On September 14, 1997, at 1745 mountain daylight time, an AS + LTD model AC-4 glider, N3021, settled after an auto launch, struck one wing tip on the ground, and crashed on the Canfield Ranch, 10 miles southwest of Sundance, Wyoming. The certified flight instructor pilot was not injured and the glider sustained substantial damage. Visual meteorological conditions existed and no flight plan was filed for this personal flight conducted under Title 14 CFR Part 91. The flight was originating at the time of the accident. According to the attached pilot statement and a telephone interview with the pilot, this was the fourth flight of the day, all of which used a vehicle tow pulley system for launch. The mechanical advantage of this system was 3 to 1, the same as used on the third launch and the wind was a right quartering head wind estimated to be 5 to 7 knots. The pilot said that during the tow, the rope became entangled in some alfalfa stubble. When it came loose from the stubble some slack occurred. When the slack was taken up it caused the tow rope to "jerk" the glider, which then pitched up. Altitude at the time was about one wing span, according to the pilot, and about 800 feet into the takeoff run. The pilot said he released the tow and attempted to set up for a landing; however, the left wing dropped and the glider struck the ground with the left wing in about a 25 degree left wing down attitude. The glider reversed heading and came to rest approximately 900 feet from where the takeoff had started. (See attached narrative and drawing). Substantial damage was done to the fuselage and both wings.