I didn’t want to hijack this thread so I thought I would present this question on its own.
The topic was submarines getting stuck in ice, and a few posters mentioned using the waste heat from the reactor to melt the ice if they became stuck.
Now, from my laymans knowledge, I realize that subs are all about stealth.
Specialized propellers to reduce cavitation, the shape and configuration of the bow and other hull parts, etc.
So my question is about the reactor.
They generate heat. Apparently lots of heat.
How do they manage this heat and not advertise themselves to anyone with their heat signature?
An interesting read.
So if I glean correctly, they do dump their waste heat into the surrounding water. It’s just that it is virtually undetectable at depth.
I guess we (the general public) won’t really know what advanced detection systems are in use today.
Pretty much. On the surface, you’d be trying to detect the heat that leaks to the air - not the water. The heat energy is not nearly as attenuated (weakened) in air as in water.
Even a couple meters down, you’d still be trying to detect it from what it sends straight up towards the air, rather than into the water.
Think about setting a very hot pan on a block of wood. Sure, the wood under the pan might char and burn, but how much is the rest of that block heated? It’s sort of the same principle.
Or even those infrared scanners they have at science museums. If you happen to wear glasses, they block your body’s heat signature. Water just happens to be pretty effective at the same thing.
Okay. I don’t doubt the accuracy of these statements.
I just can’t fathom (hehe, pun not intended) how dumping 100 degree C water into cooler surroundings can’t be detected.
What am I missing here?
Whack-a-Mole’s cite provides the technical detail without explaining, in explicit terms, the fact that water absorbs infrared radiation very well,[sup]*[/sup] so your IR camera won’t be able to see that warm water unless the camera is airborne and the warm water is practically on the surface, meaning there’s little or no cold water between the camera and the warm water that it’s trying to detect. It’s analogous to a diver trying to visually identify an object in muddy water, in that the muddy water is absorbing the wavelengths of light that he’s trying to detect, and the object can’t be detected unless it’s inches from his face.
The only other option for detecting the heat signature is by measuring the temperature of the water through which you are cruising, and hoping that you happen to drive right through the target sub’s thermal trail, at which point you can start following it like a bloodhound. Odds of encountering a thermal wake in this manner out in the open ocean are vanishingly small.
Note also that the madsci article assumes that the sub is discharging water that is at 100 degrees Celsius. Two points:
the actual discharge temperature is undoubtedly classified information, but I’ll wager it’s a lot less than 100C.
if they’re discharging their coolant water into their wake, the screws probably do a fantastic job of mixing it with ambient water, lowering the peak temperature of their wake to something very close to ambient.
[sub]* Infrared light is light in wavelengths longer than about 740 nm. [/sub]
Information from someone I know that went to Annapolis and did submarine duty:
Diesel subs can actually be quieter than nuclear subs. Seems counter-intuitive.
In real life, a sub’s nuclear reactor cannot be shut down. So there is always a signature, albeit very slight. A diesel submarine can charge it’s batteries and shut it’s engines down. When operating on only battery power there is hardly any signature.
True statement. Diesel submarines are generally running off of a battery when submerged. Very little speed and endurance, but very quiet. Batteries are very quiet, whereas a nuclear reactor generally requires pumps, etc. that generate some noise.
Untrue. The reactor can absolutely be shut down. It’s done all the time for drills. It’s just that the sub is then operating off of its battery and has the same limitations as any other non-nuclear sub. In addition, the battery on a nuclear submarine is generally smaller than those on diesel boats, because it’s really only designed for emergency use.
This answer, in a nutshell. Analogy: you’re sitting at a table on the deck of your yacht on Lake Superior enjoying a meal when you fumble and knock your salt shaker overboard. Not only does it not turn Lake Superior into salt water, but within minutes you’ll be hard pressed to detect any increased salinity in the water adjacent to your boat. Though you’re talking a lot more waste heat, you’re also talking an enormously greater volume of water for it to disperse in – and water transmits heat more rapidly than air. (Hence why you get hypothermia from water at a temperature which air would not cause it.)
I think the main issue is the absorption of radiation by water.
In the air, a hot object - or the air heated by a hot object - radiates infrared radiation. This can easily be detected from a distance because air is mostly transparent to infrared radiation. But water is opaque to infrared.
That’s a bit of a nitpick. Of course the reactor can be shut down in an emergency but that is an extreme measure. When the sub is doing what it is designed to do, whether in warfare or on maneuvers, shutting down the reactor is not part of standard procedure (unlike a diesel sub). Shutting down the reactor introduces a whole new host of problems. While it may be done in drills it doesn’t mean that the sub it still operational for it’s intended purpose. It’s more like a fire drill rather than a learned operational warfare procedure.
As I understand it modern nuclear subs are quieter than the ambient noise of the surrounding ocean. Can a diesel boat be quieter still? Maybe but not really an issue when the noise you produce cannot be discerned anyway.
I highly doubt that the that he condenser water discharge would be 100 degrees C more like 15 to 32 degrees C depending on the sea water temp and the load on the turbines.
Let’s make a basic, pedestrian analogy the may or may not be appropriate (you tell me).
You are at a rock concert and the drummer is banging away at a level that is measured at a far higher Db levels than the rhythm guitarist. Does that mean that the rhythm guitarist can’t be heard? Your ears and just about any recording device could pick up the rhythm guitarist and pick it out. Do you only hear the loudest sounds or do you hear the total composition? But if the rhythm guitarist isn’t playing while the drummer is hammering away can it be picked up by your ears or a sensitive hearing/recording device? Can that device tell you that this person in the band is even on the stage?
A little bit of noise is a lot. No noise, that’s a different story.
It can be easily detected, if you are close enough. I think there is some confusion about how we use heat to detect the heat signature of airplanes. Every object in the universe emits radiation. You do, I do, the computer does, and so do airplane engines. This radiation is called infrared radiation, and is essentially the same as visible light, just at a bit lower frequency (infra = below and red = the color red). The amount of this radiation depends on the absolute temperature of the object to the 4th power.
Say the temperature of the air is 80F (300K) and the engine is 1000F (800 K). The engine is 2.667 times as hot as the air. Taking that to the fourth power results in a factor of 50. Thus, to something that can see in the infrared spectrum, the engine is going to be 50 times as bright compared to the sky. It’s like picking out a spot light against the night sky.
So what’s the difference between a plane and a sub? Remember, this infrared radiation acts about the same as light. Since you can see the engine (really, the exhaust ports), your infrared detector can see the radiation as well. Can you see a sub when it is under water? Not really. Most ocean water does a very good job of blocking the light. Even if the sub had a spot light, you would be hard pressed to see it past 30 or 50 feet in most water. Your infrared detector is the same. So the engine might look like a very bright spotlight from 2 feet away, but after 50 feet of water you can barely see it.
This. When you’re detecting a thermal signature, you’re not detecting it like you detect warm streams in public swimming pools (i.e. through direct contact with the warmed water). The ocean’s too big for that, and the water moves around too much (and water that is a different temperature from ambient moves around even more because of convection).
Instead you’re detecting the IR emitted by the warm object, which only manages to get out a little ways before being absorbed by the water.
This is also why people think “stealth spacecraft” will be pretty much impossible. At first glance operating in space is a lot like operating in an ocean, except there’s even more room to get lost in. But without the water to absorb your IR emissions, the heat from just keeping a compartment warm enough to live in is detectable from ridiculous distances.
In theory any noise is, well, noise. Any sub has to make some if only from the water passing by the hull. The question is can an enemy discern that sound and know the sub is there? You need to be able to peel that unique sound signature out of the background noise. Modern computers can do that to an extent but there are always limits and the sound environment under water is complex.
I have heard that diesel subs are astonishingly quiet but I have also read that modern US nuclear subs are deemed black holes in the water. They are exceptionally hard to track via hearing them.
I think at this point it is splitting hairs. Diesel quieter than a nuke? Maybe but you aren’t likely to hear either one sneaking up on you.
I’ll note when I was growing up there was a naval air station in my city (Glenview Naval Air station which is now long gone). They had a variety of planes there but mostly they had P-3 Orions based there which had Magnetic Anomaly Detectors (MAD) which they used to detect submarines (you can see it in the picture…that long thing sticking out the back).
A sub is a big hunk of metal and supposedly this would distort the earth’s magnetic field which this plane could detect.
I have not heard of them in a long time though. Not sure if they are still used or other/better methods have been developed to replace them.
