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  • This episode of Real Engineering is brought to you by Brilliant, a problem solving website

  • that teaches you to think like an engineer.

  • The US military has the strongest and most diverse navy in the world.

  • The US militaries fleet of aircraft carriers is so large, it makes the US navy the second

  • largest airforce in the world, second only to the actual US Air Force.

  • A single Nimitz- class aircraft carrier like the USS Ronald Reagan, a 6.2 billion dollar

  • nuclear powered ship, can carry the twice aircraft as any other foreign carrier, which

  • makes it even more shocking that it was sunk by a single diesel powered swedish submarine

  • during war games in 2005.

  • A single submarine that cost the same as a F-35 at 100 million dollars managed to sneak

  • by an entire carrier task force, with anti-submarine defences to enter the red-zone and score multiple

  • torpedo hits on the USS Ronald Reagan, sinking it virtually, before shrinking back into the

  • vast ocean undetected.

  • This was just one of many exercises where the Swedish Gotland class submarine proved

  • too stealthy for the world's strongest navy.

  • The new submarine proved so threatening that the US military leased the Swedish sub for

  • an additional year to develop strategies to counter the silent threat.

  • So what set the Gotland apart from other subs.

  • Submarines' primary instrument to detect enemy subs is sonar.

  • Sonar is essentially a finely tuned ear that works like a whales or dolphins echolocation

  • to create a 3D map of ocean around it.

  • There is active sonar where the submarine will send out a sound pulse called a “ping

  • and listen for the reflections, but in warfare this isn't a sound strategy, as that ping

  • is detectable by enemies to give your exact location, so passive sonar is used where no

  • ping is emitted and instead you simply listen.

  • These electronic ears are so accurate that the nationality of submarines can be determined

  • based on the operating frequency of the alternating current used in it's power systems.

  • The 60 Hz alternating current of a US sub could be differentiated from the 50 hz of

  • European subs, if the transformers and other electronics were not adequately insulated

  • from the hull.

  • The Swedes managed to create a submarine so silent that it was practically undetectable

  • by passive sonar, so how did they do this at such a low cost?

  • The Gotland was the first submarine in the world to use a stirling engine as it's power

  • generator.

  • Stirling engines are not a new concept with the first being created and patented by Robert

  • Stirling in 1816.

  • Inspired by a series of high pressure steam boiler explosions at the beginning of the

  • industrial revolution, Stirling wanted to create a safer engine that did not require

  • such high pressures.

  • I can't be arsed trying to animate this old drawing, let's go for something simpler.

  • He did this by creating a closed cylinder containing a fixed mass of gas permanently

  • sealed within.

  • Here one side of the piston cylinder has a large buffer space, which allows for a relatively

  • constant pressure on this side of the piston, while the other side fluctuates in pressure

  • due to alternating heating cycles.

  • When heat is applied to the outside of the cylinder, the pressure increases causing the

  • piston to move until the pressure equalizes.

  • Now, if we cool the outside of the cylinder with a heat exchanger, the pressure will drop

  • and once again the piston will move.

  • This is our basic pressure cycle to create mechanical work.

  • But, this is an insanely inefficient system, as most of the energy we put into the system

  • as heat is lost during the cooling cycle, not to the gas, but to the actual cylinder

  • wall, which provides no mechanical work.

  • Robert Stirling solved this by adding a displacer piston, which can drive the gas from one end

  • of the cylinder to the other.

  • Allowing this end to be permanently hot, and the other permanently cold.

  • So the cylinder wall is no longer experiencing a temperature cycle.

  • The pressure cycle here works slightly differently.

  • First the air in the hot end expands and causes the displacer to move into contact with the

  • power piston, displacing more air from the cold end to be heated and expanded, allowing

  • work to be done on the power piston.

  • The air on the hot end now has nowhere to go, and so is driven to the cold end, where

  • it is cooled and contracts causing work to be done once again on power piston.

  • This is our new pressure cycle.

  • The efficiency of this system can be increased further by placing what is essentially a heat

  • battery in the tubes between the hot and cold cylinder.

  • This conserves a huge amount of heat that would otherwise be wasted during the cooling

  • cycle, and gives the heat back to the air as it travels back to through.

  • Robert Stirling dubbed this the regenerator.

  • Now we have the foundations of a useful engine.

  • By incorporating a coolant system and a heating chamber we create a larger temperature differential

  • to drive the engine, and the efficiency can be future increased by increasing the number

  • of tubes connecting the hot and cold spaces, along with the number of regenerators, and

  • adding fins to increase the surface area of these tubes to allow for heat transfer.

  • Did I say simplified?

  • Sorry I meant easier to read.

  • Maybe I like the misery

  • Stirling engines ultimately fell into obscurity as stronger steel became available to make

  • steam engine boilers safer, but have seen a resurgence in recent decades with the Gotland

  • being the most famous implementation.

  • The Gotland uses two Stirling engines that use diesel and liquid oxygen to provide heat,

  • which in turn runs it's 75kw generators.

  • These generators can run an electric motor directly, or charge batteries that can provide

  • a huge boost in speed when needed.

  • All the while the exhaust is compressed and stored on board, allowing the sub to stay

  • submerged for up to 2 weeks vastly longer than any other diesel power submarine.

  • So why is it so silent compared to other submarines, it doesn't require much explanation as to

  • why using an internal combustion engine using tiny controlled explosions for power tends

  • to cause some noise.

  • While the the multi-billion dollar nuclear powered submarines need to pump huge volumes

  • of coolant to their reactors to prevent a meltdown, causing enough noise to be detectable

  • by passive sonar within a certain range.

  • On top of this, recently declassified intelligence suggest that Russian subs are using these

  • instruments to detect the faint trail of radiation left in the wake of these nuclear powered

  • submarines.

  • Giving the swedish submarine another way of avoiding detection.

  • This is a fascinating application of the laws of thermodynamics.

  • Understanding and applying the laws of science is the closest to a real life superpower in

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This episode of Real Engineering is brought to you by Brilliant, a problem solving website

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