Putty Is Also Forced Between The Gap Of The Tang And Clevis

Assembly of the SRB could have damaged the sealing joint

. The segments of the SRB are transported to the assembly plant horizontally. Each segment is heavy and therefore with its weight, changes the shape of the segment so it is not perfectly round. The irregular shape of the segments can be distorted and stressed from previous missions, and or the effects of handling. At assembly the aft segment is lowed vertically, with the tang sliding into the clevis of the previous joint. Because of the distortions, the dimensions of the segments may have changed. Mission 51-L, was one of the missions where the dimensions had changed. Even during the assembly process of the SRB the dimensions of segments continue to change, with the amount of weight that is applied to each of the segments. As stated, the shape and dimensions change, so at the assembly plant, to make the segments fit more easily the tang shape is changed with a special tool. The important thing during assembly is whether the diameters of both segments are the same. If the difference of diameters is too big, then the sides of both the tang and clevis are flat against each other. When this happens, the inside of the joint can not be seen to access if the joint is good. When the difference in diameters are small, then the tang is slanted against the clevis but, the slant still allows the assembly of the SRB to continue. Another thing that they look for during assembly is if the centers of the segments line up. A difference of +0.25 inches is allowed for mis-aligned segments. If the difference is more than +0.25 then there is a chance of contamination. When the tang and clevis fit together and the centers are off, then there is metal to metal contact. Metal splinters can flake off and can land on the O-ring. The metal segments can also scrape against the O- rings and ruin them. There has been tests that show that contaminates with a size of 0.001 and 0.003 inches in the joint have actually passed a leak test. Thus, there is always a possibility that contamination did actually occur on the SRB of mission 51-L.

On the inside of the tang and clevis there is a gap that needs to be sealed. It is the O-rings that seal this gap. The size of the gap changes as the pressure of gases inside the SRB change. The gap gets bigger when the amount of pressure rises. Gap opening change is called Delta Gap Opening. There are two O-rings, the primary and secondary O-rings. The gap at each O-ring in the aft field joint is different, the gap at the primary O- ring is approximately 0.029 inches, and the secondary is approximately 0.017 inches. During launch the O-ring should move to seal the delta gap opening, and return to its proper state.

Putty performance is another possible cause of the joint seal failure. Putty, Zinc chromate is placed on the inside of the joints before assembly. It is there to stop the heat of combustion gas from going to the O-rings. Putty is also forced between the gap of the tang and clevis, to make sure that the seal is tight. The Zinc chromate can affect the joint in many ways, one, putty can affect the amount of pressure that is sent to the O-ring for actuation of the O-ring. The hot gases can make holes in the putty, thus letting gas go through to the O-rings which could cause damage. Having gas go to the O-ring could decrease the time it takes for O-ring actuation. Second, the putty moves by gas pressure and could go all the way to the O-rings. The putty could be blown into the grooves of the O-rings and block the O-rings from properly sealing the joint.

For most of the causes of the joint seal failure, it was assumed that the segments stay perfectly round during launch. When a shuttle is launched, the SRBs are actually subjected to a great deal of force. For the final launch of Challenger, the SRBs were bolted to the pad for 6.6 seconds after ignition. The great forces bend and strains the SRBs forward. The circular segments are changed to an elliptical shape. The side that is most flat of the elliptical is on the shortest distance between 045-315 of the right SRB. The bending and straining occurs in cycles of three per second. At launch when the SRBs were ignited, there were puffs of smoke coming from the same location, also at three puffs per second. There are other changes with the forces being applied to the SRBs. The tang and clevis joints change shape and therefore the gap opening becomes bigger, thus the O-rings have a bigger gap to seal. If the O-ring does not follow the gap opening then the seal fails.

by: alina00123

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