I had the opportunity to be on two SSBN's that did a missile launch - one as an Navigation Electronics Technician, the other as the COB and was the Diving Officer during the launch. Kirk Smith
USS Daniel Webster (SSBN-626) USS James Monroe (SSN-622) 

Launching a Missile From a Submarine Is Harder Than You Think
David Hambling, Popular Mechanics, February 13 

Launching a missile from a submarine isn't as easy as pressing a big red button. It takes a lot of engineering and rocket science to ensure a missile gets from an underwater launch tube to streaking through low-Earth orbit at more than 13,000 mph—and sometimes, that delicate dance of physics can go wrong.
As these missiles increase in lethality, like today's Trident II D-5 missile, so do emotions surrounding the ethics of using such a weapon in the first place. But knowing Trident's future, and the future of other nuclear missiles, requires a trip to the past and an exploration of how you even launch a missile from under the sea. Here's how a missile makes it way through the water, into the air, and on its way to a target.
How We Got Here
Submarines couldn't always launch airborne missiles from under the sea, of course. During both World Wars, submarines were armed with torpedoes and deck-mounted guns—heavy cannons for engaging surface ships and lighter anti-aircraft weapons. Mine-laying submarines also terrorized the sea in the first half of the 20th century.
But at the dawn of the Cold War, it became clear that nuclear missiles would decide future world conflicts. The earliest versions of this technology—such as what went into the Nazi V-1 and V-2s, proto-cruise missiles used to bomb London—were used as a design starting point. These missiles had a range of just a few hundred miles, which meant you needed an aircraft or ship to carry them within range. A submarine with a capable missile carrier would be the perfect weapon, able to get weapons of mass destruction within incredibly close range of the enemy without being detected.
In 1947, the United States launched a JB-2 Loon, a direct copy of the German V-1, was launched from the deck of the submarine USS Cusk. This test proved it could be done, but the Cusk used an unpolished, jerry-rigged system. By 1953 the USS Tunny had been adapted into a true missile submarine, but firing the Regulus cruise missile was still an awkward process. The submarine had to surface, then the missile was manually loaded from storage onto a launch rail on the submarine's deck before it could fire. During the whole process, the surfaced submarine was visible and vulnerable to attack by enemy aircraft. The Grayback class of submarines were subsequently built to launch missiles from the surface.
At the end of the 1950s, weapons systems still had yet to master the tricky science of shooting a rocket through water. But technology was progressing quickly, and at the turn of the decade, the Navy developed the Polaris A1 Fleet ballistic missile. Successfully launched by the USS George Washington, this missile was a revolutionary development because it allowed a boomer, another name for a ballistic missile submarine, to remain submerged. Subsequent missiles have all been refinements of the same basic design, invisible and unstoppable.
Decades later, the ballistic missile submarine is still considered the most secure leg of the nuclear triad (land-, air-, and sea-launched nukes). And sub-borne missiles have only increased in range, power and accuracy. The original Polaris had a range of about a thousand miles and delivered a single 600-kiloton warhead with an accuracy of around a mile. In 1972 a new version entered service (originally known as the Polaris B3 but then renamed Poseidon C3) with had a range of almost 3,000 miles, and carried up to twelve warheads. In 1979 came the sea god's most trusted weapon—the Trident C4, which carried the same payload to a distance of 4,600 miles. It meant a sub in the Pacific could hit any target in the Soviet Union.
Today, American Ohio-class and British Vanguard-class submarines are equipped with a sixth-generation Trident weapons system. But perhaps more impressive than its awesome destructive capability is the mechanical process that launches the missile in the first place.
The Physics of a Launch
Made by Lockheed Martin, the current Trident II D-5 missile is a squat, blunt-nosed, 44-foot-long cylinder weighing nearly 120,000 pounds. It's fired by a steam cannon. First, an explosive charge flash-vaporizes a tank of water into steam. As the pressure of the expanding steam drives the missile out of its launch tube, it provides enough momentum for the weapon to clear the water's surface. This cocktail of high pressure and dangerous explosives is a crucial phase of every launch. Multiple safety mechanisms are in place to deactivate the missile if it fails to get away from the sub.
The missile slows down as it leaves the water and gravity tries to pull it back down. Motion sensors monitor the changes as the missiles hang in the air for a brief moment before the first of three rocket stages ignites.
Here, things can go spectacularly wrong if you're unlucky. Trident's first test launch from the USS Tennessee in 1989 failed because the plume of water trailing behind the missile interfered with a rocket nozzle. The resulting asymmetric thrust sent the missile spiraling in a spectacular pinwheel lasting four seconds before ending in a shower of flaming debris.
If all goes well, though, the first stage rocket burns for 65 seconds. During this phase, the missile extends an aerodynamic spike to smooth the airflow over the blunt-nosed cylinder. Without this spike, the missile can't survive its brief, high-speed transit through the atmosphere.
Finding Its Target
During this first minute, the missile should now be well on its way. It will eventually reach 600 miles above sea level. The remaining rocket stages still need to ignite, separate, and remain on the correct trajectory.
Again, this isn't so easy sometimes. According to a recent leaked report, a British Trident missile launched off the coast of Florida in June 2016 as part of a testing program was supposed to head east toward a target site near Africa. Instead, the missile allegedly veered east toward the U.S. before it was destroyed.
If the missile stays on the correct path, Trident then navigates with an inertial guidance system, based on a set of sensitive accelerometers measuring precisely how much the missile accelerated and for how long. An onboard computer uses this data to calculate speed and position of the missile. In most military technologies, inertial guidance has been replaced by GPS because the older way is expensive and has a tendency lose position over time. But that's not such a huge problem when your flight is only a few minutes long. The U.S. Navy has never fired a GPS-equipped Trident, largely out of fear of possible GPS tampering.
Because of the internal guidance system's limits, Trident also has a star sighting navigation system. Like an old-time sailor, this sensor gets a location fix by measuring the position of the stars to provide fine detail correction. This correction may be needed because the orientation of the submarine may not be precisely known at launch. A compass can be thrown by magnetic disturbances, and conditions at Earth's poles (where subs sometimes operate) don't help things either. Even odd gravitational anomalies may be great enough to throw the missile miles off course, so missiles—as well as Navy seamen—are well-versed in reading the stars.
Once all those stars align, the missile finally streaks toward its deadly destination.
Coming In For a (Destructive) Landing
As the missile approaches, it ejects twelve independent warheads at different targets. Each warhead has a yield of 100 kilotons—six times greater than the Hiroshima bomb. The missile's accuracy is quoted as less than 400 feet Circular Error Probable (CEP), meaning there is a 50 percent chance it will land less than 400 feet from the target.
But everything has to function perfectly for the missile to hit a target, and a glitch at any stage can be disastrous. There is also the new hazard of cyber sabotage of nuclear delivery systems. Malicious software, or even hardware which interferes with the missile controls, is a cheap way of disabling a nuclear deterrent. Although it sounds unlikely, such an attack wouldn't be without precedent.
The Royal Navy's latest botched test has only renewed calls from nuclear opponents who would like to see these destructive monsters of the deep retired completely. But as long as nuclear weapons exist, it's likely that that the Trident is going anywhere anytime soon.

Kirk Smith