As with maritime ships, lines are essential for a wide range of uses. One difference is that some lines may be very long and need to be self-guiding. A maneuverable grappling device can be used at the end of a line to place or pick up payload containers, to secure a ship to a structure such as a station, or to pick up samples from an asteroid or comet. These smart lines will be an essential part of sailing operations. Lines a few hundred kilometers long may be used to move a ship from a space station to an orbit farther out where it could begin sailing. Other lines, called stays, may be used as rigging among mast and booms or for connecting to other parts of a ship's structure.
A sailing ship can have its structure completed and sails unfurled either at a station or after it has been started on its way by some other means, such as a rocket. For reliability, the choice of using a space station is the better one. If a sailing ship is completed in low Earth orbit beneath the radiation belts, it will usually require some assistance in moving to an orbit farther out, away from Earth's atmosphere, before it can begin sailing. This appears to be a relatively minor disadvantage compared to the advantages of operating directly from a station in low Earth orbit. The first large sailing ships may then be completed at the International Space Station or at a similar facility, possibly one dedicated to the operation of sailing ships.
Sailing ships can spiral in toward a planet or away from it. They can escape from a planet or sail into a capture orbit from interplanetary space. The time required to operate from orbits at lunar distance and interplanetary space is typically 1 to 2 weeks. Operating between low Earth orbit and interplanetary space would typically take 4 to 6 months.
Sailing ships can be steered by several means, primarily by vanes or adjusting the orientation of the sails. This latter approach appears to be the easiest to implement. Sails will typically be attached by three lines. Letting out or taking in the lines in an independent manner allows a sail to be reoriented. This results in a torque on the ship, which causes it to turn to a new orientation. The orientation of a ship determines the direction of its acceleration.
While cruising in interplanetary space, the ships turn slowly. Turning rates of about 1 deg/day are typical. The ships will have very capable onboard computers so that any ship could sail itself to its destination. Control centers on Earth will monitor the progress and condition of the ships.
|Accel (ac), mm/s2||0.4||1.0||1.5||3.0|
|Areal density, g/m2||20.7||8.3||5.5||2.8|
|Mars transfer, days||330||130||100||70|
|Payload cost, $/kg||710||2000||3100||8300|
JPL estimated the cost of the first production unit of an 820-meter solar sail at $65 million, for use in 1982. A second unit would cost a bit less, and could have been produced by about 1984. Adding some electronics to make it functionally independent might have pushed the cost to around $70 million. From this, a very rough estimate of $50 to 100 million seems reasonable for a 1-km ship design in steady production, and $70 to 150 million for a 2-km ship. These unit costs could be amortized over 15 to 30 years of operations.
Commercial charter rates might be roughly $10 to 25 million annually for ships in this size range. For a two-year voyage, the commercial shipping charges could be roughly $20 to 50 million, which for a 25-ton load averages $1 to 2 million per ton. The table shows rough estimates of shipping costs for cargo taken to Mars by a 2-km ship.
Commercial shipping companies will begin operations with interplanetary ships as soon as a commercial market exists. One could exist in the near future if NASA, ESA, and other space organizations purchased interplanetary transportation in the same way as they purchase launch services. Private organizations, most likely ones associated with the development of Mars, could also be part of the market for sailing ships. Several companies, including General Astronautics, are likely to offer shipping services on a charter basis, much like maritime shippers on Earth.