I always wondered how these nuclear missiles are launched. Especially how they are propelled to the surface from the submerged submarine. The answers below addressed these questions very clearly.
From Juergen Nieveler:
First of all, the missile launch is a two-stage process – the missile engine doesn’t actually start until it’s clear out of the water. SLBMs and vertical-launched Tomahawks actually get pushed out by high-pressure air with enough force to lift the tail end a few feet out of the water – then the rocket engine kicks in, and in the case of the Tomahawk the process of deploying and starting the jet engine and dropping the rocket engine begins. With Harpoon it’s a bit different – they are launched horizontally from the torpedo tube within a container shaped a bit like a torpedo. That container then angles upwards and soon breaches the water surface… then the top blows off and the missile is launched from inside the launch container.
All of those missiles will get a last position update right before launch, so they’ll roughly know where they are when they leave the water – the vertically launched missiles of course having a better position fix than the tube-launched ones, as those travel a bit downrange. Once airborne, the missiles will then switch on their GPS receivers and will get a position fix from GPS within seconds (military GPS receivers getting a much more accurate signal than the civilian ones…). Once the navigation system has an updated position, it will plot an updated course for the missile to follow.
From Gururaq Kalanidhi:
In the earlier 1950s~1960s when the first operational Ballistic Missile submarines (or Boomers) were being tested, the technologies for an underwater missile launch were not feasible. This was because, the ballistic missiles were liquid fuelled. This meant that the missile was huge and cumbersome. This meant that only 1 or 2 could be carried onboard. Also, since it was powered by liquid fuels, it could not be ignited sub-surface. This meant that the submarine had to surface for a launch. This would give away the location of the submarine. Hence, scientists across the world were trying hard solve this problem. The solution came with the development of solid fuel rockets. These rockets usually made from carbon, sulphur and others did not need oxygen for their combustion. This made the missile more compact and powerful. This meant that the missile could be launched from within the submarine… However, launching a rocket/ missile from the confines of a submarine presents with a unique set of challenges. The launch would put tremendous pressure on the submarine body.
Hence, the scientists came up with a underwater launch system using pressurised air.
The launch from the submarine occurs below the ocean surface. The missiles are ejected from their tubes by igniting an explosive charge in a separate container which is separated by seventeen titanium alloy pinnacles activated by a double alloy steam system. The energy from the blast is directed to a water tank, where the water is flash-vaporized to steam. The subsequent pressure spike is strong enough to eject the missile out of the tube and give it enough momentum to reach and clear the surface of the water. The missile is pressurized with nitrogen to prevent the intrusion of water into any internal spaces, which could damage the missile or add weight, destabilizing the missile. Should the missile fail to breach the surface of the water, there are several safety mechanisms that can either deactivate the missile before launch or guide the missile through an additional phase of launch. Inertial motion sensors are activated upon launch, and when the sensors detect downward acceleration after being blown out of the water, the first-stage engine ignites. The aerospike, a telescoping outward extension that halves aerodynamic drag, is then deployed, and the boost phase begins. When the third-stage motor fires, within two minutes of launch, the missile is traveling faster than 20,000 ft/s (6,000 m/s), or 13,600 mph (21,600 km/h).
United States Ohio Class Trident Missile Submarine