I can see why one would want nuclear-propelled ships; it makes it so that a ship can be at sea longer without being resupplied. That can be very useful.
An aircraft carrier isn’t going anywhere at sea without its battle group so it’s just as tethered by a traditional supply train as if it were conventionally propelled.
I can see why corvettes and similar ships wouldn’t be nuke-powered; they’re so small relative so the other ships that enough conventional fuel might be carried in a nuke-powered supply ship.
So, why bother making aircraft carriers nuke-powered if the other major ships in the battle group aren’t?
Did someone at the DoD run the numbers and figure out that the added cost of a nuke plant is lesser than the cost of fuel supply ships for the carrier but that the same doesn’t hold for cruisers, destroyers and frigates?
Or is it just a compromise that wasn’t worth it; A case of some people wanting a baby, some wanting no baby and the issue being settled by procuring half a baby?
The carrier is the one ship in the battlegroup that still has very sizeable fuel requirements even when nuclear powered. It’s air assets are thirsty. A carrier battle group would still need resupply to sustain air operations. Just conjecture but if there’s volume savings using the nuclear core versus fuel for the ship it would increase the amount of aviation fuel carried in the same size ship. Reducing the overall amount of fuel needed for the battle group to sustain itself might make maintaining that resupply easier as well. Just
Also consider that many of the surface combatants and other ships that support the carrier are not full-time in carrier strike groups. For example, a destroyer could do a deployment supporting a carrier strike group, and then later support an amphibious strike group, or do other operations entirely. Making all destroyers nuclear simply to support one of its missions doesn’t make sense on the face of it, especially considering the huge expense to build such a ship and then likely do a mid-life refueling.
By way of comparison, a DDG-51 destroyer costs roughly $1.6 billion. If it were to be nuclear powered, the rough cost of the ship would probably be about 40% greater. If it were nuclear powered, it’s operations cost would go down significantly depending on the price of fuel at the time. However, just like how middle class people are not rushing to buy $90,000 Teslas based on substantial cost savings during its operational life; there’s only so much up-front investment that one can afford in order to gain long-term savings.
I think that the nuclear power plant also provides more power than a fossil-fuel plant could, which translates into nuclear ships being faster than fossil-fuel ships of the same sort.
Carriers are a very costly scarce resource. The other ships, not so much. You can keep your carrier on station for quite a while and have other ships come and go.
(This does make one wonder why not have the carriers be conventional and have fuel brought to them.)
Not really.
Nuclear power plants are conventional steam plants bolted on to reactors instead of boilers. In fact, one of my buddies trained at a facility that was built by attaching a weird experimental reactor to a power plant scavenged from a WWII cruiser. Indeed, our basic steam plant training was given at the same school that teaches conventional machinists mates and boiler technicians.
A typical pressurized water reactor (the naval type) generated saturated steam in the secondary loop, so if anything, the plant less efficient and powerful than a similar sized 1200psi conventional plant that runs on superheated steam.
I suppose that if they need more steam in a conventional ship they can always add more boilers. The limiting factor will be fuel oil.
Much of the weight savings from not needing fuel oil is counterbalanced by the added weight of shielding and additional nuclear-only plant components / additional robustification. I used to know which was heavier, but that is lost to the mists of time.
Conventional power plants use pressurized water too? As in, the water in conventional plants is so pressurized that it’s still liquid at 300 C or such?
If I may tack on an additional question now that you’re here:
Do PWR use supercritical water or is it below the point of criticality? From what I understand, criticality helps in that the coolant remains as dense as liquid yet as gap-filling and pressure equalizing as a gas which helps it transfer the energy the turbine with less loss, right?
Notice how the steam exits the steam drum at the top and then loops back through the part labeled “superheater” in the right side of the drawing? The hot exhaust gases heat up the already-hot steam beyond the saturation point. In other words, the steam now has much more energy than steam that is still in contact with its boiling water source (a.k.a. saturated steam).
Not sure what your other question is about. Critical means two things to me in this discussion: the “critical” of a stable self-sustaining nuclear reaction and the Critical Point in thermodynamics. The former is a very real part of a normal nuclear power plant; the latter is an academic concern only (as far as I know). Perhaps you can clarify your question.
The USS Midway (1945 - 1992) was a conventional powered CV. At 15 knots it would consume about 260 gallons of diesel fuel per mile. Over an extended period of time it would consume over 100,000 gallons per day. This, in turn, required refueling every 3-4 days. Consider how many refuelings this would have entailed over its 47 years of service. On the other hand our Nimitz Class carriers get refueled only once during their projected 50 years of service. When you consider the time savings, to say nothing of the inherent danger of underway replenishment, the advantage of nuclear power is significant.
The link about boilers has some errors in it. On a “D” type boiler the top drum is the “Steam Drum” The bottom drum is the “Mud Drum” not the feed water drum. A feed water drum is external to the boiler on a open feed system. Not on a modern ship. And the feed water enters the boiler in the steam drum.
Hey Snipe, I didn’t even notice that they were running the feed water into the water drum. That shows you how much I remember of boilers (we did study them before the training paths diverged).
Anyway, it did serve to illustrate superheaters.
Mickey, I have never heard of a supercritical fluid plant, but apparently there is such a thing as a supercritical water reactor design and a supercritical steam generator for conventional use.
The wording of both of those wiki pages indicates that neither design is common.
There is a considerable size difference, and hence fuel consumption, between the carrier and the other ships in the battle group. It makes sense to use nuclear power for the oil hog.
Pretty much. The US built 9 nuclear powered cruisers over the years to accompany nuclear powered carriers. The last were a set of 4 Virginia classcruisers
The cruisers were scrapped for cost reasons; one during refitting, 3 others early, and an extra one canceled. It was found that conventional gas turbine replacements were cheaper to operate; the major cost factor being the extra manpower needed and cost of refitting/refuellings and mid life overhauls.
The Navy study found that these cruisers had a $40 million annual operating cost compared to $28 million for Ticonderogas and $20 million for the Arleigh Burkes.
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US carrier groups today will typically still include 1-2 nuclear powered vessels; it’s just that those may be underwater.
I thought the idea behind the battle group was that they were to protect the carrier. They’re ‘expendable’ in that regard, and you don’t necessarily want a bunch of reactors blowing up or sinking in even a minor engagement.
Just think of the mess is one of them caught fire and exploded in a foreign port, even if it was due to a terrorist attack.
Dont forget the other reason for a nuclear powered carrier is the amount of steam on tap for the steam catapult. I dont think a conventionally powered ship could manage that.
The JFK was originally designed to be a nuclear powered vessel, with an A3W reactor, but they changed their minds and outfitted her with 8 Babcock and Wilcox 1200psi boilers. She got along just fine for 40 years.
From a strategic standpoint, there are two reasons:
The carrier is what you need to keep going, and it’s the rarest fighting ship on the planet. Escorts are somewhat interchangeable; they can come in and out of the task force as needs dictate. On top of that, in shit really hits the fan and an honest-to-God war is brewing, the carrier task force can include warships from any number of Allied navies, most of which are quite capable and use similar technology, so you have a potentially vast array of high quality warships that could protect the carrier. If you have to peel an American destroyer off a task force in the Indian Ocean, HMAS Stuart could take its place.
It’s a hell of a lot easier to refuel a 7000-ton destroyer away from port than it is a 100,000-ton aircraft carrier that has to carry jet fuel.