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:
â€¢ Continuous force exerted by sunlight
â€¢ A large, ultrathin mirror
â€¢ 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.
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.
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