Here's what I posted at the FPSPACE forum:

There is a common denominator for both Challenger and Columbia tragedies:
The shuttle (with its thousands of fragile tiles) is attached alongside the external tank and the solid rocket boosters. That's the shuttle's greatest weakness.

New designs put the crewed vehicle on top of the rocket, away from the effects of an ET or SRB failure, and with a launch escape system.
Even in case of a catastrophic failure, astronauts would have more chances to escape from death, also because the crewed vehicle would be much smaller and compact than the shuttle. .
Rockets are going to fail sooner or later: it is better to give the crew a chance to escape from it, using a tested and known design even if it had failed before (and we do learn from failures-the SRBs have been redesigned) than to rely on a totally new, untested design, with the "belief" or "faith" that such a design will be far more reliable than the current one.

Raoul