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
earth itself believed to have formed
over 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
do not 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 four point four
times denser than Earth's and is the
only moon in our solar system to have
any noticeable atmosphere at all 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 goldmine 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 in 1997 a
collaboration between NASA the European
Space Agency and the Italian space
agency launched the Cassini space probe
on a seven-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 seven-year journey on
its own attached to the space probe was
a smaller Lander called halcon's which
was hoping to become the first
spacecraft to land on Titan several
months after entering Saturn's orbit how
can separated from Cassini and started
its journey towards Titan and began
sending back vital details about Titans
environment like the fluid properties of
the atmosphere and the nature of the
moon's surface
as it descended Howkins 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 and after
two and a half hours of descending
through the unknown how can 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
on the surface how can 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 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 Howkins
mission will be vital to its success
dragonfly is a mobile Lander fitted with
eight large rotors that will be able to
fly around the surface like a drone an
incredibly difficult engineering
challenge and the data gained from
Hawkins will be insanely valuable when
designing the drone everything from its
sensor layout battery capacity energy
source and propeller 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 a 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 analyzing elemental
compositions at landing sites using a
neutron activated gamma ray spectrometer
this instrument typically needs cryo
cooling but thanks to Titan sub-zero
temperatures this instrument can be
passively cooled it will however need to
generate its own neutrons rather than
relying on cosmic rays to generate them
as the atmosphere blocks too much
sunlight when it lands a seismometer
will give us information about quakes
and reveal the thickness and nature of
titans icy crust sitting above what is
thought to be a liquid water ocean we
think this because Cassini witnessed the
surface shifting in position by 30
kilometers in just two 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 since the air is thicker on Titan
and the gravity is one seventh of
Earth's dragonfly we'll be able to
achieve more thrust on a planet that
needs less lift drastically reducing the
energy needed for lift compared to earth
yet finding that energy to fly on the
surface of Titan is not easy
due to titans 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 thankfully we have a
lot of practice in a different type of
energy source through missions like the
Curiosity rover which was powered by a
radioisotope thermoelectric generator
RTG x' 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 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 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
but choosing a suitable radioactive
material is our first challenge with an
e spacecraft a light 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
primarily produce alpha waves as this
form of radiation is most easily
converted to heat in a compact space as
a result of these requirements
plutonium-238 strontium-90 and curium
244 are the most commonly used fuels
next we need a material that is both a
thermal insulator to maximize their
temperature differential and an electric
conductor to maximize our current these
two material properties are typically
linked materials like copper are a good
thermal and electrical conductor and a
material like iron is it worth
an electric conductor using these
materials in conjunction can create a
crude thermo electric 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 LED Tellarite and tags
which is an alloy of Tel irrilium silver
germanium and antimony the thermal
electric generator used for the
Curiosity rover could generate 110 watts
of electrical power but we will lose
some of the 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 dragonflies cruise
vehicle will need to be equipped with
radiators to bleed that heat into space
to prevent overheating
just as the Curiosity rover did we will
also lose energy to keeping the crafted
operating temperature as the surface of
Titan can reach temperatures as low as
minus 180 degrees Celsius 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
titans daylight hours so we will be
aiming to charge our batteries during
Titans nights which lasts 192 hours the
same as its daylight hours so it makes
sense to make our battery charge fully
in those 192 hours giving us a 14
kilowatt hour battery for comparison a
typical Tesla battery is about 75
kilowatt hours with a specific energy of
100 watt hours per kilogram that will
make our battery 140 kilograms in
practice a smaller battery will probably
be used and even a 30 kilogram 3
kilowatt hour battery would provide us
up to 2 hours of flight time at 10 m/s
providing a massive 72 kilometres
even more incredible when you consider
the Curiosity rover has only driven 21
kilometers over the past 7 years on its
time on marys of course dragonfly won't
fly its maximum range in one hop and
will likely take shorter safer hops
between
interesting points during Titans 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 and from
earth will be difficult the energy
requirements to transmit data rises
dramatically as a result of the inverse
square law the Hawkins 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 after falling into
Saturn's atmosphere in 2017 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 the average round-trip
communication time is around two and a
half hours making it impossible to fly
the spacecraft in real time instead
dragonfly will navigate using its own
vision much like the self flying drones
we have here on earth dragonfly will use
its cameras along with 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 originally dragonfly was
intended to fly with a single rotor but
since helicopters are mechanically
complex in the way they varied the
rotors pitch to vary lift the idea was
never developed but with the surge of
multi rotor drone technology over the
last decade the idea for a quadcopter
became much more feasible dragonfly will
feature a total of eight 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 be throttled
electronically to vary the amount of
lift generated although it's 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
since Titan's atmosphere is made up of
mostly nitrogen and is much colder than
Earth's atmosphere the viscosity is much
lower along with the higher density
this means the rotors on dragonfly will
be operating in a fluid with a much
higher Reynolds number than they would
be 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 a 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 at
the denominator here a higher density
will increase the Reynolds number and
thus increase the likelihood of
turbulent flow and a lower viscosity
will also increase it 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
buildup of surface dirt which will be a
valuable property on another planet with
maintenance crews billions of kilometers
away we also need to factor in the lower
speed of sound on Titan which is about a
hundred and ninety four meters per
second versus three hundred and forty
meters per second here on earth so
shockwave 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 450
kilograms with a fantastic range and max
speed allowing dragonfly to first land
in the sand dunes of Titans equatorial
region before eventually making its way
to sulk 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 of
humans 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 this applies to gravity electric
fields and radiation like the dragonfly
trying to send radio waves back to earth
to learn more about us you could take
this course in gravitational physics to
learn how Newton deduced that the force
of gravity obeys the inverse square law
and helped you understand some of the
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