This episode of Real Engineering is brought
to you by the Royal Air Force and the Briggs Automotive Company, who are currently holding
an engineering design competition for Young Engineers. One of the moments that will always stand
out in my life is the watching the joy and excitement of the engineers at JPL celebrating
the monumental task they had completed. They had just landed a 900 kilogram rover
the size of a mini cooper on another planet. The Mars rover is bursting at the seams with
engineering innovation. I could detail the insane landing process
and any of the multitude of engineering marvels on board, but today I want to discuss something
you may not have put much thought into. Something we use on earth everyday. Tires. The wheels of the Mars Rover have been one
the biggest technical difficulties encountered on the mission. Accruing substantial damage though the rough
Martian terrain. Surely the engineers at NASA could have predicted
this and designed something better. Well not everything is as simple as it may
and seem. Even on flat hard ground, the rover has a
top speed of just 0.15 kilometres per hour. Yet this snail's pace still had enough force
to tear holes into the aluminium wheels of the rover. The engineers have some strict criteria that
made the job more difficult. The wheels need to be stiff enough to support
the weight of the rover, at nearly 1 tonne that isn’t an easy task. The wheels need to be as light as possible
to reduce launch costs and must to be able maintain traction and navigate the unpredictable
martian terrain. The current record holder for longest journey
on an extraterrestrial body is held by the Opportunity Rover at 45 kilometres, with the
previous record being the Russian Lunokhod 2, a lunar rover. Curiosity has racked up just 20 kilometres
to date [1]. All Mars rovers have used solid aluminium
wheels. The Sojourner, Opportunity and Spirit rovers
all used the design successfully, but the curiosity rover is much heavier. Each wheel on the Curiosity rover is about
half a metre in diameter and 400 millimeters wide. Milled from a solid block of aluminium in
a similar fashion to the methods described in my Bloodhound SSC video, but unlike the
Bloodhound, the Mars rover wheels are milled down to just 0.75 millimeters thick over the
majority of its circumference. Damage here isn’t all that surprising, 0.75
millimeters thick is the same thickness as a credit card. Running a nearly 1 tonne vehicle over a sharp
rock would be expected to damage aluminium that thick. NASA simply underestimated the roughness of
the terrain, more worryingly though the load bearing threads which are about 6.4 millimeters
thick are also damaged, and if they continue to break the wheels will become useless. The Mars Rover cost 2.5 billion dollars to
develop and it employs a nuclear reactor for power. Extending the life of this project was top
priority for it’s engineers, and so finding a solution to this problem has been fairly
high on NASA’s list for future projects. So what other designs could we use? To reduce the chances of the wheels experiencing
a stress that could damage them we want them to be able to bend and conform to the terrain. This is what our rubber air filled tires do
on earth. Land rovers drive over similarly rough terrain
without any major problem, so why not use tires like this on Mars? Flip down of temperature with suitable image/footage
of Mars. Temperatures on Mars can dip as low as minus
130 degrees celsius. Temperatures this low would transform rubber
from an elastic material capable of absorbing stress to a brittle glass-like material, making
it useless for this application. Rubber would also degrade from the UV radiation
it would be exposed to on the surface of mars. On top of this, weight savings is also top
priority and rubber wheels with thick rubber, steel reinforcement and a rim, are actually
quite heavy. The lunar rover used flexible steel mesh wheels,
with a stiffer inner frame to prevent over deflection and thin strips of metal attached
to prevent the wheel from sinking into the lunar soil. This option was studied for applications on
Mars, but the Lunar Rover had a mass of just 450 kilograms compared to the 900 kilograms
of the Curiosity Rover, combine this with the higher gravity on Mars and it made the
wheel unsuitable for the application. The wheels would simply not be able to hold
the weight of the vehicle without deforming permanently, but Goodyear, who developed the
original Lunar wheel, has been working alongside NASA and in 2010 they were given the R&D 100
award for this spring tyre, which could be the solution to our problems. Being both light, capable of bending and conforming
to the terrain without permanently deforming and capable of holding the weight of a heavier
rover. So what is it’s secret. What makes these spring tyres different to
the ones used on the Lunar Rover. They use a new age material that has some
incredible properties, not just for applications in space, but right here on earth. This spring tyre uses a material called Nitinol. Nitinol is a nickel titanium alloy that has
been named a shape memory alloy, and for good reason. You can bend and deform it, and then just
apply a little bit of heat and it magically returns to its original shape. It remembers its original shape. That is incredible, but nothing is magic in
this world and everything has an explanation. Here is how it works. Typically when a material is stressed one
of two things can happen. If the stress is below it’s yield point,
the material will deform elastically like a rubber band, and return to its original
shape when the stress is released. Alternatively, if the material is stressed
beyond its yield point the material will deform permanently and when the stress is released
the material will be not return to its original shape. So how does this permanent deformation occur? If bonds are not broken, the material should
return to its original preferred crystal lattice orientation, but when sufficient force is
applied small defects in the crystal lattice are able to move. This is what is happening to the wheels of
the Mars Rover. When driving over pointed rocks and gravel
the stress exceeds the yield stress, and thus the wheels are gradually picking up permanent
damage, and eventually cracks will form and the material will fracture. Deformation occurs slightly differently for
Nitinol, as it has some unique properties due to the internal crystal structure. When Nitinol is below a certain temperature
it has a crystal structure called martensite. It’s crystal structure is arranged in such
a way that it can accommodate deformation very easily. Martensitic nitinol forms grains of twinned
atoms where the direction aligns. When stress is applied these twin grains deform
and align to best absorb the stress. Particular twin grains can grow at the expense
of others. This is called detwinning. This, like the dislocation movement in other
metals is permanent, without external energy providing the energy needed to revert backwards,
but Nitinol can get that energy from heat. Upon heating the nitinol forms austenite,
an ordered and regular crystal structure, which effectively resets the crystal structure,
and when the nitinol cools again the nitinol remembers its original shape. What makes nitinol even more amazing and useful,
is that this transformation temperature can be tailored for different applications. In general, increasing the titanium content
of the metal will increase the transformation temperature. This is an incredibly useful property and
it has found many applications across many industries. Let’s say we want the metal to remember
its shape at 37 degrees celsius, body temperature. Can you think of any applications when this
would be useful? I’ll give you one. Traditionally when stents, which are tiny
tubes used to clear blockages in arteries, are being deployed, a small balloon is used
to inflate it and plastically deform the stent into shape. However this often places force on the lining
of the blood vessel, which can damage it and cause it to form scar tissue. Many stent designs now employ nitinol which
has been tailored to remember it’s shape at 37 degrees celsius, and so when unsheathed
it automatically expands into place without placing excessive force on the blood vessel. [7] So how is this useful for the Mars rover? Do we have to apply heat to the metal so it
can recover its shape after running over a particularly big rock? No, luckily nitinol can also be induced into
that martensite to austenite transformation through stress and strain, and so when the
stress passes a threshold it actually causes the crystal lattice to transform to austenite,
and when the stress is released it returns to martensite and the metal recovers its shape. This is called super elasticity and it is
the property that allows these new Mars rover wheels to deform right down to the rim and
still recover its shape after. This combined with the interlocking coil design
allows the tyre to tolerate strains in a way no other tyre could, while surviving the harsh
Martian environment. These tyres could appear in NASA’s designs
for future Mars Rovers, like those planned for launch in 2020. It’s easy to think something as simple as
a wheel will never be reinvented, but there are always ideas and opportunities for people
willing to put the work into thinking about them. That’s why I have teamed up with the Royal
Air Force and the Briggs Automotive Company to bring you an exciting design competition
for Young Engineers. We want you to brainstorm designs for a new
Mars Transport Vehicle that improves on previous designs in some way. We want your most innovative and creative
ideas. Think from the ground up, what problems will
Mars rovers encounter, have you ever thought about how we communicate with a vehicle on
another planet. The top prize winner will have their idea
developed and made into a 3D model by the Briggs Automotive Company. They’ll also win work experience at the
pioneering BAC factory in Liverpool that produces the world’s only road legal single-seater
supercar. For more information head over to the STARRSHIP
YouTube Channel, a link for that is on screen now. As always thanks for watching and thank you
to all my Patreon supporters. If you would like to see more from me the
links to my instagram, twitter, subreddit and discord server are below.
It's a funny saying "reinvent the wheel". Contrary to the conceit, it's happened many many times and the results have been really important.
Very interesting video. How much would a wheel cost?