At least from all the preliminary studies I've seen, electric power is not really practical for all but the smallest diameters. FYI I'm referring to the use of

At least from all the preliminary studies I've seen, electric power is not really practical for all but the smallest diameters. FYI I'm referring to the use of a rock-melting penetrator rather than a traditional cutting head. This is maybe just science fiction at this point, but it came to mind as the sort of technology that would make these sorts of projects economically feasible. http://www.osti.gov/scitech/servlets/purl/4687637 http://www.osti.gov/scitech/servlets/purl/4687637 http://www.osti.gov/scitech/servlets/purl/4444905 http://www.osti.gov/scitech/servlets/purl/4444905