You've posted something interesting here on this thread.
I like the idea that you're working on
a "breakaway" cockpit for the shuttle.
You're probably thinking along the lines of an airtight, thermally-sealed "capsule" to be fitted inside the
cabin area. The liftoff/reentry survival capsule would hold upwards of upto eight crew-persons, am I right?
That capsule would have to have
at least 1.25 cubic meters of volume
per crewperson [volume equivalent to the Mercury capsule]
So 8 persons [1.25m^3/person]35.9 cubic ft/m^3 = 359 cubic ft.
That would mean sacrificing alot of volume inside the shuttle's cabin
to hold that survival capsule, not to mention the ejection mechanism and rockets.
I calculate that that survival capsule would probably weigh anywhere
from 5-10 metric tons [reducing the
shuttle's cargobay capacity to 20-25 tons, down from the max.30 tons].
I assume that such a liftoff/reentry survival capsule that you're working on would be surrounded by ablative heat shielding, right?...Thermal tiles will not do.
I also assume that not only parachutes would be installed in the
capsule, but deployable, heat-resistant [kevlar/silicone?] inflatable impact-cushioning bags?
I assume you'll design such a capsule so such bags would be deployed if and when the capsule is ejected [ to a safer distance] from the shuttle's
cabin [scenario: seconds after liftoff
with terminal 'anomoly']
is that correct?

I'll do some calculating!

A shuttle's cabin, seconds after launch, maybe 100 meters above C.C.L.S.
vf^2 = 2gh
2gh = 2[9.8m/s^2][100m]
= 1,960m^s/s^2
vf^2 = 44m/s [almost 100mph] ground impact speed]

vf^2 = vi^2 + 2ad
d = diameter of impact cushion bags
d = 5 meters [16 ft]
Vi^2 = 0
a = ground-impact acceleration with impact-cushion bags.
1,960m^2/s^2 = 0 + 2(a)(5m)
[1960m^2/s^2]/10 = a
a = [roughly]196m/s^2 = 20g's.
20g's would be survivable.

Jacob