Complex sentence: The T-15 project began in strict secrecy in 1951 and the research and testing was contemporaneous with the other concept, the much smaller and lighter 533 millimeter torpedo referred to as the T-5.
Split into simpler sentences: The T-15 project began in strict secrecy in 1951. Research and testing was contemporaneous with the other concept, the much smaller and lighter 533 millimeter torpedo referred to as the T-5.