GKN Aerospace launches new single-skin material for aircraft fuel bladders

The improved material is flexible and offers crash resistance and puncture tolerance.

As part of an initiative to reduce and remove volatile organic compounds (VOCs) from fuel tank manufacturing processes, engineers and scientists at GKN Aerospace (Portsmouth, U.K.) have developed the first single-skin flexible fuel bladder material that is able to offer crash resistance and puncture tolerance. The company recently launched this new development at Heli-Expo in Houston, Texas.

Frank Bamford, senior vice president of Business Development and Strategy, says, “We estimate that use of this material could lead to a 60 percent reduction in the use of VOCs and a 30 percent reduction in manufacturing times – while the weight of each fuel bladder could be reduced by approximately 5 percent through the removal of the adhesive coatings used in traditional bladder construction.”

To provide effective crash resistance of the necessary standard, fuel bladders for the aerospace sector have traditionally been manufactured using multiple layers of composite materials with adhesives joining the bladder to the protective textile layers. This has involved a manufacturing process using solvent-based adhesives and the application of a final lacquer coating to protect the exterior of the fuel bladder.

This new material is a composite of a woven aramid core with thermoplastic polyurethane coatings and an integrated proprietary fuel barrier layer. The fuel barrier provides the same level of fuel tolerance exhibited by traditional tanks. This means the crash-resistant layer is not applied using adhesives and no final lacquer coating is required, significantly reducing VOC solvent use. It is made using a hot-spread coating technology developed in-house at Portsmouth. The company uses similar technology for helicopter floats and some existing fuel tanks but have adapted the technology to produce this new material.

The material is designed for fuel bladders that fit into a fuel bay area but are not for "wet wing"installation as is common in civil airliners. In contrast, bladders are installed in helicopters and military aircraft typically due to their exposure to higher forces in operation.

The tank has a high level of puncture resistance due to the density of the polyurethane coatings combined with the known ballistic performance of the woven aramid. The coating process reduces the tendency of the woven aramid to move and so requires the polymer as well as the aramid to fail before a puncture occurs. This also provides the high burst strength of the material which is where the real attraction lies. Whilst helicopter tanks have a puncture load figure in their specification the real purpose of the crashworthy fuel tank is in containing the hydraulic forces that occur when the tank is subjected to a sudden shock load during a crash. If the tank was not crashworthy and punctured the resulting emission of fuel under load could be an aerosol effect which is the most dangerous situation for combustion of the fuel in a crash.

Additional environmental benefits from the new tank assembly process include the removal of ozone-depleting chemicals that are used in traditional rubber manufacture and the elimination of most fuel tank manufacturing adhesives and the solvents they contain, because the polyurethane basis of the new material will allow the use of radio frequency welding of the tank structure.

Bamford concludes: “As a company, our R&D effort in all our businesses is targeted on developing and applying new technologies to bring real and practical benefits to aerospace. Without doubt, this new material represents a huge progression in the flexible transportation of fuel for helicopter operators and others. It will bring a technological step-change into this sector with appreciable environmental and operational benefits. And this is our second new product launch at Heli Expo where our team is also discussing, for the first time, our new emergency flotation system technology.”