English units SUCK

Today, I figured out a major problem I was having at work. I’m modeling the suspension system on the next-generation lunar rover, and NASA’s Constellation program recently made the misguided step backwards of officially going to English units. I had been having really strange issues where my model rover could balance just as well on one wheel as six, and I couldn’t figure out why, until someone suggested I compare the mass properties of several of the components.

It turned out that when I imported mass properties from Pro/E (a terribly painful process, I might add…), some of the individual parts thought that force and mass were related by a specific gravity in inches per second, and some thought it was by specific gravity in feet per second. Couple this with the fact that when I emailed one guy for the masses of a couple parts, he provided me with “masses” in pounds-FORCE. The result was that some of my model components had outrageously high inertias. The model was happy to balance on one wheel because it had such high resistance to rotation.

I’ve realized that while I think intuitively in miles, feet, and pounds in everyday life, I have no intuition for English units in a technical context. Kilograms, meters, Newtons, and metric prefixes are definitely the way to go. They are simple, easy to deal with, intuitive, and they lack the inherent ambiguities of units like the pound.

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2 Responses to English units SUCK

  1. Bobby K says:

    Very ambiguous, which can potentially lead to serious problems in the future. The pound-force is either with respect to Mars or the Earth. If it is respect to Mars, than it may get confusing. I think for lbf respect to Mars you divide by Mar’s gravity to get slugs. Then to go from slugs to lbm, you would multiply by Earth’s gravity. In other words, only on Earth is one lbf= one lbm.

  2. Joseph says:

    Well, it’s not just Earth and Mars, but any planet or moon. In fact, that leads to an even worse ambiguity, especially for using English units in the space program – What’s the relationship between lbf and lbm in zero gravity?!

    Since I have seen both the definition that 1 lbm = 1 lbf on Earth and that 1 lbf = (Earth g) * 1 lbm, my standard MO is to operate entirely in SI units and then convert to English if somebody needs me to. Makes calculations and units simple and easy.

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