[zardo]

> So we can expect that it might have taken 25 unmanned missions to cause a similar loss of vehicle.

That doesn't follow. If those were unmanned test flights pushing the vehicle limits you can't just assume they would have gone as they actually did.

[pdonis]

> If those were unmanned test flights pushing the vehicle limits

As far as the launch to orbit, which was the flight phase when Challenger was lost, every Shuttle flight pushed the vehicle to its limits. That was unavoidable. There was no way to do a launch that was any more stressful than the actual launches were.

[zardo]

You can push the environmental conditions of the launch e.g. winds and temperatures.