Unit errors? What's the worst that can happen?

What is the price of making sure your product design data is correct and consistent? Well, as NASA discovered, it can be just shy of $200 Million. To some folks this will be an old tale of pain, but for those who are unaware, here is the wiki article on how the Mars Climate Orbiter was done dirty by unit conversions.

The TL;DR version is that on December 11th, 1998, NASA launched a robotic space probe to study climate on Mars. During the design of the probe, the navigation team at NASA’s Jet Propulsion Laboratory used the metric system (e.g. millimeters, centimeters, meters), while the engineers at Lockheed Martin utilized the US customary units system (e.g. inches, feet, yards). You can probably start to see how this can lead to some serious issues. And it did. The worst-case scenario occurred, the probe was lost as it passed behind Mars, and this went from being a story of great success to an example of some rather painful lessons learned very quickly.

For the mission report, you can see the official NASA page about it.

Avoid problems with mismatched units of measurement

The problem here was not the error; it was the failure of NASA's systems engineering, and the checks and balances in our processes, to detect the error. That's why we lost the spacecraft.

—Edward Weiler, NASA associate administrator for space science

If you're working on a complex assembly, like a Mars Orbiter, for example, you may be pulling in CAD files from various different sources. When you do, you need to be careful when mixing and matching parts that might be in different units systems. Most of the time, SOLIDWORKS will do the right thing — but not always. Sometimes you can end up with an assembly drawing in one system with mass properties in another. This is, as an Aerospace Engineering friend used to say, "how you crash orbiters into Mars".

Luckily, if you are using Bommer, there are a couple ways to make sure this doesn't bite you in the approach vector. First and foremost, Bommer automatically converts all measurements (dimensions, mass properties, etc.) into the units system of the top-level assembly. The entire class of unit mismatch problems that could lead to the above disaster just doesn't exist within the Bommer bill of materials. And to Dr. Weiler's point above, you can utilize the built-in Document Units property to view or export the selected units system for each file in your model, so detecting potential errors is simple and straightforward.

If you aren't yet a Bommer user, the best way to ensure that you won't have any units-related errors is to make sure all of the files you are working with use the same units system.

Changing units in SOLIDWORKS

SOLIDWORKS makes it easy to change the units system for a part, assembly, or drawing file, one file at a time. To change the units system for a part, assembly, or drawing, the file needs to be open and checked out/writeable in SOLIDWORKS.

1. From the main screen, go to the units popup menu in the lower right-hand corner of the SOLIDWORKS screen.
2. You can choose from any of the default units systems;
   • MKS (metre, kilogram, second)
   • CGS (centimetre, gram, second)
   • MMGS (millimetre, gram, second)
   • IPS (inch, pound, second)
3. You can also choose Edit Document Units to define a custom units system for your part or assembly.

You can then repeat this for each of the files that need to be fixed.

Here's a short video from Solid Solutions illustrating this process, along with some additional configuration options you can use.

Safeguard your engineering

With the right tools in place, engineering or manufacturing errors from issues like unit system mismatches can be prevented or detected with ease. Investing in an easy-to-use BOM software like Bommer will help you work faster while making sure any product "launch" goes off without a hitch.

Want to try Bommer out for yourself? Contact us for a demo!