Chapter 3 - Design background
Concept considerations
The design objective of a Land Speed Record car is to obtain the required performance as simply and safely as possible, while conforming to the rules of the game.
The present rules state that the car must have four wheels, two of them steering. The speed registered is the average over a common mile transversed in both directions and the second run must take place within an hour of the first. This means that the 'measured mile' must lie in the middle of the available track. The existing record must be exceeded by one per cent and we had to beat 622 mph set by Gary Gabelich in "Blue Flame".
Other milestones we could pass were:
- - the fastest British car and driver, Donald Campbell in "Bluebird" at 403 mph.
- - the fastest jet car, "Sonic I", driven by Craig Breedlove at 600 mph.
Top speed is determined by thrust versus drag. The ultimate speed is reached when the drag (aerodynamic plus rolling) builds up to equal the thrust. To reduce drag, the essentials are wrapped in a 'slippery' shape with minimum frontal area. Acceleration is determined by thrust versus weight. The lighter the car the faster the acceleration and the shorter the run-up distance required.
The heart of any Land Speed Record car is the engine. As the car is a "one off' it would be very expensive and unreliable to develop a purpose built unit, so usually an existing and obtainable engine is selected. Generally these are aircraft engines which have the same requirements - powerful, light, reliable.
Jet or rocket
There are benefits in favour of both jet and rocket cars, but often final selection is determined by which the contenders can lay their hands on.
The rocket does not need an air intake, so wheels, fuel, driver and engine can be placed in line, achieving a slim pencil shaped body, only spoiled by having to stick the rear wheels out for stability. The low drag combined with high power weight ratio gives very rapid acceleration enabling a short run to be used. However, a larger proportion of the vehicle weight is fuel so it becomes light and lively when this is consumed, just at top speed! This fuel is also expensive and dangerous to handle. For the rocket to maintain peak speed for an official mile' against the colossal drag encountered near Mach 1, would require drastically Increasing the fuel load. This means a heavier car and reduced acceleration, requiring a longer run-up and yet more fuel. This compound effect may be difficult to overcome, so we can expect pressure to alter the Land Speed Record rules to measure the 'peak speed' over a short distance, in one direction.
The jet car needs to suck in vast quantities of air and the intake for this pushes the other main compounds aside, making for a greater frontal area and drag than the rocket. The result is a heavier, bulkier machine with lower acceleration. However, the fuel weight is not so critical and the vehicle can give a good stable ride, allowing the performance to be fully exploited and held for the statutory mile. The fuel is readily available and easily handled allowing in a quick turn round for the return run.
The battle between the two schools could be closely fought up to the speed of sound. The decision for "Thrust" was clinched by the acquisition of a Rolls-Royce Avon jet engine complete with reheat from a Lightning fighter. This well proven and reliable unit is 25 feet long, weighs 3,700 lb (bare) and produces up to 16,800 lb of thrust in full reheat at which it consumes a gallon of fuel a second.
Configuration
Having selected the engine this and all other vital components such as driver, fuel tanks, wheels and drag chutes, have to be arranged in the best possible layout to suit many Conflicting interests, such as frontal area - safety - visibility - weight'distribution - stability - accessibility and, not least, cost. A convenient way to hold these components together can he provided by a space frame of tubular steel.
This frame acts as a base from which the rest of the car is built up providing datums to work from and attachments for all components and the outside skins.
Safety aspects have to be considered early in the design, as they can clash with those of performance and cost but are nevertheless of paramount importance, insuring the survival of the car toget the record and the driver to enjoy his success.
Safety falls into two classes:
- primary — preventing an accident
- secondary — driver protection in case of an accident
Primary safety is dependant on structural and mechanical integrity and vehicle stability and control. Stability is produced by a combination of aerodynamic shape (weather-cock effect), centre of gravity and ground reaction.
For a straight line car we have followed the 'arrow principle' — weight at the front and "feather'- fins at the rear.
Control of the car is through front-wheel steering (rack and pinion). To keep the wheels firmly on the ground we have all-round independent suspension on.rubber springs which build up their resistance progressively helping to keep ride *height and angle within strict limits under large load variations. This suspension is doubly vital for running on 'solid' wheels without a rubber tyre to iron out the surface irregularities. In the event of loss of control, a 'drag chute' can be deployed to 'straighten up' and slow down the car.
To protect against impact or fire, the driver is isolated in a strong roll cage surrounded by 2 inch thick mineral wool firewalls with stainless steel backing. This cage is located in the centre of the car alongside the engine - the safest area - away from corner impacts. The driver is firmly harnessed into a form-fitting seat of energy absorbing foam, and all controls are simple and easy to reach.
Fuel tanks are of aircraft rubber bag type, housed in aluminium containers and further protected with anti-explosive foam - all inlets and outlets are guarded by shut-off valves.
"Thrust 2"
From the melting pot of all the preceding considerations we have evolved "Thrust 2" in her present form. This is a 27ft long 4 ton 'missile' with wheels, ancillary bays, cockpits and fuel tanks housed in sidebodies either side of the central engine pod.
The driver sits on the right hand side of the engine. His forward visibility is limited but adequate and there is enough bonnet to aim the car. His controls are basic and consist of accelerator pedal, brake pedal and steering wheel with chute release buttons.
A Record car must be right first time, both in concept and detail; it is both prototype and final product. To ease the development it was decided to build "Thrust 2" in two stages:
- UK trials car
- Land Speed Record car
The object of the UK trials was to prove the car up to the limit of 260 mph imposed by available rubber tyres, to gain experience and demonstrate progress. The car was fitted with all the systems required to make her go and stop but without the final bodywork or high speed wheels. These trials on aircraft runways culminated in the British Land Speed Record of 249 mph within the tight confines of a 10,000ft runway at Greenham Common.
The next stage was to transform our rolling "bedstead" into a sleek Land Speed Record contender. This involved fitting a more powerful Mark of Avon (the 302) panelling the body to an aerodynamic shape and making solid high speed wheels.
The aerodynamics had to be investigated. At top speed "Thrust 2" would reach transonic speed when the drag rises dramatically. To predict the effects, transonic windtunnel tests were made up to Mach.86.
Lift and downthrust were also vital to estimate - too much lift and the car would take off, too much downthrust and the wheels are ploughing. This aspect and the underbody airflow was studied in a moving ground tunnel.
Tyres for 650 mph were unobtainable, so we had to design solid wheels specially for running on dry lake beds. These wheels were machined from solid aluminium alloy forgings with different 'treads' front and rear to give understeer handling characteristics. They are considerably lighter than conventional wheels reducing unsprung weight and gyroscopic forces.
When these and all the myriad of lesser jobs were completed, the car would be ready to venture out on to the salt flats and go for the Record, but little did we guess the difficulties that lay ahead.
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