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Can You Measure Up?

A Weights and Measures office in England

When I watch the International Space Station fly overhead, I am always amazed at what a piece of craftsmanship it truly is.  Is it because of its large size or how high up it is?  No, not really.  It’s the fact that each and every piece was built in over 1 dozen countries by thousands of people and brought together for the first time 250 miles up in low Earth orbit.  The chances of everything fitting together the first time would be astronomical (and it did fit together the first time!) if it wasn’t for one thing, the standardization of measurement.

With things such as the ISS, the various countries involved use the United States Customary System of measurement.  In fact, as stated on the ISS tour at KSC, the ISS is the last international space project involving the USA that will use this system of measurement.  Afterwards, all international space projects are to be done in the metric or SI system.

Currently the USA uses the United States Customary System as its standard of measurement.  The rest of the world uses the International System of Units (SI or commonly known as “metric.”)  In 1959, an international standard was agreed upon so that both standards could be easily translated back and forth.  The table below shows a good example of conversions for common measurement between the American and SI standards.

Unit Divisions SI Equivalent
Exact relationships shown in boldface
inch (in) 2.54 cm
foot (ft) 12 in 0.3048 m
yard (yd) 3 ft 0.9144 m
mile (mi) 1760 yd 1.609344 km

The standardization of measurement is a basic building block of a successful civilization.   You cannot have trade, buildings, or complicated machinery without an agreed upon standard of measurement.   From standards on length, volume, etc. societies cannot function without some standard of measurement.  The next time you go to fuel your car, look at the gas pump.  Somewhere on the pump will be a stamp from an official state office certifying that the pump meter is in accordance with standards of measurement when it calculates flow of gasoline being pumped into your car.  This ensures the fair trade in your purchase of gasoline.

Many ancient societies had standards of measurement.  Some small villages that dealt in trade would post their “standards of measurement” on a board in the village square, while larger governments and cities would actually set up standards of measurement by decree and have officials to enforce the standards.

The earliest known examples of standards of measurements came from the 4th and 3rd mellennia BC from the civilizations of Indus Valley (covering modern day parts of Pakistan, India, Iran, and Afghanistan, Egypt, and Mesopotamia (covering modern day parts of Iraq, Syria, Turkey, and Iran).  Probably the most common story of government setting standards of measurement is the story of King Henry I of England, who ruled England from 1100 to 1135.  The standard for the “foot” was supposed to have been made by measuring the King’s foot.  This practice had been going on before his rule, but it appears that new rulers would frequently want to leave their “mark” in some way in the culture, and this was one way of doing it, hence the “foot” and quite likely the term “ruler” for the stick showing a foot.

So what can happen if standards are ignored or the wrong standard is applied?   Much can happen such as unfair trade practices, building collapses, machinery that cannot have interchangeable parts, and one famous example of a multi-million dollar spacecraft being lost.

Aerospace technicians working on the Mars Climate Orbiter. Credit

The Mars Climate Orbiter was launched on December 11th, 1998 as part of a two spacecraft team (the other being the Polar Lander which was also lost) and was declared lost September 23rd, 1999.  It was discovered that the loss of the spacecraft (total program cost of $327.6 million) was due to the wrong measurement standard being used.  Lockheed Martin was responsible for the thrusters and had used United States Customary Units to calculate the thrust in pound force.  The main computer was expecting the calculations to be in newton’s based on SI standards resulting in the spacecraft underestimating it’s thruster effects by a factor of 4.5 (1 pound force is equal to 4.5 newton’s) .  The software error was never caught during ground testing and the entire spacecraft ended up dipping too low into Mar’s atmosphere during orbit insertion causing the spacecraft to burn up.

The importance of having a standard of measurement cannot be stressed enough.  Because of these standards, a meter or foot means the same throughout the world ensuring fair and accurate trade, collaboration on international projects, and someday a human return to the Moon and on to Mars.

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Friday, December 31st, 2010 History, Introduction to Aerospace, Tests and Measurements Comments Off on Can You Measure Up?