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. Sharing passion and skill is something this
channel was founded on and today I want to share with you a fantastic course on Skillshare
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skill to gain. As usual thanks for watching and thank you
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