d) that can be combined to provide torque-free force along the -Z direction only _ A thruster assembly according to claim 6, wherein the third set (105) is configured to provide a first pair of thrust forces, wherein each thrust force of the first pair forms a first angle (ßx) with the Z axis. _ A thruster assembly according to claim 7, wherein the third set (105) further is configured to provide a second pair of thrust forces, wherein each thrust force of the second pair forms a second angle (ßy) with the Z axis. _ A thruster assembly according to claim 8, wherein the first angle (ßx) and the second angle (ßy) are equal.