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Space Shuttle Refueling Operation
"NASA gave
us a requirement for dew point that we must consistently meet. We’ve
been using these instruments for a long time to obtain accurate
measurements...and our technicians have not experienced any problems."
-Mr.
Erich Schatzle, Test Engineer
Lockheed Martin
Concern for
safety may not be the most universal reason for making a reliable
dew point measurement, but it is the most important. Verifying conditions
as a test in any safety procedure, requires instrumentation you
can count on. A quality audit of a product or process is typically
used to protect profits for a business. In some cases, a quality
audit is adopted to protect people.
Here’s
one case…..
Background:
When it comes to human space flight, the track record of the United
States’ space program for safety and reliability is unmatched
around the world. The Space Shuttle program is the latest example
of the high standards set by NASA. Stringent quality requirements
dictate every aspect of the Space Shuttle’s design and manufacturing,
from major components all the way down to the smallest parts.
One area where
exceptionally high tolerances are maintained is in the preparation
of the Space Shuttle’s external fuel tank. The large orange
tank—familiar to Shuttle watchers—is manufactured and
supplied to NASA by Michoud Space Systems, a Lockheed Martin company
located in, Louisiana. Constructed from ultra-lightweight aluminum-lithium,
the external tank consists of two smaller tanks that are filled
with liquid oxygen and liquid hydrogen.
Before delivering
the tanks to NASA for use on a shuttle mission, Lockheed Martin
performs a host of final acceptance tests. One particularly important
test involves verifying the dew point level inside the tanks before
they are pressurized and shipped.
The
Challenge:
The sheer size of the Space Shuttle’s external fuel tank presents
a challenge to engineers who must perform the final acceptance tests.
The tank measures approximately 154 feet in length and 28 feet in
diameter, and holds 1.3 million pounds of liquid oxygen and 227,000
pounds of liquid hydrogen. Test technicians and engineers first
pressurize the tank to approximately 6 psig and check for leaks.
The tank is then purged with nitrogen gas over a three to four hour
period, after which a dew point reading is taken. NASA specifications
require that the dew point level of the gas be minus 15° C (+5°
F) or less. If a higher than specified dew point reading is indicated,
the purging is continued, and another gas sample is taken.
Should excess
moisture remain in the tank, after it has been purged, there exists
the potential for corrosion, which could weaken the external tank.
As such, precise, reliable measurement of dew point is a must.
The
Solution:
Lockheed Martin uses EdgeTech Moisture & Humidity's (formerly
EG&G Moisture and Humidity Systems) two-stage chilled mirror
sensor technology for processing the external fuel tank. The two-stage
sensor (S2) is selected for its performance capability of achieving
dew points exceeding the minus 15° C (+5° F) specification.
The S2 has a dew point measurement range of minus 50° C to plus
100° C. The heart of the sensor is a hermetically sealed module,
containing a rhodium mirror attached to a two stage thermo-electric
cooler. The mirror module is designed as an integral unit (unibody
construction) to eliminate the potential of intermittent signals.
Being assured
of a quality measurement is only part of the story. Consistent quality
is critical to meet the safety requirements. A dew point measurement
that drifts over time is unacceptable. Chilled mirror technology
is a primary measurement device that provides the accuracy to assure
a measurement whose quality is consistent over time. A chilled mirror
measurement does not need to be compensated for temperatures that
might vary during the dew point verification procedure.
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