by Eric Flint
Another small increment in inner system acceleration comes if you use vanadium (score 12.9) in return for a doubling of density (6.11). Vanadium is a little less expensive than beryllium.
Niobium has a density of 8.85 and a score of 22.1. It is also much cheaper than all of the high-scoring metals mentioned above.
You can't improve on niobium's sundiving capability without increasing density to that of molybdenum (10.28), tantalum (16.65) or, worst of all, tungsten (19.25).
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Landis's figure of merit can be improved upon. It doesn't take into account the ability of the mirror to reflect sunlight (a perfect reflector wouldn't heat up at all). Taking this into account, the top scorers are, in descending order: molybdenum, tungsten, niobium, and tantalum.
Sail Coatings
Emissivity (a measure of a metal's ability to radiate heat—and thus keep its temperature down) could also be considered. Unfortunately, emissivity is a function, not only of wavelength, but also of temperature.
You can improve the temperature tolerance of a solar sail by putting a high emissivity coating (which doesn't have to be transparent) on the back side. The measured emissivity of black chromium on the back of 0.9 micron aluminized Mylar is 0.4. Without the chromium treatment, the emissivity is only about 0.06.
Gray Sails
Recently, two of the basic assumptions of space sail design have been challenged. The first, that a sail should be highly reflective. The second, that letting the sails heat up is bad.
It is normally stated that a sail which perfectly absorbed light would experience only half the acceleration felt by one which was a perfect reflector. That ignores the re-radiation of the absorbed energy as infrared. As the infrared photons say goodbye, the sail recoils. And the hotter the sail, the more infrared it emits.
These "gray sails" were proposed by Robert Forward in 1999, and he suggested that they be made of sparsely aluminized carbon. The sail would initially behave like a traditional "white sail" but when the craft got to within three solar diameters of the sun, the aluminum would evaporate, exposing the carbon layer. This would absorb sunlight and give off heat. A lot of heat, since carbon has a high emissivity, and a melting point of 2,800K.
Rigging for Space Travel
A sailing ship has masts, yards and standing rigging (ropes) to hold its sails in position, and running rigging (more ropes) to control how the wind acts upon them.
While the pressure of sunlight is much less than that of terrestrial winds, solar sails are much flimsier than canvas. So they need to be stabilized in some way.
Like terrestrial sails, "sun-catchers" can be supported and stabilized by booms and lines. The booms can telescope out, fold out, roll out or inflate when the craft is ready to deploy the sail.
The thinner and larger the sail, the more support it needs against the photon pressure. Moreover, the longer the booms, the thicker they must be to resist bending. The mass of the rigging is therefore likely to be roughly proportional to the surface area of the sail. There is great variation in the estimates, but figure the rigging mass to be two-thirds to one-and-a-half times the sail mass.
A supported sail can have any of a variety of shapes: square, triangular, flower-like (with square-ish petals), or umbrella-like.
The square shape is the one most often depicted. One rigging method is to provide a central mast rising from the center of the back (non-reflective) side, from which yards extend to each corner. Stays further tie the mast to various segments of the yards and the outer rim.
Team Encounter, which hopes to launch an interstellar probe in the near future, doesn't plan to run lines from a central mast. Instead, its sail is attached at numerous points (so-called "striped suspension") to four booms extending from a hub.
In order to stretch the sail taut, one would need a substantial supporting structure. The acceleration is better if one accepts some sagging (billowing) and wrinkling in the sail, even though more sail material is needed, and the light pressure is a bit less.
The preferred method of steering the supported sail is with relatively tiny reflective vanes, attached to the periphery, which likewise are susceptible to solar pressure.
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In outer space, another way of holding a sail rigid is to spin it. This has the advantage that you don't need to provide an elaborate support structure, minimizing the weight of the sunjammer.
Geometrically speaking, you can't get any simpler than the disk sail. The problem is with controlling its orientation to the sun. The most practical method appears to be to equip the payload with some means of moving relative to the center of the disk. For example, it is given tracks to move on. Moving the payload shifts the center of mass of the craft, and the sail tilts to compensate.
The heliogyro sail, invented by Richard MacNeal and John Hedgepath in the mid-sixties, is in the form of long blades which splay outward from a hub. It is spun up much like a disk sail. However, its design advantage is that the blades can be individually rotated around their long axes to change the angle of incidence of the sunlight.
According to Wright, the blades of a heliogyro sail require edge reinforcement to withstand the centrifugal force induced by the high-speed spinning. Consequently, the heliogyro sail is less mass efficient than the disk sail.
Instead of stiffening the blades, the "UltraSail" has a tip micro-satellite with its own propulsion system attached to the end of each blade. These move out to extend and spin up the blade. The proponents believe that, for large sails, the mass requirements of these micro-satellites will be less than for conventional heliogyro blade stiffeners. Of course, unlike a pure solar sail, the UltraSail requires fuel for its tip agents.
Sail Area
The sail on Clarke's Diana racer was about five square kilometers. In Cordwainer Smith's story, set much further in the future, the early interstellar colony ships had 3,200-square kilometer (km2 ) sails. (Manhattan has an area of 59 km2 , and Central Park, 3.2 km2.)
In contrast, the real-life engineering proposals have been much more conservative. The Jet Propulsion Laboratory recently proposed use of sails with an area of about 10,000 m2 (0.01 km2 ) for inner planet missions. That is equivalent to almost two American football fields. The square sailer and heliogyro designed for the Halley's Comet rendezvous both had a size of ~0.6 km2.
The largest sailcraft on the drawing boards are those intended for interstellar flight. Forward, in his novel Rocheworld (Baen Books, 1990), described a 1,000 km diameter, laser-pushed, multistage light sail, destined to explore Barnard's Star. Similar interstellar sailcraft appear in his scientific papers.
Sailcraft Payload
The purpose of the sailcraft is to transport a payload to an extraterrestrial destination. The payload may be just instrumentation, as in the case of a probe, or it may include astronauts and their life support systems. It may also be cargo for delivery to a space base, and robotic "teamsters" for handling it.
The payload may be an integral part of the "hub" of the sail, it can be rigidly connected to it by some sort of mast, or it may be suspended from the sail by cables. The last approach, a "parachute" design, was proposed by Wiley.
Acceleration is dependent on the total mass of the sunjammer, including its payload. The advantage of large sails is that a given payload has a smaller percentage effect on acceleration. JPL assumes that its 10,000 m2 sails, together with their supporting structure, will have an "areal density" of ten grams per square meter (g/m2 ), implying a sail-cum-rigging mass of 100 kilograms.
Adding just a 100 kilogram payload (one-eight the mass of the 1977 Voyager) would double the sailcraft mass, giving it half the acceleration with the JPL sail. It would only nick by 5% the performance of a sail which was ten times as large.
For interstellar missions, JPL expects that super-sails, one square kilometer in size, will be de rigeur. The mass of the sail will then be about the same as that of the Hubble Space Telescope (11,000), and more than that of the Apollo Command Module
(5,800).
Getting the Sailcraft Into Space
It is impossible for a sailcraft to take off, under photonic propulsion, from the Earth's surface. Atmospheric pressure is such that the sails either couldn't be deployed at all, or would be torn apart.
Sailing is difficult even in low earth orbits; the deployed sails would cause so much drag that the ship couldn't accelerate to a significant velocity. In general, "atmospheric effects surpass solar pressure effects at altitudes lower than 1000 km."
Hence, the sailcraft will be carried to a higher altitude by rocket or Space Shuttle, or it will be built in space.
Sail Manufacturing, Packaging and Deployment
On Earth, it is impractical to manufacture, as a single piece, a sail which is 10,000 square meters in size. Instead, the sail material is going to be fabricated as long strips.
The prefabricated sail is then packaged into a deployment module, which is launched into space by a conventional rocket. Or the module could be carried in the storage bay of a space shuttle. Once the module is set free, it is ready to deploy the sail.
NASA expects that a 10,000 square meter sail can be stowed in a space of 1.5 cubic meters (a bit larger than an executive office desk).
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For a heliogyro sail, each blade is produced as a single strip, which is rolled as the final step of the manufacturing process. When the craft is ready to set sail, the rollers are swung out, the central hub spins up, and the blades unroll.
With a scaffolded sail, the strips are loaded into individual canisters. The hub extends the mast and yards, and then the sail strips are hauled out of their containers and hung on the yards.
Louis Friedman says that it is impractical to package a disk sail for launch from the ground, but disk sails can be made once we have spaceborne manufacturing facilities.
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Orbital factories would make it possible to build a lighter, hence higher performance, sail. As Eric Drexler said, solar sails can be improved "if you forget about folding them up and launching them from the ground."
Wiley envisioned that the sail would be built in space, alongside an artificial satellite. He thought that a thin plastic sheet could be made rigid by applying an electric charge. The metal would then be sprayed on the plastic. Now comes the really neat trick: The plastic is coated with lampblack so that, when turned to face the sun, it melts, and can be separated from the metal. Wiley even explained how to recover the plastic for reuse.
In 1977, Drexler proposed constructing a sail in outer space, on a wire mesh framework. A thin layer of metal would be vapor deposited on wax, the wax would be vaporized and recycled, and the resulting foil would be laid on the scaffold.
Solar System Missions
Since the solar sailer never runs out of fuel, it is perfect for long "continuous thrust" missions. One such mission is the "Solar Sentinel." This would "sit" seven million kilometers from Earth, angling its sail as needed to keep station as it watches for solar wind disturbances. Its sunward position would allow it to give Earth telecommunications and power networks about four hours warning of a "storm."
The solar sailer is also favored for the "South Polar Imager." This satellite, which is intended to study the sun from an unusual vantage point, is intended to take up a 0.48 AU high-inclination orbit. For a rocket ship, dramatically changing orbital inclination requires a lot of fuel.
You wouldn't use a sunjammer for any mission for which time is of the essence. Still, I can imagine them being used to ferry cargo from a near-Earth space station to a base on the moon or Mars. They would never actually land; their payload would include rocket-driven dinghies for taking the cargo "to shore." Once a space station was built in lunar or Martian orbit, these dinghies would become unnecessary, and there would be more space for cargo. Missions to Venus or Mercury are also possible, and, so far as solar sailing is concerned, are actually easier.
It has also been suggested that sunjammers could be used for asteroid mining or deflection. Again, the fact that they don't run out of fuel is important. The shadow of the asteroid could complicate towing operations.
Even though other propulsion systems are favored for short missions to the outer planets, solar sails allow one to achieve high ultimate velocities—making them attractive for missions to the edge of the solar system, or beyond.
Interstellar Missions
In the late-nineteenth century, Svante Arrhenius speculated that life could travel between stars, as spores pushed by light pressure. A century later, science fiction writers wrote about great starships which were pushed the same way.
One way to escape the solar system is to accelerate at a rate greater than you're pulled by gravity. For the photonic acceleration to equal the gravitational acceleration, the areal density would need to be about 1.6 g/m2 .
However, it is not necessary to accelerate that quickly. The acceleration can be minimal, so long as it is continued long enough to reach escape speed (which, one A.U. from the sun, is 42.1 km/sec). Further out, of course, it is lower.
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For true interstellar missions, we want to accelerate for a very long time, building extremely high velocities—we have a lot of distance to cover even to reach Alpha Centauri!
Unfortunately, the further out the spacecraft goes, the less the photonic force, and the lower the acceleration. Pluto's average distance from the sun is 39.5 A.U., and at that distance, the push of sunlight would be 1/1560th what it was in Earth's orbit.
The sun is not the only possible source of photons. A powerful laser, mounted on a space station, or perhaps on the moon, could be used to push solar sailing ships along.
This is loosely analogous to how electric trains work. The electricity is generated at a central power plant, and transmitted by overhead lines or a third rail to the train. The train can accelerate rapidly, because it doesn't have to generate the power itself, just apply it.
Of course, that means that the energy is no longer completely free; we have to generate it. Still, we retain the advantage that the spacecraft doesn't have to carry fuel.
The laser has to supply all of the kinetic energy of the probe at its terminal velocity (when the laser is turned off). For a 1000 kilogram probe traveling at 11% light speed, and driven for three years by the station laser, the latter would need a power output of 65 gigawatts. The laser, in turn, would be powered by a photovoltaic power plant generating perhaps 650 gigawatts.
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Laser beams spread out (diffract), albeit slowly, with distance. There is what is called a diffraction limited distance at which the diameter of the laser beam is equal to that of the sail. If the solar sailer ventures further out, then part of the laser energy is wasted. The diffraction limited distance is the product of the diameters of the sail and the initial laser beam, divided by 2.4 times the wavelength of the laser light.
To increase the effective initial diameter of the laser beam, a giant Fresnel lens may be used. How big? Forward proposed a lens with a diameter of 1,000 kilometers, which happens to be one-third the diameter of the moon.
It sounds as though use of short wavelengths (e.g., ultraviolet) would be a good idea, as it would reduce beam divergence. That is true, but as the wavelengths are reduced, so too, are the efficiency of the laser, the transparency of the lens, and the reflectivity of the metal sail. Landis suggests use of a wavelength of 500 nm (that is in the visible light range).
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The fact that we can control the wavelength emitted by the laser does give us an advantage; we can use a mirror optimized to reflect light of that wavelength. The most efficient mirrors aren't metals, they are multi-layer dielectrics. Dielectrics are insulators like glass, plastic, and so forth.
In a multilayer dielectric mirror ("Bragg reflector"), layers of high- and low-refractive index materials are alternated, and these layers have thicknesses, relative to the wavelength of the light, such that the reflections from all the boundaries are "in synch." There are commercially av
ailable multilayer dielectrics with reflectivities greater than 99.9%.
The ideal low refractive index "material" for a dielectric-type solar sail is vacuum (1.0).
Some of the high refractive index substances which have been considered are diamond (2.41), silicon carbide (2.65), or zirconia (2.15). These three materials have densities of 3.1-5.4, comparable to those of the metals scandium (3), titanium (4.5) and vanadium (6.1).
Unfortunately, the use of N high-index layers, even with just vacuum inbetween, still increases the weight of the sail by the factor N, whereas the improvement in reflectivity is slower. Hence, solar sail designers are content with using a single dielectric layer; with silicon carbide of the ideal quarter wavelength thickness the reflectivity is 56%.
It should be noted that with traditional dielectric mirrors the reflectivity is a strong function of the angle of incidence of the light. So the dielectric mirrors have to be optimized for a particular mirror tilt, or specially structured to make them omnidirectional.
