Solar Sail | Electrical Seminar Topic
Solar Sail
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. What could we accomplish if we could reduce our
need for so much fuel and the tanks that hold it?
International 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.
NASA is one of the organizations that has been studying this
amazing technology called solar sails that will use the sun's power to send us
into deep space.
SOLAR SAIL CONCEPT
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.
There are three components to a solar sail-powered
spacecraft:
• Continuous
force exerted by sunlight
• A large,
ultrathin mirror
• A
separate launch vehicle
A solar sail-powered spacecraft does not need traditional
propellant for power, because its propellant is sunlight and the sun is its
engine. Light is composed of electromagnetic radiation that exerts force on
objects it comes in contact with. NASA researchers have found that at 1
astronomical unit (AU), which is the distance from the sun to Earth, equal to
93 million miles (150 million km), sunlight can produce about 1.4 kilowatts
(kw) of power.
If you take 1.4 kw and divide it by the speed of light, you
would find that the force exerted by the sun is about 9 newtons (N)/square mile
(i.e., 2 lb/km2 or .78 lb/mi2). In comparison, a space shuttle main engine can
produce 1.67 million N of force during liftoff and 2.1 million N of thrust in a
vacuum. Eventually, however, the continuous force of the sunlight on a solar
sail could propel a spacecraft to speeds five times faster than traditional
rockets.
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 "crane" 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. Six
compression members define the vertical edges of the hexagonal prism. Many
tension members parallel to the base link these compression members to support
them against centrifugal loads.
Ballast masses flung further from the axis
provide additional radial tension and rigidity near the top of the scaffolding.
Other 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. If the scaffolding is oriented properly, the sun will shine
on the usual side of the sail, making it pull 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. It 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. Fortunately, even
the structure for a sail much larger than described herein can be deposited in
the Shuttle payload bay in deployable form.
Since the sail is a pure tension structure, its structural
elements can be wound up on reels. Conceptually, the grid structure can be
shrunk into a regular array of reels and a plane. With 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.
The structure will be deployed by pulling on cords attached
to certain nodes. Deployment may be controlled by a friction brake in the hubs
of the reels. By setting the brakes properly, positive tension must be applied
for deployment and certain members may be made to deploy before others.
Further
control of the deployment sequence, if needed, may be introduced by a mechanism
which prevents some elements from beginning to deploy until selected adjacent
elements have finished deploying. If 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. Each segment is fastened to the previous segment and to
the edge tension members that will frame the finished panel. This non-steady
process of panel assembly requires a length of track to serve as a buffer with
a steady film production process.
At the assembly station, the segments are transferred to
fixtures with a lateral transport capability. During transfer, each segment is
bonded to the one before along one edge. While the next segment is brought into
position, the last segment is indexed over a one strip width, completing the
cycle. Special devices bearing the edge tension members travel on tracts and
place foil tabs on the panel structure. The foil tabs linking the segments may be
bonded to one another in many ways, including ultrasonic welding, spot welding,
and stapling.
Attachment and conveyance may be integrated if the foil tabs
are hooked over pins for conveyance. The panel assembly cycle ends with a
pause, as the completed panels, now held only by their corners, are lured into
a storage region and new edge members are loaded into position.
At this point the sail's structure is deployed within
scaffolding, and panels are being produced and stored at a panel fabrication
module. The stored panels are initially loaded at a node suspended on tension
members above the center of the sail. A crane is likewise suspended, but from
tension members terminated in actively controlled reels mounted on devices free
to move around the top of the scaffolding. This makes it possible to position
the crane over any aperture in the grid.
Once panel installation is complete and the operation of
various reels has been checked, the sail is ready for release and use. It is
already spinning at a rate within its operational envelope, and is already
under thrust, hence, this task is not difficult. First, 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.
Finally, all restraints are released, and the sail rises free.
