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AERO AT 1000mph

There is much debate amongst the teams about high-mounted engine air intakes. Some say it causes lift, but the Bloodhound SSC team see it as a benefit to be exploited, utilising the fact it compresses the incoming air being drawn into the engine in much the same way as a supercharger

shave some drag off the car, and increase our understanding of the more complicated regions of the flow field. That said, as any good CFD engineer will tell you, it’s very difficult to exactly match the numbers that one would encounter in reality... but we believe the deltas will match, and any improvements seen in our simulations will be reflected in the real world. More importantly, potentially dangerous shock wave interactions can also be better understood.’

Bloodhound SSC Unlike AI 5R, Bloodhound is both jet and rocket-powered, and has a wider fuselage, reflecting a wider front track dimension. It is also considerably taller because of the high-mounted jet engine and its large intake. The project’s chief of

aerodynamics, Ron Ayers: ‘The secret of designing a trans / supersonic vehicle is to make it as slender as possible and minimise the severity of section changes down the length. In this way you greatly reduce the number and size of shockwaves. The flow under the car should

also be reasonably well behaved, as the underside is parallel and featureless – apart from the wheels... and the v-section creates less blockage.’ Alluding to the large jet

engine intake duct, Ayers continued: ‘Intakes have the job of collecting air for the engine and, in doing this, they are going to create drag. However, this is the wrong way to look at it. The intake pre-compresses the air before the engine’s axial compressor compresses it further still. When heated by combustion, this is what creates the thrust. So the intake, and its drag, is an essential part of the engine! Think of it as acting like a supercharger...’ In relation to the effect

of shock waves fluidising the running surface, Ayers remarked, ‘I used the CFD to check that the pressures on the desert surface are greatly reduced [compared to Thrust SSC]. Experience will show whether I have done enough to prevent the desert from being churned up.’ And in relation to ‘spray drag’, wherein salt and / or dust kicked up by the front wheels becomes entrained with

68 • January 2012

the vehicle at speed, adding significantly to mass and drag, he continued, ‘On Bloodhound SSC the rear suspension is not in the firing line of the spray. Also, the reduced shock waves should reduce spray. We will see!’

Silver Bullet RV1 Silver Bullet RV1, the Bullet Project’s supersonic contender again features rocket power only, but has a wider front track still, and accordingly a rather different body shape. There is a distinctive ‘wasp waist’ aft of the front wheel fairing, a very obvious application of the Whitcomb Area Rule, as confirmed by joint project principal, Paul Noone: ‘In the case of the Silver Bullet RV1, the narrowing of the fuselage behind the front wheels avoids the stack up in cross-sectional area that would otherwise have occurred where the cockpit and canard wings are located. One of the principal aerodynamic challenges is wave drag, and by keeping the cross-sectional area as small as possible and avoiding big jumps in cross section, this can be minimised. However, this has to be balanced against

compromising the dynamic stability of the car. We believe a conventional four-wheel layout is still the best platform. And twin tail fins make a car less sensitive to crosswinds than a single large tail fin.’ The twin tail fins are

essentially employed to aid yaw stability by ensuring the centre of pressure is aft of the c of g, which has been located as far forwards as possible. Various methods of generating downforce are also being considered, including nose shape, hydraulically-adjustable, disc- shaped canard fins and ground effect tunnels under the nose. The channel between the tail fins is also intended to increase downforce at speeds over Mach 1 by using ‘compression lift’, wherein air is forced against the steeply dihedral fins by the bulge created by the rocket propellant housing. Intriguingly, Noone also stated

that ‘spray drag’ would hopefully be avoided by Bullet’s front track being wider than the rest of the fuselage. He did, however, add this cautionary note: ‘If experience shows otherwise

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