Suspension and bearings specified for 1000mph

Much of a design engineer's job nowadays is about incremental improvement; solving individual parts of much larger problems. It is fairly rare that a small dedicated team of highly motivated engineers is given a phenomenal challenge and told to just get on with it. But that is exactly what happened late last year, when junior minister at the Ministry of Defence Lord Drayson and Richard Noble concocted plans to go not just supersonic, but to exceed 1000mph with the Bloodhound Supersonic Car (SSC). The car is laced with innovation and high end technology from front to rear. The wheels, for example, will be made from solid titanium forgings; essential if they are to withstand the 50,000g that will be generated as the structure they support hurtles along at more than Mach 1.4. To enable the wheels to spin at some 10,050rpm, Bloodhound is working with US based precision bearing maker Timken. The recommended bearing is actually a standard part from Timken's super precision range. However, Bloodhound will be fully physically tested at full load and speed to confirm Timken's recommendation. At present, the bearings are likely to be lubricated with oil, but this may need to be modified, depending on the temperature build up during tests. The team has managed to 'procure' a Eurofighter Typhoon EJ2000 development engine that was destined for a museum. The engine is seen by the Bloodhound team as 'perfect' for the challenge, given its considerable thrust to weight ratio – in excess of 9:1. But the 22,000lb of thrust it delivers with the afterburner fully lit still isn't enough to propel the 13m 6.5tonne vehicle to the necessary speed. So the car will also use a hybrid rocket engine to provide a further 27,000lbs of propulsive force, hopefully taking it beyond the 1000mph barrier. Not only does the rocket provide a fantastic amount of thrust, but it also avoids the need for an additional air intake – a major cause of drag on Thrust SSC, the current land speed record holder. For much of his professional career, senior design engineer Mark Chapman has been involved with large aerospace projects for the likes of Boeing and Rolls-Royce. These projects have been, on the whole, undertaken by large teams with large budgets. But his new venture into the realm of supersonic cars still sees the need for a significant amount of clever engineering and innovation, this time with a rather limited budget. But that doesn't worry him. "It is easy to be innovative when you have got a big budget," he says, "The real challenge comes when you have to be innovative under very tight budget constrictions." Going supersonic poses two significant design challenges. The first is to minimise the cross section of the vehicle, so it cuts through the air with ease. The second is to provide enough thrust to take the vehicle up to speed. But, alongside the immense aerodynamic and propulsive challenges, there is a raft of underlining mechanical issues that need to be resolved. One of the biggest design elements was the steering and suspension system. "This is not too different from a standard suspension," says Chapman. "We decided against an active suspension, as this would incur a massive amount of cost and weight. And, really, it is not necessary." The suspension mechanisms considered for the job were essentially the same as found in mainstream automotive applications, with wishbone and leading arm configurations. The main difference is that, while mainstream automotive suspensions are optimised for comfort, Bloodhound's will be optimised for stability. With solid titanium wheels, there is no pneumatic tyre to absorb bumps, so springs and dampers must be optimised for considerably higher shock loads. But, despite the colossal speeds, suspension travel will be limited to just 100mm in either direction. There is also a problem with gyroscopic effects in the steering as the 140kg wheels spin at more than 10,000rpm. This creates huge forces on the steering wheel – in some cases, making the car undrivable. To overcome the problem, the team has specified a worm and wheel gear system. "The down side to using a worm and wheel is the driver has no feedback through the steering wheel, so we are working on a system in which a servo motor attached to the steering column applies loads, giving artificial feedback," says Brian Coombs, a senior design engineer on the mechanical elements of the car. "This could also be used as a powered steering system if the steering loads become too high." Several suspension systems were assessed, including one with a narrow front track and wide rear track. This setup would use a double wishbone independent rear suspension and a leading arm non-independent front. But this was rejected because of the drag created by the wide rear suspension and the difficulties in getting the front non independent geometry to behave in a stable manner. The second option used a wide front track with a narrow rear track, with a live axle at the front and independent rear trailing arms at the back. Again, this was rejected; this time due to the supersonic shock waves caused by the front suspension. The shockwave would actually be severe enough that it would disturb the ground upon which the rear wheels would run. This lesson was learnt from Thrust SSC, where it proved difficult to get good vehicle stability from this set up. The final configuration features a front track of 1m , with a 2.39m rear track. Both front and rear systems have a double wishbone suspension, with pull rods operating the spring and damper assembly. But, as Coombs adds: "All suspensions are a compromise and it is the designer who needs to find a solution which best suits the requirements. Our double wishbone pull rod rear suspension is a prime example; we needed a suspension with good stiffness, but also with very low drag as it is in the air stream at the rear of the car. We could have gone for a small diameter beam rear suspension, which would have only one link in the airstream, and low drag, but this would have given us poor camber and toe stiffness. So we have compromised on the drag to make sure we have good control of the wheel." Bloodhound is being designed to be fully supersonic, compared to the transonic Thrust SSC, which broke the sound barrier by just a few miles per hour. Although having the same amount of thrust, Bloodhound will weigh only 75% of Thrust SSC, with aerodynamic improvements playing a vital role in allowing the vehicle to cut through the air, despite the significant shockwaves that will build up all over the structure.