Capítulo 120: Chapter 198 AIP
Although nuclear submarines are not expected in the short term, Wilhelm isn't too disappointed. After all, in the 21st century, several decades later, not every country can afford nuclear submarines. For many nations, manufacturing nuclear-powered submarines poses insurmountable technical and financial challenges. Therefore, while nuclear-powered submarines are vigorously developed, conventional submarines have not exited the stage of history. Moreover, conventional submarines are more suitable for operations in near and shallow waters compared to nuclear submarines.
However, conventional submarines have a significant drawback – they cannot perform long-duration underwater voyages. The limited capacity of the onboard batteries requires frequent surfacing to the sea for "breathing", meaning using diesel engines in the ventilation pipe state to charge the batteries. This makes them easily detectable by the enemy, and the noise from the diesel engine charging the batteries is easily picked up by sonar and other acoustic equipment. Consequently, the exposure rate of submarines is greatly increased, affecting both the stealth of attacks and their survivability.
To address this issue, countries have made long-term explorations and efforts, achieving success in the development of various Air-Independent Propulsion (AIP) systems, which have now entered the practical stage.
AIP stands for "Air-Independent Propulsion" because AIP systems can operate normally without relying on air.
Since AIP propulsion systems operate without requiring oxygen, they significantly improve the submarine's underwater endurance after installation. This allows submarines to stay submerged for 2-3 weeks, substantially reducing their exposure during patrols. Thus, the bottleneck that has historically affected the operational effectiveness of conventional-powered submarines has finally been resolved. Equipped with an AIP system, conventional submarines are metaphorically referred to as "green nuclear submarines", signifying comparable long cruising capabilities to nuclear submarines but without their potential dangers and high costs.
Even before and during World War II, countries like the Soviet Union and Germany were involved in the research of AIP propulsion systems.
In 1935, the Soviet Union's 18th Central Design Bureau proposed the use of AIP systems on submarines and conducted trials on Submarine M-92, obtaining a considerable amount of experimental data. However, at that time, the technology was not mature, and the designers only presented the use of air-independent technology as an experimental subject.
During World War II, the German company Walter attempted to use compressed hydrogen peroxide in submarines as a source of oxygen for diesel engines underwater. Chemists know that hydrogen peroxide can decompose into oxygen and water. When submarines operated underwater, the heat from the diesel engine heated hydrogen peroxide, producing oxygen to supply the diesel engine. Later, Germany constructed a few submarines with this AIP system.
However, Wilhelm didn't like hydrogen peroxide because it was extremely unstable when converting to oxygen! In 2000, the Russian submarine "Kursk", a nuclear submarine, experienced a catastrophic sinking due to the leakage of hydrogen peroxide from a training torpedo. The subsequent explosion led to a massive fire, and the high temperature caused seven torpedoes containing live warheads to explode, resulting in the largest nuclear submarine sinking disaster in history, with all 118 crew members on board perishing.
In later years, three practical AIP solutions emerged: Closed-Cycle Diesel Engines, Stirling Engines, and Fuel Cells.
Among them, fuel cell technology was impossible to achieve in this era. Using liquid oxygen or hydrogen peroxide was too hazardous. Moreover, it wasn't until 1993 that Germany successfully tested a 250-kilowatt Closed-Cycle Diesel Engine system on the retired 205-class submarine U-1. Wilhelm didn't think he could make German technology leap half a century all of a sudden. Therefore, the safest way to use existing technology was to use Stirling engines.
"General Dönitz, have you heard of Stirling engines?" Wilhelm inquired.
Dönitz nodded. "Are you referring to the Stirling engine invented by the Britishman Robert Stirling?"
"Yes."
The 17th and 18th centuries were an era of remarkable geniuses. There's a joke that in Paris, one copper coin could buy three geniuses.
Watt added a governor to the steam engine, ushering humanity into the steam era and subsequently leading to the development of trains and steamships. However, the steam engine consumed too much coal. Some believed that replacing steam with a hot gas could avoid the heat loss caused by condensation.
Based on this idea, the British scientist Robert Stirling invented the external combustion closed-cycle hot air engine, known as the "Stirling engine", in 1816. The Stirling engine operates by cycling a working medium (hydrogen or helium) through cooling, compression, heat absorption, and expansion within a cylinder for one cycle, hence its alternative name, the hot air engine.
However, at that time, there was a lack of good heat-resistant materials. Also, Nicolas Léonard Sadi Carnot, the French engineer, had not yet proposed Carnot's cycle. People's understanding of the performance of hot air engines was limited, and the known machines had low efficiency and power. After the mid-19th century, with the invention of highly efficient internal combustion engines and extensive petroleum extraction, the development of Stirling engines ceased.
This type of Stirling cycle hot air engine doesn't exhaust gases. It doesn't require additional air besides the air in the combustion chamber, making it suitable for urban environments and outer space. It is an ideal solution for ensuring conventional-powered submarines can travel underwater for extended periods without surfacing.
Dönitz seemed a bit hesitant. "But, Your Highness, we don't have the technological reserves in this area. If we want to use this technology on submarines, we'll need to start research from scratch, which might take several years."
"No need to take years, General Dönitz". Wilhelm said with a triumphant smile. "We will soon receive ready-made research results."
Dönitz was a bit surprised. "Ready-made? From where?" He thought it was a secret research department of His Highness, but unexpectedly, His Highness mentioned another country.
"The Netherlands. Their Philips company has been developing Stirling engines for several years, and I heard they've finally succeeded recently." A company known for light bulbs developing engines? Wilhelm wondered how they came up with such an idea, but now it turned out to be advantageous for him.
Of course, this Stirling engine is only in its infancy, and Wilhelm doesn't expect it to provide hundreds of kilowatts of power like in the future. It only needs to deliver tens of kilowatts, sufficient for normal illumination, electronic equipment operation, and the facilities needed for submarine life. The remaining energy can be used for propelling the submarine, maintaining continuous underwater travel for half a month at a few knots of underwater speed.
Dönitz exclaimed, "That's great! The next batch of 'Tiger Sharks' should be able to use this engine." After all, the most important thing is to give submarines the ability to travel underwater for a long time, avoiding the need to surface frequently, which is more effective in avoiding danger.
Then, the two discussed wolf pack tactics. "General Dönitz, do you think there are any shortcomings in the 'wolf pack tactics'? Don't hesitate, speak freely."
Dönitz hesitated for a moment before carefully saying, "Your Highness, there are some issues. The biggest problem is the attribution of command. Should it be commanded by the submarine commander at sea, or should it be centrally ordered by the command headquarters onshore? If onshore headquarters gives unified combat orders, it's challenging to adapt flexibly, and it may lead to countermeasures. However, if complete authority is given to the submarine commander at sea, once the enemy masters the method of locating submarines based on radio signals, our submarines may easily expose their positions."
Wilhelm understood Dönitz's concerns were not baseless. In the original timeline, British experts extensively studied Germany's "wolf pack tactics". They believed that if they could capture the signals emitted by tracking submarines and use those signals to determine the submarines' locations, aircraft could quickly and effectively attack, rendering the once formidable "wolf pack" defenseless when it became a "flock of sheep."
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