The Trident II D5 missile is powered, in the first moment of its flight, by steam. Inside the launch tube, an explosive charge vaporises a tank of water in a flash. This creates a sudden huge increase in pressure that pushes the missile – which weighs almost 60 tons and is one and a half times the length of a Routemaster bus – out of the submarine, up to the surface and a short distance into the air above the surface of the ocean. As soon as the missile senses that it has begun to fall back towards the waves, the first of its rocket engines ignites and it begins to climb into the air. The power of these engines is phenomenal; in less than two minutes, the missile reaches 24 times the speed of sound, covering five miles a second.
The last publicly known Trident missile test by the UK was in June 2016. HMS Vengeance, then submerged off the coast of Florida, released a missile programmed to head south-east across the Atlantic, crossing thousands of miles of unpopulated ocean to a point below the southern tip of Africa, but this did not happen. According to defence sources, the missile headed instead towards the mainland United States.
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