“Nominal” Soyuz launch sends Expedition 44 crew of three to International Space Station


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Apropos of nothing: How did “nominal” come to mean “as expected?”


I came to ask the same question.

It’s some weird artefact of the early space program technical language?


It means all goes as it should, nothing significantly out of ordinary. Apparently fairly common in engineering, not only in aerospace.


I figured that’s what the term means, but while I can see how a word that has to do with a name can come to mean “in name only” (e.g. a nominal fee), I was curious how it acquired the definition of “nothing out of the ordinary.” It just seems an odd stretch of language, as if there was something it was used for (actually relating to names) that the engineering use of the term grew out of.


There’s a discussion about it here:

1966 Aviation Week & Space Technology 5 Dec. 30/1
The mission is to launch the 800-lb. Prime vehicle to effect a nominal re-entry at 400,000 ft. following injection at 26,000 fps.
1970 N. Armstrong et al. First on Moon vi. 124
An example of misuse is our use of the word ‘nominal’, which most of the English-speaking world interprets as meaning small, minimal-and we usually use it in the sense of being average or normal.
1970 R. Turnill Lang. Space 94
Nominal, a favourite word, meaning within prescribed limits; anything from ‘perfect’ to acceptable.
1972 Daily Colonist (Victoria, B.C.) 26 July 3/1
As one engineer said, ‘She is phenomenally nominal’ — nominal being space jargon for operating-as-planned.


“Normal” has a special meaning within optics, mechanics, and perhaps even aeronautics.

Here’s a 1921 document that uses “nominal” as a synonym for “within expected values”


It looks like the word was mostly used in the sense of "the rated ___________(power, limits, weight, etc.), so people started to understand it as meaning ‘standard/corresponding to the nominal value’. Google gives one possible definition that links the two (presuming the term isn’t just a corruption of the word ‘normal’:

(of a quantity or dimension) stated or expressed but not necessarily corresponding exactly to the real value.
“EU legislation allowed variation around the nominal weight (that printed on each packet)”


Hmm. It may be an engineering thing in general. (I’m not an engineer).

When Nasa launches a spacecraft, they’ve already precomputed a large number of scenarios, so that the risks are minimized. If everything behaves as they predict, within certain tight tolerances, the spacecraft will be launched successfully and safely. Outside of those tolerances, they can recover by following certain procedures. And they’ve essentially rehearsed those recovery procedures to avoid disaster.

When a engine is nominally rated at 800 horsepower, an engineer will design a plane around that 800 horsepower figure. If the plane crashes because the engine could only deliver 550 horsepower when it was supposed to deliver 800, it’s the fault of whoever tested and maintained that engine, not the fault of the plane’s designer who specified a 800hp engine, plus minus some variation. The solution is not to junk the design but to improve the reliability of the engine-- unless of course the design relies on an unrealistic nominal value. This culture works well for the aeronautical community.

So, fast forward a few decades, and nominal becomes synonymous with “working as designed.” If a system departs from those nominal limits, other systems that rely on that nominal spec will have to compensate to ensure that they don’t fail as well.


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