Welcome aboard HMAS Ovens: this is the Oberon Class, Cold-War era Aussie submarine which
sank the USS Enterprise, delivered special forces to South East Asia, and stalked the
soviet navy throughout the world’s oceans.
None of these missions could have been achieved if not for her unprecedented stealth.
So, how do you hide one of Australia’s most important weapons?
Obviously, the Ovens is pretty easy to spot when on the surface, but that's not where
we tend to operate.
By going just 10 metres down, the ship’s black-painted hull combined with reasonably
choppy conditions makes spotting them pretty much impossible.
Because of this we don’t rely on visible light in order to detect submarines, but instead
use some specialised technology.
<<Give me a ping, Vassily.
One ping only, please.>>
This iconic sound - the ping not Sean Connery - is part of what's known as a submarines
active sonar system.
In just the same way as a bat uses echolocation, active sonar works by generating an intense
pulse of sound which then bounces off any nearby objects and returns to the vessel.
We record the return signal using an array of underwater microphones - or hydrophones-
embedded into key positions in the hull.
All of that incoming data is routed here into the sonar room where it is analysed by computers
and trained technicians.
By comparing the order in which our hydrophones are activated we can work out where our sound
is coming from.
We calculate how far away our object is by multiplying the known speed of sound by the
time between emitting the ping ….. and hearing it’s echo, remembering to divide by two
to account for the return path.
By using a bit of basic trig we can account for more complicated pathways that our sound
might be taking; giving us a detailed model of our surroundings.
Active sonar is great for spotting enemy warships, but they can just as easily generate their
own pings and search for us as well.
Therefore, we want to be sure that our submarine either doesn’t reflect any of the signals
or else disguises its reflection to look like something more innocent.
Anechoic tiles are attached to all external surfaces.
They are basically thick rubber sheets with an array of precisely-sized holed drilled
into them.
These holes are designed to trap incoming sound or modify the reflected signature.
These are the equivalent to the foam sound studio tiles which you may be familiar with
and are roughly as good at stopping an echo.
Unfortunately you can’t see any of them today on the Ovens submarine because they
are one of the first things that get removed.
They are very sensitive military hardware.
As a kid I remember playing Marco Polo in the pool.
If yelling Marco and waiting for the returning Polo is active sonar, then listening as your
friends run across the deck and jump in with a splash is the equivalent of passive sonar.
Since listening is a much more reliable way of spotting enemy vessels, staying quiet has
become an essential element of submarine warfare.
One of the loudest parts of a submarine is its screw – what we might also refer to
as a propeller.
In order to illustrate why it’s so loud, I’ve commandeered a friend’s boat and
lashed it to the side of a jetty.
I’m now turning on the motor, lets see what happens.
Looking very closely, you can see a thin triple helix of bubbles being formed; corkscrewing
off each of the propellor blades.
These aren't bubbles of air but instead steam.
The water around the blades is boiling.
It’s a well-known fact that water boils at precisely 100 degrees celsius, except that
isn’t entirely true.
To demonstrate I’ve filled up this bottle with water and connected it to a vacuum pump.
Turning it on we’re taking out all of the air from the top of our flask; producing a
vacuum.
After just a couple more seconds… yep, there we go, it’s starting to bubble.
This is boiling water.
It’s not boiling because of temperature, I can touch it with my hand.
Instead, it’s boiling because of pressure.
Something really interesting has to be going on.
You can think of it like being at concert where the audience is our water molecules
and security is the ambient pressure.
If we increase the energy of our audience members, analogous to increasing the water
temperature, then they can break through the security line.
If, on the other hand, we reduce the number of security, equivalent to reducing the ambient
pressure, then the audience members can again break through this line.
In both cases we say that the water has boiled and our concert goers are havin’ a rockin’
time!
If we use a more volatile liquid, like acetone, then it's able to boil much more readily.
This would be the equivalent of having your audience composed of Navy SEALs.
When a submarine’s screw spins quickly, it forms a low pressure region behind the
blade.
As we saw earlier, its this low pressure which causes the water to boil and turn into water
vapour even though no temperature change is taking place.
This is what’s known as cavitation.
After the bubbles move away from the low pressure region around the propeller, the external
water pressure crushes them back down and they implode.
As the bubbles collapse an intense pocket of 1000s of degrees centigrade and 100s of
atmospheres of pressure is formed.
With millions of these mini-shockwaves happening every second, they make a very loud, very
characteristic noise signature.
These don’t just alert enemy vessels of your presence, but also slowly eat away at
the edges of the blades.
There are a few different ways of reducing noise.
We could reduce the shaft speed, which does result in loss of cavitation, but at the same
time massively reduces thrust.
On the other hand, there are some very smart military engineers who have designed the perfect
propellor.
As to what it might look like, well we really have no idea because they’re not telling.
However, based on this satellite photograph, we can assume that they have large flat surfaces
in order to minimize cavitation while still having pretty-good thrust.
Back inside our aim is to keep the machinery as quiet as possible, starting here in the
engine room.
These massive diesels burn through a lot of oxygen.
If we were to turn them on when underwater we’d asphyxiate the crew pretty much immediately.
On top of that, they’re also pretty loud, meaning that if we were to turn them on we’d
allet the entire soviet navy of our precise location.
For these reasons, we only ever turn them on when we are near the surface and in relatively
safe waters.
These generators charge two enormous lead acid batteries which can later be used to
power the submarine for three days.
For context, these could run an Aussie home for two years or drive a Tesla around the
world three-and-a-half-times.
When we want to move the sub we need to be sure that the gears connecting the electric
motor to the drive shaft mesh exactly.
If they don’t, then we’ll get lots of unwanted noisy vibration.
Nowadays a computer will design and cut the gears, getting micrometre precision.
However, back when the ovens was built they’d have been made by hand.
While we’re on the topic of submarine propulsion and given my love for the movie “The Hunt
for Red October”, it would be silly of me not to mention the silent ‘caterpillar drive’
which features so heavily in that film.
Also known as a magnetohydrodynamic drive, this is actually a real-life technology and
I’ve managed to build my own small-scale working model using some materials I found
in my garage.
We could test it out in my pool, it is a salt pool so in theory it should work.
However, to get the full impression I’ve filled up my bathtub with a few bucketloads
worth of salt to get just the right conditions.
As you can see, the boat is travelling through the water without any moving parts.
Although it may appear to be a perpetual motion machine, trust me it isn’t one.
If it were then I would have already commercialised it and made millions of dollars.
So instead, I’m going to tell you how it works.
Here is the view from under the surface.
You can see lots of tiny bubbles being formed, but these aren’t what's propelling it through
the water.
If you’re interested in those bubbles, then I’ve discussed hydrolysis in my Martian
Caves video.
For the boat we have a battery wired into two metal plates and some very strong neodymium
magnets superglued to the bottom.
The whole thing is supported by some foam floats.
My salt-laden bath water contains lots of positive sodium and negative chlorine ions.
The sodium are attracted toward the negative plate and the chlorine toward the positive
one.
By itself that is not enough to move the boat, but remember we also have a bunch of really
strong magnets glued to the underside.
These are all aligned so that the south pole is pointed toward the water; giving us an
upwards magnetic field.
When a moving charge passes through a magnetic field a force is exerted on that charge.
For the positive we use the right hand rule to remember which direction that force is
in.
Thumb in the direction the charge is moving, index finger in the direction of the field,
and finally force on the particle is given by the direction of your middle finger.
Therefore our positive sodium ions and being forced backward and thus pushing the ship
forward and through the water.
Now, when I first started looking at this technology, I thought that obviously the negative
chlorine ions would be pushed out the front and the boat wouldn’t move at all, but that
is not what we observed.
What I was forgetting is that negative charges don’t use the right hand rule, but instead
the left hand rule.
Since the chlorine ions are moving to the port or left-side of the ship but the field
is still pointing upward, using the left hand rule we find that our negative charges are
also being pushed out the back.
Making the ship move forward.
The reason this won’t work in fresh water is that there just simply aren’t enough
ions to be moving around.
Unfortunately, even a scaled up version of the drive wouldn’t be all that good.
The current passing through the water produces bubbles as well as noise, while the strong
magnetic field would be easily detectable.
On top of that, the thrust isn’t even all that great either which means that it would
barely work to move a submarine, you saw how bad it was at moving the boat across my bathtub.
In space travel where none of these issues matter, this becomes the foundation of ion
propulsion.
But before we use it to explore the outer reaches of the universe, back on the HMAS
Oven’s we’ve spotted the soviet navy.
And I think we’re going into battle.
When stalking an enemy ship or indeed being hunted by one, sailors were told to cease
all unnecessary activity and remain silent, perhaps lying down in their bunks.
While there is some discussion about how effective this could be as compared with the noises
of the ship itself, it is undeniably an excellent way of instilling a sense of caution.
More than one war game has been lost by the unfortunate clang of a toilet seat or by someone
tripping down the stairs!
Even the most regimented crew on a perfectly designed submarine can’t be completely silent,
but there are other ways of preventing that noise from reaching the enemy.
When not being used to load torpedoes, this hatch would have been sealed shut, supported
by some thick steel plates.
It was common practice to slip up a penny between these plates so that after a dive
it could be extracted as a thin wafer.
When decimal currency came in 1996 there was a shortage in these copper pennies so instead
5c and 10c pieces were used.
These nickel coins are much harder than copper, harder even than the steel pads, and you can
still see the indent that they’ve left.
I can’t think of a better demonstration as to the extreme loads experienced during
a deep sea dive.
Officially the Ovens has a maximum diving depth of 300 metres although it’s probably
designed to go even deeper.
It achieves this by using a 25 millimetre thick pressure hull.
This doesn’t just allow the submarine to survive high pressure dives, but it’s large
mass also prevents vibration passing from inside the sub into the surrounding water.
Counterintuitively, the bigger that submarines are the quieter they become since they can
dedicate more volume to thicker and therefore more sound insulating pressure hulls.
If you can’t afford super thick steel, then you can use a bit of clever marine science
to help hide your noise.
Sound travels faster in warm water than cool and is also faster the more saline that water
is.
Temperature decreases with depth while salinity increases with depth which gives us a triangular-shaped
speed profile.
If we put our submarine at the vertex of this speed triangle, then sound that travels upward
will be bent back down, and sound going down will be bent back up.
This is similar to how a mirage works with visible light.
Our narrow sound channel makes our submarine invisible to anyone not in that channel.
It's the challenge of navigators and marine scientists to work out where that channel
can be found.
By using these techniques, the HMAS Ovens is able to remain invisible.
During the Cold War she sailed for almost 1 million kilometres, both above and below
the waves.
In peacetime she competed in the KANGAROO TWO joint naval operation with the United
States.
Among her simulated strikes, she managed to “sink” the USS Enterprise by taking a
photograph of the aircraft carrier using her periscopes without being detected.
They celebrated this victory by strapping a broom to the top of the periscope; representing
them sweeping the simulated enemy from the seas.
Nowadays that sort of achievement would be recognised by blaring by blaring Men At Work’s
‘Land Down Under’ as loud as possible.
<>
<> With a land girt by sea, submarines have become
an essential part of the Australian military.
Although we’ve covered much of the technology used to keep them silent, ultimately much
of this has to remain a military secret.
While the HMAS Ovens may today be decommissioned, she helped to guide the way for the next generation
of Australian subs.
Silently guarding us beneath the waves.
This has been James Dingley from the Atomic Frontier.
Keep Looking Up.