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Sunday, March 2, 2014

Why Go Metric?


Reprint of article from Robert Lipsett, Engineering Manager ,  Thomson Industries, Inc.

The U.S. is a multi-lingual country, in its measurement units as well as its language.  The rest of the world, however, uses one language for measurement, and that is metric.  This makes the U.S. the only industrialized country that has not standardized on this system, even though it has had over 200 years in which to join the rest of the world.   And this presents a few problems.

For example, in 1999, NASA engineers launched their Mars Climate Orbiter, a $125 million spacecraft designed to explore the surface of the red planet.  For nine months, engineers monitored the spacecraft’s flight and altered its trajectory as needed.  The engineers knew that two crucial programs spoke in different units of measure, one in metric and the other in English units.  And a simple conversion check was set up to ensure that the data from one program was compatible with the other. However, that simple conversion check was not done.  It was overlooked.

In calculating the thrust of rocket firings, engineers used feet per second.  The spacecraft’s program, however, interpreted those instructions in the metric measure of thrust, Newtons per second.  The difference between the two measures of unit is 4.4 feet per second.  Therefore, each time engineers ordered a rocket firing, the spacecraft’s orbiting error increased, resulting in a critical problem – NASA lost the spacecraft.  After months of analysis to determine what happened, engineers discovered the conversion problem and concluded that the Climate Orbiter either flew too low and crashed into the planet or it glanced off into outer space.  Either way, they have not heard from it since. This is an example of what can happen when metric and English units are mixed without proper conversion. 

Click on the link below to download this complete article.


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