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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.