dietz%usc-cse%USC-ECL%SRI-NIC@sri-unix.UUCP (11/05/83)
The latest issue of Analog has an interesting article by Keith Lofstrom on a novel electromagnetic launching device called the "Launch Loop". The launch loop is a narrow iron strip formed into a loop some 4000 km in circumference. The iron moves at some 10 km/sec, and is guided by attractive forces, mainly from permanent magnets. The loop is located at the equator. The central section rises to an altitude of some 120 km and stays there for about 2000 km, then dips back to earth. At the ends the iron strip is guided around a large turning loop (10's of km in diameter) and sent back again. At 10km/sec the loop would fly into space without some extra force to hold it down; this force is provided by masses magnetically suspended below it (attractive magnetic techniques again). Payloads are launched by putting them on top of the loop using repulsive magnetic levitation; magnetic drag from the loop accelerates the payload to orbital velocity and above. The force the loop can exert is limited by eddy current heating of the strip (you don't want to heat it above the curie point). The idea doesn't look totally impossible, although I suspect the attractive magnetic technique would cause eddy currents in the strip. The strip has a mass of 35,000 tons and stores some 1/2 x 3.5E7kg x (1.0E4m/sec)^2 = 1.75E15 joules of energy, about 3 weeks output from a 1Gw electric plant. Lofstrom points out that the loop would make an ideal long distance power transmission system: the power transmitted by a 1 kg/m strip moving at 10 km/sec is some 500 Gw, and can be extracted with high efficiency (99%). Estimated cost of the loop is $1 billion. It could launch one million tons of material a year to geosynchronous orbit. Lofstrom doesn't mention it, but his idea may provide a cheap way to store electrical energy. A 1 kg/m strip formed into a circle with a radius of 100 km and moving at 10 km/sec would store some 8.7 million kilowatt hours of energy. The strip itself would be very cheap: it's only 628 tons of iron. The expensive parts would probably be the magnets for suspending the strip against gravity and for providing centripetal force (the centripetal acceleration is 100 gee's), the control electronics and the linear motor for adding/removing energy from the loop. The loop would have tremendous angular momentum -- would the earth's rotation cause a problem? Maybe counterrotating loops are needed, or the loop can be put entirely at one latitude. For centripetal magnets of fixed strength the energy stored in such a loop is proportional to r^(3/2), while material costs rise linearly in r, so you want the loop to be as large as possible. If this idea is feasible and economical then maybe wind and ground based solar make sense after all. I don't think Lovins would approve, though. Lofstrom and friends are currently building a 3.4 meter, 170 m/sec prototype system. If you want to donate time/money to his project you'll find his address at the end of the Analog article.
judd@umcp-cs.UUCP (11/09/83)
having put up such a large loop of iron and got it spinnig et al - what hapens if something large hits it and breaks it or even distorts it enough to contact the ground?? Judd Rogers (judd@umcp-cs)