On October 15th 1997 Thrust SSC became the first land vehicle to break the sound barrier,
breaking the land speed record with an astonishing top speed of 1,228 km/h, which still stands
today.
20 years on and the team that created this wonder of engineering is now looking to break
their own record with the new and improved Bloodhound SSC, a vehicle looking to achieve
an amazing milestone of 1,000 mph, or 1609 km/h for those of us living in the civilised
world.
Creating a land vehicle capable of achieving these speeds presents some very unique engineering
challenges, and today we are going to explore a problem that even limits the top speed of
the Bugatti Chiron.
How to design a wheel capable of withstanding the extreme forces at these speeds.
When designing a high speed car there are two problems of physics that grow disproportionately
the faster you travel.
First, the drag force the vehicle experiences grows with the square of the speed.
Returns in top speed for every unit of horsepower included shrinks the faster you go, but that’s
a problem for a future video.
Today, we are going to explore the second problem, the massive inertial forces the tyre
experiences at high speed.
If you watched my video on Artificial Gravity, you will know that any spinning mass will
experience an increase in weight, proportional to the square of the angular velocity multiplied
by the distance from the rotational center.
This phenomenon can be used to create artificial gravity in space, but put too much mass on
the outside of the space ship, or spin it too fast and it could tear itself apart.
The Slow Mo Guys did an absolutely mesmerizing test of this when they spun a CD up to 23,000
revolutions per minute, before it shattered under the weight of it’s own inertia.
This effect is one of the key limiting factors currently holding cars like the Bugatti Chiron,
the Hennessey Venom and Koenigsegg Agera from the illustrious milestone of 300 mph, just
this month the Agera broke the production speed record and came the closest to that
milestone with a top speed of 277.9 mph (447 km/h).
There isn’t a tyre on earth that can withstand the inertial forces at higher speeds, the
rubber would simply peel away from the hub, but evidently we aren’t far off.
However these cars don’t come close to the speeds of the land speed record held by Thrust
SSC.
So what kind of wheel did the Thrust SSC use when it demolished the land speed record all
the way back in 1997?
Well the Thrust SSC has one primary advantage when it comes to wheels.
It has very little use for traction beyond braking and turning, as the wheels are not
used to transmit rotational motion from the engine to linear motion, they achieve their
propulsion from jet engines and rockets.
These wheels simply need to support the 7.5 tonne weight of the vehicle and allow it to
roll along the ground, and have enough lateral traction to allow the driver to steer the
car.
At higher speeds this even becomes unnecessary, as the vehicle get’s the majority of its
steering force from the force of air hitting the angled wheels.
The Bloodhound derives it power from a EJ200 jet engine, the same engine used by the Eurofighter
Typhoon, (90 kN), and an even more powerful hybrid rocket engine (122 kN).
At it’s fastest the wheels of the Bloodhound will be rotating 10,000 times per minute.
Using that equation from earlier, and with the wheel radius at 46.5 cm, we can calculate
that any mass on the outside of the rim will experience 50,000 times the acceleration due
to gravity.
A 1 kilogram bag of sugar would weigh the same as a fully laden articulated truck.
With these problems in mind, let’s begin the design process for our 1000 mph wheels.
Step one is material selection, this will determine a large portion of our design process.
As the design will change according to material properties and manufacturing techniques.
As explained before traction is not a huge concern, so we can forgo the rubber tyre and
instead go with a solid metal wheel, which can better withstand the centrifugal force
caused by the spinning wheel.
The metal needs to not only be strong enough to withstand these forces, but it also needs
to be light to minimise this inertial force.
On top of all this, the material needs to be capable of absorbing damage, which is why
a carbon fibre wheel is not an option, as at these speeds an unexpected hit from a stone
could potentially shatter the entire wheel, if the material is too brittle.
These are a very particular set of requirements, that forged aerospace grade aluminium fulfills
best.
Now that the material has been selected we can begin forging blanks.
To forge these wheels the team took these huge billets of aluminium alloy 7037 and heated
them to 390 degrees and compressed it with a 3,600 tonne forging press.
The forging clamp operator here deserves credit for the insane precision, taking this cylindrical
billet and forming it into a compressed disk, this is our blank, which will be passed to
a milling machine to mill the wheel into it’s final shape.
Transforming the cast material, which is material that was formed by pouring molten aluminum
into a cast, into this forged material makes the material vastly stronger.
When the molten aluminium is cooling to become solid, the crystal structure grows randomly
from nucleation sites, like a snowflake from a single ice crystal.
This unpredictable process gives rise to a random jumble of crystal sizes, grain directions
and voids, called dislocations, between individual crystal grains.
When this solidified cast aluminium is compressed the crystal grains increase in density and
dislocations pile up, which increases the energy required to cause expansive deformation.
This process is called work hardening, and it drastically increases the strength of the
material, but also makes it less ductile, but that’s desirable in this situation,
as we do not want the wheel expanding during use.
Now that we have our blank material, we can begin the prototyping process.
We do not want to use this expensive material and begin prototyping with it.
Failed designs would be extremely expensive.
The team will first start with some basic design parameters, like wheel diameter.
They will then determine the general thickness needed for vehicle stability and to withstand
the predicted stress.
Using these parameters a model will be generated and tested computationally.
We performed our own computational analysis on stress and deformation from inertial forces
on the Bloodhound wheel using simscale here.
Using a tool like this, the engineers will refine the design.
Once a suitable design is found, prototyping with a cheaper material will begin.
Many engineering firms today use 3D printing for this design verification step, but the
Bloodhound team used a cheap cast aluminium wheel.
They attached this wheel to a trailer that simulated the weight of the bloodhound on
a single axle, and it helped them discover that their wheel did not adequately spread
the load over the desert surface, meaning it was breaking through the crust and driving
on the hard bedrock underneath, which would damage the wheel.
Back to the drawing board to increase the contact surface of the wheel.
This design and prototyping process will be repeated until a suitable design is found.
Once the final design was decided on the expensive forged aluminium was passed to a CNC milling
machine, which uses computer guidance to cut the wheel to it’s final shape.
However, when a work hardened material is machined, it disrupts the compressive forces
that developed in the skin during the forging process.
The surface of the machined metal is like a bottle of compressed air, and a small leak
may lead to an explosion.
It wants to expand, so if a crack forms in the surface of this material, these expansive
forces will increase the chances of the crack growing, and so another surface treatment,
called shot peening, is applied by shooting thousands of tiny spherical balls at the surface
of the machined product to introduce a thin layer of higher compression, which helps the
material resist crack growth.
This final product was sent for design verification again, where it was rotated up to speed in
a controlled environments to ensure it would safety take this astounding vehicle to 1.3
times the speed of sound.
Just two months ago the Bloodhound SSC completed a 200 mph test run at Cornwall airport.
The Bloodhound reached this top speed in just 8 seconds, with a 0-60 of just 2 seconds.
That half a second faster that the Bugatti Chiron.
This run served to test the Eurofighter Typhoon engine and to demonstrate the vehicle to the
public, after all the goal is not to break the record, they already have the record with
the Thrust SSC.
The real goal here is to inspire people to get excited about engineering, which I can
wholeheartedly get behind.
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