What was the primary structural temperature goal for the aluminum skin beneath the HRSI tiles on the Orbiter's underbelly?

Answer

Below $250^ ext{o} ext{F}$.

The HRSI tiles covering the underbelly were designed to ensure that the underlying aluminum skin of the spacecraft structure did not exceed a threshold temperature of $250^ ext{o} ext{F}$.

What was the primary structural temperature goal for the aluminum skin beneath the HRSI tiles on the Orbiter's underbelly?
Aerospaceaeronauticsheat shieldthermal protection systemreentry vehicle