SEA 1000 CONVENTIONAL AIR INDEPENDENT PROPULSION
AIP for Australia In January 2008 Captain (Ret) James Patton, USN, published an article in the US Naval Submarine League’s quarterly journal reporting on a submarine conference he’d attended in Europe in late 2007. He mentioned in the article a conversation he’d had with an RAN submariner Commodore and Commander. Asked about the likely role of Air Independent Propulsion (AIP) in Australia’s future submarine force, the Commodore indicated that he didn’t think that Australia would be interested in AIP from an operational point of view. The Commander then offered a “Yeah, but …” opinion that some form of AIP would be valuable as a contingency system – like parachutes for fighter pilots or fire extinguishers and active sonars on submarines – something that wasn’t intended to be used, but when pinned down in some shallow water or bay with the battery running low, it would be nice to have a week or so of emergency propulsion to extricate oneself from adversaries. At the time the Commander represented the entirety of the RAN’s future submarine project “team” and he knew the Commodore was mistaken. In the shadow of his superior officer, and in the best interests of the RAN, he had delicately tried to correct the faux pas.
12th Dec 2011
SEA 1000
CONVENTIONAL AIR INDEPENDENT PROPULSION
Byline: Rex Patrick / Sydney
AIP for Australia
In January 2008 Captain (Ret) James Patton, USN, published an article in the US Naval Submarine League’s quarterly journal reporting on a submarine conference he’d attended in Europe in late 2007. He mentioned in the article a conversation he’d had with an RAN submariner Commodore and Commander. Asked about the likely role of Air Independent Propulsion (AIP) in Australia’s future submarine force, the Commodore indicated that he didn’t think that Australia would be interested in AIP from an operational point of view. The Commander then offered a “Yeah, but …” opinion that some form of AIP would be valuable as a contingency system – like parachutes for fighter pilots or fire extinguishers and active sonars on submarines – something that wasn’t intended to be used, but when pinned down in some shallow water or bay with the battery running low, it would be nice to have a week or so of emergency propulsion to extricate oneself from adversaries. At the time the Commander represented the entirety of the RAN’s future submarine project “team” and he knew the Commodore was mistaken. In the shadow of his superior officer, and in the best interests of the RAN, he had delicately tried to correct the faux pas.
AIP is an essential capability in modern conventional submarines. Today it is utilised in German, Greek, Italian, Japanese, Pakistani, Portuguese, Singaporean, South Korean and Swedish submarines and will soon be found on Indian, Israeli, Spanish and Turkish submarines. In the past ten year, the only countries that have signed contracts for non AIP submarines are new entrant navies or those that have or are developing a nuclear capability, itself a form of AIP.
This month’s SEA 1000 article looks at the tactical benefits that AIP affords a submarine commander and then explores available AIP options; highlighting their various pros and cons. Nuclear AIP is not considered … it being an independent topic for the next issue of APDR.
AIP and its Tactical Advantage
Without snorting, a conventional submarine can only expect to stay continuously submerged for a maximum of about 100 hours if operating at 4 knots. A conventional submarine must snort on a regular basis to preserve the charge in its main battery, as shown in Figure 1.
Conventional Submarine Indiscretion
The ratio of time spent snorting to not snorting is referred to as the indiscretion ratio and will normally be kept as low as possible. Indiscretion ratios vary from 30% during transits to 5% for a submarine in an operational area.
By and large, snorting is the Achilles heel of the modern conventional submarine, exposing it to counter detection.
First and foremost, snorting requires a periscope/optronics mast, an ESM mast and a snort induction mast to be raised, which expose the submarine to enemy visual sensors and radar.
It is possible for an enemy to detect masts and their plumes and wakes visually, particularly during the day. Visual counter-detection opportunities increase with the number of masts exposed, the speed of the submarine and the calmness of the sea. Designers try to minimise mast visual profiles by minimizing mast sizes, using camouflage to blend masts into the background environment and streamlining masts to reduce plumes and wakes. Operators try to minimise plumes and wakes by minimizing snorting speeds; a simple rule of thumb being knots = sea state + one.
Radar counter-detection is also a function of the number of masts exposed and the speed of the submarine, although good radar performance is not limited to daylight. Techniques used to minimise visual counter-detection generally work equally well in also minimising radar counter detection. Additionally, radar absorbent material and shape optimisation are used. ESM masts and systems are employed to determine the presence of dangerous radar signals and masts are lowered when rackets approach dangerous levels. “Gulping” can be used to reduce visual and radar counter-detection opportunities, particularly in scenarios where there is a heavy ASW airborne presence, but a pressing need to snort. “Gulping” involves raising the snort mast just above the surface of the water. Wave action results in the mast washing over from time to time - which causes discomfort to the crew as vacuums are pulled inside the submarine and then released on an alternating basis.
Despite all the methods employed by submariners to minimise counter-detection while snorting, modern electro-optical systems and periscope detection radars, particularly airborne, still present challenges to submariners.
Another significant snorting counter-detection source stems from running diesels and associated equipment noises. Snorting can increase a submarine’s acoustic radiate noise source level between 20 and 30 decibels. Assuming propagation losses of six decibels per doubling of range, and all other things being equal, the acoustic counter-detection ranges of a snorting submarine can increase eight- to 16- fold! Of course, submarine Commanding Officers will take advantage of any increases in ambient noise such as that caused by evening or fish choruses and heavy rain. They will also top up the battery with short snorts whenever tactically possible.
Nonetheless, snorting presents significant challenges to conventional submarine commanders.
AIP and its Tactical Advantage
AIP submarines don’t have the same Achilles heel as conventional submarines.
Whilst conventional AIP systems don’t assist submarines in transits or in high speed runs, they do allow them to operate at low speed for two to three weeks without the need to snort. Table 1 shows the submerged endurance capabilities of an AIP variant Scorpene submarine at different speeds along with the total effect of AIP coupled with a fully charged battery.
Speed Bottomed 2 Knots 4 Knots 6 Knots
AIP Effect 600 hours
(25 Days) 400 hours
(16 Days) 300 hours
(12 Days) 200 hours
(8 Days)
AIP plus Battery 840 hours
(35 Days) 550 hours
(23 Days) 400 hours
(16 Days) 250 hours
(10 Days)
Air Independent Submarine Indiscretion
There are numerous scenarios where conventional AIP can provide advantage to submarine Command.
It can allow a submarine to commence a zero indiscretion persistent incursion into enemy territory to conduct Special Force operations, mine lays, intelligence collection and other inshore operations. Removing the requirement to snort can also assist submarines conducting operations in and around straits, narrows and choke points; a submarine without AIP would otherwise have to withdraw from such an area for a period to replenish the charge in its batteries, interrupting whatever activity may have been taking place. Bottoming to conduct surveillance, or wait, can increase on-station times further. There is a possibility that localised coastal patrols can be conducted wholly and solely by air independent means. It has been reported that during its lease period to the USN, HMS GOTLAND regularly used its diesel engines only when entering or exiting port, going on two-week-plus AIP only patrols.
Expanding on one of those scenarios, to better highlight the advantages of AIP, the contrast that exists between an old conventional diesel-electric submarine, a modern conventional diesel-electric submarine and a conventional AIP submarine involved in conducting an ISR mission against a diesel-electric submarine operating in and around its home base is explained. The target submarine is assumed to have a speed of advance of four knots with the trailing submarine spending half of its time at two knots and the other half at six knots. The trail is broken once the submarine’s battery charge has reduced to 55%. Starting with 85% in the main battery, the older conventional diesel-electric submarine is capable of conducting the trail for 15 hours, the modern conventional diesel-electric submarine for 27 hours and the conventional air independent submarine for 10 days. Significantly more intelligence will be collected by the conventional AIP submarine.
AIP Options
There are three conventional AIP systems on the market; the MESMA (Module d’Energie Sous-Marine Autonome) system used on French designed submarines, the Stirling engine system used on Swedish and Japanese designed submarines and the fuel cell approach used by the Germans and the Spanish.
In the MESMA system, ethanol and liquid oxygen are mixed in a high pressure burner to a temperature of 700 degrees centigrade which acts as a heat source for a primary water loop which is pressurised at 60 bar, allowing operation to full diving depth (A bar is a measure of pressure roughly equivalent to one atmosphere; at the sea surface the pressure is one bar, at 10 metres it is two bar, at 20 metres it is three bar ... at 100 metres it rounds to 10 bar and at 600 metres it rounds to 60 bar). The primary loop passes through a steam generator. The steam, which is >20 bar and 500 degrees centigrade, spins a turbine attached to an alternator. The alternator charges the submarine’s battery. The MESMA system on the Agosta 90B submarine is capable of producing 200 kW of energy, which provides enough energy to for a light battery charging rate when operating the boat is operating at six knots. Newer versions of MESMA use diesel instead of ethanol that, like Stirling engines, limits the additional AIP fuel supply to just liquid oxygen.
The Stirling engine is a closed cycle engine that uses helium as the working fluid. Diesel and liquid oxygen are mixed in a high temperature burner to a temperature of 750 degrees centigrade, which acts as a heat source for an enclosed quantity of helium. The helium is driven through a repeating sequence of thermodynamic changes. By expanding the helium against a piston and then drawing it into a separate cooling chamber for subsequent compression, the heat from the external combustion of diesel and oxygen can be converted into work that can then be turned into electrical energy by a DC generator. The DC generator charges the battery. Gases from the process are mixed with cooling water in a special CO2 dissolver and discharged to sea. The system uses less pressure than a MESMA system, operating at 20 bar. Each Stirling engine is capable of producing 75 kW of energy, with two or more installed on each submarine.
A Polymer Electrolyte Membrane (PEM) fuel cell is an electro-chemical conversion device that combines hydrogen and liquid oxygen to produce electricity, water and heat. The water output is stored onboard to ensure submarine weight is unchanged as a result of fuel cell use. The operating temperature of this type of fuel cell is approximately 80 degrees centigrade. The Type 212 boats have nine 34 kW cells producing 270 kW of energy, with one spare cell. The Type 209, 209PN, 214 and Dolphin II submarines have two constant voltage 120 kW cells producing 240 kW of energy which pass through a DC to DC converter to match the fuel cell the voltage of 215V to the battery voltage. The Spanish use a fuel cell designed by the American company UTC Power that also connects to the submarine’s battery via a DC to DC convertor.
AIP Choice
The ideal conventional AIP system would have high efficiency, be acoustically silent, have a low magnetic signature, generate no heat or exhaust gasses, be reliable, be relatively easy to maintain and require no additional personnel. Because all AIP systems are an addition to a standard conventional submarine, weight and space are consumed and therefore the system needs to be both compact and light.
The fuel cell is the most efficient. Whilst MESMA produces the most power of the three options, it is the least efficient at around 25%. The Stirling engine has an efficiency of around 33%. The fuel cell’s beginning of life efficiency is 56% at 100% load factor and higher, around 68%, under partial loads. This trait of fuel cell efficiency is capitalised upon, with partial load used most of the time when submerged.
With respect to noise, the MESMA and Stirling engines both contain moving parts and so some radiated noise can be expected. Nonetheless, efforts are made to limit both systems’ signatures. The Stirling engine-generators, for example, are installed in double elastic mounting with a sound proof hood to guarantee that very low noise levels are transmitted into the sea. Much of the radiated noise from AIP systems is generated by the auxiliary equipment rather than by the core system itself. This is true for fuel cells too.
Note also that the Stirling engine, which operates at a pressure of 20 bars can only be operated down to 200 metres, unless a power consuming and potentially noisy exhaust gas pressure intensifier is used. The MESMA system does not suffer from this limitation as it runs at approximately 60 bars. The fuel cell outputs water - which is retained onboard for weight compensation purposes.
Both the Stirling Engine and Fuel Cell have a time between overhauls of more than 5000 hours. No data is publicly available for MESMA.
With respect to size, there is a great deal of similarity between each of the systems for any given power output. This is due to the size dominance of the liquid oxygen tank required by all systems to store the system oxidizer.
Having said that, they are all equal with respect to size. The fuel cell is the heaviest on account of the metal hydride storage containers currently required for the 99.99% hydrogen, which in turn restricts the amount of hydrogen fuel that can be carried onboard an ocean going submarine. Additionally, the refueling of the hydrogen on fuel cell submarines requires more effort because the temperature of the metal hydride has to be carefully maintained during the evolution; special refueling infrastructure not typically available in standard commercial ports is required. From a logistics perspective, the MESMA and Stirling engines are simpler to refuel with ethanol, diesel and liquid oxygen more readily available than hydrogen at more ports.
Reformer technology will eventually address refueling logistics associated with the fuel cell and its gravimetric and volumetric disadvantages for submarines weighing more than about 2,500 tonnes. Reformers consume more readily available and easily manageable fuel and reform it into hydrogen as and when required. The German manufacturer, ThyssenKrupp Marine Systems (TKMS), is working on a prototype reformer that converts methanol to hydrogen. The reforming takes place at 250 to 300 degrees centigrade and utilises the liquid oxygen already stored onboard for fuel cell operation. Onboard reforming decreases the system’s overall efficiency, but still outperforms MESMA and Stirling. The size of the cryogenic liquid oxygen tank therefore has to increase, but noting that the efficiency of the reforming process is about 90%, the increase is relatively low. The reformer is capable of producing enough hydrogen to supply two 120 kW cells on full load. The Spanish manufacturer, Navantia, is also testing an ethanol reformer system in its land based test site prior to installation in its new S-80 submarine. The S-80 uses a 300 kW PEM fuel cell and a reformer designed by Hynergreen, a subsidiary company of the Spanish ethanol supply company, Abengoa.
ThyssenKrupp Marine Systems indicated at Pacific 2010 that it has plans to produce liquid oxygen at sea during snorting for the Stirling AIP system, thus allowing a submarine Commanding Officer to transfer transit capacity to AIP capacity.
Finally, with respect to maturity, the Stirling engine system has been in operational service since 1990, the fuel cell since 2005 and MESMA since 2007.
Follow the Market
A look at market AIP uptake is interesting.
Stirling engines are installed in the Swedish Södermanland (2) and Gotland (3) class submarines; the Singaporean (2) Archer class submarines; and are being installed in the latest Japanese (8) Soryu class submarines.
There is currently only one MESMA system at sea - on the first Pakistani (1) Agosta class submarine. It is to be retrofitted to the other Pakistani (2) Agosta class submarine and is an option for the last three Scorpene class submarines being built under licence in India.
The fuel cell is in service on German (4 submarines) and Italian (2) Type 212 submarines, Greek (4), Portuguese (2) and South Korean (3) Type 214 and Greek (1) Type 209 submarines. Fuel cells are earmarked for further German (2) and Italian (2) Type 212 class submarines, further South Korean (6) and Turkish (6) Type 214s and Israeli (3) Dolphin II submarines and the Spanish (4) S-80s. The fuel cell has been touted for use in the French SMX-24 concept submarine. The fuel cell is also reportedly being offered for sale to Russian submarine customers.
Summary
In the opening paragraph the story was told of an RAN submarine Commodore who back in 2007 dismissed the need for AIP in our future submarines. With respect to SEA 1000, they were early days. After a few years of education, debate and consideration it would be difficult to find anyone who thinks that our future submarines wouldn’t have some form of AIP (even if some think it should be nuclear).
One of these systems will be adopted or adapted for Australian use. Although there have been rumblings in the submarine community about Australia developing its own AIP system, such a proposition makes little sense. The development time frame (25 years for the HDW AIP systems) is simply too long and there would seem to be little point. If Australia is going to spend our limited submarine R&D funding well, it should be on submarine capabilities that the French, German, Spanish and Swedes have yet to even think about.
AIP will tie Australia in with one of the European submarine manufacturers in one way or another. We should not reinvent the wheel.
