Challenger was killed by the small explosions in the fuel called "Combustion Instability". However of the 7 known holes in Challenger's side 5 clustered at spots (the struts) Ali calculated overstresses at -- and (see the pictures in the May 1988 Spaceflight) one of these burned a hole in the wing which WOULD have killed her.
Columbia was killed by an almost identical hole except in the left wing -- the explosions using the weakened area as a preferred path was an accident waiting to happen.
Columbia was thus killed by the Challenger Panel Majority, who buried the details & ended their "Final Review" in mid-April the day before even the chemical report on the first joint piece was available (guess what: they found NO O-ring markers in the soot. They never leaked. That flight. The critical ananlysis of cracks in the ET was dated 3 weeks later.
Now Combustion Instability has: blown holes in 3 of the 4 Shuttle-equivalent Expendables, almost killed Challenger earlier in its flight by a thrust mismatch nearly flying it sidewise to the wind -- and a mismatch occurred again on Columbia. Also (see my replies at the end of the other 2 Forums (use keyword Columbia) the SHAKING probably caused LH2 to migrate past valves in the fuel cell interconnect system, blowing the electrical system on Columbia and foiling the gutsy Pilots' restabilization of Columbia 30 seconds after the wing broke in half --- in this I am cribbing NASA's research on the last large Mars Mission (Mars Observer died from a fuel migration-past-valves problem obviously the result of violent SRB shaking).
Ali's Transient overstresses can also kill directly.
Ripping the 2068 ET seam should kill about 1% of older ETs; Accellerated wear-and-tear is Constantly being uncovered (I am glad Ali broadened his argument since last I had contact -- I pointed to the Main Engine bearings: but I had no idea there was so much more.
Nonetheless there is not as much as you believe:
1. Companies typically add 40% to ALL strengths, fearing becoming a WorldWide Pariah like the Apollo 13 Low bidders.
2. Liquid Thrust is 40% higher at altitude.
3. 10% was added to account for Transients
--- note, not assessing them was sloppy, but the whole Moon program was a race between the USA upgrading fuels & the USSR increasing engine pressure -- the Shuttle combines BOTH, and the jump to a high-pressure engine resulted in SURPRISES -- such as a 59% overrun when 10-15 was experience. Similarly "Escape off the Solids" always assumed the Liquds would shut instantly because they always had ... Challenger's orbiter's near-escape on only 1 engine shows how easily one can use them for escape and, in Idle mode & with Centaur CH4/LH2 OMS (Lunar & Zubrin's Mars Direct landers could use the SAME engines) for up to 22,000 lb net cargo (with the Al-Li ET, 4000 lb to "108-degree reverse Sun-synchronous (Military) orbit" becomes 33,000).
4. Standard 15% margin (40% where poorly understood).
... now Thiokol used the latter, +40% = 194% strength or 94% margin vs 70% Transient for the Solids ... Liquid stresses used 115% x110 (x140% for thrust), x140 = 248 or +148% margin vs. 59% -- but only 77.1% for sideways thrusts where +40% thrust, does not apply.
... now the welding flaws in seam 2068 halved the usual 40% Company extra, which was informal & optional, after all (ie they were 20 & 30% stronger than NASA specs) ...
---NOW NOTE WELL: attempts to use expendables to fly Shuttle-sized cargos DO NOT have that extra 40% margin (of course they also rarely have a 3-legged cross-stress) which explains why only one ever reached orbit -- and at that a VERY low one -- and all have had their cases thickened to where they can not take the large Miltary Satellites. --
Now if Challenger's descent to 20% margin was closer to the strut: Goodbye on the Pad ... as it was, the cracks came from above & went Through the flaws, but it still caused That seam to split Prior to the one above where the ET's problems started. That bouyed the Commission argument that flames from the visible O-ring hole hit the ET at the repressurization line, melting the tank yet dropping the Pressure (the line is on the BACK LEFT but they argued the vehicle is flying upside down -- kind of like saying when you turn around ONE of your eyes moves to the back of your head) (Feynman disagreed, suggesting SRB hole gasses which cut Strut strength in half broke it which compromised tha ET in a vague way --- BUT, the strut broke on the (unheated) ET side, implying an IMPACT faster the speed of sound in metal, i.e. ejecta from a hole higher on the side (not unlikely the very chunk of fuel whose shrinkage caused the pressure spike that blew out the side and pierced the ET -- Challenger had a LOT of ground damage from such chunks).
In short, both the fuel problem, & Transients, have to be avoided or fixed --
... there are MANY possible accidents if these two root causes are running wild, enough for the Bureaucrats to claim the next 'doesn't look the same' so it is "another NASA slipup", EVEN THOUGH vitually identical damage happened to both Challenger & Columbia, damage which any REAL investigation (not one excluding Rocket Scientists) would not have buried, but FIXED.
My nightmare is losing all 5 Shuttles, ALL blamed on "NASA" if there is 1 of each of the accidents I have cited above (hole by strong explosion, hole by moderate explosion POINTED at wings by weakened area near strut, thrust mismatch, ET seam rip on pad (the only one to be fixed -- partly: struts still might break), and: repeated explosions migrating fuel past valves.
Remember, the same man headed ALL investigations at NASA until fired by the Challenger Commission Majority, first day. Also that Rocco Petrone cancelled Challenger's Launch --the people who Fixed the Panel evidently ordered he not be informed when they called everyone else back but it appears in records because he assured Thiokol their inability to get good calculations ruling out the Cold should not dismay them -- he'd already planned to cancel it (at least partly for the Right reason: it was too much like STS-4 when they lost both solids & shook so bad any cargo would have been glitched, plus the Low orbit from Low SRB performance we saw again in Columbia & the Delta lauched just prior to it .. literally, he said "too many things close to the edge"
.
So I consider the Institutional Cause of the accident is not NASA but the EXCLUSION of the non-politicised Hard-Science NASA people, the Congressional elimination of NASA's Overrun Protection, the cut by a factor of 13 in Space Science, the loss of the NASA Administrator's Reserve, and the constant cancelling of critical upgrades ostensibly for Budget reasons, but often because FIXING anything means Admitting a Mistake...
But, again, Transients "only" caused fatal damage to Challenger.
A simple fuel explosion inside the SRB, blew her ET.
Before the Transients could kill them ala' Columbia.
Columbia saw the mainly-Transient-caused, back-of-the-Wing-roasting, return, to preempt a possibly fatal Fuel-cell leak.
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... in re Shuttle sucessors -- I love SSTOs, but they run so close to materials technology limits ... will Transients be too much, since the extra 40% the companies have tacked on our Shuttle looks impossible?
Or will air-launch or slow thrust-buildup avoid the Transient problem?
