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Our ability to build and sail systems that fly high above the ships depends in part on advanced materials that have specific strength and stiffness an order of magnitude higher than what was used in the last round of commercial sail craft a hundred years ago. Spectra, a remarkable polyethylene fiber invented in the early 1980's in Europe and currently in mass production at Allied Chemical, is stronger than steel and floats on water, making it an ideal material for tow line and other elements of large sail systems. Filament fortified film, invented in 1991 and used in our successful defense of The America's Cup in 1992, combines Spectra with carbon fibers and other space age materials to create sail material an order of magnitude better than 19th century canvas in terms of strength, stiffness, and life.
In the rapidly growing sport of traction kiting, soft sails derived from modern parafoil technology are becoming more popular than stick kites like the Kiteski. The main reason is a lower unit weight. Popular soft kites like the Peel, Sputnik, and Chevron extract 20 hp from a 20 kt wind and weigh less than two lbs. Their unit weights are of the order of .01 lb/square foot (psf), which enables them to stay aloft in relative winds down to 2 kts, where the dynamic pressure is also of the order of .01 psf. Fig 7 shows a large parafoil kite used for traction on land and water. Although small kites are easily launched, a mechanism such as the one invented in 1994 by Bill Schrems46 would be required for larger soft kites.
Fig 7 Chevron Power kite. Photo and kite by Andrew Beattie
At the end of the spectrum of soft kites is the 30,000 square foot lighter than air wing (Fig 8). This 360 x 90 x 20 foot wing would float in the sky thanks to 300,000 cubic feet of helium that would support her 30,000 lb mass. She would have 50 ribs tied to the upper and lower surfaces to maintain a good foil shape in relative winds up to 100 kts. The 100 bridle lines would attach to some of the ribs and transmit up to a million lbs of force to the ship at the other end of the tether. The bridle lines are 3/8 inch dia Spectra and have an ultimate breaking strength of 11 kips at 100 ksi. They weigh .03 lb/ft or 300 lbs total if they average 100 ft in length. The main tether is 2 inch dia and weighs 500 lbs at 1000 ft length.. The control lines might be led to winches on the deck of the ship or connect to servos in a radio control pod between the ship and the wing.
Compressed helium is released into the wing to maintain an internal pressure a few tenths of a psi greater than the external pressure. The wing inflates to a shape determined largely by the rib. Contrary to common expectation, the skin billows out on the lower surface. The four bridle lines attach to every other rib and take the internal rib loads to the tether.
The 30,000 lb weight estimate is based on a fabric weight of 10 oz/square yard. This is an order of magnitude heavier than hot air balloon material, but less than many airships. The ribs are of similar material, reinforced at the bridle attach points and tied to the upper and lower surfaces for tension loads of 100 lbs/inch associated with the inflation pressure. The projected cost of this wing is $100/lb for her Spectra reinforced Mylar surfaces. That works out to $3m for a wing capable of exerting 400,000 lbs of towing force on a ship operating at 14 kts in the typical 20 kt trade winds. The power extracted from the wind is 400,000 lbs x 24 ft/sec = 10^7 ft lb/sec or over 20,000 thrust horsepower. The cost of helium to fill this wing is $30,000, and it may need to be replenished each year during the ten year life of the system.
Semi rigid wings such as the Kiteski have unit weights of the order of .1 psf, which causes them to fall out of the sky when the relative wind drops below 5 kts. Rigid wings like the Global Hawk have unit weights of the order of 1 psf, so they must maintain a flight speed above 15 kts to stay in the sky. The goal, of course, is not just to stay aloft, but to do useful work. Staying aloft is merely a prerequisite.
The largest modern parafoil is the 7000 square foot wing being developed by the US army for flying payloads up to 35,000 lbs which exit from the back door of a C-130 cargo plane. These payloads are guided to a precision landing up to 15 miles away by a control system hooked to the risers. As a kite tethered to a ship, this wing could provide 20,000 lbs of thrust on points of sail within 45 deg of a beam reach in 20 kts of true wind. Assuming the ship was making 14 kts along her course, the wind energy would be 14 x 1.69 x 20,000/550 = 860 hp, or about the same as that of the Condor described in our abstract. The difference is that the soft sail weighs two orders of magnitude less than the rigid wing, can stay aloft in much less wind, and can be stowed and deployed from a tidy little bag on deck. The down side of the soft sail is that it is not very weatherly, having significantly less lift to drag ratio (L/D) than the Condor, and would not be able to tow the ship to weather in more than about 20 kts of true wind. The Condor would be able to exert her 10,000 towing force in relative winds up to about 100 kts, although the turbulence associated with these once a year storms may cause a rigid wing to break up.
A great deal of operational data is being obtained on rigid wings thanks to the aerosonde39, Predator47, and other UAV's. Although rigid wings have the best lift to drag ratio of all types of kites, they are more expensive per unit area and are more difficult to build and fly at the low speed end of the useful wind spectrum. They are best in storm conditions, where soft sails may need to be furled. By flying larger patterns in the sky, the rigid wings can be competitive with soft sails having much more area. Rigid wings such as the one used in San Diego in 1988 to defend the America's Cup are only marginally more efficient than soft sails attached to a wing mast. While a small speed advantage in a sailing race is worth the trouble, a commercial shipper may opt for the lower cost, lighter weight, and other advantages of soft sails.
By combining aerospace technology with that used on the successful America's Cup defender in 199250, we could build a wing of 360 ft span, 30,000 square feet in area, which would weigh less than 30,000 lbs, cost less than $3M, and would develop up to 400,000 lbs of lift at 60 kts. (Fig 9) We might start by laying a two inch thick plate of carbon/epoxy tape using our automated tape laying machine, then cure it in our autoclave. It could be sliced into 16 bars each 2 x 2 inches by 90 feet in length. These could be bonded into a set of 4 honeycomb wing sections each 90 ft in length with a 10 ft chord.
Spar weight 360 x 12 x 8 x 2 x .056 = 4 kips Sail wgt = 3,000 square yds x 1 lb/ square yd = 3 kips .050 skins weigh .2 x 10 x 360 x 144 x .06 = 6 kips. 2 inch core adds 5 kips,
Fig 9 Wing based on America's Cup Technology
This type of construction is similar to that of the Boeing Condor and the wing masts of many ocean going multihulls. It is related to AmericaÕs Cup by the tubular carbon battens, external bracing and advanced sail laminates.
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