WO1991008949A2

WO1991008949A2 - Furlable sheet structures and methods of furling

Info

Publication number: WO1991008949A2
Application number: PCT/GB1990/001919
Authority: WO
Inventors: Gerald Edward Johnson; Gordon Kenneth Andrew Oswald; Stephen Temple; Mark Richard Shepherd
Original assignee: Cambridge Consultants Limited
Priority date: 1989-12-08
Filing date: 1990-12-10
Publication date: 1991-06-27
Also published as: WO1991008949A3; GB8927867D0

Abstract

The sail of a solar spacecraft or related structure can be furled by rotation about a central hub. Alternatively, hill and valley folds extend in a logarithmic spiral from the hub and the sail can be furled without strain, without multiple folds and without departing to any great extend from the plane of the unfurled sail. Support ribs every n'th hill fold take the form of flattenable tubes which are wound spirally about the hub in the furled structure. The set of the unfurled sail can be controlled by programmed flexure of the ribs to adjust the attitude of the craft relative to the direction of solar radiation.

Description

FURLABLE SHEET STRUCTURES AND METHODS OF FURLING

This invention relates to furlable sheet structures and in an important example to the use of such structures in solar spacecraft propelled by photon pressure. The invention also provides methods of furling.

A solar spacecraft, while being large (several hundred metres) in diameter, must have a low mass per unit area. Also, it should be packaged into a small diameter suitable for transit by rocket into earth orbit, and be capable of automatic unfurling or deployment in space.

Preferably, the craft should be capable of refurling for return to earth or for the close approach of other craft to enable repair or recharging.

There exist prior proposals for solar spacecraft and reference is directed in this regard to:-

Solar sailing engineering development mission Hoppy W Price (Jet Propulsion Lab) A1 AA Student Journal Summer 1981 p14

The development of a continuous manufacturing method for a deployable satellite mast in CFRP.

Aguirre-Martimez (ESRTC) D H Bowens R Davidson R J Lee and T Thorpe

(AERE Harwell)

ESA 22 p107-1 10

The problem has already been recognised of folding the metal coated foil so that it can be readily deployed or unfurled in space. Certain prior proposals assume that it is necessary to first build the supporting structure and then spread the foil in earth orbit. In the Price reference above, the foil is folded and refolded for packaging in the rocket and deployed (as in a sailing ship) by running the folded foil up two masts YY of substantial length, and subsequently unfurling the foil as a sheet on two masts ZZ. This proposal is limited to the use of four masts, and the foil or sail cloth is folded over and over in packaging so that shearing or tearing during deployment remains a problem which is not avoided.

SUBSTITUTE SHEET The following statements of invention describe methods and apparatus which in certain aspects meet the objectives of the above requirements, but which are not limited thereto.

It is one object of this invention to provide an improved method of furling a generally planar sheet, such as the sail of a solar spacecraft.

Accordingly, the present invention consists in one aspect in a method of furling a generally planar sheet about a central hub, comprising the steps of forming alternating sets of hill folds and valley folds in the sheet with each fold extending from the hub to the periphery of the sheet, and effecting relative rotation between the hub and the periphery of the sheet to cause the sheet to furl about the hub generally in the plane of the unfurled sheet.

Advantageously, the sheet is furled about the hub to occupy a volume the extent of which normal to said plane is less than 5% and preferably less than 1 % of the mean dimension from the hub to the periphery.

In another aspect, the present invention consists in a furlable sheet structure comprising a central hub and an annular sheet disposed about the hub, there being formed in the sheet alternating sets of hill folds and valley folds extending from the hub to the periphery of the sheet, such that on relative rotation between the hub and the ^'periphery of the sheet, the sheet is furled about the hub generally in the plane of the unfurled sheet.

Advantageously, said folds are formed at constant angular intervals about the hub of less than 5 degrees and preferably less than 2 degrees.

Preferably, the structure further comprises support ribs secured to the hub and extending along at least some of the hill folds, the support ribs having resistance to flexure normal to said plane and being adapted to wrap spirally aboux the hub on furling of said sheet.

Suitably, the structure further comprises a subsidiary hub mounted for powered rotation on said hub and having means engaging the sheet to effect furling thereof on said powered rotation.

SUBSTITUTE SHEET In a further aspect, the present invention consists in a rib capstan hub mounted on said central hub for rotation coaxially with said relative rotation of the hub and the periphery of the sheet, and a plurality of rib guides disposed radially to the rib capstan hub, fixed for rotation with the rib capstan hub and in sliding engagement with respective ribs.

Advantageously, there is provided an attitude controller and on at least four of said ribs, rib control means for effecting controlled flexure of the rib, the attitude controller acting through the respective rib control means to alter the set of the unfurled foil means.

In still a further aspect, the present invention consists in a method of controlling the attitude of a solar spacecraft having generally planar foil means supported on a plurality of substantially radial ribs, comprising deflecting some or all or the ribs in a direction normal to the plane of the foil means to cause the foil means to adopt selectively a shape and orientation with respect to the -direction of solar radiation which is selected from the group consisting of planar, concave conical, convex conical, saddle shaped and fluted.

In yet a further aspect, the present invention consists in a method of manufacturing a furlable structure comprising a hub, at least four ribs extending from the hub and adapted to be spirally wound about the hub and a sheet supported on said ribs, comprising the steps of arranging about said hub a plurality of rib spools each spool having wound thereon one of said ribs; attaching a free end of each spool wound rib to the hub so as to define an annulus of exposed ribs; applying one or more discrete sheet portions to said annulus of exposed ribs; effecting relative rotation between the hub and the array of rib spools so as to wrap said exposed rib portions with the applied sheet portions spirally about the hub and simultaneously exposing fresh rib portions from the rib spools and repeating said steps of applying sheet portions and effecting relative rotation until the desired diameter of sheet is attained.

The invention will now be described by way of example by reference to the accompanying drawings, in of which:-

SUBSTΠΓUTE SHEET Figure 1 shows diagrammatically a solar sail according to this invention, on axis and in the unfurled condition;

Figures 2a) and 2b) show a section of the rib used in the solar sail of Figure 1 , in its furled and unfurled condition respectively;

Figure 3 illustrates a method of furling according to this invention;

Figure 4 illustrates the shape of the fold lines, in a structure according to this invention;

Figure 5 is a section illustrating a mechanism according to a further embodiment of this invention for mechanically furling or unfurling the spacecraft;

Figure 6 illustrates a plan view of the spacecraft of Figure 5, during deployment;

Figure 7 illustrates in a part plan view, a method of assembling the spacecraft in a piece-by-piece manner without requiring an assembly floor comparable in area to the space sail when deployed;

Figure 8 shows a modified rib .section illustrating a method of flexing the rib normal to the folding axis;

Figures 9a), b) and c) shows various forms of set of the sail that may be achieved using flexure of the ribs, being respectively a cone, saddle and sawtooth or turbine profile;

Figure 10 illustrates the stability of the sail set into a convex or concave sectional profile.

SUBSTITUTE SHEET Figure 1 shows a solar spacecraft 10 in the form of a disc viewed on axis in the deployed or unfurled condition. The spacecraft 10 comprises a central hub 12 and a plurality of ribs 14a....141 deployed generally tangentially from the hub. Between the ribs is supported a metal coated foil 16 on which solar photon pressure acts to provide thrust for spacecraft navigation.

The hub 12 houses the active functioning components of the spacecraft 10 including energy supply, communication and control functions. The ribs 14 are attached to the hub and function as structural members to support the solar pressure and dynamic forces between the hub and the foil.

For a solar sail in orbit which is sufficiently large compared with the

2 total mass of the spacecraft (more than 600m per kilogram) thrust from photon pressure which may amount only to a few Newtons (N), will suffice over long periods to effect propulsion equivalent to that of a chemical rocket. The desired large area and low mass is obtained by using a very thin foil typically 1 -2/_m thick, which is supported on ribs that are both extremely slender and very light. Although not evident from the diagram, the length of the ribs is typically one or a few hundred times the diameter of the hub 12. The ribs are typically of the type described in the Price and Aguirre-Martimez references above or as described below.

The number of ribs 14 illustrated in Figure 1 is twelve. Although a spacecraft in accordance with the mvention may include any number of ribs above three and preferably above six, it is explained below that a multiple of twelve ribs is a practical arrangement and some upper number of ribs in the region of 96 obtains due to limits of complexity and weight of the spacecraft.

One feature of a spacecraft in accordance with the invention is that it is capable of being furled or folded up for transit from the Earth by rocket into earth orbit and unfurled with the form shown in Figure 1 on arrival into orbit. Preferably, the sheet can be furled into a volume the depth of which normal to the plane of the unfurled sail is less than 5% or, better, less than 1 % of the sail diameter. A particular feature is the form of the folded structure, enabling the sail to be packed efficiently without introducing multiple folds and without inducing shear in the foil. A further feature is the folding procedure which enables the ribs 14 and the coated foil 16 to be progressively assembled and furled onto the hub.

SUBSTITUTE SHEET A further feature of the spacecraft in accordance with the invention is that it is agile when deployed in space, so that it is able to extract a turning moment about its principal axes in space from the solar pressure. One feature is that the solar sail is controllable to acquire either a stable or an unstable attitude in space. When it is unstable it can be rotated about a diameter to alter its attitude in space and to take up a different stable attitude and therefore to alter the direction of the thrust vector on the spacecraft. A consequent feature is that the spacecraft is controllable for navigation under solar photon pressure in space. A further feature is that the spacecraft is controllable to extract angular momentum about the axis of the hub from solar pressure and thus to alter its speed of rotation.

A further feature of the solar spacecraft is that the structure including the ribs 14 and the foil 16 can be unfurled or furled up again in space. This enables the spacecraft 10 to be approached by another spacecraft and therefore collected to be returned to earth in a suitable enclosure: or alternatively to be repaired or recharged. These and other related features are described in the following paragraphs.

Figure 2 shows sectional views of the ribs 14. In the deployed condition the rib has a lenticular (lens shaped) profile 18 formed from two sheets of thin resin bonded carbon fibre reinforced plastic (CFRP) 20 united at the flanges 22 as illustrated in Figure 2b). By employing a thermoset resin such as poly-ether-sulphone (PES) the sheets can be preformed with the desired profile along the length of the rib and the rib may also be tapered in a direction away from the hub, to effect weight reduction. The lenticular profile is chosen so that the flexural rigidity of the rib section is approximately equal about its principal axes.

Owing to the characteristics of the lenticular profile, the ribs 14 can also be rolled up (or furled) round the hub 12 to package the structure for transit by rocket from the Earth into orbit. When the spacecraft is located in orbit it can then be unfurled or deployed by allowing the ribs 14 to unroll and can also be rewound or refurled onto the hub. The section of the ribs when furled is shown in Figure 2a). In the furled state the collapsed ribs are elastically stressed and, when unfurling, the ribs release strain energy and assume their original profile of Figure 2b). Deployment therefore can occur passively, using stored energy obtained during furling, the ribs being allowed gradually to relax during unfurling. In particular a plurality of ribs can be wound in nested spirals in the hub, and automatically unwound in collaboration with one another, thus representing a convenient design for furling and unfurling a solar sail. The ribs when partly unfurled from round the hub 12 are illustrated in part perspective in Figure 3.

Figure 4 illustrates the method of folding a flat sheet 30 about a central disc 32. Fold lines which are alternatively hill folds 34 (heavy lines) and valley folds 36 (light lines) are formed on the flat sheet. The fold lines have the intrinsic equation -

ds = Hs. (r + nts ) dj> nt 2ttf

Where s is the length along the fold or rib

r is the radius about the central disc or hub

n is the number of folds

t is the thickness between folds or ribs

<j) is the tangent angle of the fold line.

It can then be demonstrated that the flat sheet can be folded up spirally round the central disc by rotating the central disc in the manner illustrated in Figure 3. The fold lines above can be shown to be a type of logarithmic spiral and are such that a stiff sheet, when fold lines are formed, can be rolled up round the central disc without inducing shear in the sheet plane or tearing during folding or unfolding. In this manner the radius is reduced from radius approximately s to a radius of about

(r + st )

SUBSTITUTE SHEET In the application to a solar sail, tl2$τ is a small number of the order

-3 -4 of 10 to 10 , so that the sheet being a foil of radius s, when furled reduces in size to a radius only a little greater than to the radius of the hub. The locus of the ribs is that of the fold lines which depart in practice only a few degrees from straight when unfurled and when furled are wrapped with hill and valley folds of the foil repeatedly, typically 10-12 times, round the hub 12. The angle between neighbouring fields is preferably less than 5 degrees and, better, less than 2 degrees.

Figure 3 illustrates the fold structure. It consequently illustrates the method of deployment or unfurling of the solar spacecraft in orbit. The spacecraft is delivered into orbit by rocket in the furled condition. In this state the coated foil is attached to the ribs in locations representing the hill folds 34 and drawn down axially relative to the hub forming the valley folds. One arrangement incorporates a rib 14 at every hill fold 34, so that there is only one valley fold 36 between each rib. Alternatively a rib may be placed at every second, third or n'th hill fold so that there are n valley folds and n-1 unsupported hill foldjs between each rib. Other forms of fold which do not extend to the periphery of the sail all the way to the hub may also be employed.

To deploy the spacecraft, a retaining ring (not shown), which holds the ribs during packaging is removed when in orbit by igniting explosive bolts. The ribs 14, which as described above are furled elastically and tend to unfurl by releasing stored fiexural energy, then begin to unfurl. Accordingly the solar sail of the spacecraft is or may be automatically deployed extending from its initial furled radius somewhat above r to the fully deployed span of around 2s.

A mechanism for automatically furling and unfurling the ribs is illustrated in Figures 5 and 6. In these figures the coated foil has been omitted for clarity. The hub is modified, providing a main hub 40 which supports the energy supply, control and communications modules; and a rib capstan hub 42. The hub 42 is mounted on roller bearings 44 so that it is capable of rotation relative to main hub 40, being driven by a motor driven worm gear 46 and planetary pinion 48. Extending from the main hub are arms 50, supporting guides 52 against which each rib 14 rests. There exists a guide 52 for each rib. It will then be evident that by energizing the worm drive, the ribs can be actively furled or unfurled round the rib capstan hub. An axial view is shown in Figure 6.

SUBSTITUTE SH It will be evident that alternative mechanisms can be devised to furl and unfurl the spacecraft. Such systems are a prerequisite to approach the solar spacecraft (e.g. with a conventional chemical thrust engine) to repair or deliver supplies to the craft or to collect it for a return journey to earth.

Assembly of the solar spacecraft can be accomplished on the ground progressively in sections without the necessity for an open space of the size of the unfurled spacecraft in orbit, as illustrated in Figure 7.

Here, the hub assembly 60 is mounted centrally, so that the main hub is rotatable and the rib capstan hub fixed. A number of rib spools 62 are then set up at intervals radially round the hub assembly, each rib spool having the rib wound round it, the number of rib spools being equal to the number of ribs on the spacecraft. Only four rib spools 62a) to 62d) are illustrated.

Each rib is partially unfurled from its spool and attached substantially tangentially to the rib capstan hub at a predetermined angle according to the intrinsic equation of the fold line, this angle being the intercept of the spiral of the folded ribs on the hub, with the hub tangent. After attaching the ribs it is straightforward to wind the ribs onto the hub even in a relatively confined space. This is performed gradually as the foil is progressively attached in sections to the ribs.

Starting with the ribs unfurled (but wound on the spools) sections of foil are attached to the ribs in successive turns, each section having a predefined shape and folds. The sections can be supported or draped over filaments drawn from filament spools 63 between the rib spools, the filaments assisting to hold the fold lines. The sections in the first turn are attached to the ribs so that the foil is substantially flat, but successively, as the ribs are furled to attach sections of further turns, the sections are prefolded and may be draped over the filaments. As further turns are added, progressively deeper folds are formed in the sections. Clearly, having regard to the thin section of the foil, all motions are performed very gradually.

SUBSTITUTE SH ET After the last turn of sections are attached and wound onto the hub, the tips of the ribs are secured to the arms so (or to a securing ring) holding the ribs and the foil firmly in a package of predetermined radius.

An important feature of the ribs 14 is that they are capable of selective flexure, so that the ribs can each be bent in a controlled manner into the plane

0 containing the axis of the hub. The amount of flexure (say 1 per metre length) is small compared with the elastic limit of the rib. However, if programmable flexure control is provided in each of many longitudinal sections extending in total several tens of metres along each rib, then the set of the sail can be programmably varied as will later be described with reference to Figure 9.

One manner of causing the ribs to flex is illustrated in Figure 8. In this diagram a longitudinal section 70 of a rib 14 is provided with wires 72,73 extending in tension between anchorages 74. If either wire is heated, it will relax and cause a nat bending moment. In this way there is effectively created a hinge providing flexure normal to the rib folding axis. The temperature of the wires can be measured by sensing their electrical resistance and thus the attitude of the hinge can also be monitored.

One method of differentially heating the wires 72 and 73 relatively is by incorporating into each wire an electrically heated resistive layer and applying a Joule heating current until in equilibrium with the space environment. An alternative approach is to provide an eiectrochromic coating to vary the emissivity of the wires. The wires are illustrated external to the rib 14 but may also be placed internal to the rib structure.

Programmable rib flexure enables profiles such as those in Figure 9a), b) or c) to be accomplished and this provides a means of causing the spacecraft to spin in space about its three axes and thus to be actively controlled in attitude relative to the sun.

SUBSTITUTE SHEET Figure 10a) shows diagrammatically a sail convex in section due to flexure near the hub. Solar radiation, if the metallized foil is reflecting to maximise the available force, acts normally to the surface of the foil. The moment of the radiation force has also a greater magnitude on the right (as illustrated) than on the left. Thus the sail when convex receives a corrective moment tending to set the axis of the hub pointing to the sun. The convex configuration is stable.

Conversely, the concave configuration shown in Figure 10b) receives a destabilising moment and is unstable. This configuration would be expected to flip over.

Considering Figure 9a) a conical set thus tends to cause the sail to be pointing towards the direction of the sun regardless of initial condition. The saddle setting of Figure 9b), however, defines a stable and an unstable diameter and this generates a diametrical axis of rotation. Further, by alternating the setting from that of .Figure 9a) to that of Figure 9b), at selected intervals, it is possible to set the sail such that the axis has a predefined offset angle relative to the direction of the sun, which causes a net propulsive force offset at the angle.

It has further been realised that when the circular profile of the sail is set with a sawtooth profile there is a net rotation effective on the sail which can be used to spin the spacecraft about its hub axis, and thus can be employed to stabilise the craft by centrifugal forces, or to correct for prior manoeuvres. Such a profile is best obtained with twelve or some other multiple of three ribs.

Whilst the invention has so far been described with a circular hub and a circular sail, these are not the only alternatives. The hub can, in the plane of the unfurled sail, take the form of an ellipse or other continuous curve. Moreover, the hub might in special circumstances be rectangular or polygonal. The unfurled sheet will typically be planar but it is to be understood that the invention encompasses conical, hemispherical or other surfaces which are generally planar in this context. In certain applications the sheet need not be continuous.

SUBSTITUTE SHEET In addition to the described solar spacecraft, this invention will provide a wide variety of furlable structures. These may include a solar panel, a wind sail and a retractable cover. Myriad further applications will occur to the reader in which it is desired to furl a planar sheet to a much reduced area without departing substantially from the plane and without introducing multiple folds or strain in the sheet material.

SUBSTITUTE SHEET

Claims

1. A method of furling a generally planar sheet about a central hub, comprising the steps of forming alternating sets of hill folds and valley folds in the sheet with each fold extending from the hub to the periphery of the sheet, and effecting relative rotation between the hub and the periphery of the sheet to cause the sheet to furl about the hub generally in the plane of the unfurled sheet.

2. A method according to Claim 1 , wherein the sheet is furled about the hub to occupy a volume the extent of which normal to said plane is less than 5% and preferably less than 1 % of the mean dimension from the hub to the periphery.

3. A method according to Claim 1 or Claim 2, wherein one set of folds lies in the plane of the unfurled sheet as the sheet is furled.

4. A method according to any one of the preceding claims, wherein said folds are the only folds in the furled sheet.

5. A method according to any one of the preceding claims, wherein the hub describes a continuous curve in said plane.

6. A method according to any one of Claims 1 to 4, wherein the hub is circular in said plane.

7. A method according to Claim 5 or Claim 6, wherein said folds are disposed substantially tangentially to the hub.

8. A method according to any one of the preceding claims, wherein each fold generally, describes a logarithmic spiral in said piane.

9. A method according to any one of the preceding claims, wherein each fold is generally defined by the equation:-

ds = 21s (r + nts )

SUBSTITUTE SHEET Where:- s is the length along the fold or rib

r is the radius about the central disc or hub

n is the number of folds

t is the thickness between folds or ribs

is the tangent angle of the fold line.

10. A method according to any one of the preceding claims, wherein said folds are formed at constant angular intervals about the hub.

1 1 . A method according to Claim 10, wherein said angular interval is less than 5 degrees and preferably less than 2 degrees.

12. A method according to any one of the preceding claims, wherein the hub comprises an integral portion of the sheet.

13. A furlable sheet structure comprising a central hub and an annular sheet disposed about the hub, there being formed in the sheet alternating sets of hill folds and valley folds extending from the hub to the periphery of the sheet, such that on relative rotation between the hub and the periphery of the sheet, the sheet is furled about the hub generally in the plane of the unfurled sheet.

14. A structure according to Claim 13, wherein the furled sheet occupies a volume the extent of which normal to said plane is less than 5% and preferably less than 1 % of the mean dimension from the hub to the periphery of the unfurled sheet.

15. A structure acording to Claim 13 or Claim 14, wherein, with the sheet furled about the hub, one set of said folds lie in a plane.

16. A structure according to any one of Claims 13 to 15, wherein said folds are the only folds in the furled sheet.

S

17. A structure according to any one of Claims 13 to 16, wherein the hub describes a continuous curve in said plane.

18. A structure according to any one of Claims 13 to 16, wherein the hub is circular in said plane.

19. A structure according to Claim 17 or Claim 18, wherein said folds are disposed substantially tangentially of the hub.

20. A structure according to any one of Claims 13 to 19, wherein each fold generally describes a logarithmic spiral in said plane.

21 . A structure according to any one of Claims 13 to 20, wherein each fold is generally defined by the equation:-

ds = 2τfs (r + nts )

Where:- s is the length along the fold or rib

r is the radius about the central disc or hub

n is the number of folds

t is the thickness between folds or ribs

φ is the tangent angle of the fold line.

22. A structure according to any one of Claims 13 to 21 , wherein said folds are formed at constant angular intervals about the hub.

23. A structure according to Claim 22, wherein said angular interval is less than 5 degrees and preferably less than 2 degrees.

24. A structure according to any one of Claims 13 to 23, wherein the hub comprises an integral portion of the sheet.

SUBSTITUTE SHEET

25. A structure according to any one of Claims 13 to 24, further comprising support ribs secured to the hub and extending along at least some of the hill folds, the support ribs having resistance to flexure normal to said plane and being adapted to wrap spirally about the hub on furling of said sheet.

26. A structure according to Claim 25, wherin the ribs comprise flattenable tubes.

27. A structure according to Claim 25 or Claim 26, wherein the ribs are adapted such that said spiral wrapping about the hub effects resilient deformation of the ribs.

28. A structure according to Claim 27, wherein on release of said spirally wrapped ribs, the energy in said resilient deformation is sufficient to effect unfurling of the sheet.

29. A structure -according to any one of Claims 25 to 28, further comprising rib capstan means provided on said hub and cooperable with at least some of the ribs so as on actuation to effect spiral wrapping of the ribs about the hub.

30. A structure according to Claim 29, wherein said rib capstan means comprises a rib capstan hub mounted on said central hub for rotation coaxially with said relative rotation of the hub and the periphery of the sheet, and a plurality of rib guides disposed radially to the rib capstan hub, fixed for rotation with the rib capstan hub and in sliding engagement with respective ribs.

31. A structure according to any one of Claims 25 to 30, wherein there are provided on at least some of said ribs, means for controlled flexure of the rib in a direction normal to said plane.

32. A structure according to any one of Claims 13 to 31 comprising a subsidiary hub mounted for powered rotation on said hub and having means engaging the sheet to effect furling thereof on said powered rotation.

33. A structure according to any one of Claims 13 to 32, in the form of a solar spacecraft.

34. A solar spacecraft comprising a hub; a plurality of ribs extending from the hub and adapted to be spirally wound about the hub and foil means for receiving solar photon pressure to be communicated to the hub through said ribs, the foil means having alternative hill and valley folds so as to furl about the hub on said wrapping of the ribs.

35. A spacecraft according to Claim 34, wherein each said fold generally describes a logarithmic spiral.

36. A spacecraft according to Claim 34 or Claim 35, wherein each rib is adapted to undergo resilient deformation on wrapping about said hub.

37. A spacecraft according to any one of Claims 34 to 36, further comprising rib capstan means mounted for powered rotation relative to the hub and cooperable with said ribs to effect furling of the foil means on said powered rotation.

38. A spacecraft according to any one of Claims 34 to 37, wherein there is provided an attitude controller and on at least four of said ribs, rib control means for effecting controlled flexure of the rib, the attitude controller acting through the respective rib control means to alter the set of the unfurled foil means.

39. A spacecraft according to Claim 38, wherein said attitude controller is programmed through controlled flexure of said ribs to induce a conical set in the unfurled foil means optionally convex or concave to a specified direction.

40. A spacecraft according to Claim 38, wherein said attitude controller is programmed to induce a saddle-shaped set in the unfurled foil means.

41 . A spacecraft according to Claim 38, wherein said attitude controller is programmed to induce a fluted set in the unfurled foil means.

SUBSTITUTE SHEET

42. A method of controlling the attitude of a solar spacecraft having generally planar foil means supported on a plurality of substantially radial ribs, comprising deflecting some or all or the ribs in a direction normal to the plane of the foil means to cause the foil means to adopt selectively a shape and orientation with respect to the direction of solar radiation which is selected from the group consisting of planar, concave conical, convex conical, saddle shaped and fluted.

43. A method of manufacturing a furlable structure comprising a hub, at least four ribs extending from the hub and adapted to be spirally wound about the hub and a sheet supported on said ribs, comprising the steps of arranging about said hub a plurality of rib spools each spool having wound thereon one of said ribs; attaching a free end of each spool wound rib to the hub so as to define an annulus of exposed ribs; applying one or more discrete sheet portions to said annulus of exposed ribs; effecting relative rotation between the hub and the array of rib spools so as to wrap said exposed rib portions with the applied sheet portions spirally about the hub and simultaneously exposing fresh rib portions from the rib spools and repeating said steps of applying sheet portions and effecting relative rotation until the desired diameter of sheet is attained.