On March 5, 1994, at 1315 central standard time, a Bell 47-G2, N6711D, operated as an instructional flight, sustained substantial damage when it landed hard while practicing autorotations at the airport in Aurora, Missouri. The Airline Transport Pilot/Certificated Flight Instructor (CFI) and the Commercially certificated dual student reported no injuries. Visual meteorological conditions prevailed for the flight, no flight plan was filed. The flight operated under 14 CFR Part 91, and originated from Aurora, Missouri, approximately 1230. Both pilots reported the purpose of the local flight was to practice autorotational landings. The CFI stated the student satisfactorily completed several normal and 180 degree autorotational landings during the flight. The student initiated another 180 degree autorotation. The CFI stated everything was normal as the autorotation was initiated. He reported the rotor RPM climbed during the turn, but this was corrected by the student. The CFI realized the closure rate was too fast and the flare was not sufficient to slow the rate of descent. As he reached for the controls, the helicopter contacted the ground on the rear one third portion of the skids, then bounced. The CFI reported the main rotor blades struck the tail rotor drive shaft, and the helicopter began to rotate to the right. The helicopter bounced again and came to a stop approximately 220 degrees from the touchdown heading.