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ABSTRACT

Hundreds of space missions have been launched since the last lunar mission, including

several deep space probes that have been sent to the edges of our solar system. However, ourjourneys to space have been limited by the power of chemicalrocket enginesand the amount

of rocket fuel that a spacecraft can carry. Today, the weight of a space shuttle at launch is

approximately 95 percent fuel. hat could we accomplish if we could reduce our need for so

much fuel and the tanks that hold it!

"nternational space agencies and some private corporations have proposed many methods of

transportation that would allow us to go farther, but a manned space mission has yet to go

beyond the moon. The most realistic of these space transportation options calls for the

elimination of both rocket fuel and rocket engines ## replacing them with sails. $es, that%s

right, sails.

&olar#sail mission analysis and design is currently performed assuming constant optical and

mechanical properties of the thin metali'ed polymer films that are projected for solar sails.

(ore realistically, however, these properties are likely to be affected by the damaging effects

of the space environment. The standard solar#sail force models can therefore not be used to

investigate the conse)uences of these effects on mission performance. The aim of this paper

is to propose a new parametric model for describing the sail film%s optical degradation with

time. "n particular, the sail film%s optical coefficients are assumed to depend on its

environmental history, that is, the radiation dose. *sing the proposed model, the optimal

control laws for degrading solar sails are derived using an indirect method and the effects of

different degradation behaviors are investigated for an example interplanetary mission.

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CONTENTS

1. "ntroduction

+. &olar &ail oncept

-. &olar &ail onstruction

. &olar &ail /ynamics and control

.0 ruising by &unlight

5. &olar &ail (aterial

5.0 1luminium as (aterial

5.0.0Titainum as reinforcing material

5.0.+&iliconmonoxide as reinforcing material

5.0.-2oron as reinforcing material

3. &olar &ail 4aunch

. "nvestigated &ail /esign

6. osmos#0 &pacecraft /esign

9. 1dvantages

07. 4imitations

00. (isunderstandings

0+. 8uture utlook

0-. :eferences

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1. INTRODUCTION

Hundreds of space missions have been launched since the last lunar mission, including

several deep space probes that have been sent to the edges of our solar system. However, our

journeys to space have been limited by the power of chemical rocket engines and the amount

of rocket fuel that a spacecraft can carry. Today, the weight of a space shuttle at launch is

approximately 95 percent fuel. hat could we accomplish if we could reduce our need for so

much fuel and the tanks that hold it!

"nternational space agencies and some private corporations have proposed many methods of

transportation that would allow us to go farther, but a manned space mission has yet to go

beyond the moon. The most realistic of these space transportation options calls for the

elimination of both rocket fuel and rocket engines ## replacing them with sails.

;1&1is one of the organi'ations that has been studying this ama'ing technology called solar

sailsthat will use the sun%s power to send us into deep space.

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2. SOLAR SAIL CONCEPT

;early 77 years ago, as much of

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3. SAIL CONSTRUCTION

The strategy for near#term sail construction is to make and assemble as much of the

sail as possible on earth. Thus, while the delicate films of the sail must be made in space, all

other components are made on earth. The sail construction system consists of the following

elements> a scaffolding ?to control the structure%s deployment@, the film fabrication device, a

panel assembly device, and a BcraneB for conveying panels to the installation sites.

The scaffolding structure rotates at a rate within the operational envelope of the sail itself, to

facilitate the sail%s release. &ix compression members define the vertical edges of the

hexagonal prism. (any tension members parallel to the base link these compression

members to support them against centrifugal loads. 2allast masses flung further from the

axis provide additional radial tension and rigidity near the top of the scaffolding. ther

tension members triangulate the structure for added rigidity. Tension members span the base

of the prism, supporting a node at its center. The interior is left open, providing a volume for

deploying and assembling the sail. The top space is left open, providing an opening for

removing it. The face of the sail is near the top of the scaffolding, and the rigging below. "f

the scaffolding is oriented properly, the sun will shine on the usual side of the sail, making itpull up on its attachment point at the base of the prism. The total thrust of the said is then an

upper bound on the axial load supported by the compression members. "t is clearly desirable

to make the scaffolding a deployable structure.

The sail%s structure consists of a regular grid of tension members, springs, and dampers, and

a less regular three#dimensional network of rigging. This is a very complex object to

assemble in space. 8ortunately, even the structure for a sail much larger than described

herein can be deposited in the &huttle payload bay in deployable form.

&ince the sail is a pure tension structure, its structural elements can be wound up on reels.

onceptually, the grid structure can be shrunk into a regular array of reels and a plane. ith

each node in the lid represented by housings containing three reels. The rigging can be

sunken into a less regular array, and the nodes containing its reels stacked on top of those of

the grid.

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The structure will be deployed by pulling on cords attached to certain nodes. /eployment

may be controlled by a friction brake in the hubs of the reels. 2y setting the brakes properly,

positive tension must be applied for deployment and certain members may be made to

deploy before others. 8urther control of the deployment se)uence, if needed, may be

introduced by a mechanism which prevents some elements from beginning to deploy until

selected adjacent elements have finished deploying. "f detailed external intervention is

deemed desirable, brakes could be rigged to release when a wire on the housing is severed

by laser pulse.

The film fabrication device produces a steady stream of film triangles mounted to foil spring

clusters at their corners. The panel fabrication device takes segments of the stream and

conveys them along a track to assembly stations.

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nce panel installation is complete and the operation of various reels has been checked, the

sail is ready for release and use. "t is already spinning at a rate within its operational

envelope, and is already under thrust, hence, this task is not difficult. 8irst, the sail%s path

must be cleared. To do this, the film fabrication device, its power supply, the panel assembly

device, and the crane are conveyed to the sides of the scaffolding in a balanced fashion. The

top face is cleared of objects and tension members. Then, the members holding the corners

of the sail are released, and the remaining restraint points are brought forward to carry the

sail out of the scaffolding. 8inally, all restraints are released, and the sail rises free.

A four quadrant, 20-meter solar sail system is fully deployed duringtesting at NASA Glenn Research Center's lum !roo" facility in

Sandus"y, #hio$

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4. SOLAR SAIL DNA!ICS AND CONTROL

There are essentially two modes for operation and control of the solar sail.

"n the first mode, the tilting of panels produces control forces.

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unreeling the shrouds in a coordinated fashion as the sail turns. 8or the sail discussed above,

and the probable range of sail performances, this arrangement implies precession rates of 0-

to +3 radA077 minutes, when the sail is flat with respect to the sun. This provides a generous

margin in turn rate, even from maneuvers in low earth orbits. This active control permits

damping of nutation. This is important, since nutation would otherwise be initiated by rapid

changes in precession rate. "t should be noted that during precession the payload is offset

from the axis of rotation in a direction fixed in inertial space.

8or missions involving both interplanetary cruise and circumplanetary maneuvering, a

vehicle able to operate in both modes is desirable. The first mode has a decisive advantage

near planets ?because of its maneuverability@, but cannot enter a passive cruise mode. The

greater distance between the payload and sail in this mode precludes balancing the tor)ue on

the sail resulting from absorbed light with a reasonable amount of concavity, as is done in the

first mode. "nstead, the tor)ue must be countered in the same manner as the sail is precessed>

by active manipulation of shroud tension. hile control of shroud tension might be made

redundant by placing reels at both ends of the lines, reliability still favors a passive system on

long missions. 8ortunately, interconversion seems simple. The second mode control can be

maintained as the shroud lines +7+ and +7 are reeled in, so long as the sail is properly

ballasted for mode one. hile the payload reaches the mode one position, the reel can be

locked and mode one control begun.

4.1 Cr"#s#n$ %& S"nl#$h'

(aneuvering a solar#sailspacecraft re)uires balancing two factors> the direction of the solar

sail relative to the sunand the orbital speed of the spacecraft. 2y changing the angle of the

sail with respect to the sun, you change the direction of the force exerted by sunlight.

hen the spacecraft is in orbit around the

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The force then decelerates the spacecraft, decreases the speed of its orbit and the spacecraft

drops into a lower orbit.

The pressure of sunlight decreases with the s)uare of the distance from the sun. Therefore,

sunlight exerts greater pressure closer to the sun than farther away. 8uture solar#sail

spacecraft may take advantage of this fact by first dropping to an orbit close to the sun ## a

solar fly#by ## and using the greater sunlight pressure to get a bigger boost of acceleration at

the start of the mission. This is called a po(ere) per#hel#on *ane"+er.

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,. SOLAR SAIL !ATERIALS

hile solar sails have been designed before ?;1&1%s had a solar sail program back in the

097s@, materials available until the last decade or so were much too heavy to design a

practical solar sailing vehicle. 2esides being lightweight, the material must be highly

reflective and able to tolerate extreme temperatures. The giant sails being tested by ;1&1

today are made of very lightweight, reflective material that is upwards of 077 times thinner

than an average sheet of stationery. This Balumini'ed, temperature#resistant materialB is

called CP-1. 1nother organi'ation that is developing solar sail technology, theClanetary

&ociety?a private, non#profit group based in Casadena, alifornia@, supports the Cosmos 1,

which boasts solar sails that are made of aluminum#reinforced (ylar and are approximatelyone fourth the thickness of a one#ply plastic trash bag.

1luminium being manufactured for the &olar &ail.

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The reflective nature of the sails is the key. 1s photons ?light particles@ bounce off the

reflective material, they gently push the sail along by transferring momentum to the sail.

2ecause there are so many photons from sunlight, and because they are constantly hitting the

sail, there is a constant pressure ?force per unit area@ exerted on the sail that produces a

constant acceleration of the spacecraft. 1lthough the force on a solar#sail spacecraft is less

than a conventional chemical rocket,such as the space shuttle, the solar#sail spacecraft

constantly accelerates over time and achieves a greater velocity.

,.1 Al"*#n"* as Solar Sa#l !a'er#al

The thin metal film, according to the preferred embodiment of this invention, is an aluminum

film. 1luminum films have high reflectivity, low density, a reasonable melting point, and a

very low vapor pressure. The reflectivity and transmissivity of aluminum film is a function of

its thickness. Denerally, reflectivity for short wave lengths falls off faster with decreasing

film thickness than for longer wave lengths. onse)uently, any aluminum film thick enough

to reflect well in the visible wave lengths should reflect even better in the infrared, where

roughly half the sun%s power output lies.

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degrees =elvin. However, the analogous temperature for aluminum is a mere -6 degrees

=elvin. ;evertheless, aluminum films have survived fifteen minute anneals at 3- degrees

=elvin, and two hour anneals at77 degrees =elvin. The reason for this discrepancy is the

presence of an oxide layer on the aluminum, which armors the surface with a rigid, refractory

skin, thereby inhibiting surface diffusion and preventing changes of shape.

&ince the film is to be hot and mounted under tension, creep is of concern. The interior of a

small droplet will be in compression, because of its surface energy and resulting force of

surface tension. "n like fashion, the interior of a thin film will be in compression, unless the

mounting tension exceeds its surface tension. onsidering the oxide#coated film, elongation

not only breaks the oxide skin ?which may be very strong@, but also creates a fresh, uncoated

aluminum surface. To shrink, on the other hand, it must somehow crush or destroy the outside

surface, which it clearly cannot do. "n fact, shrinkage would manifest itself as agglomeration,

as discussed above.

The strength of a variety of thin metal films and thicker vapor deposited sheets has been

measured experimentally. (etals in thin films have mechanical properties differing from

those of the bulk material, because of the close proximity of all parts of the film to the

surface. The yield and fracture stresses of aluminum film increase as the film gets thinner.

1luminum films show substantial ductility, and a variable degree of deformation before

failure.

1luminum films of the minimum thickness re)uired for reflectivity may prove too weak to

support the stresses imposed upon them during fabrication and operation, or may creep under

load at elevated temperatures. "f so, it is possible to strengthen them, not by adding further

aluminum, but by adding a reinforcing film of a stronger, more refractory material. 1 good

reinforcing film should be strong, light, and easy to deposit. "t need not be chemically

compatible with aluminum, since a few nanometers of some other material can serve as a

barrier to diffusion. 1 reinforcing film is apt to have a high modulus such that it will act as

the sole load bearing element in the composite film. The aluminum film could help contribute

tear resistance, however. The use of a metal as a reinforcing film could reduce the amount of

aluminum needed to give good reflectance. &ome metals, such as nickel, may reflect well

enough to be of interest by themselves.

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,.1.1 T#'an#"* as re#nor/#n$ *a'er#al

8ilms of pure titanium from 057 to +,777 nanometers thick were found to have strengths of

37 to 3+7 ;Ca, while vapor deposited foils of Ci#31l#E from 7,777 to +,777,777

nanometers thick had tensile strengths of 97 to 0+77 ;Ca. Titanium has enough strength and

temperature tolerance to make it an attractive choice as a reinforcing film.

,.1.2 N#/0el as re#nor/#n$ *a'er#al

The strength of nickel film exceeds +,777 ;Ca at a thickness of 7 nanometers or less,

dropping to 0577 ;Ca on annealing. ;ickelFs density is a disadvantage for use in sails of the

highest performance, which should prove acceptable for bulk transport sails.

,.1.3 S#l#/on !ono#)e as re#nor/#n$ *a'er#al

&ilicon monoxide is a popular thin film material with many uses. n aluminum, these films

have found extensive use as satellite thermal control coatings, and have demonstrated their

stability in the space environment. (ounted on fine metal meshes, unbacked &i films as

thin as +.5 nanometers have found use as specimen supports in electron microscopyG such

films are described as having Bgreat strength,B and are so stable at high temperatures that they

may be cleaned by passing them rapidly through a flame. &ince silicon monoxide is easy to

evaporate, is refractory, has a low density, is apparently of high strength in extremely thin

film form, and is of known space compatibility, silicon monoxide shows promise as a

reinforcing film material.

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,.1.4 Boron as re#nor/#n$ *a'er#al

Eapor deposited boron film has a strength of 3+7 (Ca. &ince it is light and refractory, boron

may prove desirable as a reinforcing material. arbon forms amorphous films of Bexceptional

strengthGB those used in electron microscopy are made as thin as nanometers. &ince carbon

is strong, light, refractory, and easy to deposit, it is a promising material for reinforcing film.

8or a wide variety of reasons, the sail surface will not be one big piece of film, but rather

many smaller sheets mounted on a structure. &ince the fabrication device, as described

hereinafter, will produce strips, natural choices for the shapes of the sheet include long strips,

shorter rectangles or s)uares cut from strips, and triangles cut from the strips. The sheets

must be tensioned, and should be planar. &ince a triangular sheet will be planed if tensionedat its corners, and since triangular sheets will fit well into a fully triangulated structure, they

will be used as a basis for further design.

"n +777,

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1 more natural approach to tear#stopping is to subdivide the film, convert it from a

continuous sheet to a redundant network of small, load#bearing elements. "n such a structure,

a large manufacturing flow or a gra'ing micrometeoroid impact is free to initiate a tear##but

the tear will cause the failure, not of an entire sheet, but of a small piece of film, perhaps +5

s)uare millimeters in area. Catterns of cuts and wrinkles can de#tension areas of film to

isolate stress to smaller regions.

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. SOLAR SAIL LAUNC

ith just sunlight as power, asolar sailwould never be launched directly from the ground. 1

second spacecraft is needed to launch the solar sail, which would then be deployed in space.

1nother possible way to launch a solar sail would be with microwave or laserbeams

provided by a satellite or other spacecraft. These energy beams could be directed at the sail to

launch it into space and provide a secondary power source during its journey. "n one

experiment at ;1&1%s et Cropulsion 4aboratory?C4@, sails were driven to liftoff using

microwave beams, while laser beams were used to push the sail forward.

nce launched, the sails are deployed using an inflatable boom system that is triggered by abuilt#in deployment mechanism.

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. IN5ESTI6ATED SAIL DESI6NS

The highest thrust#to#mass designs known ?+77@ were theoretical designs developed by

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"n the 097sC4did extensive studies of rotating blade and rotating ring sails for a mission

to rende'vous with Halley%s omet. The intention was that such structures would be stiffened

by their angular momentum, eliminating the need for struts, and saving mass. "n all cases,

surprisingly large amounts of tensile strength were needed to cope with dynamic loads.

eaker sails would ripple or oscillate when the sail%s attitude changed, and the oscillationswould add and cause structural failure. &o the difference in the thrust#to#mass ratio was

almost nil, and the static designs were much easier to control.

C4%s reference design was called the BheliogyroB and had plastic#film blades deployed from

rollers and held out by centrifugal forces as it rotated. The spacecraft%s altitude and direction

were to be completely controlled by changing the angle of the blades in various ways, similar

to the cycle and collective pitch of ahelicopter. 1lthough the design had no mass advantage

over a s)uare sail, it remained attractive because the method of deploying the sail was simplerthan a strut#based design.

C4 also investigated Bring sailsB ?&pinning /isk &ail in the above diagram@, panels attached

to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five

percent of the total area. 4ines would connect the edge of one sail to the other. eights in the

middles of these lines would pull the sails taut against the coning caused by the radiation

pressure. C4 researchers said that this might be an attractive sail design for large manned

structures. The inner ring, in particular, might be made to have artificial gravity roughly e)ual

to the gravity on the surface of (ars.

1 solar sail can serve a dual function as a high#gain antenna. /esigns differ, but most modify

the metalli'ation pattern to create a holographic monochromatic lens or mirror in the radio

fre)uencies of interest, including visible light.

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7. COS!OS-1 SPACECRA8T DESI6N

The first solar#sail spacecraft, called osmos#0, has been developed, built and tested by The

Clanetary &ociety,a private, non#profit organi'ation whose goal is to encourage the

exploration of our solar system. The Clanetary &ociety contracted a :ussian spaceorgani'ation, the 2abakin &pace enter, to build, launch and operate the spacecraft. The cost

of the project is about I#million and is funded by osmos &tudios, a new science#based

media company.

The spacecraft itself weighs 66 lb ?7 kg@ and can sit on a tabletop. 1fter a first#phase testlaunch, the spacecraft will be launched into

&olar sail

made of alumini'ed (ylar thickness of 7.777+ inches ?5 microns@

area of 3,05 s)uare feet ?377 s)uare meters@

arranged in eight triangular blades>

each about 9 ft ?05 m@ long

consist of inflatable plastic tubes that support the sail ?a foam may be

used inside the tubes to hold them rigid once inflated@ can be pivoted ?like a helicopter blade@ by electric motorsto change its

angle relative to the sun

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The Clanetary &ocietyOne solar-sa#l %la)e

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Solar-sa#l )eplo&*en'# 1 pressuri'ed gas#filling system inflates the plastic tubes.

Po(er# 1 small array of solar cellssupplies all of the electrical power.

Na+#$a'#on# "t is essential for the spacecraft to know where it is and where the sun

is at all times.0. 1 sensor detects the position of the sun.

+. 1 global positioning system ?DC&@ receiverdetects the spacecraft%sposition. ?8rom the ground, the spacecraft orbit will be determined from/oppler tracking data with the aid of on#board accelerometers, whichwe%ll discuss later.@

-. The information from the sun sensor and the DC& receiver arecontinuously relayed to the spacecraft%s on#board computer.

. The on#board computer operate the motors that turn the sail blades to

maintain the proper orientation of the sail blades with respect to the sun.5. The on#board computer can accept corrections or override commands

from the ground.

ommunications# :edundant radiosystems are used to communicate with flightcontrollers on the ground.

one *H8 band, 77 megahert'

one band, ++07 (H'

n#board computer Two -63

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9. AD5ANTA6ES

1 solar sail is a spacecraft without a rocket engine. "t is pushed along directly by light

particles from the &un, reflecting off its giant sails. 2ecause it carries no fuel and keeps

accelerating over almost unlimited distances, it is the only technology now in existence that

can one day take us to the stars.

The major advantage of a solar#sail spacecraft is its ability to travel between the planets and

to the stars without carrying fuel. &olar#sail spacecraft need only a conventional launch

vehicle to get into

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1:. LI!ITATIONS O8 SOLAR SAILS

&olar sails don%t work well, if at all, in low

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11. !ISUNDERSTANDIN6S

ritics of the solar sail argue that solar sails are impractical for orbital and interplanetary

missions because they move on an indirect course. However, when in

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12. 8UTURE SPACE TRA5EL

&olar sail technologywill eventually play a key role in long#distance;1&1missions. ;1&1

believes that the exploration of space is similar to the tale of the BTortoise and the Hare,B with

rocket#propelled spacecraft being the hare. "n this race, the rocket#propelled spacecraft will

)uickly jump out, moving )uickly toward its destination. n the other hand, a rocket less

spacecraft powered by a solar sail would begin its journey at a slow but steady pace,

gradually picking up speed as the sun continues to exert force upon it. &ooner or later, no

matter how fast it goes, the rocket ship will run out of power. "n contrast, the solar sail craft

has an endless supply of power from the sun. 1dditionally, the solar sail could potentially

return to

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13. RE8ERENCES

www.howstuffworks.com

www.wikepedia.org

www.SeminarsTopics.com

http://www.howstuffworks.com/http://www.wikepedia.org/http://www.howstuffworks.com/http://www.wikepedia.org/