[Moonman62]

The foam had been changed due to environmental concerns to a foam that tended to break away in larger chunks.

...

IIRC, the foam that broke away had nothing to do with any EPA rule changes. I think Rush Limbaugh perpetuated the myth.

The bottom line is the Shuttle was always an unsafe design starting with the Orbiter being place on the side instead of the top.

[Cboldt]

-- IIRC, the foam that broke away had nothing to do with any EPA rule changes. --

NASA was (still is) exempt from those EPA regulations as a matter of law, and insulaiton from the previous foam insulation was also known to break off.

[chimera]

When I first saw the design for the shuttle orbiter my thought was, man, that heat shield is just hanging out there, nothing covering it. Until then all of our capsule-type spacecraft kept the heat shield covered during powered flight. Gemini and Apollo kept the heat shield protected up until almost the moment it was used (except for a brief time on Apollo 13, after SM jettison the CM and LM flew for the only time as a joined unit). The Mercury capsule heat shield was partially covered by the retro pack after release from its launch vehicle. Seemed kind of a risk to leave that heat-deflecting/absorbing surface just bare to whatever would befall it.