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LAND SPEED RECORD BLOODHOUND SSC BELOW Bloodhound SSC will harness a jet and hybrid rocket in an audacious attempt to raise the existing land speed record by no less than 31 per cent
1,000 MPH! Chris Pickering examines the scientific and engineering challenge behind Bloodhound SSC, a craft designed to obliterate the existing Land Speed Record
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T’S ANOTHER glorious day on Hakskeen Pan, in the north eastern tip of South Africa. The horizon stretches out as far as the eye can see, the sun-baked brown surface meeting the deep blue desert sky for a full 360 degrees. The midday heat is a toasty 35 degrees and the intense light causes the air to shimmer with heat haze. It’s late 2011, and the desert silence is about to be well and truly shattered. The distant rumble of a jet engine shakes the ground as a plume of dust appears on the horizon. At its head, Bloodhound SSC is fast approaching. Over 40 feet of metal, composites and jet fuel, weighing in at six and a half tons, is about to hit 300 mph, but things are only just getting started. Slung underneath the Eurojet EJ-200, which has so far powered the car, is a hybrid rocket
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engine. As it fires, the total thrust more than doubles, accelerating the craft across the desert at over 2.5g. Another mile or so down the road, it breaches the sound barrier, sending a rumble of thunder across the plain. The team pushes on: 800 mph comes and goes... 900 mph... 1,000 mph. The speed peaks at 1,050 mph (roughly Mach 1.4) before first the rocket, and then the jet engine, shut down. Sheer wind resistance, followed by air brakes and a series of two parachutes, slow the car down to around 200 mph, where conventional hydraulic disc brakes take over and bring it to a halt. That may sound like fiction, but if all goes to plan it’s exactly what a small British team hopes to achieve in around two years time. And the deeper you look, the more the project
takes on an air of the surreal. We’re used to hearing supersonic speeds quoted at high altitudes, but down in the comparatively dense air of the African plain it’s a whole different ballgame. Bloodhound SSC won’t just be the fastest car on the planet; it’ll more than likely be the fastest manned craft full stop. Its driver, Wing Commander Andy Green – himself seemingly escaped from a comic book, with a career flying jet fighters, several land speed records to his name and a first in Mathematics from Cambridge – assures us that nothing he’s encountered in his day job would keep up at that altitude. In fact, even the munitions might struggle: at 1,050 mph the car’s projected top speed is already faster than the bullet from a hand gun. It’s not just the velocities that are impressive.
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LAND SPEED RECORD
‘ At top speed, the tips of the wheels (rotating at a not-inconsiderable 10,300 rpm) will be subject to more than 50,000 radial g. Meanwhile the aerodynamic loads on the bodywork are expected to peak at over 12,500 kg/m2. The task of controlling a car at 1.4 times the speed of sound is also pretty mammoth – unchecked, a crosswind of just 5 kph would send it 120 metres off course over the run. Fortunately, with the team planning to clear a straight track 18 km (11.18 miles) long and 1.5 km (0.93 miles) wide, Green will have a certain margin for error. The idea, however, is to run within a few metres of a series of painted lines. Solid wheels on a baked mud surface will leave ruts, so with each run the team will move onto a new set of lines. So why 1,000 mph? Initially the idea for Bloodhound SSC came out of competition. Green and project director Richard Noble (along with various other members of the Bloodhound team) had set the previous record in 1997 with Thrust SSC. So, when much missed American adventurer Steve Fossett declared his intention to mount a challenge with Craig Breedlove’s Spirit of America, they hatched the plan to build a defender. “Instead of using 30-year-old engines and a processor from a derelict tank, which is what happened with Thrust SSC,” Noble recalls,
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Bloodhound SSC won’t just be the fastest car on the planet; it’ll be the fastest manned craft full stop
“we thought, ‘We’ve earned the right to do this properly.’” And it was at this point that he arranged a meeting with Lord Drayson, the UK ‘s Minister of State for Science and Innovation: “Our main objective was actually to talk him into lending us a Eurofighter engine, but there was this awful pause when I thought the whole thing was going to collapse, then he just changed everybody's lives by suggesting we take it into the schools… ‘And by the way,‘ he added, ‘I'm not going to give you any money.’” This turned out to be a master stroke. Not only has it raised awareness of the project itself, it’s also become an instant hit with schools and colleges across the UK and beyond.
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UNIQUE AERO CHALLENGE Perhaps not surprisingly, aerodynamics has tended to dominate the proceedings. Specifically, the team faces the unique challenge of creating a ground vehicle that can operate equally well at subsonic and supersonic speeds, as well as managing the transition between the two. In a nutshell, the problem with supersonic aerodynamics is shock waves. At conventional racing speeds, aerodynamicists can treat air as an incompressible fluid, but above about Mach 0.3 this assumption starts to break down. Put simply, there’s a limit to how fast the air can be pushed out the way of an
ABOVE Bloodhound SSC being modelled in CFD. The experience with Thrust SSC is proving invaluable as the team enters uncharted territory
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LAND SPEED RECORD BLOODHOUND SSC
approaching vehicle. As this limit approaches, the air particles simply don’t have time to fully re-distribute themselves around the vehicle, and instead they end up crammed into a limited volume, which causes a sudden and dramatic rise in pressure. The resulting wave front breaks up downstream in a turbulent wake that creates a lot of drag, and the high pressure area itself can generate unwanted lift if trapped under part of the car. So how do you go about reducing this? Well, broadly speaking, the key to minimising shock waves is to have a long slender profile and a pointed nose, and it’s this which gives rise to Bloodhound SSC’s characteristic shape. “Many of the design evolutions that we’ve gone through have been intended to lessen the strength of the shockwaves and control exactly where they form, so we can manage which surfaces the resulting pressure field is going to act upon,”
ABOVE Andy Green has the unenviable task of controlling the car at over 1,000 mph. The V12 engine from MCT shown here will drive the fuel pump and power the ancillaries
a relatively conventional double wishbone suspension. The limited dimensions did present somewhat of a challenge in the kinematics, however. “It's important to have as little camber change on lock as possible and no camber change in bump,” comments Bloodhound SSC’s engineering director John Piper. “The gyroscopic effects of two wheels, each weighing 137 kg and rotating at over 10,000 rpm, are huge. “Currently we have 8 degrees king pin inclination, 6 degrees of caster, 2 mm ground offset, and 100 mm of trail. The trail is quite a bit higher than you would see in a road car as this would normally give a high steering load, so to counteract this load we have a steering ratio of 30:1.” Fortunately, there are no sharp corners to negotiate on the record run, so the steering lock can be restricted to just 5 degrees, but, even with a 30:1 steering ratio, the gyroscopic loads in a conventional rack and pinion steering system would be too high
BELOW The long slender profile and pointed nose is the key to minimising shock waves
explains the team’s CFD engineer Ben Evans. “One of the key areas is the back end of the car, where you’ve got two big wheels stuck out in the airflow, which generate strong shockwaves. You have to make sure that they interact positively with the car and don’t try and pick it up.” In order to tackle the procedure of modelling the car, Evans and the team break the design down into reduced models, such as the front wheels, the winglets used to trim the aerodynamic forces, and the jet intake. With each design iteration these were optimised in isolation and then fed into the full car model to check that they still work in the overall system. But what of the fundamental task of CFD modelling a supersonic car? It is, to some extent, unknown territory but the team can claim to be the world’s leading experts on supersonic land vehicles, thanks to experience gained with Thrust SSC. “It means we’re the only people
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in the world with a validated CFD model of a supersonic car,” says Evans, ”and although exact details are hard to predict we’re confident on the basic issues.” From its pointed snout, the bodywork expands just far enough to accommodate the two front wheels. The team had originally pushed for staggered front wheels to reduce
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for Green to control. To solve this, the team designed a worm gear system, which has no feedback. In order to provide some degree of information to Green the team is currently working on a system using a servo motor attached to the steering column to apply some artificial feedback. The front suspension is attached to a metal
The car’s projected top speed is faster than the bullet from a hand gun
the area even further, but the resulting asymmetry proved hard to overcome and parallel wheels were adopted for the sake of safety. As it is, a front track of just 1 metre is still miniscule for a vehicle 12.8 metres long, with an 8.9-metre wheelbase. Yet, within this space, Bloodhound SSC manages to package
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subframe, which, in turn, bolts onto the carbon composite monocoque and provides the mid section of the structure. This is effectively a twin structure, with a stressed shell on the outside and a second composite assembly inside forming the cockpit’s safety cell. Overall, it’s still not entirely unlike the
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LAND SPEED RECORD BLOODHOUND SSC
technology used in a formula car. From there back, however, things start to get a little different. From the outset the team had been keen to use rocket propulsion. Historically this has come in one of two forms: solid fuel rockets, with a premixed block of fuel and oxidiser, are comparatively simple and reliable, but virtually impossible to regulate once ignited; liquidfuelled rockets, meanwhile, with separate supplies of fuel and oxidising agent pumped independently into the combustion chamber, offer more sophistication but carry a higher safety risk. The solution was to use what’s known as a hybrid rocket. This takes its fuel from a solid – in this case rubber – and pumps in a liquid oxidiser – here hydrogen test peroxide (or HTP) – to create combustion. It produces a rocket that’s comparatively safe to handle, easy to shut down and yet still relatively straightforward to engineer (providing you happen to be a rocket scientist, of course). ROCKET SCIENCE Although originally conceived as a pure rocket design, the challenge of precisely controlling a single large rocket engine – even a hybrid – meant it was rapidly dropped. Instead the team decided to go with a smaller rocket mounted above a Eurojet EJ200 jet engine. But the changes were just beginning. Initially, the plan was to use twin air intakes for the jet, positioned either side so a structural beam could be led down the centre of the car, maximising rigidity. However, this made it far more difficult to achieve the required flow into the engine and the aerodynamicists promptly vetoed the idea. As the project progressed the thrust requirements for the rocket got bigger and bigger. This meant the size (and hence weight) of the unit began to increase as well. Consequently, the vehicle dynamics benefit that came from placing the heavier jet engine below the rocket started to shrink. What’s more, even with a single duct, engineering the flow of a low mounted intake was proving difficult. The sheer thrust of the enlarged rocket – now at 27,500 lbs – also stood to generate a considerable pitching moment, placed so high up on the design. And so, in late 2009, the decision was taken to swap the two units round.
ABOVE A CAD section of the projected record breaker. The front of the car, which features a carbon safety cell for the pilot, is the nearest thing to conventional motorsport technology – apart from a ton of HTP! BELOW Flow patterns around the side-by-side front wheel configuration that was adopted after initial ideas for a staggered layout had been discarded
ABOVE The rearmost section is exposed to some of the highest loads in the car
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LAND SPEED RECORD BLOODHOUND SSC
LEFT The geometry of the nose, cockpit canopy and intake duct (far left), allied to the sci-fi style rear wheel covers, give Bloodhound SSC a distinctive appearance
The rocket’s thrust line now runs pretty much through the centre of gravity; the jet doesn’t have this luxury, but unlike the rocket (which engages quite violently) it can be throttled far more progressively. V12 RACE ENGINE That’s not to say the rocket doesn’t have its challenges. It consumes no less than a ton of HTP during the run, which comes from a tank located in the rear section of the monocoque. Never known to do things by halves, the Bloodhound team plans to employ a 750 bhp V12 race engine to drive the fuel pump. The
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We’re the only people in the world with a validated CFD model of a supersonic car
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4.2-litre Menard Competition Technologies (MCT) unit will need to empty the tank in a little over 17 seconds, as well as powering various ancillaries, such as the steering system and actuators for the winglets. The V12 and its pump are mounted on a metal spaceframe behind the monocoque. This is assembled in two halves, allowing easy access to the EJ200, which is secured to the top half of the frame using mounts specifically designed to mirror the behaviour of those on the original aircraft. This arrangement means
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the top frame can simply be lifted complete with the EJ200 for engine changes. The rocket, meanwhile, is supported by a structural tube that runs along the lower half of the frame. A structural flange, located at the nozzle end of the structural tube, channels the thrust straight into a final onepiece spaceframe at the rear of the car. “You want to take all the thrust out next to the nozzle,” comments senior mechanical design engineer Mark Chapman. “If you can imagine a long tube, with 27,500 lbs of thrust produced at one end and the reaction force at the other, it’d be like standing on a Coke can. To avoid collapsing it we take the
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entire load out at the beginning.” This rearmost section of the structure is exposed to some of the highest loads on the car. As well as the thrust from the rocket and the parachute anchoring points, the aerodynamic loads from the fin act onto it. It also provides the mounting points for the rear suspension; again double wishbones, but this time mounted externally and connected via pull rods to inboard springs and dampers. This section’s proximity to the nozzles means it also has to cope with much greater heat, as well as
the risk of acoustic fatigue from the sheer sound pressure level of 200+ dB. And so, as we roll into 2010, the Bloodhound SSC Project is well underway. The final design is expected to be frozen in the spring, after which manufacturing starts in earnest. All being well, the car is expected to make its debut in May 2012. The limiting factor, as ever, is money. “Financing the car has been tricky at times, but we have a tremendous amount of grassroots support,” explains Noble. “We've got to make £6.3 million over the next 20 months to get the car out of the door. In order to generate revenue there's a supporters club, and amongst other things members of the public can buy a space for their name on the fin. These have deliberately been placed at a low level (£10) to make it accessible to everyone and there are 333,000 spaces, with around a thousand signed up already.” The target of 1,000 mph was chosen because it’s such an awe-inspiring figure that people can’t help but take note of the science and engineering behind it. When Bloodhound SSC captures the record it’ll cement its place in history, but in a way it’s already achieved half its objective. Noble points out that the US space program spawned a massive rise in PhD applications, thanks to what became known as The Apollo Effect. With 2,500 schools and colleges signed up and the number still rising, it seems The Bloodhound Effect is already in evidence. RT
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