A supersonic car requires nothing less than super materials. Melanie Rutherford talks to Roland Dennison, Engineering Lead in Stress Analysis for the Bloodhound Project, about the design and testing of the UK’s latest attempt to break the world land speed record.
In October 2008 at the Science Museum in London, Lord Drayson, UK Minister of State for Science and Innovation, officially announced the Bloodhound Project. The British supersonic car is aiming to not only break the land speed record but also the sound barrier, by reaching speeds of 1,000mph. But speed is not the only objective – it is hoped that the design and build of Bloodhound SSC will boost the popularity of STEM subjects and further advance automotive and aerospace technology R&D.
British engineering already has a good track record in fast cars. Indeed, the current land speed record was set by a team of UK engineers in September 1997, when Thrust SSC breached the sound barrier at a top speed of 763.035mph, in the Black Rock Desert, Nevada, USA. Should Bloodhound SSC achieve its 1,000mph goal, this record would not only beat the current land speed record by 31%, but also break the low-altitude air speed record, which lies at 994mph.
Six years after its official launch, Bloodhound SSC is starting to take shape. With the help of numerous sponsors and partner companies, a 42-strong engineering team has searched for and tested the materials and technologies that will stand up to the challenges of travelling faster than the speed of sound. Lead Engineer and specialist in stress analysis, Roland Dennison, reveals the materials that made the cut and how the team is putting them through their paces.
What challenges are involved in designing and testing a supersonic car?
The main challenges are the forces acting on something going at speeds of 1,000mph at low altitude. Bloodhound is not so much of a car as a supersonic truck – this thing is going to weigh about 7.5 tonnes and be the fastest vehicle that has ever travelled at that altitude – faster than any plane has been – so the loads themselves are quite substantial.
How great are these forces?
The aerodynamic forces experienced at low altitude are in the order of 30 tonnes suspension loading – so at 1,000mph, you’re getting 10 tonnes of air pressure per square metre over the car. We use computational fluid dynamics to mathematically map the airflow over the car. That kind of science has developed enormously over the last 15 years and the computer processing available has evolved massively, so we’ve got a very good understanding of how thin the air is and how to design a car to be most effective in those conditions. The challenge has been to find the materials and components that will safely withstand those forces when we repeat the test run several times.
Did any materials or test methods pose a particular challenge?
There’s one material I hadn’t worked with before, which is used for the intake duct. Instead of using pre-preg carbon composite, we’ve used a resin-infused, quadaxial carbonfibre structure that precludes the use of an autoclave. The fibres are loosely stitched at 0, 45, 90 and 180 degrees, formed over a structural foam core and injected with resin. This cures at room temperature. I hadn’t had much experience of this type of structure, so we carried out a few structural tests on some representative panels and coupons to obtain some initial material properties to work with. We’re also going to do a pressure test on the intake duct itself within the next few weeks. That’s going to be an interesting challenge.
Another area we’re going to study is the airbrake doors – basically, two large panels that stick out of the side of the car to slow it down from supersonic speeds. As you can imagine, the forces here are significant at three tonnes per door. We’ve got a few schemes for the doors but we haven’t actually narrowed down a suitable material. Numerous holes are drilled into the doors to reduce the strength of the vortices coming off them – it’s almost a structure holding a load of holes together. One interesting scheme that we’re starting to consider is a composite hybrid structure with a 3D woven carbon core. It’s a fairly new technology but it has been demonstrated quite a lot, so we’re going to do some studies on the material with US technical fabric manufacturer Sigmatex.
Weight 100kg each
Materials Aluminium was chosen for its relatively low density and its strength-to-weight ratio. Rotating up to 10,300rpm, the wheels will produce a significant centrifugal load. ‘We considered titanium first of all, but the density was a little high,’ says Dennison. ‘We decided on a 7037 series alloy that Otto- Fuchs manufactures and tests for Airbus.
Strength properties tend to drop off through the thickness of the material, but we need the strength right at the centre of the wheel because that’s where the highest stresses are. This particular alloy has 7075-type properties all through the thickness that we require for the wheels.’
Design challenges Due to stress concentration, the wheels cannot have holes in the centre – instead, the sides of the wheels are bolted onto the hubs, which ‘presented a bit of a challenge,’ says Dennison.
Testing During the forging process, German company Otto-Fuchs performed cut-up and tension tests to ensure there were no weaknesses in the metal, ‘as they would any aircraft component,’ says Dennison. After machining by Castle Engineering, in Scotland, the wheels will be subjected to nondestructive tests, including X-ray, charpy and dyepen tests. The final wheel will be spintested by Rolls-Royce in Derby, in exactly the same way they test a jet turbine blade. The test will show how much the wheel deforms when subjected to 50,000 radial g.
Carbon composite monocoque (2)
Materials The shape and sweeping curves of this large structure lent itself to the use of carbon composite. The core is aluminuim honeycomb with seven outer and five inner carbon skins, with some UD fibres in the floor for added stiffness. Dennison notes, ‘The actual lay-up itself is nothing special – we’re using a T700 2x2 woven fabric of quite a high density, almost like a woolly jumper in carbon-fibre terms. Stiffness was a priority for the monocoque itself, due to its sheer size.’
Design challenges ‘The guys who designed it were designing monocoques for Formula 1,’ he adds. ‘This is two or three times the size of a Formula 1 chassis, and you’ve got around three tonnes of car attached to the back of it. That presented a big challenge.’
Testing Structural coupon tests have been carried out on the composite structures and more are planned for the main sections. The monocoque will be fully strain gauged while the car is being run, the results of which will be compared to those predicted in the finite element models.
Materials The rear upper chassis is skinned in a high-strength, cold-formable titanium, while the rear lower chassis has a steel skin and the stringers comprise Ti-15AI-3V.
‘This was a bit of an interesting one,’ says Dennison. ‘We had looked at the skin on the previous world record car and saw that it had worn away quite a lot with the abrasion from the salt and dust, so initially we decided to go for a steel floor for the rear chassis, for abrasion resistance.’ The team had all but decided on an aluminium skin for the upper chassis to save weight, however, ‘the different expansion rates of the two materials would mean that in the heat of the desert, the aluminium would expand at a much greater than the steel in the lower chassis,’ he adds. ‘We predicted that the car would turn almost into a banana shape, so we instead chose titanium, which has a much closer expansion rate to steel. In fact, it almost offsets the sag on the weight of the car by expanding the other way.’
Design challenges Forming the shape of the stringers – ‘they’re in a section resembling the shape of a top hat, so we had to choose a fairly lazy bend radius to reduce the likelihood of the stringers cracking when forming.’
Testing As the material is fairly well known, little is required in the way of testing, although the entire chassis will be fully strain gauged.
Steering wheel (4)
Materials The team 3D-printed titanium to make the steering wheel, though not for mechanical reasons. ‘We’re not making a production line of this car, it’s a one-off, a prototype, so we can use it as a showcase for technology,’ explains Dennison. ‘While 3D printing has been around for a while, it hasn’t necessarily been used by industry very much.’
Testing Currently at prototype stage, the steering wheel has not yet been tested for structural strength, ‘but we certainly will do before it’s used in the car.’
Rocket system (5)
Materials The hybrid rocket uses a liquid oxidiser and a solid fuel. The liquid oxidiser is concentrated hydrogen peroxide (H2O2 – also known as high-test peroxide, or HTP). The liquid is pumped through to a catalyst pack where it splits to form a gas. ‘It does this quite violently and that’s what creates the thrust for the car,’ says Dennison. ‘The trick is to stop this H2O from decomposing into hydrogen and oxygen before it reaches the rocket’s combustion chamber, which it wants to do every time it comes into contact with impurities.’ As such, the team spent time working on decontamination of the stainless steel components and piping that the HTP will come into contact with. The technique, known as passivating, involves dipping a material in a dilute solution of nitric acid.
Testing The team has partnered with Norwegian hybrid rocket company Nammo, which manufactures small booster rockets for the European Space Agency that propel smaller sections away from the main rocket. ‘They just so happen to be developing a rocket that’s of the right power using hightest peroxide, so they will provide one rocket running the 800mph runs in 2015 and a further cluster of three or four for running in 2016,’ says Dennison. Both Nammo and the Bloodhound team will test the rocket system, the latter taking place at a test site in Newquay, Cornwall. ‘We’ve got an empty hanger down there where we can carry out the passivation testing, pumping the deionised water through the system to ensure the system is tight.’
Brake discs (6)
It goes without saying that stopping a 7.5t car from supersonic speeds calls for very powerful brakes. The team is using two types of brake disc – one for testing on the runway in Newquay, and another for the actual run in the desert.
Materials The runway requires the wheels to have tyres, so for this, aircraft-type carbon disc brakes will be used. Dennison explains, ‘We don’t need such high braking forces for the desert wheels because we haven’t got the coefficient friction between the aluminium and the desert surface – if we used carbon brakes, they would tend to lock up the wheels because the friction is quite low. The problem is, in the desert the disc brakes will have to spin at the same speed as the wheels – 10,300rpm. We’ve already tested some aircraft carbon disc brakes up to that speed and they tend to fly apart as they cannot withstand the centrifugal forces, so for the desert brakes we’ll be using steel. We’re still deciding on the type of steel, but at the moment we’re using EN36, a nickel/chromium case-hardening steel that won’t harden without a carbon-rich environment. We need something that will keep its properties under heating, which you don’t get with a hard steel.’
Testing The team has already conducted high-energy spin brake testing on the EN36 steel at the AMRC Testing Centre, in Sheffield, which it is currently analysing for any changes in the properties.
Chassis adhesives (7)
Materials Redux-type 322 and 312 film adhesives are used on the upper and lower chassis respectively, using a bonded and riveted structure. ‘The adhesives have to be heat-resistant to some extent, because the latent heat from the jet engine inside the chassis tends to soften it,’ says Dennison. ‘For the redux 322 on the upper chassis to withstand these temperatures, we need to cure it in an oven at around 170˚C.’ While the intention was to use the same adhesive for the lower chassis, the resulting expansion rates from curing at that temperature would have caused some panels to buckle. As such, the slightly lower cure temperature, redux 312 adhesive was chosen for the lower chassis.
Testing The biggest question for the team was, if the glue film that bonds the riveted and bonded structure fails, how will that affect the rivet strength? ‘To find out, we did some two-prong coupon tests on some bonded and riveted joints,’ says Dennison. ‘While the bond produced the required stiff structure, we discovered that the glue would fail before the rivets. While the adhesive provides stiffness, it is the rivets that provide the ultimate strength of the chassis, so even if the glue fails the riveted structure retains its full strength.’
Watch a video of the fastest wheels in history being manufactured at Otto- Fuchs, in Germany, and see which other cars are being developed to challenge Bloodhound SSC in the race for the 1,000mph land speed record on our blog.