By Ron Ayers
The World’s Air Speed Record for low altitudes is 994 mph. We are proposing to exceed that speed on land. It is a measure of the challenge we are taking on. Apart from the difficulty of achieving the performance, discussed elsewhere on this website, the engineering challenges are many and varied. For instance, the dynamic air pressure at maximum speed is in the order of 12 tonnes per square metre. Thus the structure must be incredibly strong and rigid.
The wheels must cope with stresses corresponding to a radial acceleration of 50,000g. They must also be robust enough to withstand stone impact at over 1000 mph. The design of these wheels will push current materials technology to its limits.
The aerodynamic forces could easily lift the car off the ground or could as easily crush the suspension, so very precise control of these vertical forces is essential.
The static and dynamic stability of the car will enter new and uncharted regions, and we are employing skills from automobile engineers and from aircraft engineers to tackle them. When the car is travelling at low speeds, gravity is the dominating force. But aerodynamic forces increase in proportion to the square of the vehicle speed while gravity remains constant, so by 1000 mph, the aerodynamic forces are the dominating ones. For instance, when the front wheels are steered, the aerodynamic sideforce they experience will be much greater than the sideforce from wheel/ground interaction. At intermediate speeds the gravitational and aerodynamic forces may be comparable. The gyroscopic effect of steering wheels at these high speeds must also be considered. From the above it is clear that the factors influencing stability and control are complex and constantly changing.
In order to achieve the necessary vehicle acceleration, we are developing a large hybrid rocket motor (that is, one with solid propellent and liquid oxidant) that is powerful, efficient and safe. It will probably be the largest and most powerful hybrid rocket ever built in this country. To achieve this we are conducting an extensive test firing programme supported by a theoretical analysis of the complex combustion processes implicit in a hybrid rocket.
When running Thrust SSC we encountered severe problems when shock waves from the vehicle penetrated the desert surface and ‘fluidised’ it – making the surface nearly impossible for the wheels to run on. For BLOODHOUND SSC we are using CFD to design the underside of the car to minimise the damage to the ground. Time will tell us how well we have succeeded!
Consideration must also be given to the problem of driving the car. Assuming a total track length of 10 miles, the driver will accelerate at an average of more than 1.5g, then traverse the measured mile in less than 3.6 seconds before averaging more than 1.5 g during deceleration. He must then repeat the procedure in the reverse direction within one hour. The peak accelerations and decelerations could well exceed 3g.
The turn-round procedure for such a complex car will itself be a challenge. At the end of the first of a pair of runs, we must turn the car round and position it at the start position for its second run, refuel and replenish the oxidant, check on-board computer data, replace parachutes, complete visual inspection of the car and carry out count-down procedure.
The above are just some of the problems we are tackling. In all of them we are making extensive use of computer modelling. But those are just the problems we currently know about. During the development and testing programme we will undoubtedly find more. In addition to being an Engineering Adventure it is also an Engineering Exploration.