Solar Sails

Name : RAMAKANTH.K USN :1SG05EE041 Department Of Electrical Engineering Sapthagiri College Of Engineering

Introduction A solar sail is a spacecraft without an engine - it is pushed along directly by light particles from the Sun, reflecting off giant mirror-like sails. Because it carries no fuel and keeps accelerating over almost unlimited distances, it is the only technology now in existence.

What is a solar sail? A solar sail, simply put, is a spacecraft propelled by sunlight. Whereas a conventional rocket is propelled by the thrust produced by its internal engine burn, a solar sail is pushed forward simply by light from the Sun. This is possible because light is made up of packets of energy known as “photons”, that act like atomic particles, but with more energy.

COMPONENTS OF A SOLAR SAIL

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Solar sails are composed of large flat smooth sheets of very thin film, supported by ultra-lightweight structures. The side of the film which faces the sun is coated with a highly reflective material so that the resulting product is a huge mirror, typically about the size of a football field. The force generated by the sun shining on this surface

SOLAR SAILING Solar sailing is a method of converting light energy from the sun into a source of propulsion for spacecraft. In essence, a solar sail is a giant mirror that reflects sunlight in order to transfer the momentum from light particles (photons) to the object that is propelling. The phrase "solar sails" is often confused with "solar cells", which is a technology for converting solar light into electrical energy.

THE CONCEPT OF SOLAR SAILS Nearly 400 years ago, as much of Europe was still involved in naval exploration of the world, Johannes Kepler proposed the idea of exploring the galaxy using sails. Through his observation that comet tails were blown around by some kind of solar breeze, he believed sails could capture that wind to propel spacecraft the way winds moved ships on the oceans. While Kepler's idea of a solar wind has been disproven, scientists have since discovered that sunlight does exert enough force to move objects. To take advantage of this force, NASA has been experimenting with giant solar sails that could be pushed through the cosmos by light.

WORKING PRINCIPLE OF SOLAR SAILS How Does Light Push a Solar Sail? 1-Electromagnetism James Clerk Maxwell developed the laws describing electromagnetism and concluded that light is an electromagnetic wave. Maxwell predicted that when light hits an object and is absorbed or reflected, the light wave pushes on electric charges in the surface of the object, which in turn push on the rest of the object. If the light is reflected, the object gets pushed twice as hard, just like a ball bouncing of the wall. In this process the photons transmit their momentum to the surface twice-once by the initial impact, and again by reflecting back from it.

2- A Very Very Gentle Force Sunlight exerts a very gentle force. A square mirror 1 kilometer on a side would only feel about 9 Newtons or 2 pounds of force. Fortunately, space is very empty and clean compared to Earth, so there is plenty of room for a 1 kilometer wide sails to maneuver, and there is no noticeable friction to interfere with your 9 Newtons of thrust.

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SOLAR SAIL MATERIALS While solar sails have been designed before, materials available until the last decade or so were much too heavy to design a practical solar sailing vehicle. Besides being lightweight, the material must be highly reflective and able to tolerate extreme temperatures. The giant sails being tested by NASA today are made of very lightweight, reflective material that is upwards of 100 times thinner than an average sheet of stationery.

Another organization that is developing solar sail technology, the Planetary Society (a private, non-profit group based in Pasadena, California), supports the Cosmos 1, which boasts solar sails that are made of aluminum-reinforced Mylar and are approximately one fourth the thickness of a one-ply plastic trash bag.

ALUMINUM AS SOLAR SAIL MATERIAL 

The thin metal film, according to the preferred embodiment of this invention, is an aluminum film. Aluminum 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.  High deposition rates, near-normal vapor incidence, and a good vacuum favor high reflectivity.

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Consequently, 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.  The reflectivity of aluminum films varies with the deposition conditions.

Aluminium being manufactured for the Solar Sail.

USAGE OF REFRACTORY MATERIALS 

Aluminum films of the minimum thickness required 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. If so, it is possible to strengthen them, not by adding further aluminum, but by adding a reinforcing film of a stronger, more refractory material. A good reinforcing film should be strong, light, and easy to deposit.

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The use of a metal as a reinforcing film could reduce the amount of aluminum needed to give good reflectance. Some metals, such as nickel, may reflect well enough to be of interest by themselves.

AREA OF CONCERN IN CONSTRUCTION 

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Tears are a critical concern in the use of thin films for solar sails. While even sheets of extremely thin material have adequate strength to support the load expected during fabrication and operation in the absence of stress concentrations, the inevitability of manufacturing flaws and micrometeoroid damage makes this a small comfort. The most obvious method of limiting tears is to mount the film on a supporting mesh. the mesh adds mass to the sail and, because it must be fabricated, transported into space and attached to the film, adds cost as well.

REMEDY

A more natural approach to tear-stopping is to subdivide the film, convert it from a continuous sheet to a redundant network of small, loadbearing elements. In such a structure, a large manufacturing flow or a grazing 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 25 square millimeters in area. Patterns of cuts and wrinkles can de-tension areas of film to isolate stress to smaller regions. Each wrinkled region is fabricated with enough extra material to avoid being

EXAMPLE OF SOLAR SAILS 5.1 Cosmos 1 Cosmos 1 is a small solar sail intented only for a short mission. Nevertheless, once it spreads its sails even this small spacecraft will be 10 stories tall, as high as the rocket that will launch it. Its eight triangular blades are 15 meters (49 feet) in length, and have a total surface area of 600 square meters (6500 square feet). This is about one and a half times the size of a basketball court.

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The spacecraft was built in Russia by the Babakin Space Center under a contract to the Society. It was launched and operated from Russia.

The purpose of the mission was to conduct the first solar sail flight. Solar sailing is recognized as a future planetary flight technology on the pathway to interstellar flight (using laser instead of solar photons).

ADVANTAGES A solar sail is a spacecraft without a rocket engine. It is pushed along directly by light particles from the Sun, reflecting off its giant sails. Because 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. Solar-sail spacecraft need only a conventional launch vehicle to get into Earth orbit, where the solar sails can be deployed and the spacecraft sent on its way. These spacecraft accelerate gradually, unlike conventional chemical rockets, which offer extremely quick acceleration.

Solar sails will set new speed records for spacecraft and will enable us to travel beyond our solar system.

LIMITATIONS OF SOLAR SAILS

Solar sails don't work well, if at all, in low Earth orbit below about 800 km altitude due to erosion or air drag. Above that altitude they give very small accelerations that take months to build up to useful speeds. Solar sails have to be physically large, and payload size is often small. Deploying solar sails is also highly challenging to date.

MISUNDERSTANDINGS Critics of the solar sail argue that solar sails are impractical for orbital and interplanetary missions because they move on an indirect course. Another false claim is that solar sails capture energy primarily from the “solar wind": high speed charged particles emitted from the sun. These particles would impart a small amount of momentum upon striking the sail, but this effect would be small compared to the force due to radiation pressure from light reflected from the sail. The force due to light pressure is about 100 times as strong as that due to solar wind.

FUTURE SPACE TRAVEL  

Solar sail technology will eventually play a key role in longdistance NASA missions. NASA believes that the exploration of space is similar to the tale of the "Tortoise and the Hare," with rocket-propelled spacecraft being the hare. In this race, the rocket-propelled spacecraft will quickly jump out, moving quickly toward its destination. On 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. Sooner or later, no matter how fast it goes, the rocket ship will run out of power. In contrast, the solar sail craft has an endless supply of power from the sun. Additionally, the solar sail could potentially return to earth, whereas the rocket powered vehicle would not have any propellant to bring it back.

REFERENCES

www.howstuffworks.com www.wikepedia.org www.answers.com

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Wind sailing

COSMOS-1 MISSION The mission of Cosmos-1 occurred in two phases. Phase 1 will test the deployment of two solar-sail blades, and Phase 2 will launch the Cosmos-1 spacecraft into Earth orbit

Launch Vehicle To get Cosmos-1 into Earth orbit, the spacecraft was loaded into a modified intercontinental ballistic missile (ICBM) of Russian design, called the Volna. The ICBM was launched from a Russian submarine in the Barents Sea. Typically, the Volna ICBM does not have enough thrust to reach orbit, but the missile used for Cosmos1 will have an added rocket engine (kick stage) that is used to deorbit satellites. The kick-stage engine will provide the additional thrust required to get Cosmos-1 into orbit.

The Planetary Society Cosmos-1 will be launched from a submarine.

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Phase 1 of the Cosmos-1 Solar Sail Project was launched on June 21,2005. The goal of Phase 1 was to test the deployment of the solar sails. To do this, a payload consisting of two inflatable solar-sail blades and a solar-sail platform with an imaging camera was packaged inside a Volna ICBM and launched from a Russian submarine in the Barents Sea. The flight was a suborbital flight that lasted about 15 minutes. At about 248 mi (400 km) high, the two solar-sail blades were deployed. The camera in the platform imaged the sail deployment. This test spacecraft used an aerobrake to slow down in the upper atmosphere and an additional inflatable braking device as it approached the ground.

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Phase 2 was an orbital flight of the actual Cosmos-1 spacecraft. Again, it was launched from a Russian submarine on a modified Volna ICBM, but the Volna rocket failed, and the spacecraft failed to reach orbit. A solar sail would have been used to gradually raise the spacecraft to a higher earth orbit. The mission would have lasted for one month. A suborbital prototype test by the group failed in 2001 as well, also because of rocket failure.

Laser assisted light sailing Light sailing works well for inner planet missions and for activities extending out to the Mars orbit. However, the solar flux falls off as the inverse square of the distance from the sun. Thus for missions beyond the Jupiter orbit, an alternative to solar propulsion is to use directed light from a high power laser. As a pioneer inventor in the field of interstellar propulsion, Robert Forward has an avid interest in developing methods for boosting the intensity of light that can be delivered to a light sail. His goal is to reduce the cruise duration of a trip from our solar system to the nearest star from 6500 years to a time frame on the order of 40 years.

5.3 Recent Developments No solar sails have been successfully deployed as primary propulsion systems, but research in the area is continuing 1. On August 9,2004 Japanese ISAS successfully deployed two prototype solar sails from a sounding rocket. A clover type sail was deployed at 122 km altitude and a fan type sail was deployed at 169 km altitude. Both sails used 7.5 micrometer thick film. 2. A joint private project between Planetary society, Cosmos Studios and Russian Academy of Science launched Cosmos 1 on June 21,2005, from a submarine in the Barents Sea, but theVolna rocket failed, and the spacecraft failed to reach orbit. A solar sail would have been used to gradually raise the spacecraft to a higher earth orbit. The mission would have lasted for one month. A suborbital prototype test by the group failed in 2001 as well, also because of rocket failure 3. A 15-meter-diameter solar sail (SSP, solar sail sub payload, soraseiru sabupeiro-do) was launched together with ASTRO-F on a M-V rocket at 21:28, February, 2006 UTC and made it to orbit. It deployed from the stage at 21:46 UTC but opened incompletely

Where Solar Sails Can Take Us By the end of this decade, there's a good chance solar sails will be used for a longdistance NASA mission. Flight demos took place in early 2005, with a sail-propelled craft launched five years later, according to Sarah Gavit, program manager for JPL's Solar Sail Technology Program. But just how far will these solar sails be able to take us and how fast will they get us there?

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Here rocket-propelled spacecraft will quickly jump out, moving quickly toward its destination. On 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.

Sooner or later, no matter how fast it goes, the rocket ship will run out of power. In contrast, the solar sail craft has an endless supply of power from the sun. Additionally, the solar sail could potentially return to Earth, whereas the rocket powered vehicle would not have any propellant to bring it back. As it continues to be pushed by sunlight, the solar sailpropelled vehicle will build up speeds that rocket powered vehicles would never be able to achieve. Such a vehicle would eventually travel at about 56 mi/sec (90 km/sec), which would be more than 200,000 mph (324,000 kph). That speed is about 10 times faster than the space shuttle's orbital speed of 5 mi/sec (8 km/sec).