Re: What was the biggest problem for each of the 2 destroyed US space shuttles?

From: Jay Windley (webmaster_at_clavius.org)
Date: 09/01/04 

Date: Wed, 1 Sep 2004 17:53:29 -0600

"Brian Thorn" <bthorn64@cox.net> wrote in message
news:nkicj0di98qivsn7ou5qdtk1eddaufb81n@4ax.com...
|
| Neither problem was fundamentally fatal and doomed to occur. Had
| management pulled their heads out of their asses to come up for breath
| once in a while, both accidents could have been avoided. Warning signs
| were ignored in both cases.

It's unfair to say they were "ignored". From the very beginning, for
example, the problems with the SRB aft field joint were examined, even
before flights began. The joint was known to rotate incorrectly. The
problem was studied and everyone agreed that the design could be modified by
adding shims etc. to reduce joint rotation.

The erosion problem was not connected to joint rotation except in hindsight.
It was known to be caused by blowholes in the putty, and the modified leak
check procedure was known to create those blowholes. The joint had to be
pressurized up to 200 psi in order to make sure the putty wasn't masking a
leak in the primary O-ring.

Much has been made of Feynman's analysis of the O-ring problem. Not his
immersion of the rings in cold water -- I think he was spot-on with that
part. But the criticism of the redefinition of risk to allow for O-ring
erosion lacks a bit of refinement.

The joint separated. The O-ring extruded into the resulting gap. This is
normally not how O-rings are supposed to operate. But they can. They went
to the manufacturers and asked if that would be okay. The manufacturer
said, "You'd better just test it." So they did, and found that the
extrusion seal mode was effective and redundant as the plan called for.
This is not out of the ordinary for this type of engineering.

The O-ring eroded. This is definitely not how O-rings are supposed to
operate. But again, rather than make a wild guess, the engineers tested
what happens to O-rings in the joint when they are eroded. They found that
the O-rings could tolerate up to 0.090 inch erosion before they failed to
seal. This is not trying to "get around" or "avoid" problems. It is an
attempt to understand the nature and extent of the problem. Without flight
experience you have to rely on test experience. As the SRBs flew, flight
experience was accumulated and it confirmed everyone's suspicions about the
O-ring erosion. This is how knowledge is gained. You get a feel for how
the SRB joint *really* works by watching it work.

There was nothing to connect erosion and sooting with joint rotation, and so
they were not considered related problems.

The problem is not that management didn't "pull its head out". The problem
was that the concerted efforts of both management and engineering had an
incrementally evolved construct of risk that understandably normalized the
behavior of the joint.

Practice differs from prediction all the time. When it does, you try to
understand the reasons behind what physically happens. That, in some cases,
is more valuable than the assumptions and computations that went into the
predictive model.

I've rambled on too long, I realize. The same sorts of normalization
arguments can be brought to bear on the Columbia accident.

The point is that it's easy to try to find individual scapegoats and imply
that they are indolent or ignorant or immoral. That doesn't often tell the
whole story. It's easy to wag a finger at NASA managers and say, "You mean
you flew a design you *knew* to be risky?" There is no such thing as zero
risk, especially in manned space flight. Some risk must be accepted, and so
the risk management scenario has to look at ways to quantify each potential
risk before deciding how to deal with it.

That said, let me hasten to agree with you that many individual NASA
managers who testified expressed a fairly unrealistic concept of the risk of
flying the space shuttle. These tend to be the ones on which the disaster
was pinned, but there were more credible risk assessments available, and
those show a defensible pattern of trying to understand the problems and
trying to apply the best overall solution to them. To try to pin the
problem on a few spaced-out managers runs the risk of failing to see what I
believe to be the real problem. 

-- 
                                          |
The universe is not required to conform   |  Jay Windley
to the expectations of the ignorant.      |  webmaster @ clavius.org
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