Solar sail
Solar sails (also called light sails, especially when they use light sources other than the Sun) are a proposed form of spacecraft propulsion. The spacecraft deploys a large, lightweight sail which reflects light from the Sun or some other source. The radiation pressure on the sail provides thrust by reflecting photons. Tilting the sail at an angle from the Sun produces thrust at an angle that bisects the angle between the Sun and the spacecraft. Steering is usually with auxiliary vanes. The concept was first proposed by Friedrich Zander in the late 1920s and gradually refined over the decades.
NASA study of a solar sail. The sail would be half a kilometer wide
A number of demonstration projects have proven this method's feasibility. Some unmanned spacecraft have been constructed with reflective panels that can be used instead of small rocket motors, to conserve fuel for maneuvering and attitude control. Solar collectors or sun shades can also serve as crude solar sails, and can help a spacecraft correct its attitude and orbit without using fuel.
Sails fall off in efficiency by the square of the distance from the sun. Beyond the orbit of Mars, other forms of propulsion become more practical.
Most missions that make good use of a solar sail therefore pass very close to the sun. For this reason, a good solar sail should resist high temperatures.
| Table of contents |
|
2 Sail Materials 3 Interstellar ships 4 References 5 See also 6 External links |
"Parachutes" would have very low mass, but theoretical studies show that they will collapse from the forces placed by shrouds. Radiation pressure does not behave like aerodynamic pressure.
The highest thrust-to-mass designs known were developed by Eric Drexler, in an MIT master's thesis, He designed a sail using reflective panels of thin aluminum film (30 to 100 nanometers thick) supported by a purely tensile structure. It rotated and would have to be continually under slight thrust. He made and handled samples of the film in the laboratory, but the material is too delicate to survive folding, launch, and deployment, hence the design relied on space-based production of the film panels, joining them to a deployable tension structure. Sails in this class would offer accelerations an order of magnitude higher than designs based on deployable plastic films.
The highest-thrust to mass designs for ground-assembled deployable structures are square sails with the masts and guy lines on the dark side of the sail. Usually there are four masts that spread the corners of the sail, and a mast in the center to hold guy wires. One of the largest advantages is that there are no hot spots in the rigging from wrinkling or bagging, and the sail protects the structure from the sun. This form can therefore go quite close to the sun, where the maximum thrust is present. Control would probably use small sails on the ends of the spars.
In the 1970s JPL did extensive studies of rotating blade and rotating ring sails for a mission to rendezvous with Halley's comet. The thought was that such structures would be stiffened by centripetal forces, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile structure were needed to cope with dynamic loads. Weaker sails would ripple or oscillate when the sail's attitude changed, and the oscillations would add and cause structural failure. So the difference in the thrust-to-mass ratio was almost nil, and the static designs were much easier to control.
JPL's reference design was called the "heliogyro" and had plastic-film blades deployed from rollers and held out by centripetal forces as it rotated. The spacecraft's attitude 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 a helicopter. Although the design had no mass advantage over a square sail, it remained attractive because the method of deploying the sail was simpler than a strut-based design.
JPL also investigated "ring sails," panels attached to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five percent of the total area. Lines would connect the edge of one sail to the other. Weights in the middles of these lines would pull the sails taut against the coning caused by the radiation pressure. JPL 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 equal to Mars.
A solar sail can serve a dual function as a high-gain antenna. Designs differ, but most modify the metallization pattern to create a holographic monochromatic lens or mirror in the radio frequencies of interest.
Artist's conception of a solar sail
The best known material is thought to be a thin mesh of aluminium with holes less than 1/2 the wavelength of most light. Nanometer-sized "antennas" would emit heat energy as infrared. Although samples have been created, it is too fragile to unfold or unroll with known technology.
The most common material in current designs is aluminized 2μm Kapton film. It resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminium reflecting film is on the Sun side.
Robert Forward proposed the use of lasers to push solar sails, providing beam-powered propulsion. Given a sufficiently powerful laser and a large enough mirror to keep the laser focused on the sail for long enough, a solar sail could be accelerated to a significant fraction of the speed of light. To do so, however, would require the engineering of massive, precisely-shaped optical mirrors or lenses (wider than the Earth for interstellar transport), incredibly powerful lasers, and more power for the lasers than humanity currently generates.
A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "Starwisp" interstellar probe design would use a microwave laser to drive it. Microwave lasers spread out more rapidly than optical lasers thanks to their longer wavelength, and so would not have as long an effective range.
No solar sails have been successfully deployed as primary propulsion systems, but research in the area is continuing. The Russian space program has attempted to deploy at least one prototype test unit in Earth orbit.
Books:
Investigated Sail designs
Sail Materials
Interstellar ships
References
See also
External links