A new form of AIP system?

bottomcrawler · 03-26-09, 02:55 PM

Non-nuclear submarines typically have a submerged endurance of up to a couple of weeks with the latest AIP systems.

For a sub at the larger end of the size range, something like a Kilo, what would the power requirement be for a submerged crawl speed of a 2-3 kts?

See, I had an idea...

A nuclear reactor is the ultimate in AIP, as we all know, since they can operate uninterrupted for years, so submerged endurance is effectively only limited by available supply storage and crew considerations.

There's quite a gap between the best non-nuclear AIP subs and the fully nuclear ones. I have an idea for how to make the gap a little smaller. I think.

RTG - Radioisotope Thermoelectric Generator. A single device is a feeble power source, but like any power cell, it can be connected in series/parallel to reach pretty decent power levels.

They're compact, lightweight and require very little shielding. With the right choice of radioisotope, they produce power for many years.

And - they're not mechanical, so they're completely quiet!

I can imagine having a room full of RTGs, with a clever power management system that connects banks of cells as they are needed for propulsion, and having them do other chores (recharging batteries, produce fresh water, et c) when not required for propulsion. The heat would have to be dissipated somehow, but I imagine that you would get quite far with non-mechanical convection cooling using vertical water pipes.

This would not be the sole AIP system, but rather augment other systems like LOX/Diesel Stirling engines or advanced fuel cells. Those would be needed for higher power requirements.

But for just doodling along at a couple of knots trying to stay undetected, RTGs should work very well.

All right, whaddaya think?

TLAM Strike · 03-27-09, 12:42 PM

Canada is already considering this for their Victoria class SSKs to give them under ice capablity.

TLAM Strike · 03-31-09, 11:46 AM

After doing some research I've found some problems with using an RTG as an AIP system, the weight. The best RTG today produces one kW of energy per 200 Kg of equipment. This is because thermoelectric generators today have very low efficiency (very little energy radiated from the nuclear fuel is converted to wattage). Lets see how this stacks up against the power requirements of a modern sub, a Kilo class submarine's electric motor produces 4,400 kW worth of torque on the screw. 4,400 * 200 = 880000 Kg, 880000 / 1000 = 880 tons of weight. Thats quite a bit of equipment on a 3000 ton sub. If the whole battery and diesel engine system was swapped out for RTG then it might work but there is another problem getting the fuel. RTGs run on Pu-238 (Plutonium 238) while nuclear reactors run on U-235 (Uranium 235). Uranium is naturally occurring (you dig it out of the ground as an ore) while Plutonium is man made (its made out of Uranium in a nuclear reactor) Am-241 (Americlum) which is also used in RTGs is also man made so time and expense becomes a problem.

So a sub powered by RTGs is possible but expensive and wasteful compared to the alternatives. RTGs are better for powering things that need power for decades even centuries which at this time is only deep space probes. Maybe future sea floor research platforms might use RTGs but I think it would be simpler to use hydrogen fuel cells for those if a effective way to distill hydrogen from sea water becomes feasible, or just run a cable to the surface for power.

Go check out the beginning design page over at projectrho.com's page on atomic rockets for the numbers I've used for my calculations on RTGs.

LoBlo · 03-31-09, 08:47 PM

An alternative to a pressurized water reactor would be a gas cooled reactor, even a pebble-bed reactor. Supposedly they require much less bulk and pebble-bed reactor is reported very safe even in the event of a reactor core breech.

I wish the US would experiment with one.

bottomcrawler · 04-02-09, 03:58 AM

TLAM, it's a bit difficult reading white text on a light-grey background.

What I meant, however, was not a complete AIP system for all propulsive need, but rather a "crawl" system, so that you can save your stored oxygen and keep the batteries fully charged for bursts of speed, and not waste them just swimming about at a few knots.
You don't need 4.4 MW to propel the sub at 2-3 kts! Imagine instead a system capable of, say, 100-200 kW. Using your numbers, that would mean no more than 20-40 tons. But I also think that your numbers relate to stand-alone units with all subsystems included. In a large array, some parts are likely only needed once or a few times, lowering the total weight.

TLAM Strike · 04-02-09, 11:19 AM

Quote:

Originally Posted by bottomcrawler

TLAM, it's a bit difficult reading white text on a light-grey background.

What I meant, however, was not a complete AIP system for all propulsive need, but rather a "crawl" system, so that you can save your stored oxygen and keep the batteries fully charged for bursts of speed, and not waste them just swimming about at a few knots.
You don't need 4.4 MW to propel the sub at 2-3 kts! Imagine instead a system capable of, say, 100-200 kW. Using your numbers, that would mean no more than 20-40 tons. But I also think that your numbers relate to stand-alone units with all subsystems included. In a large array, some parts are likely only needed once or a few times, lowering the total weight.

Sorry about that I cut and pasted from a word processor doc and it was all black. Highlight to see better.

The problem is that RTGs need to be in an array since one unit can't exceed 1 kW of power.

bottomcrawler · 04-02-09, 04:24 PM

Yes, that's what I said - they would need to be in an array in fairly large numbers. So?

If we imagine each RTG as a regular battery (technically a "cell", but let's ignore that), then we simply connect them in series and parallel as needed, and as many as needed, to get the power required to drive the propeller and move the sub at 2-3 kts.

And who knows? Perhaps it's possible to use a different design, more efficient in terms of specific power and specific volume, if the application is submarine propulsive power instead of power generation for a interplanetary spacecraft.

TLAM Strike · 04-04-09, 03:27 PM

Quote:

Originally Posted by bottomcrawler

Yes, that's what I said - they would need to be in an array in fairly large numbers. So?

If we imagine each RTG as a regular battery (technically a "cell", but let's ignore that), then we simply connect them in series and parallel as needed, and as many as needed, to get the power required to drive the propeller and move the sub at 2-3 kts.

And who knows? Perhaps it's possible to use a different design, more efficient in terms of specific power and specific volume, if the application is submarine propulsive power instead of power generation for a interplanetary spacecraft.

No power is power. It doesn't matter if its on a submarine or a space probe. All that might be needed is a converter to change power to the same voltage as the sub's electric motors utilize. However there is another downside of an RTG that I just realized, RTGs are designed for a low continuous drain of power while submarine batteries are designed for rapid power surges. In most any contingency a submarine's battery would probably be sufficient, if held down by an aircraft it could bottom or hover until its forced to RTB, in the case of a surface ship the sub has the advantage in weapons range (discounting embarked helos or ASROCS), but the sub still has the option of waiting for the surface ship to leave.

An RTG could drive a sub at one knot at about the same noise level at batteries indefinitely. But whats the point of running at 1 knot forever? The sub is not going to get anywhere nor can it pursue a target, batteries can probably keep a sub going until its safe to snort. If open ocean SSKs (like the Tango) were still being built then maybe having half the batteries replaced by RTGs for long duration creeping could be useful. But in today's littoral environment any submarine encounter is going to be fleeting so the option of months traveling at a few knots isn't necessary and a loss of power isn't likely to result in plunging below crush depth just bottoming and in such a situation emergency lighting is already provided by borosilicate-tritium glow lights, O2 by Oxygen generating candles, and CO2 scrubbing by Lithium Hydroxide canisters.

03-26-09, 02:55 PM	#1
bottomcrawler Mate Join Date: Jan 2009 Posts: 53 Downloads: 15 Uploads: 0	A new form of AIP system? Non-nuclear submarines typically have a submerged endurance of up to a couple of weeks with the latest AIP systems. For a sub at the larger end of the size range, something like a Kilo, what would the power requirement be for a submerged crawl speed of a 2-3 kts? See, I had an idea... A nuclear reactor is the ultimate in AIP, as we all know, since they can operate uninterrupted for years, so submerged endurance is effectively only limited by available supply storage and crew considerations. There's quite a gap between the best non-nuclear AIP subs and the fully nuclear ones. I have an idea for how to make the gap a little smaller. I think. RTG - Radioisotope Thermoelectric Generator. A single device is a feeble power source, but like any power cell, it can be connected in series/parallel to reach pretty decent power levels. They're compact, lightweight and require very little shielding. With the right choice of radioisotope, they produce power for many years. And - they're not mechanical, so they're completely quiet! I can imagine having a room full of RTGs, with a clever power management system that connects banks of cells as they are needed for propulsion, and having them do other chores (recharging batteries, produce fresh water, et c) when not required for propulsion. The heat would have to be dissipated somehow, but I imagine that you would get quite far with non-mechanical convection cooling using vertical water pipes. This would not be the sole AIP system, but rather augment other systems like LOX/Diesel Stirling engines or advanced fuel cells. Those would be needed for higher power requirements. But for just doodling along at a couple of knots trying to stay undetected, RTGs should work very well. All right, whaddaya think?

03-27-09, 12:42 PM	#2
TLAM Strike Navy Seal Join Date: Apr 2002 Location: Rochester, New York Posts: 8,633 Downloads: 29 Uploads: 6	Canada is already considering this for their Victoria class SSKs to give them under ice capablity. __________________

03-31-09, 11:46 AM	#3
TLAM Strike Navy Seal Join Date: Apr 2002 Location: Rochester, New York Posts: 8,633 Downloads: 29 Uploads: 6	After doing some research I've found some problems with using an RTG as an AIP system, the weight. The best RTG today produces one kW of energy per 200 Kg of equipment. This is because thermoelectric generators today have very low efficiency (very little energy radiated from the nuclear fuel is converted to wattage). Lets see how this stacks up against the power requirements of a modern sub, a Kilo class submarine's electric motor produces 4,400 kW worth of torque on the screw. 4,400 * 200 = 880000 Kg, 880000 / 1000 = 880 tons of weight. Thats quite a bit of equipment on a 3000 ton sub. If the whole battery and diesel engine system was swapped out for RTG then it might work but there is another problem getting the fuel. RTGs run on Pu-238 (Plutonium 238) while nuclear reactors run on U-235 (Uranium 235). Uranium is naturally occurring (you dig it out of the ground as an ore) while Plutonium is man made (its made out of Uranium in a nuclear reactor) Am-241 (Americlum) which is also used in RTGs is also man made so time and expense becomes a problem. So a sub powered by RTGs is possible but expensive and wasteful compared to the alternatives. RTGs are better for powering things that need power for decades even centuries which at this time is only deep space probes. Maybe future sea floor research platforms might use RTGs but I think it would be simpler to use hydrogen fuel cells for those if a effective way to distill hydrogen from sea water becomes feasible, or just run a cable to the surface for power. Go check out the beginning design page over at projectrho.com's page on atomic rockets for the numbers I've used for my calculations on RTGs. __________________

03-31-09, 08:47 PM	#4
LoBlo Subsim Diehard Join Date: May 2006 Location: Texas! Posts: 971 Downloads: 78 Uploads: 3	An alternative to a pressurized water reactor would be a gas cooled reactor, even a pebble-bed reactor. Supposedly they require much less bulk and pebble-bed reactor is reported very safe even in the event of a reactor core breech. I wish the US would experiment with one. __________________ "Seek not to offend or annoy... only to speak the truth"-a wise man

04-02-09, 03:58 AM	#5
bottomcrawler Mate Join Date: Jan 2009 Posts: 53 Downloads: 15 Uploads: 0	TLAM, it's a bit difficult reading white text on a light-grey background. What I meant, however, was not a complete AIP system for all propulsive need, but rather a "crawl" system, so that you can save your stored oxygen and keep the batteries fully charged for bursts of speed, and not waste them just swimming about at a few knots. You don't need 4.4 MW to propel the sub at 2-3 kts! Imagine instead a system capable of, say, 100-200 kW. Using your numbers, that would mean no more than 20-40 tons. But I also think that your numbers relate to stand-alone units with all subsystems included. In a large array, some parts are likely only needed once or a few times, lowering the total weight.

04-02-09, 04:24 PM	#7
bottomcrawler Mate Join Date: Jan 2009 Posts: 53 Downloads: 15 Uploads: 0	Yes, that's what I said - they would need to be in an array in fairly large numbers. So? If we imagine each RTG as a regular battery (technically a "cell", but let's ignore that), then we simply connect them in series and parallel as needed, and as many as needed, to get the power required to drive the propeller and move the sub at 2-3 kts. And who knows? Perhaps it's possible to use a different design, more efficient in terms of specific power and specific volume, if the application is submarine propulsive power instead of power generation for a interplanetary spacecraft.