Q: Why do they say "nominal" instead of “normal” when they launch rockets?

The relevant definition of “nominal” is, “as named.”

Nominal, in this technical context, does not mean “normal.” It means as planned, as named, or as written (in the mission plan). Often, it means “within acceptable or expected boundaries.”

When we are talking about complex systems, “normal” doesn’t mean much, but “nominal” means “according to plan, given these conditions, at this stage of the game.” For example, a rocket engine might by operating at 109% of its rated thrust, or might be idle either of which might be nominal at different times. Tank pressure might be high or low, a failure might have occurred—but in an anticipated way and at an acceptable time. Any of these might be nominal—if they contribute to keeping the mission within planned parameters.

During the launch of Apollo 13, the center engine of the second stage shut down prematurely. Originally, it was not meant to shut down at all, as its operation would not cause excessive G forces to build up—the reason for the early shut down of the center engine in the first stage. But, early Saturn flights experienced a dangerous POGO oscillation in the second stage, so the center engine was programmed to shut down to reduce the energy available to feed into this oscillation.

But during the Apollo 13 launch, the second stage center engine shut down earlier than planned. Later analysis determined that this was because the POGO oscillation had already set in—the engine shut down because it was shaken to bits. This was a failure, but it didn’t affect any other systems and it occurred within the limits of when a shut down could occur without pushing the rocket’s trajectory out of acceptable limits. That is to say, the launch remained nominal, even though it most definitely was neither normal or uneventful. Subsequent flights programmed the shut down a bit earlier and had no further problems.

This aerospace usage derived from engineering, where “nominal” refers to the named (written) specifications for a component. So for example, a nominal 12 volt battery might charge to 13.8 volts and drain down to 8 before it fails. A nominal 4 ohm resistor will actually have any resistance within a certain range, and that resistance will vary with temperature, etc.

[plot showing nominal aircraft roll, actual aircraft roll, and the pilot inputs used to achieve it.]

This illustrates why technical fields often use very specific technical language rather than everyday language. Often, such language conveys nuanced information far more concisely than common or colloquial equivalents.

If you like science, you might enjoy my free award-winning scifi sampler.