Author Topic: 51L O-ring failure mechanism  (Read 3845 times)

Offline Kaputnik

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51L O-ring failure mechanism
« on: 09/08/2006 10:46 AM »
First off I would have thought this would have been posted already but I couldn't find it on the forum.
A short and simple question: I would have thought the grain pattern of an un-fired SRB would not initially expose any of the casing wall to combustion, and this would only become exposed towards the end of the burn. If this were true, combustion products would not be able to have escaped the combustion chamber area and damage the O-rings. I recently saw the footage of 51L leaving the pad, with black smoke coming from the SRB- so clearly even at the very start of the burn there is a way for the combustion products to reach the casing. How does it do this?
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Offline spacecraft films

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Re: 51L O-ring failure mechanism
« Reply #1 on: 09/08/2006 11:43 PM »
You've gotten to the very problem with 51-L. The black smoke at liftoff was at the joint, where between each segment the casing/joint was protected only by putty and the o-rings. Once the putty was blown through and the o-rings burned, the combustion products could get out the joint and burn a larger and larger hole in the casing. At each of the joints the combustion products don't have to get through the remaining solid fuel to get to the casing. Now there is an additional o-ring and a "capture feature" that helps seal the joint. There is also an additional "flap" of insulation that protects the joint.

There's a good bit of discussion of the joint and how it operated on our 3-DVD Challenger set available at www.spacecraftfilms.com.

Best,
Mark

Offline emarkay

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Re: 51L O-ring failure mechanism
« Reply #2 on: 09/12/2006 01:13 AM »
Yes, what SF said above.  As the casing expanded (called "joint rotation" as I recall) the segment joints 'bowed" outward, (picture an inflating balloon) and allowed excessive clearance to the 2 O-rings.  Also the segments have to have a gap, to allow for stacking tolerances and predictable combustion performance, and that's where the putty comes into play - The putt is NOT supposed to seal, just provide a transition between segments (again as I recall).  The current modified design has 3 O-rings, one where this joint rotation actually compresses and seals more tightly.  Also heaters are installed to keep the O-rings pliable in colder temps.

Basic research is what I am doing when I don't know what I am doing.                                            W. von Braun, 1957

Offline emarkay

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Basic research is what I am doing when I don't know what I am doing.                                            W. von Braun, 1957

Offline Kaputnik

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Re: 51L O-ring failure mechanism
« Reply #4 on: 09/15/2006 07:14 PM »
Thanks for the replies, it makes more sense now. I had assumed that the (apparently 'rubbery') propellant would make a contact fit between each segment and there would be no gap.
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Offline Do Shuttles Dream

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Re: 51L O-ring failure mechanism
« Reply #5 on: 09/16/2006 12:22 AM »
There is also some comments on the STS-51L thread.

Offline shuttlefan

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Re: 51L O-ring failure mechanism
« Reply #6 on: 09/26/2006 01:59 PM »
I THINK they experienced one of the stongest wind shears during the 51-L launch. If it weren't for that might the debris that sealed the gap when both o-rings blew, held for the rest of SRB burn?

The shuttles probably would have been grounded for a long time anyway, because Thiokol would have discovered the near-miss after the SRBs were recovered, correct?

Offline ryan mccabe

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Re: 51L O-ring failure mechanism
« Reply #7 on: 09/26/2006 02:39 PM »
There were other incidents of seriously damaged O-rings once the SRB from other flights were recovered, so who is to say?

Offline kevin-rf

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Re: 51L O-ring failure mechanism
« Reply #8 on: 09/26/2006 05:59 PM »
Quote
shuttlefan - 26/9/2006  8:42 AM

I THINK they experienced one of the stongest wind shears during the 51-L launch. If it weren't for that might the debris that sealed the gap when both o-rings blew, held for the rest of SRB burn?

The shuttles probably would have been grounded for a long time anyway, because Thiokol would have discovered the near-miss after the SRBs were recovered, correct?

The SRB was leaking long before the windshear. It was the burn through/failure of the lower strut the led to the SRB breaking free and pivoting into the ET. Did the wind shear contribute? Don't know, it did not help. At this point in flight the LH tank was already leaking and the controls where at or near the limit from the off balance thrust due to the SRB problem.

On the previous shuttle flight one of the O-Rings actually failed. Challenger launched less than month later.

Just like the foam ramps where causing problems before Columbia, O-Rings where causing problems before Challenger.

You know I have never seen any real discussion on this board of control problem caused by the unbalanced thrust from the two SRB's.

Hate to edit  and revise like this, but
The http://spaceflightnow.com/challenger/timeline/ timeline has the first measured drop in SRB pressure at T+5.674 seconds. A full two seconds before Challenger cleared the tower. The wind shear was at T+36.990 seconds. You are right though, the plume isn't observed until T+58.788 seconds.
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Offline Jim

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Re: 51L O-ring failure mechanism
« Reply #9 on: 09/26/2006 06:06 PM »
It was still under control at the point the strut broke.  The thrust imbalance wasn't that great

Offline psloss

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Re: 51L O-ring failure mechanism
« Reply #10 on: 09/26/2006 06:18 PM »
Quote
kevin-rf - 26/9/2006  1:42 PM

The SRB was leaking long before the windshear. It was the burn through/failure of the lower strut the led to the SRB breaking free and pivoting into the ET. Did the wind shear contribute? Don't know, it did not help. At this point in flight the LH tank was already leaking and the controls where at or near the limit from the off balance thrust due to the SRB problem.
What's your source for that?

The most widely cited document I know of is the Rogers' Commission report (here's one of many links), which says:

Quote
At approximately 37 seconds, Challenger encountered the first of several high-altitude wind shear conditions, which lasted until about 64 seconds.
According the timeline of evens, that's pretty much before the the visual indications of a leak or changes in the LH2 tank ullage pressure.

("First evidence of flame on RH SRM -- 58.788 seconds")

Offline kevin-rf

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Re: 51L O-ring failure mechanism
« Reply #11 on: 09/26/2006 06:29 PM »
Jim,

I have seen some comments on the thrust inbalance in the past. I have never seen any references to what percent of the control authority was being used to compensate. Just one of those makes you wounder, if the strut had not failed would the stack rotated and done loops if control authority was exceeded. Kind of like those that feel if the strut had been stronger the shuttle would have ended up in an abort mode instead.

All this ignores the fact that the ET was coming apart before the strut failed.

If I remember right the strut was never recovered.
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Offline shuttlefan

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Re: 51L O-ring failure mechanism
« Reply #12 on: 09/26/2006 06:30 PM »
I also know that, if it wouldn't have been for 'debris' sealing the joint when they blew on liftoff, the vehicle would've exploded on the pad.

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