Complex sentence: Entering flight trials in 1953, BOAR proved satisfactorary; twenty test firings during the course of 1955 were conducted without a single failure, and in 1956 the rocket entered operational service.
Split into simpler sentences: Entering flight trials in 1953, BOAR proved satisfactory. Twenty test firings during the course of 1955 were conducted without a single failure, and in 1956 the rocket entered operational service.