Commander Hewson joined the ISDC from the Royal Navy, where he led a successful career as a marine engineering officer. After a number of tours aboard the Royal Navy's nuclear attack and ballistic submarines, Hewson was appointed as Chief Engineer of HMS Triumph, a position he held for four years before electing to undertake advanced study in nuclear physics at the University of Manchester. On completion of his doctorate, Hewson was appointed to a joint Anglo-US engineering group tasked with identifying advanced new reactor designs for next generation of naval and space exploration vessels. During his time with the joint engineering group Hewson became one of the first Royal Navy officers to undertake active duty on a US nuclear submarine, spending six months as an engineering officer aboard the USS Miami. Hewson also taught at the Imperial Staff College in London during this time.