Thrust SSC - Media Zone

Thrust SSC
Wheels of Fortune

Art Arfons' Green Monster was approaching a terminal velocity of 610mph as he sped down the long black line painted across the vast white expanse of salt flat at Bonneville on November 17, 1966. A successful challenge to Craig Breedlove's existing land speed record of 600.601mph (966.547km) seemed imminent.

Suddenly, Arfons felt the Monster twitch to the left, and instantly applied a small correction. Then it twitched to the right. This time he couldn't control it, and in a moment the 22 foot (6.7 metre) long green and red missile, little more than a jet engine on wheels with a rudimentary cockpit slung either side, was tumbling end over end down the salt. It came to rest a crushed and battered hulk which had disintegrated over a distance of more than four and a half miles (seven and a quarter kilometres).

Amazingly, 40 year-old Arfons himself was still alive in the remains of the cockpit on the left hand side of the General Electric J79 turbojet engine. Even more incredibly, he was unhurt. He was the world's fastest survivor, for no man before or since has ever been able to walk away from a crash that had occurred at the cruising speed of a Jumbo jet.

No accident in the lengthy and illustrious history of land speed record breaking has illustrated more graphically the crucial role of lubrication in a high-speed projectile. That incident had a profound effect on all who have attempted the record since, for it was triggered by seizure of one of the most relatively simple parts of a car: a wheel bearing. As the bearing in the offside front wheel destroyed its lubricant, it was another reminder that, though bearings might seem almost insignificant, they are one of the most vital components. So too is the lubrication whose role is to ensure that they keep within defined operating temperatures. The pure geography of land speed record breaking - the amount of space that nature can ever put at man's disposal for his attempts - will always be one of the greatest influences on ultimate speed. But every bit as important as one of the limiting factors to higher velocities is the rotational speed of a car's wheels. This is itself limited by metallurgy and the ability of man to create lubricants that can withstand the sort of friction, and therefore heat, generated at speeds of over 600mph (965kmh).

At its maximum speed of 650.88mph (1047.46lkmh), achieved during its record run of 633.468 (1019.440) across the Black Rock Desert in 1983, the 30 inch (76.2 centimetre) solid aluminium alloy wheels on Castrol-sponsored Thrust 2 rotated at almost 8000rpm. That's faster than engines rev in most road cars, so it's easy to appreciate just how crucial good lubrication is. Failure of the lubricant, which in turn would destroy the wheel bearings, must be avoided at all costs.

Thrust 2 used compact taper roller bearings capable of withstanding high loads, but the very high speed at which they turned presented a problem because traditional grease would be thrown away from the bearing faces by centrifugal force, the force which tries to tear apart objects rotating quickly. Oil would churn round and become frothy. Either way, the lubricant's efficiency and performance would be dangerously compromised. A new synthetic grease developed from the Concorde supersonic airliner technology was employed, and surprising though it may seem, just a smear proved sufficient. Throughout its life, Thrust 2's wheel hubs and bearings never gave cause for a moment's concern. Test showed that they could be run at 7000rpm for 15 minutes without their temperature ever exceeding 122 degrees F (60 degrees C).

Now, Thrust SSC's design team faces an even greater challenge with the supersonic car's wheels and bearings and, naturally, they have turned to Castrol for technical expertise and guidance.

Everything on the new car is placed under greater pressure, because of the significantly higher speed range in which the vehicle will operate. At its envisaged maximum of 850mph (1368kmh), Thrust SSC's aluminium alloy wheels will rotate at 8500rpm, which means they must be tested beforehand on a dynamometer rig at 9500. 'That,' says design engineer Ron Ayers with notable understatement, 'is pretty fast!'

The new wheels are the responsibility of Glynne Bowsher, whose careful use of a computer- ised technique called finite element analysis has resulted in an unusual contour which has substantially reduced the peak stress they will have to endure. 'They may look like they were designed by Fred Flintstone' says Ayers, 'but they are in reality very clever. And Glynne has managed to lower the stress imposed on them so that it is now well below the limit for aluminium alloy.' Nevertheless, the wheels could be subjected to forces in the region of 44,800 pounds per square inch (3,149 kilogrammes per square centimetre).

Working in close collaboration with Bowsher and SKF, Castrol is developing a lubricant that will ensure that Thrust SSC's wheels rotate without any risk of excessive heat build-up. Centrifugal force increases as the square of speed. That means that it is 50 per cent higher at 500mph (804kph) than it is at 400 (643). Thus the special Castrol lubricant used in Thrust SSC must face an even more demanding operational task than that used in Thrust 2.

Complementing Andy Green's peace of mind as he rockets across the Black Rock Desert in Nevada is the knowledge that Castrol lubricants will also be maintaining the efficiency and peak performance of each of the 250001b (11340kg) thrust Rolls-Royce Spey turbojet engines.

In the final analysis the integrity of the wheels, bearings and lubrication will underpin the entire Thrust SSC venture. And while the development of such critical new technology is essential for record breakers, it also generates data, experience and further understanding that is also of direct benefit to a much wider series of applications throughout the world of engineering.

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