Although it would be easier and less expensive to carry the tank to the ISS than to dump it the Indian Ocean, making it useful and outfitting it in space is not considered to be economical nor practical and preparing it for such use prior to launch has been deemed an unjustifiable risk to the safety of the shuttle launch system.
Used tanks are free to anyone able to take delivery of and responsibility for their safe use in space without jeopardizing a space shuttle mission.
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