Bloodhound – firing the imagination

A wet and and blustery day in Cornwall seems an incongruous setting for a display of cutting edge engineering, but it was here, at RAF St Mawgan in Newquay, that the latest step in the saga of the Bloodhound Supersonic Car was taken. This is because it was here that the rocket that, it is hoped, will propel the car at 1,050mph or Mach 1.4 across the Hakskeen Plain in South Africa was test fired. Developed by Falcon Project, this rocket was to be the biggest fired in the UK for over 20 years. Indeed, it was hard to move for all the statistics being thrown at one on the day. At 4m long and 45.7 cm in diameter and 450kg in weight, Bloodhound's rocket is the largest of its kind ever designed in Europe. Equally, one was assured, the sound made would be the largest man-made sound in the world that day – equivalent to 30 Jumbo jets taking off. For all these claims, however, one thing was repeatedly made clear: this was to be an experiment rather than a demonstration. No-one was sure what was going to happen when the rocket was fired. This was made clear by the Bloodhound's eventual pilot Squadron Leader Andy Green, who said: "This is the first big test of the technology…we've got to push the tech hard to find if there's a weak point. If the rocket blows up after five seconds, then at least we still have five seconds of hard data." The possibility of very public failure was a very real one, but as Green pointed out, this was very much inherent to the nature of the project as a public enterprise. "Not since the Apollo mission has anyone undertaken this level of experimental engineering in public. Normally, people do these things behind closed doors and then invite the press and public in," he said. One of the key reasons for this level of public demonstration, according to project leader Richard Noble, is that Bloodhound is very much designed to be an inspiration to the next generation. In fact, this is at the forefront of the mission as far as the team is concerned. All data relating to the car is being shared with 5,300 schools across the UK, as well as universities and other educational establishments. In fact, the value of Bloodhound as a means of attracting young people into engineering has recently been acknowledged by the MoD in the form of a 'concordat' between the two signed by Defence Minister Philip Dunne and Bloodhound Director Richard Noble, which outlines their commitment to work together to achieve common goals. Naturally, these goals include promoting Science, Technology, Engineering and Mathematics in the UK and raising the profile of Science and Technology in Defence, with Dunne saying: "Bloodhound is an inspirational project that will have a lasting legacy for the UK by inspiring future generations into careers in Science, Technology, Engineering and Maths. These are essential skills to British industry, particularly within the Defence sector, and it is vital that we nurture them." However, any benefits bestowed by Bloodhound in this regard will ultimately be reliant on the success of the technology. And in this regard, the test proved a (literally) roaring success. During the test, which was streamed live to the web, the rocket burned for 10 seconds, generating 14,000lb of thrust – the equivalent of 30-40,000 hp. Sound levels at the rocket nozzle reached 185dB. The term 'hybrid' used to describe this rocket stems from the fact that Bloodhound's rocket combines solid fuel (a synthetic rubber) with a liquid oxidiser (High Test Peroxide, or HTP) reacting with a catalyst (a fine mesh of silver) to produce its power. Although technically demanding, the Bloodhound team believes the approach to be the safest and most controllable option, allowing driver Andy Green to shut off the flow of oxidiser and extinguish the rocket, if required. During the test, the Cosworth F1 engine revved to 16,600 rpm in order to fire HTP into the rocket at a pressure of 820 psi – equivalent to holding a large family car on the palm of your hand and with enough flow to fill a bath in five seconds. Initial results show that the peak thrust of 14,000lbs was achieved with the Cosworth F1 engine at a lower throttle position, delivering 20psi more than the engineers were expecting, giving them even more confidence in the system. The rocket's steady, smooth combustion is the result of a ground breaking Computational Fluid Dynamics (CFD) study that mathematically mapped the burning fuel grain within the rocket chamber. This resulted in a unique, star-shaped rubber fuel grain that produced perfect 'Mach diamonds' in the rocket's plume (Mach diamonds are a formation of standing wave patterns that appear in the supersonic exhaust plume of an aerospace propulsion system). Bloodhound's engineers were able to evaluate the performance of the complete rocket system for the first time, comprising of the Cosworth CA2010 F1 engine, High Test Peroxide oxidiser tank, custom designed gearbox and software and Falcon Hybrid Rocket, designed by 28 year-old self-trained rocketeer Daniel Jubb. But there is more to Bloodhound than just the rocket, of course. The whole project is one where the engineering is operating at the limits of knowledge and performance. Some idea of just the levels of strain under which even the most fundamental components will operate can be seen from the car's wheels, which, by virtue of the fact that they will need to travel faster than any other wheels in history, have to be unique. The 90kg, 900mm diameter solid aluminium wheels will spin up to 177 times per second at top speed, withstanding a load of 50,000 radial G at the rim – in other words, a 1kg weight on the rim will be equivalent to 50 tonnes when the car reaches top speed. Manufactured by Glasgow-based Castle Engineering, they are the product of a three year design study by Bloodhound engineers, Innoval Technology and Lockheed Martin UK. The challenges the team had to overcome included creating a design that would not fly apart when turning 10,200 times per minute and that could be manufactured to incredibly tight tolerances with zero distortion. According to Andy Green: "They illustrate the extraordinary nature of Bloodhound: at the speeds we're aiming for, nothing is straightforward. Even the simplest aspect is challenging. So this is a case of people reinventing the wheel...we had to!" Of course, the wheels also have to be able to provide grip, something that will be in short supply at certain points of the run, as Green explains. "At high speeds, [Bloodhound] will be dominated by aerodynamic forces, while at very low speeds there will be enough grip from the wheels. The problem is at intermediate speeds of around 300-400mph, where the wheels will be sliding all over the place. The wheel grip is at its lowest and the aerodynamic grip is very low, so that's where we'll fire the rocket – maximum acceleration, maximum dynamic changes on the vehicle. That's the bit where understanding which end of the vehicle is sliding and how to correct that – either with steering or power – becomes vital." The aerodynamics, of course, are an area where the supersonic nature of the flow and in particular the presence of shock waves and their interaction with the ground create a unique set of circumstances. Bloodhound will, after all, be the only land-based vehicle in history to travel for a sustained period of time well above the speed of sound. Shock waves form because of the nature of the propagation of sound waves through the air. Under normal subsonic conditions, the object transmits pressure disturbances ahead of it in the form of sound waves. These waves carry the 'information' to air molecules ahead of it that the body is coming and that they need to start moving out of the way. However, when the body itself is travelling at the same speed (sonic) or faster (supersonic) than the speed of these sound waves, it can no longer transmit this information forward and the waves 'bunch up' right in front of the vehicle forming a shockwave and audible 'sonic boom'. The flow properties around the vehicle no longer vary smoothly but change rapidly under these shock waves. An understanding of how these shock waves interact with the vehicle and with the desert surface has been a crucial part of the aerodynamic research for the Bloodhound programme. The aerodynamic behaviour of individual components of Bloodhound SSC has been studied in conjunction with studies of the vehicle as a whole. For instance, a modelling of the flow around the base of the wheels under various configurations has been very important in understanding how they will interact with the desert surface. The overall dimensions of the wheel had been fixed, at the outset, by structural integrity specifications. Aerodynamic considerations were then applied to design of the finer detail of the wheel profile. Analysing stand-alone components, such as intake, duct, or winglets is of value. However, it is not until all of these components fit together, and the full vehicle aerodynamic behaviour analysed, that the team will truly understand how these components interact with each other aerodynamically. The fact that such fundamental design imponderables still exist at this stage of such a massive undertaking only serves to bear out the experimental nature of this engineering project. As Green puts it: "This is experimental science. No-one has ever done this before. Which Apollo unit was going to land on the moon? They decided that during the Apollo programme because they were developing the technology as they went. We're in the same boat."