On September 9, 2015, at 1450 central daylight time, a Mark Brown Spacewalker amateur-built airplane, N43904, was substantially damaged during landing at the St. Louis Downtown Airport (CPS), Cahokia, Illinois. The private pilot and flight instructor were not injured. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight. Day visual meteorological conditions existed at the time of the accident and no flight plan was filed for the local flight. The flight's purpose was to allow the front seat (private) pilot to become familiar with the handling characteristics of the tailwheel airplane. The front seat pilot had recently completed construction of an experimental amateur-built airplane of the same make/model and was preparing for his first test flight. After completing area maneuvers, the front seat pilot executed two touch-and-go landings. During the third landing attempt, the flight instructor noticed a high sink rate develop and attempted a go-around. The airplane touched down hard, which damaged left main landing gear and tailwheel attachment spring. The flight instructor was unable to maintain directional control and the airplane departed the left side of the runway, which further damaged the landing gear and empennage. Examination of the left main landing gear revealed a failure of a multi-tube group weld which had been previously repaired. Further examination revealed that the lower tube of the multi-tube group weld was only partially welded. Records of this repair were not available to the investigation.