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August 28, 2016 in Real Space Modeling
Not bad, but a couple corrections concerning Challenger and Columbia's accidents. First of all, the O-rings themselves were never intended to see exhaust gases from the SRBs. They were intended as a final seal for the stacked SRB segments, but the original design of SRB joint was weak and they had a phenomenon known as "joint rotation" where when the SRBs ignited and pressurized, the casing joints themselves flexed out ever so slightly causing the exhaust gases to expend out the gap and get to the O-rings. Now the O-rings were flexible and pliable enough to seal things under normal temperatures, but not quite so much at cold temperatures. If it was just the O-rings that were the problem, they would have fixed those and moved on with the original joints, but the SRB joints were completely redesigned with a more robust seal that didn't flex as much and a third O-ring (and joint heaters were added). Now for Aries 1 and I believe SLS, ATK (the company formerly known as Thiokol) is using a new O-ring compound which is just as pliable at cold temperatures, so SRB joint heaters will no longer be needed.
Concerning the fireball, when the LOX tank ruptured and the contents mixed with the leaking LHX, the contents did NOT explode. There was no concussive force associated with it. Essentially we just saw was combustion and burning to form a big vapor cloud, but it would be about like seeing a high school science teacher (or the Mythbusters) setting soap bubbles filled with hydrogen gas on fire. They burn... but they don't explode. What destroyed the Challenger itself is when the tank ruptured the two SRBs still attached were now thrusting in different directions and it was ripping open almost like a giant empty beer can. So Challenger came off the structurally compromised tank almost top first into the supersonic slipstream and got ripped apart by the aero-pressure loads at that speed.
As for Columbia and the foam, chances are the continued use of the white FRL (fire retardant latex) paint wouldn't have made much difference. First of all, the foam was used because it has to flex with the structure of the tank because chilled LHX and LOX causes the ET to shrink by a certain amount. So the paint layer on top has to flex a little with the foam and if it can't, there could be problems. In the lead up to STS-1, there was a problem with some painted foam peeling off the tank near the top of the intertank, so that is why some pictures of Columbia during its Flight Readiness Firing (FRF) show a large net being fitted where the problem was to keep any foam chunks from falling and damaging the orbiter. After the FRF, they patched the foam and repainted the area. Ultimately the FRL was left off not so much as a weight savings, but rather because it wasn't needed. The white paint was originally put on as there were concerns that UV light exposure, which darkens the foam to the orange and brown shades, also might be making the foam more brittle. But further testing revealed that wasn't the case and so the decision was made to keep the foam unpainted. The 500 pound weight savings was an added bonus.
The foam by itself did its job well. What is believed to have doomed Columbia though were trapped air pockets in the large pieces of hand sprayed and hand sculpted foam used on the Bipod and PAL ramps. It is believed that there was a trapped air pocket inside the ramp and during ascent as outside air pressure got lower and lower, the air bubble expanded, cracked the foam almost along a fault line and blew the ramp clean off while acceleration of the stack under SRB power carried the chunk into the wing at the right velocity and the rest is history. NASA calls that when it happens in smaller areas "pop-corning" and you can see it happen in smaller areas on the ascent imagery for many post-Columbia flights with the ET camera. The Bipod ramps were indeed used to help keep ice from building up on the struts there and the solution was to replace them completely with heaters. The PAL ramps were designed to reduce the aerodynamic loads on the lines that ran up the side of the tank. After a big section of PAL ramp blew off Discovery's tank on STS-114 (which resulted in another delay to the program), NASA did more wind tunnel study and found they could leave the PAL ramps off and that plus the bipod ramp removal reduced the potential for a big foam chunk strike by quite a bit.
So good info there!
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