WO2004098994A2

WO2004098994A2 - Attitude determination and control system for a solar sail spacecraft

Info

Publication number: WO2004098994A2
Application number: PCT/US2003/040100
Authority: WO
Inventors: Bong Wie
Original assignee: University of Arizona; Arizona's Public Universities
Current assignee: University of Arizona; Arizona's Public Universities
Priority date: 2002-12-13
Filing date: 2003-12-15
Publication date: 2004-11-18
Anticipated expiration: 2005-06-13
Also published as: WO2004098994A3

Abstract

A solar sail spacecraft with microthrusters approximate edges of the solar sail. The microthrusters provide attitude control. The microthrusters are generally mounted along spar, or boom, tips, providing increased moment arm, thereby increasing control torque while reducing propellant usage. Some embodiments additionally include control vanes also approximate the spar tips.

Description

LOW-COST, LOW-MASS, LOW-VOLUME AND LOW-POWER ATTITUDE

DETERMINA ATTIIOONN AANNDD CCOONNTTRROOLL SSYYSSTTEEMM ((LL⁴⁴ADCS) FOR

SOLAR SAIL MICROSPACECRAFT

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Patent Application No. 60/433,053, filed December 13, 2002, entitled "LOW-COST, LOW-MASS, LOW-VOLUME AND LOW-POWER ATTITUDE DETERMINATION AND CONTROL SYSTEM (L4ADCS) FOR SOLAR SAIL MICROSPACECRAFT", the disclosure of which is incoφorated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to spacecraft, and more particularly to attitude control of solar sail spacecraft.

Solar sail spacecraft generally are considered to include a relatively large reflective surface providing thrust for the spacecraft. The thrust provided to the spacecraft is generally dependent on light reflected from the reflective surface. The orientation of the reflective surface with respect to a light source therefore is significant for thrust considerations, as well as generally for requirements imposed by desired use of a payload carried by the solar sail spacecraft.

In conventional sail control systems, a gimbaled control boom system (or a control vane system) is considered as the primary control system while a commercial microspacecraft bus ADCS is considered as the secondary control system. However, typical attitude control actuators, such as small reaction wheels and thrusters with a short moment arm length (< 0.5 m), employed for a typical commercial microspacecraft (inertia < 100 kg-m²) generally will not be suitable for controlling sailcraft with a large moment of inertia (< 5,000 kg-m²).

A gimbaled control boom system of a previously proposed NMP ST7 40-m sailcraft provides only two axis (pitch and yaw) control without redundancy; i.e., the gimbaled boom system has a single-point failure problems and also requires a separate roll-axis control system. Furthermore, its gimbal angular motion and/or steady state tilt angle may interfere with a payload/bus pointing system (e.g., sensors, cameras, solar arrays, etc.) because the sail payload/bus is mounted at the tip of a 2-m gimbaled control boom.

SUMMARY OF THE INVENTION

The invention provides a attitude control system for a solar sail spacecraft. In one aspect the invention provides a solar sail spacecraft comprising a plurality of spars, with tips of the spars defining an outline of a polygon; a solar sail mounted on the spars; and at least one microthruster mounted approximate at least some of the spars. In another aspect the invention provides a control system for a solar sail spacecraft, the solar sail spacecraft comprising a plurality of substantially linear spars having tips defining an outline of a polygon and a solar sail mounted to the spars substantially within the outline of the polygon and a payload approximate a center of the polygon, the control system comprising microthrusters mounted approximate the tips of a plurality of the spars. In another aspect the invention provides A control system for a solar sail spacecraft, the solar sail spacecraft comprising a plurality of substantially linear spars having tips defining an outline of a polygon and a solar sail mounted to the spars substantially within the outline of the polygon and a payload approximate a center of the polygon, the control system comprising means for providing thrust approximate the tips of a plurality of the spars.

These and other aspects of the invention are more fully comprehended in view of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a solar sail spacecraft including aspects of the invention;

FIG. 2 illustrates a storage box and associated spar tip end elements in accordance with aspects of the invention; and

FIG. 3 further illustrates a storage box and associated spar tip end elements along with a portion of a spar in accordance with aspects of the invention.

DETAILED DESCRIPTION

A solar spacecraft is illustrated in FIG. 1. The spacecraft includes a solar sail 11. Payload items are located approximate a center of the solar sail. The payload illustrated in FIG. 1 includes star cameras 13, sun sensors 15, and a payload bus 17. The solar sail is mounted on four spars 17, or booms, whose end points 19, or tips, somewhat form an outline of a polygon in the form of a parallelogram.

The solar spacecraft has an attitude control system including thrusters 21, for example microthrusters, or jets. The microthrusters are mounted approximate tips of the spars, some embodiments, and as illustrated in FIG. 1, the microthrusters include orthogonally mounted microthrusters. In one embodiment, one set of a first set of somewhat linear spars include orthogonally mounted roll/yaw microthrusters, and a second set of somewhat linear spars include orthogonally mounted roll/pitch microthrusters, with the roll axis being substantially peφendicular to the solar sail.

In some embodiments the microthrusters are mounted on storage boxes 23 at ends of the spars. In various embodiments, extending from each storage box is a support boom 25. A trim tab 27 is at the end of the support boom. The trim tab provides a control vane.

FIG. 2 illustrates a storage box 31 and extending support boom 33. Microthrusters 35 are mounted on the storage box. A trim tab 37 is at the end of the support boom. The trim tab includes photovoltaic cells 39 integrated into membranes of the trim tab. The photovoltaic cells provide for local electric power for electronics associated with the trim tab, microthrusters, and storage box.

At times, any reaction-jet system that is applicable to a microspacecraft of mass less than 160 kg is called a microthruster. Thrust levels and impulse bits of a typical microthruster are approximately 1 to 50 mN and 1 to 50 μNs, respectively. Thrust levels of a typical microthruster are approximately 0.01 to 1.0 mN.

The microthruster modules are mounted at the spar tips to maximize the control torque produced by a microthruster of an extremely low thrust level, thus minimizing total propellant usage. For example, a 1-mN microthruster with a moment arm length of 50 m produces a control torque of 50 mN-m and it would reduce propellant requirements by a factor of 50 over the same 1-mN microthruster with a moment arm length of 1 m. Fuel- optimal thruster firing logic, which is properly integrated or coordinated with control vane steering logic, for the tip-mounted microthrusters to damp out the rigid-body as well as structural mode oscillations.

In some embodiments external disturbance torques, including the solar-pressure imbalance torques caused by a center-of-mass and center-of-pressure offset uncertainty, are generally counteracted by the solar-pressure control surfaces (trim tabs), instead of by thruster firings. However, certain solar sail missions may require only the primary microthruster control system if the solar pressure imbalance torque can be counteracted by microthrusters with reasonable propellant usage. For such cases, the control vanes may not be provided, or they can be replaced by deployable solar arrays to provide electric energy to the solar sail bus system and/or the microthruster system equipped with ion-thrusters (e.g., a 3 -cm diameter micro-ion engine or micro Hall thrusters).

Various microthrusters are used in different embodiments. The various microthrusters include digital microthruster arrays, vaporizing liquid microthruster (VLM) fabricated on a silicon chip, micron-sized cold-gas thrusters, micro Hall thrusters, micro-ion engines, free molecule micro-resistojets, (FMMRs) and micro pulsed plasma thrusters (μPPT). VLMs of extremely low thruster weight and size provide a very low thrust on the order of 1 mN thrust level. ULMs generally vaporize a liquid propellant such as water, ammonia or hydrazine inside a micromachined, thin film deposited heater assembly. FMMRs generally utilize a water propellant and operate at very low stagnation pressures (50 to 500 Pa). FMMRs may be described as light weight MEMS-fabricated thrusters consisting of thin film heating elements and multiple exhaust slots. A long and narrow slot (e.g., 1 cm x 100 μm) is chosen to loosely prevent the possibility of plugging a nozzle throat with contaminants. Furthermore, the free-molecule condition may be chosen to reduce the propellant storage pressure requirement. An FMMR assembly of 40 slots, which produces total thrust levels of 4 to 6 mN at a low power level of 15W, uses approximately a 2.5 x 2.5 cm surface area. μPPTs use an inert solid propellant (Teflon) with no moving parts and no leaking problems. Digital microthruster arrays (e.g., consisting of 10,000 micro fabricated, single-shot thrusters placed onto a 1-cm silicon wafer) include multiple thrusters, with each thruster with 1.6μg propellant mass fired only once to produce about 10-mN thrust, and about lOmW power will be required for a single shot firing. A major advantage of digital microthruster arrays over other microtlirusters is their simplicity, not requiring micromachined valves, complex feed systems, and separate propellant tanks. The arrays represent a complete modular propulsion system. However, overall array dimensions may be quite large for some cases; i.e., a 10 x 10 cm surface area may be needed for 10⁵ single-shot thrusters (cavities). In some embodiments such digital microthruster arrays are distributed along the sail spars for distributed attitude control actuation and/or active damping control of sail structural vibrations.

The system can also be employed for most spin-stabilized sails more effectively with less propellant usage. It can also be employed for a semi-passively stabilized sailcraft such as the Team Encounter sailcraft currently under development. The tip-mounted thrusters or the active pitch/yaw control vanes can effectively damp out the troublesome rigid-body mode oscillations of a passively stabilized sailcraft such as the Encounter sail. For certain solar sail missions, only the primary microthruster control system may be needed if the solar pressure imbalance torque can be counteracted by microthrusters with reasonable propellant usage.

The system can also be used with a gimbaled control boom system instead of control vanes. For certain solar sail missions, only the primary microthruster control system may be needed if the solar pressure imbalance torque can be counteracted by microthrusters with negligible (or reasonable) propellant consumption.

FIG. 3 further illustrates items associated with an end of a spar tip in some embodiments along with a portion of a spar. A spar, which is a deployable or inflatable boom 41, extends, for example, from approximate a central section or payload section (not shown) of a solar spacecraft. A stowage box 43 is at the end of the boom. The stowage box is used for mounting and deploying a trim tab 45 coupled to the storage box by a support boom 47. The support boom is moveable using a two-axis actuator. The stowage box additionally stores a trim tab deployment mechanism, actuator, control electronics, and deployable solar array. The stowage box also has mounted thereon or therein microthrusters 47, and the storage box may additionally store a propellant tank.

The invention therefore provides means for attitude control of a solar sail spacecraft. Although the invention has been described in certain embodiments, it should be recognized that the invention includes the claims and their equivalents supported by this disclosure.

Claims

I. A solar sail spacecraft comprising: a plurality of spars, with tips of the spars defining an outline of a polygon; a solar sail mounted on the spars; and at least one microthruster mounted approximate at least some of the spars.

i 2. The solar sail spacecraft of claim 1 wherein the at least one microthruster is mounted approximate tips of at least some of the spars.

3. The solar sail spacecraft of claim 2 wherein multiple microthrusters are mounted approximate at some of the tips of the spars.

4. The solar sail spacecraft of claim 3 wherein the microthrusters approximate any one of the tips of the spars are orthogonally mounted with respect to one another.

5. The solar sail spacecraft of claim 4 wherein the microtlirusters approximate any one of the tips of the spars are mounted on a storage box.

6. The solar sail spacecraft of claim 5 wherein a plurality of tips of the spars have storage boxes mounted thereto, each storage box having orthogonally mounted microthrusters.

7. The solar sail spacecraft of claim 6 wherein a first set of tips of the spars include storage boxes with orthogonally mounted roll/yaw microthrusters and a second set of tips of the spars include storage boxes with orthogonally mounted roll/pitch microthrusters.

8. The solar sail spacecraft of claim 7 further comprising a support boom extending from each storage box.

9. The solar sail spacecraft of claim 8 further comprising a trim tab at the end of the support boom.

10. The solar sail spacecraft of claim 3 wherein the spars are substantially linear.

II. The solar sail spacecraft of claim 10 wherein tips of the spars substantially define corners of a parallelogram.

12. The solar sail spacecraft of claim 11 further comprising a payload approximate a center of the parallelogram.

13. The solar sail spacecraft of claim 1 wherein the at least one microthruster comprises multiple microthrusters mounted along the spars in a distributed manner.

14. The solar sail spacecraft of claim 1 wherein the at least one microthruster is an ion thruster.

15. The solar sail spacecraft of claim 1 wherein the at least one microthruster is a digital microthruster arrays.

16. The solar sail spacecraft of claim 15, wherein the digital microthruster arrays are distributed along the spars.

17. The solar sail spacecraft of claim 1 wherein the at least one microthruster is a vaporizing liquid microthruster fabricated on a silicon chip.

18. The solar sail spacecraft of claim 1 wherein the at least one microthruster a micro- sized cold-gas thruster.

19. The solar sail spacecraft of claim 1 wherein the at least one microthruster is a micro Hall thruster.

20. The solar sail spacecraft of claim 1 wherein the at least one microthruster is a micro- ion engine.

21. The solar sail spacecraft of claim 1 wherein the at least one microthruster is a free molecule micro-resistojet.

22. The solar sail spacecraft of claim 1 wherein the at least one microthruster is a micro pulsed plasma thruster.

23. A control system for a solar sail spacecraft, the solar sail spacecraft comprising a plurality of substantially linear spars having tips defining an outline of a polygon and a solar sail mounted to the spars substantially within the outline of the polygon and a payload approximate a center of the polygon, the control system comprising: microthrusters mounted approximate the tips of a plurality of the spars.

24. The control system of claim 23 wherein the microthrusters are mounted orthogonally with respect to one another.

25. The control system of claim 24 wherein the tips of the plurality of spars have storage boxes mounted thereto, each storage box having orthogonally mounted microthrusters.

26. The control system of claim 25 wherein a first set of tips of the spars include storage boxes with orthogonally mounted roll/yaw microthrusters and a second set of tips of the spars include storage boxes with orthogonally mounted roll/pitch microtlirusters.

27. The control system of claim 26 further comprising a support boom extending from each storage box.

28. The control system of claim 27 further comprising a trim tab at the end of the support boom.

29. The control system of claim 28 wherein the tips of the spars substantially define corners of a parallelogram.

30. The control system of claim 29 further comprising a payload approximate a center of the parallelogram.

31. The control system of claim 23 wherein the at least one microthruster comprises multiple microthrusters mounted along the spars in a distributed manner.

32. The control system of claim 23 wherein the at least one microthruster is selected from a group comprising ion thrusters, digital microthruster arrays, vaporizing liquid microthrusters fabricated on a silicon chip, micro-sized cold-gas thrusters, micro Hall thrusters, micro-ion engines, free molecule micro-resistojets, and micro pulsed plasma thrusters.

33. A control system for a solar sail spacecraft, the solar sail spacecraft comprising a plurality of substantially linear spars having tips defining an outline of a polygon and a solar sail mounted to the spars substantially within the outline of the polygon and a payload approximate a center of the polygon, the control system comprising: means for providing thrust approximate the tips of a plurality of the spars.

34. The control system for a solar sail spacecraft of claim 33 wherein the means for providing thrust approximate any spar tip provides thrust in substantially orthogonal directions.

35. The control system for a solar sail spacecraft of claim 33 wherein the means for providing thrust, across the plurality of spar tips, provides thrust along three substantially orthogonal axes.