Metric versus Imperial

You might not think it is an exciting subject, but unit conversions are fundamental to spacecraft design and all of physics and engineering. If they are not done correctly then the consequences can be dramatic. In particular since space projects often span over several generations of people, you are likely to find that the earlier parts of a project may have been calculated in one unit, such as Imperial, and the more modern measurements in metric.

To illustrate the dramatic consequences of getting this wrong, on the 11th December 1998 the US Space Agency NASA launched the Mars Climate Orbiter spacecraft to Mars. The spacecraft was designed to study the Martian climate and atmosphere from orbit. However, on the 23rd September the spacecraft was lost and instead it entered the atmosphere of the red planet and was destroyed. After an examination of the cause, NASA concluded that the failed mission had been due to a failure to correctly convert between SI units (which NASA used) and the Imperial units of the customer who had constructed the spacecraft (Lockheed Martin). US Customary units includes the foot, yard and mile for example, whilst in the SI system metres and kilometres are used.

In principle, it shouldn’t matter what units are being used, provided they are converted correctly. However, if many different systems are designed with different units, then the chances of errors accumulating are increased. This could be due to rounding errors or simple conversions.

Another illustration of how complicated things can get is in the mass of a spacecraft. Let us take the Mars Climate Orbiter as an example, which had a mass of 638 kg. If we convert this to 1b then it is equivalent to 1,406.54 1b, since there are 2.2046 1b in 1 kg. But what would this be in tons? Well, it depends on the type of unit we are using since there are actually three different definitions.

There is the long ton, which is 2,240 1b or 1,016 kg. Then there is the short ton which is 2,000 1b or 907.2 kg. Then there is the metric tonne, which is 2,204.6 1b or 1,000 kg. Using these conversions, we calculate that the Mars Climate Orbiter mass would be equivalent to 0.628 long ton, 0.703 short ton, or 0.638 metric tonne. This situation would be made even more complex when you are dealing not only with different design teams and different manufactures, but also with different nations around the world.

This is why the International Space Station in particular is such an achievement in the exploration of space. Although it has not gone anywhere, but been confined to Earth orbit since the first module was put up in 1998, it has involved many countries around the world and yet over this period astronauts have successfully lived within it without serious fatality. This is a remarkable achievement in systems engineering design.

Even with our complex technology, know your units!