Building the bullet

The technical team working on the UK’s new world land speed record bid has given The Engineer an exclusive insight into the huge challenges facing the project.

Andy Green will defend his title as fastest man on land in 2011 when he attempts to break the land speed record for a second time.

In 1997 the former RAF pilot whizzed across the Nevada desert in a jet-propelled car at 763mph. Now he is looking to reach 1,000mph.

The £10m bid has attracted support from some of the UK’s top engineers and scientists under the umbrella of a private venture called the Bloodhound Project. The three-year mission, led by former land speed record holder Richard Noble, will build a 12.8m long, 6,422kg fuelled, jet and rocket-powered vehicle that will be as tough as a submarine and faster than a speeding bullet.

Researchers from the UK’s National Physical Laboratory have been working hard for the past year to make sure that the wheels do not, literally, fall off the entire project. The NPL, with researchers at the Atomic Weapons Establishment and Fluid Gravity Engineering (FGE) have advised the world-record bid team on the Bloodhound supersonic car’s wheel and rocket designs, two of the most high-risk aspects of the world record attempt.

Brian Chapman, project leader from the NPL, said the wheels are the most important design feature of the vehicle. In order to get up to 1,000mph, he said, the wheels need to rotate at 10,500rpm and they will experience 50,000 times the force of gravity at their rims. They would need to withstand any damage by the surface or stones they run over. Yet they would also need to be as light as possible to minimise steering and suspension forces.

The wheels on the previous world record-holding vehicle, called the Thrust SSC, would be inadequate for this project, he said.

‘The Thrust SSC used aluminium alloy wheels and we didn’t believe that material would be suitable to go another 30 per cent faster,’ he said.

Materials experts at the NPL in Middlesex researched a choice of metals and composites that could be used in the design, providing reports on titanium and aluminium alloys and metal composites. Each design was put through an advanced computational fluid dynamics program developed at the facility.

The researchers considered the effects shockwaves would have on the wheels as the vehicle sped up and broke through the sound barrier. They decided the sturdiest design for the wheel was a 90cm diameter titanium disc.

The remaining challenge for the Bloodhound team is finding someone who can manufacture it. ‘One of the problems with the titanium disc is we can’t find anybody to forge it because it is so big,’ said Green, a mathematician who is not only a driver but also an essential part of the Bloodhound design team.

‘We are looking to get that forging done fairly soon so we can then find somebody else who can spin something that big and heavy to 10,500rpm to prove all the stress modelling works out.’

If the titanium discs do not materialise, the Bloodhound team is considering a lightweight composite version of the wheel with a carbon fibre interior and aluminium exterior. ‘No one has ever made anything like that before,’ said Green.

While the wheels are obviously vital, but the only way the vehicle will reach 1,000mph is with a new propulsion system, thus the concept of a new mixed powerplant of a hybrid rocket motor and a jet engine that is now used on the Eurofighter Typhoon.

Green said there has only been one other vehicle to break the land speed record using rocket propulsion — the US-designed Blue Flame in 1970 that used liquid fuels.

The new rocket motor, termed a hybrid because it uses solid and liquid fuel, ‘is going to give us the raw power combined with the jet to get us up to a new record-breaking speed,’ he said.

The hybrid rocket motor, which measures 46cm in diameter and 4.5m long, is the largest of its type the UK has ever produced. The jet will take the car from a standstill to 350mph, then the rocket will be used for a short burn time of 20 seconds with both engines accelerating the car to its peak speed.

The Bloodhound car

The motor will rely on high test peroxide as its oxidiser and an aromatic rubber substance called hydroxyl-terminated polybutadiene as the fuel.

A pump, powered by a V-12 petrol engine, will inject 50kg of peroxide, enough to fill four household buckets, into the motor.

A new modelling tool, developed by the NPL and FGE, enabled the design team to understand the hybrid combustion process and simulate the internal motor ballistics. This data was then compared to tests done with 15cm diameter models of the rocket.

Chapman said this information has helped the Bloodhound team optimise such details as the injector design, oxidiser streams into the fuel grain, radiation transfer and rocket motor exhaust.

‘That’s something you could only do by understanding the propulsion system and modelling it using algorithms and computational fluid dynamics.’
Whether all this work will be enough to help the Bloodhound car reach 1,000mph remains to be seen.

‘However,’ said Green, ‘we have every reason to believe that all the technical solutions we’ve found so far can do that.’

Siobhan Wagner