Serious wheel spin: Making sure the Bloodhound SSC's wheels are up to the job

It is perhaps one of the most ambitious engineering projects of our time. For the last six years, Bloodhound SSC has been steadily and methodically developed, and it is now coming together ahead of initial test runs earmarked for September next year.

While the project has experienced delays from its initial schedule, it is understandable in a project of this ambition: a car that will travel at 1000mph. I repeat, a car that will travel at 1000mph! It is easy to become slightly blasé to the concept and immune to the challenge that's being undertaken. But aside from rockets, turbojets and V12 engines (that's for the fuel pump), there is a fascinating materials story around what it takes to get the whole thing rolling, literally.

The wheels of the Bloodhound are just under a metre in diameter and weigh around 95kg. They're big things, and are perhaps more attune to flywheels. But spinning 175 times a second is well out of the comfort zone of most flywheels, especially at this size and weight.

Because of this, they are made from an aerospace grade alloy and involve around 20 companies and 100 processes to go from raw material to the 10,000rpm or so that will take the vehicle into the record books. They are made from 7037 aluminium alloy as the material has the very specific properties needed, in particular, the limiting ruling section on heat treatment.

This is the one of the most important factors associated with the choice of metallic for the wheels, ensuring that the desired mechanical properties are obtained throughout the section of the material after it has been heat treated. The limiting ruling section determines the maximum diameter in which the specified properties can be achieved by a given heat treatment. It is vital the wheels copes with the enormous centrifugal forces, expected to be 50,000 radial 'g' at the rim. However, the highest stress is actually at the centre of the hub.

Conor La Grue, engineering lead for commercial and product sponsorship for the Bloodhound Project, said: "If you imagine spokes of a wheel coming to the centre, it is the focal point of the high stress and it is also where the wheel is thickest, so we needed to find an alloy where we could get the heat treatment and hence material properties all the way through the wheel at that particular point, as well as having a wheel that could cope with the loads we are looking at."

To make sure the wheels do not fly apart under the huge centrifugal loads they will experience, Rolls Royce recently helped the team complete a spin test up to nearly 10,500rpm. Though this is slightly faster than the wheels are likely to see in anger, it was another necessary step in ensuring success of this ambitious project.

The rig is normally used by Rolls Royce to test compressor discs. For Bloodhound normal atmospheric temperature and pressure were used, as one of the primary effects causing the wheel to heat up is the friction from the air.

The test was essentially a validation for the simulation work carried out by the team. The wheels have been carefully spec'd and produced by dozens of expert hands, so it was time to see if all the calculations were correct and the hard work paid off.

"If the wheel wasn't the right alloy or we didn't get the right processes, at this stage, it would not have survived the spin test," said La Grue. "What we were looking for was known behaviour. What we didn't want to see in the data were surprises. We wanted to be able to explain all the behaviour we saw as that meant that the FEA models we were basing our predictions on, and the performance of the alloy, was right and the wheels were made correctly."

In particular, the expansion of the wheel from thermal and centrifugal forces were key measurements. Friction from the air actually heats the wheel to nearly 100°C, and combined with centrifugal forces, the wheel diameter grows by around 1.6mm. It might not seem like a lot, but at 1000mph, ride height is a very sensitive issue.

Measuring data
To accurately measure and record data from the spinning wheel, the team used a suite of sensors from Micro Epsilon. These included both thermal cameras calibrated to measure temperatures across the surface of the wheel, so to assess the expansion from temperature increases. Laser sensors were also in place to measure the overall change in diameter to enable the expansion from centrifugal forces to be calculated.

"We created this frame around the wheel so we can measure expansion at the rim, but also measure it across the body of the wheel and look at what was happening at the hub as well," said La Grue. "Mounting the wheel on the rig mimics the way it will be mounted on the actual hub of the uprights on Bloodhound. We wanted to make sure the test reflected the way it will behave on the hub on the actual car and we were especially interested in the performance of the hub area for the attachment of the wheel itself."

The connection of the wheel to the car is of particular interest, as the hub is made from steel, while the wheel is 7073 aluminium alloy. Aluminium usually has around twice the rate of thermal expansion to steel, so it was a key area of interest and design optimisation.

The connection is a raised area around the hub and in the centre is a collar on the wheel that also has a steel sleeve over the top. The collar sits inside the hub meaning that the centrifugal loads experience are not actually taken on the studs, but instead, are focussed on this centre part inside the hub.

"The studs are stopping the wheel from coming away from the mounting, but they are not taking shear load," said La Grue. "They're taking tension load as the wheel wants to come off as it spins that quickly. The shear load is all being reacted out by this little mounting in the centre of the wheel that has a steel sleeve over it."

The test accelerated the wheel to 10,496rpm and held it there for a brief moment before decelerating the wheel back down. The reason is that Bloodhound does not really have a steady state and is always accelerating or decelerating. During the test, friction from the air heated the wheel up to peak temperature at the rim of 96°C.

"It was pretty much in line with what we had conceived," said La Grue. "It is an exciting time and we have less than a year to go now and are 50% of the way through the primary structure build, so it is all really starting to take shape and come together."

The process
From a process point of view the wheels of the Bloodhound SSC are nothing short of a modern day engineering masterclass. The wheels have to support the 7.5 tonnes of the vehicle itself, as well as 50,000g at the rim.

The process starts with 6 tonnes of liquid aluminium. This is delivered, from smelters Trimet Aluminium in liquid form, in giant crucible on the back of a lorry, believe it or not. Amazingly, that is the most efficient way to transport it.

As soon as it delivered, it is immediately blended with the appropriate constituents to make the necessary 7037 alloy and cast. It is poured in to a 7m deep cylinder and left to set.

Once hardened, it is then chopped up into barrels and samples of the blended are tested, as well as the whole thing having NDT inspection to make sure it is free of defects and is exactly as specified.

The cooled aluminium has small dendrite crystals form in the structure as well as tiny voids. These are all potential points of failure so further processes are needed to optimise them. The barrels are then forged by Otto Fuchs that take the relatively 'loose' crystalline structure of the cast material, and squash the dendrites and squeeze out all the voids to leave a microstructure that is tightly packed and much stronger than the original cast material. This is essential to make sure the wheels can survive when travelling at 1000mph.

The barrels are heated to nearly 400°C where a 3,600 tonne open die press squashes the barrels into cubes and then into 'cheeses', where they begin to resemble wheels.
These then get rough machined and heat treated. One wheel from each batch will be cut up and tensile tested and also looked at under an electron microscope for its properties, with all the others ultrasonically inspected.

Once this process is completed, they go to Caste Precision in Glasgow where they machine the wheel forms. These then go for further NDT at Amfin to make sure no significant stresses have been imparted into the material from processing. Then Curtiss-Wright Surface Technologies shot peen the wheels before they finally go back to Amfin for anodising. Only then do you have the finished wheel.

Class dismissed.

Author
Justin Cunningham

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Do you have any comments about this article?
Hello all, I stand corrected, its incredible and true according to the manufacturers site. Amazing.

Josh


Comment Josh Deetz, 01/04/2015
I love the detailed stories about all the issues encountered when trying to make a car that can survive 1000mph. While most is amazing one point is incredible, questionable even. You write that the aluminum was transported in liquid form on the back of a truck, that's the most efficient way to transport it ? So you want us to believe 6 tons of molten aluminum was sloshing around in a huge crucible on the back of a truck and move about on roads? Or is this a cute story about an inter plant transport method? Maintaining the material in liquid state while transporting seems inefficient, and all other safety critical casting alloys are transported in ingot form without inefficiency to aircraft builders, train makers. etc. So you have a 6 ton load in what I assume is a 10 or 12 ton capacity crucible, flux covered and travelling around? It seems more a ripley's believe it or not.

Comment Josh, 31/03/2015
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