On December 20, 2007, about 1715 mountain standard time, an experimental home built Aircamper SDE, N7229R, operated by the pilot, touched down hard on runway 25 at the Flying Diamond (uncontrolled) Airport, Sahuarita, Arizona. The airplane veered off the runway and came to rest in the adjacent dirt terrain with a bent airframe. The airplane was substantially damaged, and the private pilot was not injured. Visual meteorological conditions prevailed at the time of the personal flight, and no flight plan was filed. The private pilot was not injured during the personal flight that was performed under the provisions of 14 CFR Part 91. The flight originated from Sahuarita about 1615. The pilot reported to the National Transportation Safety Board investigator that his approach to the runway was normal. On short final approach he observed an automobile approach the runway. The car appeared to be proceeding on a nearly perpendicular course to the runway. The pilot opined that if it did not stop it would cross the runway about midfield, in front of his landing airplane. The pilot stated that, in response to the potential hazard, he elected to go around. The pilot reported that he quickly advanced the engine's throttle, but the engine did not respond. Thereafter, the airplane impacted the ground in a level attitude and departed the runway to the right.