On February 13, 2015, about 0900 eastern daylight time, an experimental amateur-built Velocity STD/RG-E, N112TH, was substantially damaged during collision with terrain after takeoff from Sebastian Municipal Airport (X26), Sebastian, Florida. The private pilot/owner was seriously injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight, which was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.According to the chief pilot of Velocity Aircraft, the airplane was recently purchased by the pilot. The sale was brokered by the previous owner, and the airplane was flown to the Velocity facility for a conditional inspection, the installation of new door latches, and delivery to the new owner. The pilot was provided with 3.6 hours of familiarization training in a Velocity company airplane over the two days prior to the accident. The accident flight was witnessed by the chief pilot and others, and their descriptions of the event were consistent throughout. The airplane departed runway 28 after a ground roll of approximately 1,000 feet and passing the intersection of runway 05/23. Immediately after rotation, the witnesses watched the right cabin door open. The airplane maintained runway heading briefly, then began a "slow, lingering" left turn back towards runway 05. One witness described the engine sound as increasing and decreasing between idle and full power. The airplane would alternately roll left, then wings level, and roll left again before it ultimately descended to ground contact south of runway 05. The airplane impacted trees near the airport perimeter, where the right wing separated and the fuselage was substantially damaged. Immediately after the accident, a responding police officer asked the pilot what happened. The pilot reported that he was taking off, and when the airplane reached about 50 feet, at approximately 80-90 knots, "the door blew open." In a subsequent interview with a Federal Aviation Administration (FAA) aviation safety inspector, the pilot said the airplane "pitched up violently" on takeoff, and wouldn't respond to nose-down pitch trim. Examination of the airplane by the FAA inspector revealed that control continuity could be established from the flight control surfaces on the left wing to the cockpit area and from the flight control surfaces in the right wing to the break. The control stick was broken off in the cockpit. The latch mechanism on the right cabin door was found intact and fully functional. The pitch trim was found in a nearly full nose-up setting. Electrical power was applied, and when activated the pitch trim motor ran through its full range at its "normal" rate. The pilot held a private pilot certificate with ratings for airplane single-engine land and instrument airplane. Examination of his logbook revealed 935 total hours of flight experience, of which 9 hours were in the accident airplane make and model. His most recent FAA third class medical certificate was issued on January 31, 2015. The airplane was manufactured in 2007 and was equipped with a Superior IO-360 series, 180 hp, reciprocating engine. The airplane's most recent conditional inspection was completed on December 16, 2014 at 79 total aircraft hours.