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Space Shuttle Refueling Operation Case Study

"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.