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
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