What it takes to go 1000mph – Part 2

Last article examined the major components of the Bloodhound SSC car – the engine and body. However every part of the needs to be optimised to a level of perfection, and this article examines the wheels and braking systems, as well as the effects Bloodhound SSC will have on its driver.

They seem like an insignificant part of the car, however the wheels are a critical component. Traditional wheels and rubber tyres just cannot withstand the forces generated by the speeds that land speed record cars travel at, necessitating solid wheels. Thrust SSC used solid aluminium wheels, however the team found that stones were getting embedded in the metal, and the forces was making the metal warp and literally grow around the stones, making them a permanent part of the wheel. Obviously for Bloodhound SSC, something stronger was required.

Cut away image of Bloodhound SSC showing the rear wheels and close-set front wheels. Image from Evo magazine

When Bloodhound SSC runs at 1000mph, the wheels will be spinning around 10,000 times per minute, and undergo forces of around 50,000g. To withstand these forces the wheels are made of solid titanium and weigh around 137kg each. So unique are these wheels and the forces they will undergo, they are being tested at a facility which normally tests power station turbines, and will also have stones fired at them to test how well they resist debris getting embedded.

Regular car-type brakes are really only effective below around 250mph, so other mechanisms to slow the car are required. Probably as you suspect, the team will use parachutes to slow the car, however, again, these are only safe to use below around 600mph. To bring the car down to a speed appropriate for parachutes, Bloodhound SSC will rely on airbrakes, similar to those found on an aircraft.

Thrust SSC testing its parachute braking system in the 1990's. Image by Bloodhound http://www.bloodhoundssc.com

The problem with aerodynamic drag braking, such as airbrakes and parachutes, is that drag changes depending on speed – the faster an object is going, the higher amount of drag it will have. The flipside of this is that if you’re relying on drag to slow a car, the slower the car is travelling, the less braking force you will have. As a result, Bloodhound will deploy more braking systems as the car slows to maintain a constant level of deceleration.

Driver Andy Green explains the braking procedure:
“1000 mph: close the throttle – deceleration rate is 3’g’ initially, then falls off rapidly
800 mph: start to deploy the airbrake, gradually increasing its area to try and maintain 3’g’ deceleration through the transonic region (800 down to 650 mph)
Below 600 mph: deploy a ‘chute to increase the deceleration rate back up to 3’g’.
Below 400 mph: deploy a second ‘chute if required.
Below 250 mph: apply the wheel brakes as required to stop at the end of the track, ready for the turn round.”

Under normal circumstances the braking procedure will take 4.5 miles to bring the car to a standstill, however in an emergency the car can be stopped in less distance. Should the airbrakes fail the parachutes can still arrest the car in the 4.5 miles by themselves, however should the parachutes fail the car will overrun the test track (extra space is allocated as an emergency run off area).

Cmdr. Green sums up the development of the car, “This is an engineering adventure. We don’t know exactly what’s going to happen. All sorts of things might occur, but they’re unlikely to be catastrophic.”

Effects on the driver
Catastrophic, unlikely, but exciting? Definitely. When the rocket is engaged at around 300mph, Andy Green will experience around 3g of acceleration. This level of acceleration will force his blood into his head, and will fool his body into making physiological changes to try to reduce the blood pressure, such as increasing the size of his blood vessels. While this effect will not have any major effects on Green at the time, when the car starts deceleration he will undergo around 3g of force in the opposite direction – forcing blood into his feet. Because his body has made the physiological changes to reduce blood pressure including the dilation of his blood vessels, this effect will be larger than normal with more blood draining from his brain than he would normally experience under a similar force. So while the actual forces he will undergo wouldn’t normally be enough to cause him to black out, in combination there is the chance his brain will become starved of blood and he will pass out.

During acceleration blood is forced into the head. Image by Bloodhound http://www.bloodhoundssc.com

During deceleration blood is forced towards the feet. Image by Bloodhound http://www.bloodhoundssc.com

The other effect of these forces is that Green is at risk of becoming extremely disorientated. Human balance is controlled by fluid in the ears. Under these acceleration and deceleration forces the fluid in Green’s ears will be forced around, with the effect that under acceleration he may think the nose of the car is lifting, and under deceleration that the nose is digging down. While these forces do actually exist in a normal car, they won’t in Bloodhound SSC. To overcome this disorientation Green will literally have to remind his body that these feeling are an illusion and to try to ignore them.

Adding to this disorientating effect by the acceleration and deceleration forces is the extreme vibration going to be felt by Green. There are two sources of vibration in the Bloodhound car, the engine and the ground. From these two sources, and with effectively no consideration given to driver comfort, the vibrations will be so bad it is likely that Green will not be able to focus and track any of the dashboard displays. While the team has tried to design the cockpit to overcome this effect, it certainly won’t be a smooth, or easy, ride.

Cut away image of Bloodhound SSC highlighting the driver's position. Image from Evo magazine

While Green’s career as a fighter pilot in the RAF and previous drives in Thrust SSC has helped him prepare for the forces he will experience, he has been doing extra training by flying his plane around England… upside down.

Artists impression of the Bloodhound SSC. Image by Curventa and Siemens

On October 4th 1983 after his record breaking Thrust 2 run Noble told the world he had done it “for Britain, and for the hell of it.” With Bloodhound inspiring the next generation of scientists and engineers his intentions are more – ahem – noble, and certainly more ambitious. But it would a brave person to bet against him and his team when they begin their attempts in 2012.

For more information about the Bloodhound project, visit www.bloodhoundssc.com

Some interviews for these articles were carried out by Rob Widdows for Motorsport magazine and Ollie Marriage for Evo magazine.


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