Testing conducted by Chinese scientists has found that a solid rocket engine can ignite at 200 metres (656 feet) underwater – far deeper than submarine-launched missiles – suggesting new possibilities for deep-sea weapon systems.Ballistic missiles that can be launched from subs are central to strategic deterrence for nuclear powers – for example, the M51 used by France, America’s Trident system, and China’s Julang series.With these missiles on board, submarines, moving stealthily through deep...