X-ray spec -- Thunderbolt

ThrustSSC's designers discuss the lost record car of George Eyston with Keith Howard

You'd think that someone who had broken the Land Speed Record for Britain two years in succession, 1937-38, raising Sir Malcolm Campbell's mark by 19 per cent in the process, would have ensured himself a place in public affections alongside the greats of this country's speed-seekers. But the exploits of Captain George Edward Thomas Eyston and the massive Thunderbolt — the heaviest car ever to hold the record prior to ThrustSSC — have somehow been lost to the collective consciousness. Although Eyston is still revered by aficionados like Ron Ayers and Glynne Bowsher of the ThrustSSC project, who comment on his car here, on the street Eyston's name has little currency

What makes this all the more surprising is that Eyston didn't 'merely' drive Thunderbolt, he took the lead in designing it. It was assembled in a mere six weeks in 1937 and, without even testing in the UK, shipped over to Bonneville, where on its very first run it recorded a flying mile speed in one direction of almost 310mph, over 9mph ahead of Campbell's existing mark — an astonishing testament to Eyston's mix of self-confidence and bravery. On the return run the car suffered a problem with its clutch, at which Eyston coolly sat down, redesigned it and had the parts manufactured in California, allowing him to set new mile and kilometre records of 311.42 and 312.20mph some weeks later. In the interim he took his second car, the front-wheel-drive Speed of the Wind, on to the salt flat's circular course to set a new 12-hour record of 163.3mph.

A year later, in a titanic struggle with John Cobb's much subtler Railton Mobil Special, Eyston made running changes to Thunderbolt — removing the radiator and tail fin — to keep it just ahead of its four-wheel-drive competitor, raising the mile record to 357.50mph. And he might have done still better, perhaps even challenging Cobb's 369.74mph of the following year, but for a spectacular rear wheel failure, reportedly at over 360mph, which meant the end of Thunderbolt's career.

One reason for this stirring chapter in Britain's LSR history being forgotten is perhaps that the car was badly damaged in a fire in New Zealand, so doesn't wow the crowds today in any of our motor museums. Unless Thunderbolt's remains really can be exhumed from an old Wellington rubbish dump as hoped, all that now exists of it are the two Rolls-Royce R engines which once powered it: one (R25) in the RAF Museum, Hendon and the other (R27) in the Science Museum in Kensington.

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Information about Thunderbolt is nothing like as rich as for Cobb's Railton-Napier. Contemporary sources say it weighed almost seven tons, but not how that was distributed.

Bowsher: They called Thunderbolt rear-engined but only because the engines are behind the driver: relative to the wheels the engines are pretty central. I would guess that the weight distribution ended up something like 45/55 per cent front/rear." Which goes to explain one of the car's striking features: four steerable wheels -- a complication Eyston would surely had resisted had he been able.

"The tyre technology was right at the limit for the time: after a single run they would change all the wheels and tyres. Eyston needed four tyres at the rear for traction. If the weight was much the same at the front as at the back this would have forced him into using four wheels there too. Even with only 40 per cent of that great weight over the front, I suspect tyre limitations would not have allowed him to use just two."

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Ayers: "Eyston hoped for a maximum speed of 380mph and bemoaned the fact that he did not achieve 360mph, as that would have made him the first to travel at more than six miles a minute. All record-breakers are surprised when they don't achieve their target speed (except Reid Railton, who had an uncanny knack of succeeding -- but he was a genius). There are two main reasons. One, each vehicle is a prototype. Did you ever encounter a first prototype that achieves its design spec? Eyston certainly had his share of teething troubles, like clutch and carburetor problems. Two, people underestimate tyre rolling resistance. The rule of thumb used by pre-WWII Brooklands engineers was that rolling resistance was 100lb per ton weight, equivalent to a rolling resistance coefficient of about 0.044. Having analysed the performance of many record cars I know that rolling resistance is dependent on speed, and that by 350mph on salt or mud flats the coefficient has gone up by about 0.12. At that speed it would account for as much as 38 per cent of Thunderbolt's total drag."

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Developed for Schneider Trophy aircraft, Rolls-Royce's V12 39-litre 2500hp R engine "was probably the most powerful engine available anywhere," says Bowsher. But given that Eyston needed two of them, was it the right decision to use them side by side ? "For the alternative layout you have to look back to Kaye Don's Silver Bullet, built by Sunbeam. It had two engines in tandem, so it was a much slimmer vehicle, but they still drove only the rear wheels.

But the R engines are much bigger and heavier, so the beam strength of Thunderbolt's chassis would have had to be enormous to use them this way. If I put myself in Eyston's position I would have known that the safer engineering concept was to put them side by side, despite the increase in cross-section."

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Ayers: "I can find no evidence that Eyston tested Thunderbolt's shape in a wind tunnel. He did tests in the Vickers tunnel at Weybridge in 1934 and '35. No pictures or model descriptions now exist but the date of the tests, plus certain details mentioned, lead me to suspect that he was testing his other car, Speed of the Wind. That would have given him a clear idea of what geometric features would minimize drag and provide stability. He also consulted Jean Andreau, a French aerodynamicist. The car was designed and built so quickly that I imagine there wasn't much time for research, and Eyston appears to have been a great hands-on, intuitive, engineer, happy to back his own judgement. If anyone has evidence of wind tunnel tests, I would like to see it."

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So what was Thunderbolt's likely aero performance? Ayers: "With eight wheels nearly 8in wide, and a large air intake, the drag would be high. By comparing with other record-breaking vehicles, plus a few sums of my own, I believe the CdA (drag coefficient times frontal area) would have been about 11 sq ft. This would give drag at 300mph of about 1 ton. For the 1938 runs Eyston reduced the car's weight from 6.85 tons to 'about 6 tons', and for his last run he removed the fin and created a smooth round nose by replacing the radiator with an enlarged cooling tank. My estimate for the CdA for this version is 9 sq ft." If Ron's figures are correct, Thunderbolt will have needed 2230hp just to cleave the air at 311.4mph in '37, and 2760hp for 357.5mph in '38 -- ample evidence, with rolling resistance still to be accounted for, that he needed both those mighty Rolls-Royce engines.

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Bowsher: "This is a staggering car, not the least because it lacked all the progressive development that Railton had had with the Bluebirds. Cobb's car was an extension of that, but this one came from nowhere. Campbell's last Bluebird was beautifully streamlined and very efficient, so where do you go next? To achieve 357mph, as Thunderbolt did, Bluebird would have required 67 per cent power than for Campbell's 301mph record. So you have to use two engines. But Thunderbolt had a bigger cross-sectional area as a result, so its drag factor would rise: it had eight tyres, so tyre drag was also increased; and it was a heavier vehicle. So I reckon Eyston was probably just as efficient at using the power available to him as Railton and Campbell had been at 301mph. That's an amazing achievement."

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Ayers: "Although Eyston removed the tail fin for his fastest runs in 1938, I would say the car needed it. He had enough power to spin the wheels at over 300mph, so any unexpected happening could have started a spin. Lee Bible's accident (in the Triplex Special), for instance, was reputably because he took his foot off the power too suddenly. Eyston may have reasoned that, with a wide track and low centre of gravity, if he lost control he would slide rather than roll, but wheel gyroscopic forces make that a dangerous assumption. Perhaps he was just exhibiting the attitude of the 1920s and '30s epitomized by Henry Segrave's reply when asked what made the British so good at the Land Speed Record" "It's because we are prepared to keep our foot down and keep it there."

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Our thanks to Mike Stanton of the Speed Record Club for his help with this feature.