Reinout P. Kroon, a retired engineer and educator who helped develop the first American-designed turbojet engine, died Tuesday at Chester County Hospital in Media, Pa. He turned 85 that day. He lived in Kennett Square, Pa. His family said he died of injuries suffered in an automobile accident. Kroon led a team at Westinghouse Electric in Philadelphia that developed the turbojet engine in the 1940s that enabled the United States to catch up with its European competitors. His leadership on the turbojet project won him the Spirit of St. Louis Medal from the American Society of Mechanical Engineers in 1950. In 1960 he left Westinghouse to become a professor of engineering at the University of Pennsylvania. He retired in 1975.