SPF for Space Tank Applications

Some space tanks have to carry fuels like hydrazine which are chemically aggressive and very dangerous for human beings. The best material to withstand the corrosion hazard is Ti6Al-4V. This material is also well analyzed in terms of detrimental effects regarding "poisoning" of the fuel with alloying elements. There are a number of tanks in launchers and satellites built the conventional way from Ti6-4, which is to start from forged shells. The material loss from extensive machining is considerable and mostly far beyond 90%. Cost for material and machining gives some opportunity to think about cost-saving technologies like Superplastic Forming (SPF). Two major approaches have been investigated: SPF of a "thick" hemisphere and post-machining to final shape in 3D and pre-machining in the flat condition and SPF to final shape without any post-machining except chemical milling to get rid of the embrittled surface layer (alpha case). It is obvious that the machining of a hemisphere in 3D down to a wall thickness of around 0.9mm and a diameter of 750mm is a far more critical and cost-intensive job compared to machining a flat plate with thicker gauge. The production sequence has been developed and qualified with diameters of 480mm and 748mm. Up to now ~400 hemispheres have been produced and successfully launched towards space. Cost saving and reduction of delivery schedule have perfectly met the expectations.