All the nukes in the world are useless unless you have a way to get them to their targets. Aside from outlandish and potentially suicidal methods like suitcase nukes or nuclear artillery, there are three main ways of doing this: bombers, intercontinental ballistic missiles (ICBMs), and submarine launched ballistic missiles (SLBMs).
The only nuclear weapons ever used in anger were delivered by B-29 bombers, the Enola Gay and the Bockscar. Because the Allies had attained near total air-superiority over Japan at the time of the bombings, it was possible for these bombers to go in without any real escort. They were accompanied only by weather reconnaissance and observation planes.
In a modern nuclear exchange, total air superiority would probably be required for a country to be able to openly deliver a bomb. If a nuclear bombing is attempted with anything less than total superiority, the attacker can expect to have all remaining anti-air defenses thrown at them.
Absent total air superiority, the only reasonable way to deliver a nuclear weapon via bomber is stealth. Conventional bombers can make a play at this by flying below radar, but this is very risky and doesn’t guarantee success. In states of heightened alert, the bomber would have to avoid both radar and settlements where residents might alert the target to the coming attack.
Stealth aircraft can make a better go at delivering bombs, especially when the target is on high alert. All new combat aircraft are billed as having some stealth capabilities, but the only true stealth bomber is the US B-2 Spirit. Its virtually invisible to most means of detection (it shows up about as well on radar as a large bird or a sphere a foot in diameter would) and can carry 18 tonnes of bombs 11,000km (this gives it a range of about 5,000km, neglecting in-air refueling).
The main disadvantage of bombers then, is the difficulty that they all (except the B-2) face in arriving at their target both intact and at the correct altitude to drop a bomb. There are a couple of significant advantages to bombers though.
They’re controlled by people. Really well-trained, smart people who know what they’re doing. Tell a pilot that it’s up to her to prevent a retaliatory strike on her home country and she’s going to be very motivated to carry out her mission. Maybe she’ll come up with a new trick and make it to her target. If she doesn’t, maybe one of her squadron mates will. Send in a dozen bombers and you force your enemy into a problem of resource allocation. They may end up spread too thin to stop them all. And if just one gets through…
Having pilots also makes bombers fairly accurate. The steep fall-off in weapon intensity with distance from the target (as discussed earlier in §5.1) demands a fair amount of accuracy and pilots can be significantly more accurate than some ICBMs.
Bombers are also the delivery method that’s hardest to stop at the source. During periods of heightened alert, countries will always keep nuclear armed bombers in the air. Any sign of a nuclear attack and they’ll be released for bombing runs on their prearranged targets. Unlike ICBs, there is no silo you can take out to stop them.
Intercontinental ballistic missiles are missiles capable of delivering nuclear warheads at least 5,500km from their launch site. Intercontinental ballistic missiles are distinguished from cruise missiles by their use of rocket engines and their suborbital trajectories. They leave the atmosphere during their boost phase and come back down on top of their target, using a guidance system and fins for limited maneuvering and final targeting once back inside the Earth’s atmosphere.
US and Russian ICBMs use multiple independent targetable re-entry vehicles (MIRVs). A single ICBM can carry many MIRVs, which allow it to simultaneously strike several targets. MIRVs were invented in response to anti-ballistic missiles, nuclear tipped missiles that detonate near incoming enemy missiles, causing them to fizzle or frying their electronics. The advent of MIRVs made missile defense mostly pointless, because it is much easier to build bigger ICBMs with more MIRV capability than it is to expand missile defense systems to deal with more incoming MIRVs.
MIRVs were one of the main factors pushing down bomb yields. The scaling factors we went over earlier mean that smaller weapons do damage much more efficiently than larger ones. It was MIRVs that made the delivery of these smaller weapons economical. Without MIRVs, launching 8 smaller warheads would require launching 8 smaller missiles, which would end up much more expensive than launching one really huge missile. With a MIRV, you get the best of both worlds – one missile, many warheads.
Improvements in US and Russian missile accuracy over the last 50 years have also driven the decrease in the warhead yields commonly fielded by those countries. Early ICBMs were accurate to within about half a kilometer, which necessitated big warheads if you wanted to be sure of annihilating your target (this is especially important when the target is hardened, like your enemy’s nuclear arsenal is liable to be). Modern US and Russian missiles are now accurate to something like 200m, which (using cubic scaling to approximate destruction) allows for the same change of destroying the target with about 1/15 the yield.
China lags behind on ICBM technology, with Chinese missiles only accurate to within 800m of their target. This results in China using much larger warheads (4-5 Mt) on its ICBMs than the 475kt US MIRV capable W88 or the 200-300kt warheads the Russians favour. It is counter-intuitive yet true that routinely using larger warheads is the sign of a less capable nuclear power.
ICBMs are stored on mobile launching units, or in missile silos away from large cities and built into mountains or underground. Both strategies are designed to protect missiles from enemy attack. Keeping ICBMs mobile makes them very hard to target with other missiles – where they are when the missile is launched may not be where they are when it arrives. Add this to the problem of finding the damn things in the first place and you have relatively good protection.
Missile silos were most useful when missile accuracy was much worse than it currently is. Missile silos are supposed to be impervious to everything but a direct hit from another nuclear weapon. Once a difficult task, this is now fairly possible. If nothing else, missile silos are useful because they will divert some enemy attacks away from populated areas. Still, the expense and relative uselessness of nuclear silos mean that no one is clamouring to build more of them. The Russians are pivoting towards mobile ground based launchers, while the USA is focusing its efforts on SLBMs.
ICBMs have several advantages over bombers. Missiles can endure g-forces, altitudes, and heat that would kill humans many times over. Taken together, this lets missiles launch with very rapid acceleration, travel at very high speeds, and use very efficient trajectories; basically they arrive on target much more quickly than a bomber can. They’re also much harder to intercept. Modern ICBMs come with a variety of countermeasures, from dummy MIRVs, to aluminum balloons, all of which make countering them extremely difficult.
The Iron Dome suggests that >90% missile interception might be technically possible, but intercepting ICBMs remains much more difficult than intercepting short range rockets, so don’t expect much progress anytime soon.
One disadvantage of ICBMS are their highly visible launches and re-entries. A successful stealth bombing provides no warning at all and a successful SLBM attack might provide only 2-10 minutes of warning. ICBMs take something like 30 minutes to cover intercontinental distances (say, Russia to the USA, or China to the USA). During this time targets can be evacuated and retaliatory strikes prepared.
6.3 Submarine Launched Ballistic Missiles
At one point, it made sense to have a firm distinction between SLBMs (which had fairly small ranges) and ICBMs. For modern weapon systems, the ranges are effectively the same. Gone are the days where submarines had to sneak right up to the coast to attack their targets. If you were to look at a globe centred on a modern US or Russian ballistic missile submarine, you’d see all the targets it can hit.
Modern nuclear armed submarines are also very stealthy. As long as they remain submerged they cannot be found by satellites. SONAR and thermal imaging remain about the only things that can find them. Active sonar will smoke a sub out pretty quick – at the cost of letting the submarine know it’s been found and giving away the position of the intercepting vessel. Passive SONAR can occasionally find them, but the ocean is big and nuclear submarine captains know a whole bunch of tricks for masking their SONAR signature.
Thermal imaging can find nuclear powered submarines by the wake of warm water they leave behind (warmed after cooling their nuclear reactors), but savvy captains (and all of them are) can minimize their changes of detection by cruising along boundaries between zones of different temperatures. These zones are so irresistible to subs that a French and British submarine managed to collide as they both (probably) followed one. This was obviously pretty embarrassing to both countries, but should stand as a testament to how damn stealthy these things are. Had either captain realized what was going on, he would have changed course.
Submarines powered by nuclear reactors can run for decades without any refueling. The only practical limit on how long a nuclear sub can stay submerged is the food supply. Water is collected from the ocean and air is recycled much as it would be on the ISS.
SLBMs have all the advantages of ICBMs, with the added advantage of stealth. The US has embraced them wholeheartedly. Its SLBMs are more accurate and more heavily MIRVed than its land based missiles; the Ohio class nuclear submarine carries 24 Trident II SLBMs with up to 8 W88 475kt warheads, each with an effective range of 11,000km and an average error less than 90m. The Russians, Chinese, and others have been less enthusiastic with their nuclear subs. Russia in particular has been having a lot of trouble getting new SLBM systems into active use.
This highlights the one real disadvantage of SLMBs. They’re expensive and technologically complicated. Just getting a missile to successfully launch from beneath the ocean is challenging enough. Add in stealth requirements and a nuclear reactor and it’s no wonder that the sea floor is littered with sunken nuclear submarines, primarily of Soviet origin (one unlucky Soviet nuclear sub actually sunk twice).
You could reduce the technical complexity by using a more traditional diesel-electric submarine, but you’d sacrifice much of the submarine’s endurance and ability to stay submerged. Batteries can only power a submarine for a few days (or hours, if it’s moving as fast as it possibly can) and diesel engines will quickly suffocate the crew if run underwater. Add to this the limited amount of diesel any sub can carry and nuclear submarines become the only real way to do long duration submerged deterrence patrols.
Additional Reading: Nuclear subs collide in Atlantic, Submarine launched ballistic missile, Ohio Class Submarine, Trident II, Borey Class Submarine, List of sunken nuclear submarines, Nuclear submarine accidents, and Bulava missile troubles