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via minimized Survivability by ginger gardiner fuel tank designed to disintegrate upon reentry. / karl reque Source: Chomarat frequency of satellite replacement and eliminating the need for controlled reentry planning, training and contingency operations. Although the GPM Core is not fully demiseable — it is a low-debris spacecraft, and some propellant will be needed for reentry control — its fuel tank is. "About 10 years ago, we started looking at how to develop a demiseable tank," says Estes, who heads the GPM tank effort at NASA. Initial analyses showed that tanks made of traditional titanium or stainless steel — either as a monolithic material or as a liner for a compositeoverwrapped pressure vessel (COPV) — would not demise, but an overwrapped aluminum liner would. Four tank vendors were contracted to produce a variety of realistic, flightworthy tank designs based on projected GPM requirements. Cobham Life Support (Westminster, Md.), the breathing and life support equipment supplier for every U.S. astronaut since John Glenn, won the contract to help finalize the design and then build and qualify the tank. The baseline Cobham started with included a thin (less than 0.05 inch/1.27 mm) 6061-T62 aluminum liner overwrapped with T1000 carbon fiber (Toray Carbon Fibers America Inc., Flower Mound, Texas) impregnated with EPON 862 bisphenol-F toughened epoxy resin (Momentive Specialty Chemicals, Columbus, Ohio) to an average thickness of 0.111 inch/0.281 cm. These materials were chosen because they were already qualified for space applications and had a history of successful use. "This is a traditional materials combination for firefighter breathing tanks," says Robert Grande, Cobham Life Support's business manager. But that did not make it challenge free. He explains, "Aluminum has never been used before in a Propellant Management Device on a spacecraft." A Propellant Management Device (PMD) is a low-gravity wick that uses surface tension forces to collect propellant and draw it to the tank outlet for illustration Demiseable at Earth's atmosphere re-entry Cobham Life Support's (Westminster, Md.), and NASA Goddard Space Flight Center's (Greenbelt, Md.) spacecraft fuel tank. Designed to disintegrate upon reentry, it is also the first spacecraft fuel tank to use a carbon fiber composite-overwrapped aluminum liner. expulsion. Cobham found that with appropriate surface treatment, aluminum functioned well as the PMD material, but as a monolithic tank material it couldn't meet demiseability, pressurization, structural load and weight requirements. An aluminum liner thin enough for demiseablility necessitated a composite overwrap. engineering the overwrap … Although the overwrap was conventionally filament wound and oven cured, the winding design presented challenges. "The tank had to handle stresses from the weight of the fuel and pressurization of the tank," says engineering manager Rich Pemberton, "but also maintain a very careful balance between the stresses in the carbon fiber and those in the aluminum liner to assure sufficient fatigue life and burst strength while still remaining thin enough to meet weight and demiseability requirements." Cobham's solution was to tailor the overwrap so the stresses were optimized and the burst strength of the tank was achieved without adding unnecessary material. Tailorability provided solu- tions to other problems as the design matured. Pemberton gives an example: "Formed and welded metallic liners often result in stress concentrations that can't be ignored but are necessary for performance or manufacturability. Our solution was to add a small amount of carbon fiber to critical areas — like controlling build up at the bosses or adding hoop winds in the weld zones — to reduce strains, mitigate risk of fracture and ensure the fatigue life of the tank." Because fastener holes can't be drilled in the pressure vessel itself, tanks are typically attached to the spacecraft by means of a skirt, which is bonded to the tank and then mechanically fastened to the spacecraft. "We needed a very stiff, lightweight means of attachment while meeting the overall tank assembly weight of 100 lb [45 kg], plus the skirt transmits all of the weight and loads of the tank to the satellite," says Pemberton. … and a miniskirt Composites enabled Cobham's skirt design for several reasons. Pemberton explains: "First, the skirt must match marCh 2013 | 63