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This episode of Real Engineering is brought to you by Brilliant, a problem solving website
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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
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
Giving the swedish submarine another way of avoiding detection.
This is a fascinating application of the laws of thermodynamics.
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