AIP only submarines

[Adam Reynolds]

Has there ever been a proposal for an Air Independent Propulsion only submarine as opposed to a mix of AIP and diesel-electric ? Such a thing is clearly possible, but I don't think it's ever actually been done.

I've also heard of somewhat recent proposals for Hydrogen powered shipping, so that is further evidence in favor of this being plausible, but it hasn't really been done for naval vessels. Such a design would almost certainly trade range for greater stealth.

[madd0ct0r]

Aip only? Batteries and electric motors only?

Well, it was good enough for Nemo. But in reality.. Batteries only with no way to recharge at sea would be cripplingly short ranged I think.
It would only make sense if you expected you sub to make short forays and be attempted to be tracked from the second of leaving harbour. The gulf being such a location?

I toyed with the idea of a sub that could surface and unfurl some lilypad solar panels but you'd be visible when basking which goes against the Ultra stealth ideal. Not done the calcs but I think the space volume of the solar panels would be equivalent to the diesel engine and fuel tank. If stealth is not a concern for most of the journey, just chugging along on a diesel engine seems easier.
The lilypads would only pay off if you needed very long missions, such that the sub's diesel tank could not be large enough.

So next I thought about seperating the functions, so you could have a floating solar array that the aip sub's return to to refuel. This feels even more vulnerable to spotter planes. You could have the recharge point being a battleship or a nuclear submarine - a little like in-air refuelling.

Then you have super stealthy aip sub's covering and ranging around the slightly less stealthy nuclear one. It feels rather all in on sub warfare.

[Adam Reynolds]

AIP doesn't just have to be batteries. It can also be hydrogen fuel cells or Stirling engines. These systems are generally slightly lower power than batteries and can't run at full power, but they have longer endurance and allow the sub to stay down longer without snorkeling.

There are a few special operations submarines that do run on batteries alone for nations like Korea, but they are in something of an unusual situation.

[madd0ct0r]

You realise fuel cells need a source of oxygen to oxidise the hydrogen right? If you are assuming a canister of oxygen why not the same approach for conventional engine? Use the cooled exhaust fume as a buffer to dilute the oxygen to atmospheric thickness.

Stirling engine needs a source of heat and somewhere to dump it. Ocean is latter, but you need something for the heat and im not seeing energy storage density better then nuclear or batteries. Some dehydrated salts give off heat during hydration, but I don't think high energy density.

[Elheru Aran]

Stirling engine needs a source of heat and somewhere to dump it. Ocean is latter, but you need something for the heat and im not seeing energy storage density better then nuclear or batteries. Some dehydrated salts give off heat during hydration, but I don't think high energy density.

If you used a Stirling engine to propel a submarine and dump excess heat into the ocean, wouldn't fairly basic infrared become quickly popular to detect such submarines?

[TimothyC]

Has there ever been a proposal for an Air Independent Propulsion only submarine as opposed to a mix of AIP and diesel-electric ? Such a thing is clearly possible, but I don't think it's ever actually been done.

I've also heard of somewhat recent proposals for Hydrogen powered shipping, so that is further evidence in favor of this being plausible, but it hasn't really been done for naval vessels. Such a design would almost certainly trade range for greater stealth.

At the risk of being exceedingly flippant, there are 68 American, 49 Russian, 16 Chinese, 11 French, 10 British, & 2 Indian nuclear powered boats, and almost all of these still have some form of diesel engine as a backup. Diesel is a relatively compact power source, and one that if you don't always need to haul half of your energy source for. One thing to remember is that all submarines (and all non-sailing ships) have to carry their own store of energy for motive power. For most conventional ships, that's in the form of chemical energy. For nuclear powered ships, that's nuclear energy. Having to haul around all of your oxygen in addition to your fuel gets very big very quickly relative to the trade-off of having some time you run on diesel engines on/near the surface.

In short you could do what you asked, but the trade-offs to doing so make it not economic. There are lots of navies out there, and if it made sense in a particular set of circumstances that exists, I'm sure someone would have done it.

[madd0ct0r]

Stirling engine needs a source of heat and somewhere to dump it. Ocean is latter, but you need something for the heat and im not seeing energy storage density better then nuclear or batteries. Some dehydrated salts give off heat during hydration, but I don't think high energy density.

If you used a Stirling engine to propel a submarine and dump excess heat into the ocean, wouldn't fairly basic infrared become quickly popular to detect such submarines?

I don't know if infrared penetrates very far under water. The thought crossed my mind, but nuclear submarines dump heat the same way so I assume not a significant problem?

[Eternal_Freedom]

Pretty sure navies already use IR detectors (specifically FLIR) to spot subs near the surface already.

[Sky Captain]

There is Swedish Gotland class submarine
https://en.m.wikipedia.org/wiki/Gotland-class_submarine

that has onboard LOX tanks and Stirling engine for underwater endurance of several weeks. That system generate only 75Kw of power so the submarine won't brake any speed records in AIP mode , but it is good enough for slow stealthy cruise or battery charging to achieve short bursts of 20 knot speed. On the surface regular diesel engine is used.
I suppose it should be possible to design more powerful Stirling engines and not have diesel engine at all, just use atmospheric air on the surface and onboard LOX in AIP mode.

[JI_Joe84]

Are we not considering nuclear power as a AIP? I don't understand why nations are still launching diesel/ electric boats in the nuclear sub age. Nuclear power just seems like God mode IMHO

[Batman]

Nucs tend to be on the biggish side and fiendishly expensive. Bluewater only ships are little use to a costal/littoral navy even if they can afford it.

[JI_Joe84]

Nucs tend to be on the biggish side and fiendishly expensive. Bluewater only ships are little use to a costal/littoral navy even if they can afford it.

Hhhmmm well I watch sub brief by Aaron Amick on YouTube and on the whole nuclear subs don't seem to be that much more expensive than diesel subs but that is mostly gov. bureaucracy screwing things up like it's a sport.
Then again maybe they don't want to seem too threatening so they choose a sub that can defend /keep an eye on their coast but would be useless out in the open ocean

[aerius]

It's not just the cost of the sub. It's the cost of the entire nuclear industry & infrastructure needed to build & support it, plus you need to train all those people along with the ones who are operating the subs. The subs themselves may cost $3 billion a pop or whatever it is, but what you need to build & support them probably costs 10-100 times as much.

[JI_Joe84]

It's not just the cost of the sub. It's the cost of the entire nuclear industry & infrastructure needed to build & support it, plus you need to train all those people along with the ones who are operating the subs. The subs themselves may cost $3 billion a pop or whatever it is, but what you need to build & support them probably costs 10-100 times as much.

Ohhhh I didn't think about getting the nuclear material and storing the waste compared to just getting some diesel engines. I see now. Thanks