Technofile - the bag tank

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Fire was once the number one killer in Formula One, now it’s almost unheard of. The life-saver is the fuel safety cell, better known as the bag tank. By Keith Howard

Fire: the word alone used to be enough to send a chill through every motor sport watcher, let alone participant. For if a driver survived a major impact that was only the first hurdle: the second was the fuel fire which all too frequently foIlowed.

Many, of course, didn’t survive it. Lorenzo Bandini, Roger Williamson and Piers Courage were three fiery fatalities from just a small slice of Formula One history. Survival against the odds, enormous bravery and even tragic farce form part of F1’s fire history also: Niki Lauda’s almost miraculous recovery from his Nurburgring blaze in 1976; Mike Hailwood’s George Medal-winning rescue of Clay Regazzoni from his burning BRM at Kyalami in 1973, David Purley’s unsuccessful effort to save Roger Williamson at Zandvoort the same year and Regazzoni’s own attempt, with James Hunt, to pull Peterson from his Lotus at Monza in 78; and Torn Pryce’s death the previous season not by fire but by fire extinguisher when he collided with a marshal sprinting to reach Renzo Zorzi’s burning Shadow. Of course, the marshal perished too.

Today’s Grand Prix audience, by contrast, could be forgiven for forgetting the projectiles circulating before it are powered by highly flammable liquid. The last time a television audience watched in horror as a crashed F1 car caught fire with the driver inside was in 1989 when Gerhard Berger’s Ferrari crashed at San Marino – an incident with a happy ending. The fire in the Benetton pit in 1994 and Pedro Diniz’s Ligier catching alight on track in 1996 were more recent reminders, but neither was the result of an impact. Truth is, we’ve become used to race cars having spectacular shunts without any subsequent blaze.

Numerous inter-related regulatory and design enhancements have brought this transformation but central to the story is the fuel safety cell, colloquially known as the bag tank. No practicable fuel container can be completely puncture proof but the modern fuel cell – protected within a strong part of the monocoque, immediately behind the driver in the modem F1 car – is as close to that ideal as modem materials can make it.

The earliest use of flexible fuel tanks in a racing context that I can trace was in the C and D-type Le Mans cars of the 1950s. Manufactured by ICI and drawing on aircraft practice, these were used because they made for a lightweight fuel container that remained leak-free through a gruelling 24 hours’ racing – something that not easy to guarantee using a fabricated aluminium tank. Probably there was some small safety benefit too but those early bag tanks were simple constructions made of rubber, so their rupture resistance, to take one important safety requirement, was necessarily poor. The same applies to the early bag tanks used in F1. In It Was Fun Tony Rudd recalls a bag tank was chosen for the mid-engined Type 48 BRM in 1959 simply because, “The sums I did showed that we would get more fuel in a given space with them…”

The origins of the fuel safety cell proper come somewhat later, towards the end of ’60s, and begins, appropriately, with an ordeal by fire. Peter Regna, an American mechanical engineering student, was practising for an SCCA meeting in his Austin-Healey when he rolled it. As the car scraped along the track, leaking fuel caught fire setting alight to both car and Regna’s overalls. He escaped and extinguished the flames by rolling in the grass, but the experience altered the course of Regna’s life. Reckoning there must be a way to contain fuel in an accident he began the research that led to him founding Aero Tec Laboratories (ATL), the oldest manufacturer of fuel safety cells for the motorsport industry and supplier of today’s entire F1 grid. A few years later the FIA made its first fuel cell regulation, later designated FT3, which in 1990 was supplemented by the more stringent FT5 standard (required for F1) and more recently by the intermediate FT3.5 which has yet to be mandated.

Many features distinguish a modern tank from those early examples in the ’50s Jaguars. First, the flexible walls of the tank are reinforced by a strong but resilient fibrous core, around which the impervious elastomer is moulded. Originally nylon or polyester were the fibres of choice, combining fair tensile strength with the high elongation required to resist puncturing; today all manner of exotic fibres are used, Kevlar and carbon fibre among them, in various proprietary combinations,with the necessary elongation now provided more by their carefully contrived weave than by the fibres themselves.

The elastomers have developed significantly too, not least to cope with the increasing aromatic content of racing fuels, which in the most potent brews can reach 60 or 70 per cent. Nitrile and fluorocarbon rubbers or polyurethane are typically used to provide the required strength, fibre adhesion, abrasion and chemical resistance. Even so, like all rubber products, bag tanks have limited lives. The FIA now mandates five years,which can be extended for another two years following inspection and approval by the tank’s manufacturer.

Although it is the subject of a waiver in F1, modem bag tanks usually also incorporate an open cell foam filling which performs a number of important functions. It prevents fuel slosh and surge and, by limiting the speed at which a flame front can propagate, guards against explosion of the fuel vapour. The latest foams are also electrically conductive to prevent charge separation during rapid refuelling accumulating a dangerous static charge.

A puncture resistant tank is of little benefit if fuel can spill out, of course, so vent and filler check valves are also fitted and frangible unions inserted in the fuel lines. These break at a predetermined point and close off at both ends should the engine be ripped from the chassis. And talking of ripping out, the tank itself has to be designed so the nut ring flanges which hold it in place aren’t tom from the bladder under the high deceleration of a major impact.

Michael Schumacher’s recent crash at Silverstone was far from the most spectacular we’ve seen, but it underlines the revolution that’s been brought by the fuel safety cell. The incident will be remembered for the fact it scotched Schumacher’s championship hopes for the season. Not so long ago, the gurgle of escaping fuel would likely have signalled that was the least of his worries.

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