ET: I think its because the SSME was originally designed to a certain thrust leavel (what is now 100%) and they were later able to build more capability and robustness into the system (likely the high pressure turbo-pumps) that allows continous ops at a higher thrust level, reliably. Pre-Challenger, IIRC, they aere using 109% rated thrust during ascent on a nominal ascent. That all changed due to Challenger.

TJ: it depends which "problem" you speak of. The pre-Columbia problem of lots of foam debris has, for the most part, been cured. Eileen Collins (commander STS-114) was just at MAF on Thursday for a general assembly. She noted that post entry and landing, when she did a walk-around of Discovery, that it was by far the cleanest Orbiter she had EVER seen, post landing. We flew the cleanest ET ever. VERY few dings on the Orbiter. The big issue, of course, was the single large chunk of PAL Ramp that broke off on ascent. In well over 100 STS flights, only one other PAL Ramp event has ever been noted (though there were several flights with no post-ET separation photos, so on those no one will ever know for certain). The Program just decided yesterday to proceed to the next flight by removing the PAL Ramp altogether. I was lead structures guy on PAL Ramp removal for months now when we were supposed to have 1.5 years to do this. Not we have to ship the next flight's ET in March. No Christmas AGAIN this year due to this.

Oh well, maybe buy a turbine with the overtime $$$...... Get me a completed, redesigned, AND tested GlassCat and maybe I'll buy that!