Conference 7.286::space

    >The problem was not discovered until July 17, four days after Discovery
    >lifted off on its own mission. Discovery's two solid-fuel boosters have
    >yet to be inspected for O-ring damage; results should be available Friday.
         
    I've been watching this as information has developed. In fact this 
    morning before I left the house, I heard on NPR that in fact some
    preliminary results did find some similar problem with Atlantis, and
    that they were awaiting more detailed inspection reports to be
    forthcoming.
    The answer so far given to the media is that the o-ring seals and
    joint design in the nozzle area are a different design than the
    o-rings and field joints that failed in the Challenger.
    This would be a significant set-back if they have to ground the fleet.
    
    Bob

    To further add uncertainty to the situation, it was reported by AP in
    the Boston Globe today that since 51L there have been 11 instances of
    gas blowing through the insulative putty to the O ring (thus leaving
    soot on the ring).  The report doesn't cite where these occurances have
    been whether at the field joints or at the nozzle joint.
    
    The booster redesign after Challenger was primarily on the field joints
    where an additional tang and O ring was added.  There were some mods
    done to the nozzle and booster skirt (particularly on insulation),
    however the changes were not as radical.

    I recall while watching the post-landing conference last Saturday,
    when Brewster (last name escapes me), was asked about the problem.
    He mentioned that there was something they would be looking at, and
    that was the way the putty is being packed in there. He alluded to,
    and prefaced it by saying it was pure speculation by him, and that
    was that perhaps more putty would either be needed, or it would have
    to be packed in a different manner. Total speculation, but he said
    that these are, amongst other things what would be looked at. He did
    conclude by saying that the "blowing through" of the putty isn't a
    big deal in of itself, as long as the o-ring does its job and prevent
    further degradation. In any case, we'll know more by some time late
    tomorrow or into the weekend, as NASA may have more to tell by tomorrow
    when more details from the tear-down of the Discovery SRB nozzle's will
    have been completed.
    
    Bob

    Any news on the O-ring problem?  The media reporting seems to have subsided
    considerably since the first ka-fuffle.  I have not heard if NASA
    intends to ground the shuttle until there is a fix or what if any
    action will be taken.  Has anyone heard anything else on this matter?
    
    
    
    Rod

    The problem appears to be related to bubbles forming in the RTV
    silicone as it put in place (it is the first "line of defense" in the
    nozzle bearings, before the O rings). NASA have been working on a means
    to detect the bubbles with ultrasonics and repairing them, which
    involves someone standing inside the base of the SRB and injecting more
    RTV. It is not yet clear if this can be done on the pad or if the stack
    needs to go back to the VAB. Doing it on the pad allows other launch
    prep to proceed in parallel.
    
    NASA has to check all of the SRB nozzle sections they currently have on
    hand.
    
    gary

By the way, the fleet is grounded until repairs/changes are made or a better
understanding of the problem exists. This is not expected to have a major, long
term affect on the launch schedule, though it may delay the next launch.
						-- Tom

    The press conference they had two Fridays ago discussed the method
    used to inject the RTV. It involves injecting RTV at places
    approximately every 2 inches as the operator walks around the
    circumference of the joint. What apparently happens is that as
    the RTV is injected it kind of fans out into the insulation filling
    voids. Well, when the RTV spreads into the voids of the insulation,
    it will eventually encounter the RTV injected at the previous injection
    position. What is intended to happen is that any air pockets are
    supposed to be displaced with RTV. What they said appears to be
    happening is that at the locations where the RTV encounters more RTV,
    there is blockage formed which prevents the escape of air trapped
    behind the RTV which traps it (at its ambient temperature). Now when
    the nozzle area is exposed to the high temperatures of the exhaust,
    the air that was trapped by the injected RTV heats up and expands.
    The trapped air heats up sufficiently to push its way out through some
    weak path within the RTV/insulation. This then creates the "gas-path"
    which then allows hot exhaust gases to penetrate into this hole/path
    created by the trapped air expanding. The gas path is enlarged and
    erroded by the hot exhaust ofo the booster. This exhaust has a lot of
    aluminum particles in it and this exhaust is extremely corrosive. Thus
    the hot gas path will enlarge to the point where it will allow more
    hot exhaust and more particulate of the exhaust to reach the inner
    areas where the primary o-ring is. The damage to the o-ring that they
    have seen is comparable to the following analogy (in relative sizes).
    Picture the o-ring being about the thickness of a mechanical pencil,
    and the depth of the pits on the o-ring to be about the thickness of
    the pencil lead used in the mechanical pencil. This is the analogy
    I reacll used by Brewster Shaw at the press conference 2 Fridays ago.
    
    Bob

re .7:  That makes sense.  However, my interpretation from the AW&ST article
this week is that they now believe the air is "extruded" with the RTV as it is
applied.  I suppose this may really be a different wording for the same thing.

Burns

    Burns yes that is the expected and intended action that the RTV
    is *supposed* to have on any air trapped in the foam insulation.
    However, as Brewster said, what they saw was that some air gets
    trapped behind the areas where the RTV overlaps and it seals in the
    trapped air. Ergo, not all the room temp air is being extruded
    and being displaced as a result of the application of the RTV.
    The trapped air escapes out when it is super-heated and expands during 
    the rocket ascent, and when the air all escapes from a given place,
    the hot rocket exhaust starts to impinge into the same escape path
    used by the trapped air that escaped.
    
    Bob

Aha.  I did misunderstand.  When I said that air was extruded, I meant that
there were bubbles in the RTV and when the RTV was laid in (extruded through a
nozzle of some sort) that the air was extruded out the nozzle with it and the
RTV just cured there with the bubbles in it.  Your explanation makes more sense.

Burns
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