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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.
Over the last 100 years, advancements in science and technology have allowed us to learn so
much about where we came from and how our planet has developed over time.
With our discoveries of old fossils, we know that life on Earth has existed for at least
3.5 billion years.
But with the Earth itself believed to have formed about 4.5 billion years ago, we still
have very limited knowledge on what caused life to form in the first place, and much
of the evidence on how it may have developed has actually been destroyed by the very life
that it created.
Sadly, we don’t have a time machine, but we can look for places in our solar system
that emulate early earth.
That’s where Saturn’s largest Moon ‘Titan’ comes into the picture.
Although it’s about half the size of Earth, Titan has the characteristics that we think
are very similar to Earth in its early days.
Titan has a thick atmosphere at around 4.4 times denser than Earth's, and is the only
Moon in the solar system to have any noticeable atmosphere at all[1].
Titan also has large pools of liquid which follow a similar cycle to the rivers and seas
we have here on Earth.
But, instead of water, Titan has seas of methane which evaporate into clouds, causing it to
rain liquid methane.
In terms of scientific information, Titan is a gold mine for scientists.
But sadly, it’s extremely difficult to get to and because it’s entirely covered in
clouds of methane, it’s nearly impossible to study the surface from a distance[2].
In 1997, a collaboration between NASA, the European Space Agency and the Italian Space
Agency launched the Cassini space probe on a 7 year journey to reach Saturn.
The probe was designed to study the entire Saturn system including its rings and natural
satellites.
But Cassini didn’t make the 7 year journey on its own.
Attached to the space probe was a small lander called Huygens which was hoping to become
the first spacecraft to land on Titan.
Several months after entering Saturn’s orbit, Huygens separated from Cassini and started
its journey towards Titan[3].
And began sending back vital details about Titan’s environment, like the fluid properties
of the atmosphere and the nature of the moon's surface.
As it descended Huygens recorded accelerometer data, which could be used to deduce properties
like density of the fluid as we knew the aerodynamic properties of the probe.
It took temperature and pressure readings to teach us about the thermodynamic properties
of the atmosphere.
[16] After two and a half hours of descending through the unknown, Huygens successfully
landed on the surface, making it the furthest spacecraft landing from Earth ever completed.
Although it was only designed to survive for about 90 minutes whilst on the surface, Huygens
successfully recorded and sent back 350 images, revealing a world eerily similar to ours,
with sharp hills and valleys, and rivers of methane cutting their way through the landscape.
[4].
As our quest to learn more about the origins of life continues, Nasa’s new mission called
‘Dragonfly’ will begin its journey to Titan in 2026, and the work of the Cassini
and Huygen mission will be vital to its success.
Dragonfly is a mobile lander fitted with 8 large rotors that will help it fly around
the surface like a drone.
An incredible difficult engineering challenge, and the data gained from Huygens will be insanely
valuable when designing the drone.
Everything from it’s sensor layout, battery capacity, energy source and propellers design
will be dictated by what we learned, and those are exactly the engineering challenges we
are going to investigate today.
Dragonfly will have many of the same scientific instruments as the curiosity rover.
It will have skid mounted drill to take soil samples and run it through a mass spectrometer
to learn more about the soil composition.
It will be capable of quickly analysing elemental compositions at landing sites before landing,
using a neutron-activated gamma-ray spectrometer.
This instrument typical needs cryocooling, but thanks to Titan’s subzero temperatures,
this instrument can be passively cooled.
It will however need to generate its own neutron’s rather relying on cosmic rays to generate
them, as the 0atmosphere blocks too much sunlight.
When it lands a seismometer will give us information about quakes and reveal the thickness and
nature of Titan’s icy crust sitting above what is thought to be liquid water ocean.
We think this because Cassini witnessed the surface shifting in position by 30 kilometers
in just 2 years, indicating that the crust is floating on top of some kind of liquid
layer.
We can also look forward to incredible photos of Titan’s surface, just like the photos
we are currently getting from Mars.
[5]
Since the air is thicker on Titan and the gravity is one sevenths of earth’s, dragonfly
will be able to achieve more thrust on a planet that needs less lift.
Drastically reducing energy consumption compared to earth.
Yet finding that energy to fly on the surface of Titan is not easy.
Due to Titan’s distance from the Sun and its thick atmosphere, the sunlight on Titan’s
surface is around 100 times weaker than it is on Earth, making solar panels impractical.
[6]
Thankfully we have a lot of practice in a different type of energy source through missions
like the Curiosity Rover, which was powered by Radioisotope Thermoelectric Generator.
RTG’s work by converting the heat from the natural decay of a radioisotope into electricity.
Now this isn’t traditional nuclear energy like I have mistakenly said in the past.
[7] The RTG does use radioactive materials to generate electricity, but not through nuclear
fission.
It uses a simple principle called the Seebeck Effect to generate electricity.
[8] The seebeck effect essentially allows us to generate an electric current through
a heat differential, as charge carriers will move from hot to cold.
So if we have a heat source and a way of cooling we can generate a sustained electric current.
Thankfully radioactive substances generate heat as they decay.
Choosing a suitable radioactive material is our first challenge.
With any spacecraft a lightweight compact design is paramount, but we also need the
material to have a long half life to ensure a long lived energy source.
We also need it to primary produce Alpha waves, as this form of radiation is most easily converted
to heat in a compact space.
[8] As a result of these requirements Plutonium-238 (Pu-238), Strontium-90 (Sr-90), and Curium-244
are the most commonly used fuels.
Next we need a material which is both a thermal insulator to maximise our temperature differential,
and an electric conductor to maximise our current.
These two material properties are typical linked.
Materials like copper are both a good thermal and electrical conductor, and a material like
iron is a poor thermal and electrical conductor.
Using these materials in conjunction can create a crude thermoelectric generator, but the
efficiency is very low.
If we can create a material with the best of both properties, then we can achieve a
higher efficiency.
Leading to the use of materials like lead telluride and tags, which is an alloy of Tellurium
(Te), Silver (Ag), Germanium (Ge) and Antimony (Sb).
[9]
The thermal electric generator used for the curiosity rover could generate 110 Watts of
electrical power.[10] But we will lose some power generation capability during Dragonfly’s
8 year journey to Titan, as we cannot turn a radioactive element on and off on demand
to conserve energy.
In fact Dragonfly’s cruise vehicle will need to be equipped with radiators to bleed
that heat energy into space to prevent overheating, just as the Curiosity rover did.
We will also lose energy to keeping the craft at operating temperature, as the surface of
Titan can reach temperatures as low as -180°C, [11]and to keep some vital systems and scientific
experiments running, leaving us with about 75 watts to charge while on the ground in
a best case scenario.
All of our activities will occur during Titan’s daylight hours, so we will be aiming to charge
our batteries during Titan’s nights, which lasts 192 hours, the same as their daylight
hours.
So, it makes sense to make our battery charge fully in those 192 hours.
Giving us a 14 kWh battery.
For comparison, a typical tesla battery is about 75 kWhs.
With a specific energy of 100 Wh/kg, that will make our battery 140 kilograms.
In practice a smaller battery will probably be used, and even a 30 kilogram 3 kWh battery
would provide up to 2 hours of flight time at 10 m/s, providing a massive 72 kilometre
range.
Even more incredible when you consider the Curiosity Rover has only driven 21 kilometres
over the past 7 years of its time on Mars.
[12]
Ofcourse, Dragonfly won’t fly it’s maximum range in one hop and will likely take shorter
safer hops between interesting points during Titan’s day and one of the most impressive
things about this mission is how the spacecraft will navigate its way around the surface.
Since Titan is so far from Earth, just sending basic information to Earth and from Earth
will be difficult.
The energy requirements to transmit data rises dramatically as a result of the inverse square
law.
[13] The Huygen probe had the advantage of being able to relay information to the Cassini
space probe, which had a larger antenna and higher power.
Unfortunately for the Dragonfly mission, Cassini is no longer in orbit around Saturn falling
into Saturn's atmosphere in 2017.
[14]So Dragonfly will need to dedicate both precious power and weight to a large high-gain
antenna in order to communicate with Earth’s Deep Space Network.
On top of the additional energy and weight requirements,
Titan B-Roll (E3) The average round trip communication time
is around 2 and a half hours, making it impossible to fly the spacecraft in real time.
Instead Dragonfly with fly using its own vision, much like the self flying drones we have here
on Earth, Dragonfly will use its cameras along with the onboard gyroscopes and accelerometers
to travel from one point to another.
Dragonfly will be trained to identify suitable landing sites that are flat and free from
any obstacles like large rocks and rough terrain.
[15]
Originally, Dragonfly was intended to fly with a single rotor, but since helicopters
are mechanically complex in the way they vary the rotor’s pitch to vary lift, the idea
was never developed.
[15]But with the surge in multi-rotor drone technology over the last decade, the idea
for a quadcopter became much more feasible.
Dragonfly will feature a total of 8 rotors mounted in pairs in a quadcopter layout.
Unlike a helicopter rotor—which is designed to spin at a constant rate—the speed of
each rotor can been throttled electrically to vary the amount of lift generated.
Although its less efficient to have rotors in this over-under configuration compared
to a regular quadcopter, it does provide additional lift while giving some redundancy, as the
aircraft will be able to achieve stable flight even with the loss of one motor or rotor.
[15].
Since Titan’s atmosphere is made up of mostly Nitrogen and is much colder than Earth’s
atmosphere, the viscosity is also much lower.
Along with the higher density, this means that the rotors on Dragonfly will be operating
in a fluid with a much higher Reynolds number than they would if they were operating on
Earth.
Reynolds number is essentially a quantity that informs engineers whether laminar or
turbulent flow will develop.
It’s a little confusing as the number is not constant for every situation and depends
largely on the application, but in general it can be described by this equation for flow
in a pipe.
Where inertial forces, trying to keep the fluid flowing, are the numerators, and viscous
forces, trying to slow down the fluid, are the denominator.
Here a higher density will increase reynolds number and thus increase the likelihood of
turbulent flow, and a lower viscosity will also increase it.
[22]
Exactly the scenario we are encountering on Titan.
As a result, the propellers had to be designed differently to work as efficiently as possible
on Titan.
They are in fact much closer in design to wind turbine blades than normal propellers,
with a large amount of twist in the aerofoil.
These propellers are a metre in diameter, much smaller than any wind turbine blade here
on earth, but as a result of that higher reynolds number the same design principles are applicable.
Wind turbines are also designed to be resistant to a build up of surface dirt, which will
be a valuable property on another planet with maintenance crews billions of kilometres away.
We also need to factor in the lower speed of sound on Titan, which is about 194 m/s
versus 340 m/s, so shock wave formation can occur much sooner at the tips of our propellers.
This means we have to be mindful of tip speeds which are determined by prop diameter and
rotational speed.
Even with these unique environmental factors Dragonfly will have a mass of around 450kg
with fantastic range and max speed.
Allowing Dragonfly to first land in the sand dunes of Titan’s Equatorial region before
eventually making it’s way to Selk Crater, an impact crater thought to contain all the
building blocks of life that we are familiar with here on earth, and may just give us some
clues about Human’s ultimate question.
How did we get here.
You may have noticed that I mentioned the inverse square law, but did not fully explain
it.
This is a law that applies to a huge number of physical properties and it simply states
that the intensity of a point source of energy will decrease with the square of the distance
away from it.
[25] This applies to gravity, electric fields and radiation like the dragonfly trying to
send radio waves back to earth.
To learn more about it you could take this course in gravitational physics to learn how
Newton deduced that the force of gravity obeys the inverse-square law, and help you understand
some of the terms I use in my videos.
This is just one small chapter in a course on Brilliant, and you can choose from many
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