X-ray spec: Mobil Special

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Two men who thrust SSC through the sound barrier have spent hours in quiet contemplation of Reid Railton’s masterpiece. By Keith Howard

It was a bit early for flying saucer fever, but when John Cobb’s land speed record car, designed by Reid Railton, first took to the Bonneville salt in 1938, onlookers might have been forgiven for thinking aliens had landed. It was unlike anything ever seen before, with or without wheels.

Shaped like an elongated teardrop in plan and elevation, it was the most perfectly streamlined LSR car ever made. But it was much more than an unworldly form intended to cleave the air with unprecedented low aerodynamic drag. In its overall conception and execution it was the distillation of everything Railton had learnt since first working under Parry Thomas at the close of WWI.

Cobb wasn’t the only British contender at Bonneville that year; Capt George Eyston, who the previous year had beaten Malcolm Campbell’s existing record of 301.13mph, had returned with his behemoth Thunderbolt, which he knew could go faster. It and the Railton car could hardly have been more different, and bald statistics appeared to show that Eyston’s held the aces. Whereas the two supercharged broad-arrow Napier Lion engines in the Railton generated around 2500hp between them, the twin Rolls-Royce R-Type V12s in Thunderbolt delivered more like 6000hp. If sheer power were everything then Cobb was clearly outgunned.

But Railton’s car was an exemplar of efficiency, of carefully considered and meticulously realised design decisions. Although it had less power at its disposal, the shortfall was offset in various ways, of which aerodynamic superiority was only one. Railton had equipped the car with four-wheel drive by the novel expedient of using the two engines side by side, one driving the front wheels and the other the rears. Cobb was thereby assured a significant traction advantage over the rear-wheel-drive Thunderbolt. Careful control of the car’s all-up weight — about 7050Ib dry — also meant that four tyres were sufficient, whereas Thunderbolt, tipping the scales at around 15,340lb dry, required eight. So, along with its lower aerodynamic drag, Cobb’s car also had significantly less rolling resistance.

The proof of Railton’s superior design choices is recorded in the LSR history books, although success was not immediate or easy. Eyston enjoyed the initial glory in late August when he increased the record to 345.49mph. Less than three weeks later he was eclipsed when Cobb recorded 350.20mph. But in one of the most famous head-to-heads in LSR history, Eyston responded with 357.50mph, sealing the issue for that year. Not until 1939 was Cobb able to put his car’s superiority beyond doubt with a figure of 368.85mph — a slim advantage that was built into a crushing one when the car was wheeled out again in 1947, basically unchanged, to achieve 394.196mph, a wheel-driven record that would stand for 17 years.

Railton died in his adopted California in 1977, aged 82. So to assess his design — the one he declared himself most proud of in a lifetime of high achievement — we turned to two of the principal contributors to Thrust SSC: Ron Ayers, the project’s aerodynamicist, and Glynne Bowsher, its mechanical designer. Both are avowed fans of Railton’s unique combination of flair and meticulousness — there seems to be nothing he didn’t consider, and nothing for which he could not conceive a novel solution.

The pity is that much of the paperwork documenting the development of the car appears to have been lost. Ayers — who has followed the trail from Brooklands to the National Physical Laboratory, where the car was wind tunnel tested, to the Royal Aircraft Establishment and National Maritime Institute, where the NPL records were transferred when its aerodynamics department closed — has failed to find key documents, which may well have been ‘culled’. If anyone knows differently, he’s anxious to hear from them.

X-ray spec: Mobile Special

One of the most controversial elements of Railton’s aerodynamic design was his eschewal of a tail fin. Ayers: “I don’t know why Railton made the decision not to use one. Obviously, if you put a fin on you’re going to increase the drag. If you control your downforce sufficiently such that you’ve got download on all four wheels you can absorb quite a lot of your instability, but if you are at the friction limit of your tyres as a result of traction forces then you lose that stability, so it’s a bit of a balance. The side force on this shape would be quite low in a crosswind, so that although it’s unstable it would not be subject to a big destabilising force. I’m guessing that he was relying on that. He’s saying: ‘I can keep the yawing moment down by limiting the side force’.”

 

Although a large vacuum-operated air brake was fitted, it was never used and duly removed. The slot for it remains and is highlighted here. Ayers: “Reid would be aware that when you are far from home, having spent a fortune to get there, you have to make it work. If in doubt you play safe, and he no doubt fitted the air brake to be on the safe side. After all, there are two big unknowns he had to consider: he would be operating wheel brakes at a speed never tried before, and the rolling resistance was uncertain. In fact, it increases rapidly at high speeds, so that the rolling drag at 400mph was nearly as big as the aerodynamic drag. That would be a problem while accelerating but a pleasant bonus when slowing, so he could have found that the brake was unnecessary.”

 

To eliminate the aerodynamic drag penalty of a radiator, Railton’s design used a 75-gallon tank of iced water to cool the engines and transmission brakes. Originally positioned behind the front-left wheel, it was moved astern — perhaps to improve traction —when the air brake was removed. Ayers: “The idea of having no radiator was not new. I’ve found a report, dated June 1931, from the Vickers wind tunnel, where they tried a version with a smoothed-in nose on the Napier-engined Bluebird. It knocked the drag down by about 15 percent — useful but not dramatic. Around that time the Rolls-Royce engine came along, making a much bigger difference, so the idea was dropped. But Railton was playing with the concept back then —and a supplementary cooling system of this sort had already been used on Golden Arrow.”

 

To obviate an engine stall problem on gearchanges, Railton eventually had to add a clever anti-stall mechanism driven from each engine’s propshaft. Bowsher: “While I’ve praised Railton for having a quick solution to this, it was probably covering up a mistake. The engine flywheels had been left off to save weight, but these are the energy store that helps prevent the engine from running down during gearchanges. So it would seem that Railton’s idea of a supplementary drive back to the engine, running below engine speed and effectively de-clutched by the use of a freewheel device, simply replaced the flywheel that had been omitted in the first place. Perhaps Railton won anyway because each assembly would still be lighter than a flywheel!”

 

You might imagine that the ‘icthyoid’ shape of the car was chosen because it most closely resembled the aerodynamic optimum of a teardrop, but Ayers’ research has revealed a more complex story: “Brooklands was a hi-tech business park in the 1920s and ’30s. All the great people of aviation and motorsport worked there: so Railton had access to Captain Irving, designer of the Golden Arrow, Rex Pearson, legendary chief designer of Vickers-Armstrong who used to do all the wind tunnel tests for him, and he also spoke with R J Mitchell at Vickers Supermarine. Mitchell was an obvious person to ask because he was the only one in the world who’d designed something faster – the Schneider Trophy aircraft. What seems to have happened is that they all proposed their own shape. Hence five models were made and wind tunnel tested. There are no pictures, no drawings of them, but Thomson & Taylor gave them quite evocative names: cigar, three-wheeler (which is baffling as that wouldn’t have been legal), BB with troughs (BB is clearly Bluebird; troughs I interpret as making it look like Golden Arrow), long tail (which I can’t quite envisage) and bun. The latter was chosen because although it did not have the smallest frontal area it had by far the smallest surface area and, therefore, the lowest skin friction.”

 

Cobb’s forward driving position was positively bus-like – ahead of the front axle and with an almost horizontal steering wheel – but Bowsher doubts this hampered the car’s controllability: “In Thrust SSC Andy Green was put within a small distance of the car’s centre of gravity and Richard Noble’s comment was that he was in exactly the right place for the best feel of the vehicle. Whereas Andy, being a pilot, said no, I’d rather be up in the front so I can feel what’s going on. The further away you are from the centre of turning, the greater the accelerations on your body and the greater the seat-of-pants feel. So my own feeling is that Cobb was probably in a very good position. I’d love to drive a vehicle like that to find out.”

 

Although there are superficial similarities between the Railton car and the later Bluebird CN7, there are also major differences. Such was the structural strength of CN7 that Donald Campbell survived a major accident at around 350mph — an event which would surely have reduced Cobb’s car, the bodywork of which lifted off for easy serviceability, to a kit of parts. Bowsher: “I think they were of the opinion in those days — and on water it really happened, of course — that if you were doing these high speeds and something went wrong, well, it was curtains anyway. Some of Railton’s comments imply this. You can give the driver the best view possible, as Railton did, but then he’s in a very dangerous position if anything goes wrong.”

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