During a space shuttle abort, it would have been vitally important not to allow the vehicle to droop below 265 kilofeet during powered flight, as excessive aerodynamic heating below 265 kft would have caused the external tank (ET) to explode (see page 18 of the linked PDF); if the shuttle could not have been kept from drooping below 265 kft, its engines would have had to have been shut down immediately and the ET jettisoned before it burst. Due to this constraint, low-energy situations where the orbiter would otherwise have been able to successfully reach a TAL site (for instance, two engines out, or one engine out and one stuck at 67% throttle, early in second-stage flight) would have instead ended in a forced early MECO and a bailout over the open Atlantic, and even single-engine aborts during the middle third of second-stage flight (where the orbiter would be capable of reaching a TAL site) would have had their margins of safety seriously degraded by the need to fly at a very high angle of attack during powered flight to stay above 265 kft.
If the ET had been equipped with pressure-relief valves to vent boiling hydrogen safely overboard and prevent a rupture, this constraint would have been eliminated, and lower-energy aborts would have been able to concentrate on gaining horizontal speed rather than having to worry about minimizing their sink rate, resulting - in many cases - in the orbiter making it to a runway rather than coming down in the drink.
Why, therefore, wasn’t the ET equipped with pressure-relief vents to eliminate the thermally-imposed 265-kft droop constraint?
