Realised that most modern submarines (and some aircraft carriers) use nuclear power, but destroyers and frigates don’t. I don’t imagine it’s a size thing, so I’m not sure what else it could be.
In: Engineering
The most simple answer is that a submarine survives by staying undetected.
Surfacing to refuel makes that difficult. So having a functionally limitless power source makes the need for surfacing (at least for fuel) unnecessary.
It also vastly extends their range as there is no need for refueling infrastructure/pre-positioning.
By comparison, surface ships are already on the surface, and are more plentiful in number than submarines. The cost to outfit that many would be significantly higher, while the advantage gained would be lower.
It’s an operational thing.
A submarines biggest advantage is its ability to submerge and hide, it’s biggest disadvantage is having to surface.
Conventional engines need oxygen for the combustion cycle, they can only stay under on battery power. Nuclear subs don’t have this limitation.
So long as they have enough food and air for the sailors aboard, a nuclear sub could stay submerged indefinitely.
The US [did once have nuclear powered surface ships](https://en.wikipedia.org/wiki/Nuclear-powered_cruisers_of_the_United_States_Navy) but they were deemed too expensive to maintain compared to more conventional hydrocarbon means of power.
Many submarines are still diesel powered, but if your submarine is meant as a nuclear deterrent then staying hidden for long periods just in case it needs to nuke someone is its entire job. Nuclear power is the only way to stay submerged for months at a time.
A lot of subs meant solely to attack other vessels are still running on diesel because they only need to stay submerged long enough to do so. Some countries like the US and Russia still make those submarines nuclear powered due to range requirements and because they want them to be able to hunt down the subs with the missiles, but most other countries are less interested in those things so they stick to diesel.
It’s hideously expensive to build and maintain nuclear powered vessels. The benefits are that they offer more power so you can power a huge and power hungry ship, fuel lasts a long time so you can have great operational range, and that it doesn’t require oxygen so you can use it under water.
This explains why subs and carriers use nuclear. They have reason to use nuclear that outweighs the drawback (cost).
A destroyer or smaller wouldn’t have the same calculus. Conventional power is so much cheaper and they can have support ships to refuel them. The question would become: do you want one nuclear powered destroyer or two conventional? And quantity is the quality then. Nuclear wouldn’t add enough benefit so it’s not chosen.
Submarines and super carriers (the only 2 types of nuclear fueled ship other than a Russian ice-breaker that may no longer be in service) are designed for long duties over very long ranges. For a submarine, being able to stay submerged for long periods greatly enhances its functionality. A nuclear powered carrier is large enough that resupplying it on the high seas is broadly feasible as a doctrine (since there are only a dozen or so in service). And the demand for power is so high that nuclear reactors make a lot of sense.
Smaller ships would run out of ammunition and supplies fairly quickly (in any sort of conflict) and resupplying so many ships on the high seas is impractical, dangerous and ludicrously expensive to implement. Since smaller ships need to be in port to resupply, they can just as easily refuel. Hence the benefit of nuclear power is just not there.
But who knows, if the technology for directed energy weapons (lasers) ever matures, it might make sense to have nuclear reactors on ships to supply the electricity needed which is what nuclear power is really good at.
Nuclear powered vessels also pose a logistical and diplomatic problem – many ports and countries refuse to allow them in. This further limits their flexibility.
Nuclear reactors are very, very, VERY expensive. Submarines use them because it allows them to be really, really, REALLY good at being a submarine–able to stay submerged for weeks/months, out to sea without refueling for even longer, quiet, etc.
Those are not big qualities that you need in a surface ship. It doesn’t matter if your destroyer is quiet, and it doesn’t need to submerge where it can’t get oxygen or be refueled. So, since you don’t need the extra capability you save the money and power it with a traditional power plant
That said, there are some ships that are nuclear powered–US aircraft carriers. These are huge and require a ton of power, especially electrical power for the catapult. Going with a nuclear reactor allows you to save room vs. the large diesel engines and generators and gas tanks you’d need to generate that kind of electricity
We *did* have nuclear surface combatants that weren’t carriers for a good part of the cold war, meant to escort nuclear carriers moving at high speed. Fantastic guided missile cruisers in the Virginia and California classes, to say nothing of my beloved *USS Long Beach,* the boxiest box that ever sailed the open seas. They were too expensive to maintain/refuel after the cold war ended, and modern COGAG systems like the four turbines on the Arleigh Burke class allow for sufficient hustle and range to keep up with a Nimitz.
One problem is also the number of nuke engineers that you would need to operate more reactors. It’s hard to qualify for it and even harder to make it through the schooling. It’s around a 50% drop rate. In my few years of recruiting, I managed to get maybe 7 enlisted in the nuke program.
Submarines need oxygen. If you are using your oxygen for both burning fuel and the crew, you run low on oxygen, you need to resurface before you run out, and both your engine and crew die.
Submarines need to stay undetected, so if you use a fuel that doesn’t need oxygen (nuclear), you extend the amount of time you can stay under significantly.
Aircraft carriers are big and use a lot of power, so they can effectively make use of a nuclear reactor.
Smaller ships don’t have the niche need for a Submarine sized nuclear reactor and aren’t big enough to need an aircraft carrier nuclear reactor, so fossil fuels are sufficient to meet their needs.
It’s basically just a matter of “why use a sledgehammer when a hammer works just fine” except you don’t have the risk of a nuclear disaster at sea
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