Failure Is Always an Option
By HENRY PETROSKI
 URHAM,
N.C. — Scientists seek to understand what is, the aerospace pioneer Theodore
von Kármán is supposed to have said, while engineers seek to create what
never was. The space shuttle was designed, at least in part, to broaden our
knowledge of the universe. To scientists the vehicle was a tool; to engineers
it was their creation. With the release of the report of the Columbia
Accident Investigation Board, there is a new focus on the "culture" of NASA.
Engineers have played a prominent but not a controlling role in that culture,
both in the design of the shuttle and in the planning of its missions. When
the report speaks of NASA's "broken safety culture," the particular failure
it cites is "a consistent lack of concern" that Columbia may have been damaged
by debris at takeoff. But perhaps NASA can be better understood by examining
the culture that arises from the inevitable — and healthy — tension among
scientists, managers and engineers.
A common misconception about how things such as space shuttles come to be
is that engineers simply apply the theories and equations of science. But
this cannot be done until the new thing-to-be is conceived in the engineer's
mind's eye. Rather than following from science, engineered things lead it.
The steam engine was developed before thermodynamics, and flying machines
before aerodynamics. The sciences were invented to explain the accomplishments
— and to analyze their shortcomings.
The design of any device, machine or system is fraught with failure. Indeed,
the way engineers achieve success in their designs is by imagining how they
might fail. If gases escaping from a booster rocket can lower efficiency
or cause damage, then O-ring seals are added. If the friction of re-entry
can melt a spacecraft, then a heat shield is devised.
Much of design is thus defensive engineering: containing, shielding and fending
off anticipated problems on the drawing board and computer screen so that
they cannot bring down the design when it flies. Obviously, total success
can only come if every possible mode of failure is identified and defended
against.
Engineering is also very much about numbers. O-rings must be sized; the thickness
of heat shields specified; the weight of insulation calculated. Often, the
numbers work at cross purposes, as when increasing shield material decreases
available payload. Engineering design is ultimately the art of compromise.
What results from the design process is a thing that has unique characteristics.
It can withstand the conditions for which it was designed as long as it maintains
its integrity. There is usually some leeway allowed, for engineers know that
operating conditions cannot be predicted with absolute certainty. Until it
fails, how far beyond design conditions a system can be pushed is never fully
known.
But engineers do know that nothing is perfect, including themselves. As careful
and extensive as their calculations might be, engineers know that they can
err — and that things can behave differently out of the laboratory. On the
space shuttles, O-rings got scorched, heat tiles fell off, foam insulation
broke free. To engineers, these unexpected events were incontrovertible evidence
that they did not fully understand the machine.
Engineers do not feel comfortable with things they do not understand. It
is at this point that they begin to act more like scientists. In the case
of the scorched O-rings, the engineers studied burn patterns. They looked
for a correlation between damage and temperature, and they warned about launching
when the temperature was outside the bounds of their experience and scientific
study.
If engineers are pessimists, managers are optimists about technology. Successful,
albeit flawed missions indicated to them not a weak but a robust machine.
When engineers and managers clashed over the 1986 Challenger launch, the
managers pulled rank. In the case of Columbia, engineers who worried about
damage that the spacecraft may have suffered during launch were ineffective
in getting it properly inspected before reentry.
No one knows a machine or its failure modes as well as the engineers who
created it, and even they know it only as well as it reveals itself to them.
Because they are so humbled by their creations, engineers are naturally conservative
in their expectations of technology. They know that the perfect system is
the stuff of science fiction, not of engineering fact, and so everything
must be treated with respect.
The Columbia Accident Investigation Board has recommended that NASA establish
an independent Technical Engineering Authority. This would put responsibility
for technical matters where it rightly belongs — with the engineers who,
because they know how the space shuttle was designed, also know best how
it can fail. Without that knowledge, another fatal accident is inevitable.
Henry Petroski, professor of engineering and history at Duke
University, is author of the forthcoming "Small Things Considered: There
Is No Perfect Design."
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