In Transportation Economics of Extraterrestrial Resource Utilization", Cutler and Hughes state:
It is assumed that the utilization of space shuttle external tanks as an extraterrestrial resource is economical, as it would be ridiculous to discuss the use of lunar or asteroidal resources if this were not true.I disagree. The Space Shuttle has proven to be quite uneconomical in that it costs significantly more to launch a kilogram to a useful orbit using the space shuttle than using an expendable launch vehicle. To assert that shuttle external tanks as an extraterrestrial resource is economic when the system it is based on is not economical is a dubious assertion. Given that Cutler and Hughes wrote the statement above in 1985, shortly before the 1986 Challenger explosion, it is understandable how they could have been under the mistaken impression that the Space Shuttle is economical.