Agree Etudiant,
Not following through with the X-33 tests of represents (to me) when NASA sort of lost their 'mojo' in terms of cutting edge launch vehicles/aerodynamics. They've still got it with their exploration spacecraft and science goals, but I guess the baton has now been passed to innovators like Space X. At least NASA have got behind Space X and that arrangement seems to be working pretty well.
Actually, the Lockheed team did build a metal internal tank, which eventually came within the required weight, so it could have been used. The testimony of NASA director Ivan Bekey was pretty much the final nail in the coffin. There is something about the whole project, which is sadly reminiscent of the dumping of the TRS 2