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Ejector seats working to Mach 3 nearly bridge the gap of the SRB - which fall at mach 5. It is the only viable escape system to work on the shuttle; everything else is too heavy or too expensive.
What about the possibility of making the nose section of the Orbiter into a capsule like a big Apollo CM? Everyone crams in to a rather tight space wearing spacesuits, seated on launch acceleration couches.
With any kind of abort that would result in losing the main Orbiter anyway, the capsule ejects as a whole, lifted off by an escape tower or solids embedded in the bottom sides or some such. The capsule has a heat shield in case of a need to capsule-abort from orbit or high suborbital speeds (such as the Columbia burn-up). When all goes well, the heat shield has a hatch in it and this opens into the expanded crew habitability spaces and the cargo bay beyond them; in an abort where the capsule comes off, they won't be confined to it long, because the capsule is coming down to Earth somewhere--near the launch site for an early abort such as what Challenger would have required, near the designated landing site in such a case as the Columbia reentry failure.
I suppose this option, which I know was considered in some versions or other, comes under the rubric of "too heavy/too expensive." But I'm not sure why it has to. The basic layout of the Orbiter had the crew confined to limited volumes for launch and landing anyway; reconfiguring so the nose of the Orbiter is a cone (or Soyuz-type return capsule shape, which is said to be the most efficient) containing the crew packed in closely would leave other spaces that could be filled with what got displaced from the nose. The big addition is the heat shield and escape rocket systems plus parachutes for the capsule--these costs are the price of a working escape system, it seems to me.
I just can't fathom how ejection seats are supposed to be more than a partial solution no matter what. Surely they are better than nothing; they might have saved the crew of Challenger. But they could do nothing for Columbia, whereas a capsule with its own heat shield could have saved them too. Or taken them down from an Orbiter that arrived in orbit just fine but suffered some terminal mishap up there--meteor damage, a fuel tank explosion or some such--not to worry, they sadly abandon the main body, enter the escape capsule just as they would when descending anyway, close it up just as they would normally--then eject into a return trajectory.
Gemini, I gather, dispensed with the escape tower and went with crew ejection seats. Mercury and Apollo used towers, as did and do Soviet/Russian manned ships. None of these craft had a backup way of returning their crews to Earth if something went wrong with the return capsule's heat shield. Going with ejection seats strikes me as a step backward from the prevailing standards of crew escape; omitting them too is more than two steps backward to a putting fingers in one's ears and singing "la la la nothing bad is going to happen!" Designing in a separable escape capsule the crew routinely rides up and down in into a spaceplane, be it a 100 tonne cargo-bearing orbiter or a 20 tonne space taxi, is a step
forward as for the first time there is redundancy in how to get a human crew safely down from orbit (or a failure of the main system in mid-reentry as happened to Columbia).
All I'm saying is, it surely costs money and payload mass, but it isn't obvious to me it has to cost a fatal amount, especially on a big beast like an Orbiter, and the benefits, while only coming into play if something goes terribly wrong, are quite large to offset the costs, though they are certain and mount up each successful mission too.
So it seems odd to say the ejection seats are the
only option, when in fact they are at best a partial solution to the whole problem while other more costly options bring much bigger benefits if they are needed.