Designing any car usually involves compromises of one sort or another and in that sense, although the rules allow almost unlimited freedom of choice, record-breakers are no different. As Richard said in the foreword quoted on the previous page, the most powerful engine available is a good starting point, but what about wheel size and layout, weight, fuel load, driver location and the surface on which the completed projectile which take a shot at writing it's name into the record books? No matter where you start in any of these areas, an optimum solution for one major aspect of the design usually leads to a less than perfect solution in at least one of the others. Such is the lot of record-car designers.

Let's try and follow the thought processes that led to the design that finally emerged as Thrust SSC. And since Richard mentioned engines, then that's as good a place as any to start. From the very earliest days of record-breaking the most affordable and reliable way to secure the abundant power necessary has been to use an aero-engine. Very few engines have been designed specifically for record-breaking since the time in 1910 when Fiat installed a massive 28 litre airship engine in a car and set a trend that continues to this day. When piston-engined, wheel-driven cars gave way to thrust, two distinct camps appeared, those who favoured rockets and those who favoured jets.

In 1970 Gary Gabelich set a rocket-propelled record of 630.388mph for the kilometre in Blue Flame and ushered in a design concept that many felt pointed the way forward towards the Sound Barrier. The advantages of rockets are enormous in that the engine itself is a model of simplicity and lightness compared to a jet and since it does not need an air-intake to feed it's chemical reaction, the cross sectional area of the car is limited only by the width of the driver's body. Thus the concept of a long needle nosed, lightweight projectile with closely couled front wheels and outrigged rear wheels was born.

But for all their many benefits, rockets also have enough drawbacks to give any potential LSR aspirant many a sleepless night. For a start their appetite for fuel is gargantuan, which means that most of the given length of the vehicle will comprise fuel tankage. As speeds increase, so more and more fuel is needed, which in turn increases weight, which in turn means that more fuel is needed, which means..........and so on and so on. Another unwelcome by-product of the rocket's fuel thirst is the dramatic change in weight from full to empty in a few seconds, a weight change likely to make a rocket-car more unstable rather than more stable the faster it gets. All of which leads to the inevitable conclusion that jets are an altogether more feasible proposition.

With that decision made, the next step was to calculate the sort of power that would be needed to travel ground level supersonic. This is where the first of the trade-offs occurs in that one big engine in order to reduce cross-sectional area would be ideal. However, that means that the driver either has to be located way out in front of the engine or to one side as with Thrust 2. Option one was out not only for reasons of safety, but also because it is very difficult for the driver to sense changes in pitch and yaw sitting that far forward. Option two was also a non-starter this time because it would mean a major increase in cross-sectional area and therefore drag.

The answer when it came was straightforward: follow the maxim of "too much power is just enough" by using two engines and then mount the driver between them. This central driving location in the middle of the car gives good feel for what's going on and if built as a single integral unit with the engines, provides the best possible level of secondary safety for the driver in the event of an incident. Although secondary safety is vital, it is far more important to place the emphasis on primary safety by designing a vehicle that runs straight and true in the first place without any tendency to lift it's nose at speed.

The way this is achieved for Thrust SSC is to locate most of the weight - that is the engines - towards the front of the car and then use a tailplane assembly mounted right at the rear to exert enough aerodynamic drag to pull the car straight if it strays off-line. The combined effects of the centre of gravity towards the front of the vehicle and the aerodynamic drag exerted on the tailplane assemply at the rear, gives the design the same sort of basic stability that ensures that a dart hits the dartboard pointy bit first! The principle is exactly the same.

Fixed front wheels on either side of the engine pods give Thrust SSC a stable track, while the offset, tandem, steered rear wheels mounted in the narrow chassis section at the back keep them well out of the way of the engines' exhaust and dispense with the need for any sort of outrigging device on which to mount them.

The final piece of the design jigsaw are the wheels themselves. Hard-won experience gained running Thrust 2 at Bonneville and Black Rock on solid aluminium wheels provided a fund of data that proved invaluable when Glynne Bowsher sat down to pen those to be used this time. I well remember a conversation with Bluebird designer Ken Norris when he explained how easily people forget that total drag is the sum of aerodynamic drag exerted on the vehicle itself and the rolling drag caused by the contact between the wheels and the track surface. In many cases rolling drag can account for 50% of the total drag.

The trick, of course, is to design a wheel that will give sufficient grip to exert control but not dig into the surface enough to incur an unacceptable drag penalty. This a highly complex area not totally governed by the design of the wheel, since the weight of the vehicle itself and the downforce needed to keep it on the ground also tends to push the wheels into the surface. Conversely, at speed, air rushing under the car produces lift and effectively lightens loads causing the wheels to plane across the surface, thus reducing drag - that is if you get it right! As with every other aspect of Thrust SSC the team are confident that they have got it right.

The acid test comes when the big black car runs for the first time, but if the phrase "if it looks right, it is right" ever applied to anything then surely it applies to Thrust SSC, the car that literally should be faster than a speeding bullet.

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