Captain Jim Patton is President of Submarine Tactics and Technology, Inc., a consulting firm in North Stonington, CT. While on active duty he commanded USS PARGO (SSN 650). He is a frequent contributor to THE SUBMARINE REVIEW.
Much has been printed regarding the impact Air Independent Propulsion (AIP) has had, and will have, on Undersea Warfare (USW). Simplistically, AIP frees a submarine from the necessity to ingest air, at or near the surface of the ocean, to support some energy-generating process that then both propels the vessel and powers its sensors and life support systems. The ultimate in AIP, of course, is a nuclear reactor.
In addition to nuclear power, many lesser forms of AIP exist. There are:
- Stirling engines, an external-combustion engine fired by diesel fuel and oxygen (O2 )stored aboard in some form, whose low-pressure products of combustion are pumped overboard.
- Closed-cycle diesels that essentially reuse exhaust air after stripping out (then pumping overboard) the carbon dioxide (CO2) and replenishing the oxygen (0 2) consumed by combustion.
- Closed cycle turbines (the French MESMA system) with similar CO2 and O2 issues.
- Fuel cells, which produce electricity and water by allowing stored O2 and H2 to combine through a permeable membrane.
Although there are significant differences between these various AIP systems, one significant commonality is that a large amount of O2 must be somehow stored aboard, and is generally the limiting factor to the total quantity of energy stored.
Everyone deals with plumbing issues every day, and complex systems can sometimes be simplistically modeled and understood more easily if represented through a hydraulic analog. Figure (1) is just such an analogue meant to represent, in a general sense, a submarine AIP system. In explanation, fluid (energy) is expended through two paths- the relatively small but steady drain due to hotel loads (lights, heating and cooling, sensor suites and combat systems, etc.) and propulsion loads which can vary over a very wide range. This fluid can come from three sources- a battery, which stores a significant quantity of energy and can supply hotel loads plus maximum propulsion power for perhaps an hour; an AIP system representing many times the battery’s capacity but limited in the rate at which power can be generated (it can supply hotel loads and some limited degree of propulsion for several weeks); and a large capacity pump (diesel) which draws from a very large source (embarked fuel oil) and can supply hotel loads, propulsion and replenish the battery, but not the AIP system. To put things in perspective for a theoretical system, the fuel oil might represent a total of 300 MWHR of usable energy, the AIP system some 30 MWHR of (non-rechargeable at sea) stored energy, and the battery about 3 MWHR when fully charged.
As stated there are several different types of (non-nuclear) AIP systems. The first to operate at sea was installed on six Swedish submarines and is based on the Stirling engine. The reciprocal motion from this repetitive external combustion cycle is mechanically converted to rotary motion to drive a generator. The Swedish Got land class submarine (the GOT LAND itself now operating under contract out of San Diego as a target for U.S. ASW forces) has two Stirling units, each rated at 60 KW. Tested contemporaneously with the Swedish Stirling AIP boats in the late 80s but only recently deployed is the German fuel-cell based AIP system as installed on their recently delivered 212 class (the export version will be designated the 214 class). This system, drawing heavily from NASA- based research on fuel cells- particularly as regards PEM (Proton Exchange Membrane) technology, which reduced costs while greatly enhancing the efficiency and safety of fuel cells. The 212 has two fuel cell modules each rated at 120 KW. The closed-cycle diesel was also tested by the Germans in the early 90s, and it’s reported that MESMA, a similar but steam turbine-based system of French design is currently being developed aboard a Pakistani test submarine. There are several things that all AIP systems have in common. All involve a low-power conversion device, all require that a significant supply of0 2 be stored aboard- typically cryogenic ally, and except for fuel cell-based systems, all need to pump some gaseous products of combustion overboard, which means that increased back pressure reduces the usable power with depth.
To best appreciate the operational limitations of an AIP- equipped submarine, consider Figure (2). This again is a hypothetical, though credible, upper echelon boat of about 1400 or so tons, maximum submerged speed of about 21 KTs with a main propulsion motor of about 3300 KW, 50 days worth of fuel oil (assumed to support 10 days transit at 10 KTs, 30 days on-station at 5 KTs or less, and I 0 days transit home at 10 KTs, and two 120 KW fuel cell modules with about 30 MWHRs of stored energy in the form of liquid 0 2 and hydrogen (H 2 – in the form of off-hull cylinders of metal hydrides). It’s battery is capable of supporting maximum submerged speed for about an hour (although battery capacity can be 2-3X greater at substantially lower discharge rates. This boat essentially represents the 300/30/3 MWHR model described earlier.
What is readily apparent is that above 5-7 knots, the use of AIP makes little sense for several reasons- its maximum output quickly becomes but a small part of the power required at that speed (there is a cubic relationship between speed and power required-doubling the speed requires eight time the power); a much higher depletion rate of AIP consumables since the advertised several weeks of air independence is based on carrying hotel loads and small (less than hotel load) propulsion demands; and the expenditure of a valuable tactical asset for little apparent gain (at higher speeds there would be only a marginal difference in the time between having to recharge batteries by snorkeling. Some likely operational truths would emerge from this logic.
- Transits of any length will likely be conducted in a classic manner, with AIP secured and somewhat frequent, but short snorkeling period to keep the battery at, say, 50% full charge or so.
- When on station, be fully on AIP at very low speeds (2- 4 KTs ), with battery kept at I 00% to best support attack maneuvers or to evade prosecution. A developmental goal clearly would be the ability (stored AIP capacity) to maintain such a covert stance for an entire on-station period (i.e.-30 days).
Both the battery and AIP capacity used in the above examples are probably a bit high, particularly if the concern is what an AIP submarine would look and act like if it were a third world older model brought back to its builders yard for an AIP plug to be installed. In that case, there would more likely be but one module, and economics would likely dictate that it be Stirling engine-based vice fuel cell. Clearly a limiting path to all AIP submarine is just how much 0 2 (plus H 2 for fuel cells) can be carried, and in what form. Some fairly recent and dramatic experimental evidence exists that carbon nanotubes are capable of storing literally hundreds, if not thousands as much H 2 in an equivalent space and/or weight of other methods. If the same is true for 0 2 storage, there could be dramatic developments in the wings. Presently, however, the production costs of carbon nanotubes of the specific sizes and diameters that would be required are in the order of thousands of dollars per gram. Furthermore, there are emerging some medical concerns that these nanotubes, being incredibly tiny and non-biodegradable, represent even more of an asbestosis-like threat to human lungs than asbestos itself, and may find far less industrial exploitation than is now projected.
Hotel loads are also liable to vary significantly, but some truths do exist- solid state electronics in the sensor and combat systems in themselves use less power per circuit element, but the vast reduction in component volume has resulted in huge increases in total processing power that not only consume large quantities of power in themselves, but more significantly, are very intolerant of high ambient temperature and humidity. Perhaps the most demanding aspect of hotel loads for non-nuclear AIP concepts is the issue of atmosphere control- not just for people, but more stringently forthe electronics. The mental image of hot and sweaty (or wet and freezing) submariners effectively fighting their ship is a thing of the past. Add to this the requirement to keep the air breathable for up to 30 days divorced from the atmosphere, and hotel loads are non- trivial. Many concepts use stored AW 0 2 for internal atmosphere replenishment- a use which draws down on this critical AIP consumable at a rate of about I standard cubic foot per person per hour. Every submarine casualty atmosphere study shows that C0 1 is the limiting parameter, and that a submarine’s atmosphere can become incapable of sustaining life very quickly (a day or so) if this product of respiration is not removed. Choices include the absorbent lithium hydroxide (LiOH) in spreadable granular form (messy), canisters through which fans circulate the air, or by closed cycle machinery called scrubbers where such as cold monoethylamine absorbs C0 2 from an air stream, is then heated to boil off the gas which is pumped overboard, then cooled before being sprayed into the air stream again. Alt of this, of course, increases the electrical hotel load.
Back to Figure (1) for a moment for a diversion, it would appear that an extremely simple algorithm could model the propulsion dynamics of any non-nuclear AIP. Given the fixed maximum capacities of the three energy storage bins (fuel oil, AIP, battery), the values associated with its hotel load, diesel (pump) rating, and maximum AIP conversion rate (orifice size}, all that would remain to have a continuous state of the plant would be to specify whether or not the diesel was running, whether or not AIP was on-line and how open the propulsion throttle valve was. Real-time outputs of the model would be how much fuel oil and AIP (consumables) were left, and what the state of charge of the battery was. Accepting the fact that a modem US nuclear submarine is quieter than any SS at equivalent speeds (yes Virginia, it’s true!), an option would exist to then acoustically augment the SSN to credibly emulate that specific AIP (SS) class it is representing. This would not be a difficult task, when one considers that of the millions of MWs running around inside of a modem SSN, alt that ends up being coupled to the ocean as acoustic energy is measured in only milliliters. Exercise sponsors would direct the tactics the SSN/AIP (SS) would employ, such as “transit at a speed of 10 KTs, snorkel to recharge batteries when they drop to 50% capacity, but never snorkel longer than 2 hours, go on AIP at minimum steerage way when on station, keeping the battery at 100% except as necessary to attack or evade, and conduct continuous passive (listening) communications to support ASCM launch within 2 minutes of receiving targeting data” … or whatever was needed to be experimented with or exercised against. A huge collateral benefit of such operations would be that US submariners faced with having to act as an AIP (SS) for an extended period, with all its pros and laws of physics cons, would emerge with a far greater appreciation of what their prospective adversaries can and cannot do, and which of these platforms’ limitations can be exploited, and how.
Non-nuclear AIP submarines are a reality, and are on the verge of rapidly proliferating as older boats are upgraded by the installation of an additional hull section. This does not necessarily represent an overwhelming ASW challenge as long as it is realized just what AIP is and what it is not. It is a means by which an individual unit, having reached its patrol area, can become very stealthy for some significant period of time if it remains at very slow speeds. It is, by no means, a warship aspired to by an entity interested in its contributing to global maritime influence, but is of high value (when coupled with the proper weapons and operational concepts) to an entity interested in contesting maritime influence by others in its own waters.