Alternative Fueled Subs
by Frank "TORPEX" Kulick  
 July 2000

Prior to nuclear power, submarines were merely torpedo boats that could submerge. They spent the majority of their time on the surface where their powerful diesels could breathe. Once the stalking began or when under attack, the crew would dive the boat to evade detection. However, the endurance and speed of a submerged boat running on batteries was very limited. The surface forces could count on the submarine to be in the area and the need to surface or snorkel could be measured in hours.

With the advent of the nuclear reactor, a submarine could remain underwater for months and there are no restrictions on its power consumption. But nuke boats are very expensive and more detectable than a conventional sub running on batteries. Engineers and military designer search for a better solution.

Nuclear Powered! Diesel-Electric! Stirling Engine Air Independent Propulsion (AIP)? What is this new method of propulsion, and how potent will it prove to be? Application and dedication may be the key to proving their effectiveness in the underwater war of the future. First allow me to introduce this alternative propulsion system, and try to explain how it works.

The main feature is the use of the Stirling Engine. Burning pure oxygen and diesel fuel in a pressurized combustion chamber, the Sterling engine is a heat engine. Heat is produced in a combustion chamber separated from the actual engine. The heat is transferred to the engine’s gas, usually helium, operating in a completely closed system. The working gas forces the pistons in the engine to move, thus producing mechanical energy to turn the generator.


The combustion pressure is higher than the surrounding seawater pressure, allowing the exhaust products, to be discharged and dissolved overboard without the use of a noisy compressor. The oxygen is stored in liquid form (LOX) in cryogenic tanks, the diesel fuel is stored in standard tank systems. All this adds up to a submerged endurance that is limited by the amount of stored LOX.


Who invented this unique and useful engine? A man named Robert Stirling who was a minister of the Church of Scotland. He was interested in the health of his parishioners bodies in addition to the wellbeing of their souls. He invented the Stirling engine (he called it an "air engine") because steam engines of his day would often explode killing and maiming those who were unlucky enough to be standing close by. Robert Stirling's engines couldn't explode and produced more power than the steam engines then in use. In 1816 he received his first patent for a new type of "air engine".


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HMS Gotland with AIP

In the mid-1980s a Stirling engine and a LOX (Liquid Oxygen) system was installed aboard the French 500 ton civilian research submarine Saga, which has an operating depth of 600 meters! Intense Research & Development and the experiences gained from the Saga project paved way for the installation of a Stirling engine in the Royal Swedish Navy submarine Näcken in 1988. The submarine was placed in dry-dock and cut in two. A fully outfitted eight-meter AIP section was then inserted. The years of practical sea-trials that followed were extremely satisfactory and resulted in the installation of Stirling AIP systems in the new Swedish Gotland Class submarines. With the Stirling AIP system the submarine doesn't have to surface to charge the batteries and thus increases the submerged endurance from days to weeks!


The future "Viking" submarine is the result of the R & D carried out by Kockums Shipyard in Norway. On assignment from the Viking Project Group representing the countries of Denmark, Norway and Sweden, the mission was to combine and integrate the requirements of three nations into a submarine, which in the best possible way, would meet their common requirement for low cost. It promises an unlimited patrol endurance, and state of the art weapon systems. I’d say the future looks bright for the "Viking", and time will tell us more.

The single most important advantage is underwater endurance, however there are more. The Stirling AIP system is practically vibration-free, silent, wake less, and its infrared signature is very low. When the LOX supply is exhausted, the submarine remains a powerful conventional submarine. All this adds up to a friend or foe who now has the ability to remain stealthy for weeks in an open ocean or littoral environment, very much like her nuclear counterpart. Now, victory in the underwater battlefield will boil down to crew quality, experience, endurance, and in my experience, attitude. After weeks of silent running, fatigue and exhaustion creep into a crew and one simple human mistake could mean counter-detection and death. Are we prepared to deal with this new technology which will be available to any one who can afford the 200-300 million-dollar price ticket for a new submarine? What about upgrading an existing diesel electric submarine’s with the AIP system at a reduced cost? This new life, into a perhaps obsolete boat, could present itself in an unpleasant encounter with US forces who are ill prepared to deal with this stealthy opponent. Waiting for him to snorkel to detect him may not happen within weapons range anymore!

Present day nuclear power had its growing pains yet rose above it with dedication and funding. AIP will improve at a rate relative to funding and attention. I have avoided the nuke-vs-conventional debate, but I do believe that based on economics and sound tactical reasoning, having 2 to 4 of these boats in the US arsenal would provide numerous tactical training advantages, and keep us on the cutting edge of technology. An option to offset their transit speed limitations in times of crisis may be "piggy-backing" to war zones by larger ships. It hasn’t been done yet, but never say never. Production would be much more rapid in the event of national emergency due to size and absence of nuclear issues. This absence of a nuclear reactor would afford various shipyards the opportunity to compete and widen the opportunity for innovation and fresh ideas. Keeping all our "eggs" in one basket (EB/Newport News) reduces the country’s flexibility and increases the probability that they would become key targets of our opponents. It also re-creates a similar situation encountered during WWII with the Newport Torpedo Station. Did we learn our lesson regarding the single source problem?

I appreciate nuclear powered submarines, they are a must and have established a firm foundation in today’s joint service expeditions and operations. There is no doubt that their size, firepower, and propulsion power makes them an inseparable asset. But should we ever decide to couple our stealth technology and industrial capacity with this AIP system, the stigma of the "noisy-limited endurance diesel boat" will be gone. Revealed will be a low cost, variable mission submarine that is as deadly as her nuclear counterpart at a fraction of the cost in production and maintenance. The time to "sharpen the sword" is before battle!

©2000 SUBSIM Review

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