For more than five decades the Royal Navy has maintained Britain’s continuous at-sea nuclear deterrent – the longest mission in the history of the services.
During that time at least one ballistic missile submarine has been on patrol beneath the waves of the world’s oceans as part of Operation Relentless.
Here are some of the questions and answers surrounding the details, history and facts of the UK’s strategic nuclear deterrent.
– How long has Britain had a continuous at-sea nuclear deterrent?
Since April 1969 – for 24 hours a day, seven days a week, 365 days a year – there has always been a Royal Navy ballistic missile submarine at sea.
The mission is known as Operation Relentless and according to the Government is designed to deter against aggression towards Britain.
– What were the first submarines and missiles?
Four Resolution-class submarines, known as Resolution, Repulse, Renown and Revenge, each carried up to 16 Polaris nuclear missiles between 1969 and 1996.
– When and what were they replaced by?
The first of the Vanguard-class nuclear submarines went on patrol in 1994. Called Vanguard, Victorious, Vigilant and Vengeance, the boats are due to be in service until 2030.
They carry the American-built Trident 2 D5 nuclear missiles and replaced Polaris when it was scrapped. Trident is the mainstay of Britain’s strategic nuclear deterrent.
– What can Trident do?
A Trident missile can be fired at targets up to 4,000 miles away and at its fastest can travel at more than 13,000 miles an hour, according to the Royal Navy.
They are 44ft (13m) long, weigh 130,000lb (58,500kg) and are ejected from the submarine by high-pressured gas before they fire as they reach the surface of the water.
– How many missiles do the submarines carry?
Each Vanguard-class submarine can hold up to 16 intercontinental ballistic missiles, but will only carry up to eight Trident rockets and up to 40 nuclear warheads.
The warheads can be independently targeted to hit specific areas.
– How big are the Vanguard-class submarines?
The boats are 492 ft (150m) in length, have a top seed of 25 knots and weigh 16,000 tonnes. They are home to more than 160 crew, with patrols usually lasting around three months.
They are powered by a nuclear reactor and based at HM Naval Base Clyde in Faslane, Scotland.
– What is the Dreadnought programme?
The Vanguard class are due to be replaced by the bigger Dreadnought-class submarines in the 2030s. The four new boats will have a lifespan of at least 30 years, according to the Government.
Once built they will carry Trident missiles, with the estimated cost of the Dreadnought build set to be £31 billion with a contingency of £10 billion set aside.