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It was brought to the attention of the community that right before the crash of Antares the commentator noted:
It was noted that other engines were also reported to work at levels higher than 100% during launch, such as Space Shuttle operating at 104%.
Why is that and why is it considered nominal?
The original design called for say 500,000lbs of thrust.
After years of development, tweaks, changes in the real world (bonuses, like the pressure of the fuel in the line from the entire length of the tank boosts performance (SLS has a sort of issue with this)) means the production engine actually produces 540,000lbs of thrust.
Thus full power is now 108%. It is a bit silly, but easier to keep the specs than constantly redefine 100%.
If they said "engines at 500,000lbs thrust" instead of "engines at 100%" there would be no need to change.
100% is usually the power level where the turbo-machines operate at their design flow and pressure (head), which means they operate at maximum efficiency.
At launch it is often favorable to go above this value to reduce gravity losses. Although the turbo-machinery does not work at peak efficiency, the specific impulse may actually increase as well because running at a higher chamber pressure lessens the over-expansion in the nozzle at sea level.
Small glossary:
Chamber pressure: The pressure in the combustion chamber.
over-expansion: Because of the lessening atmosphere along the rocket's path, the nozzle is only perfect at one point of the mission. At launch it is too big, and at altitude it is too small.
Specific impulse: Kind of a fuel efficiency of the engine.
Turbo-machinery: The pumps that are used to feed the propellants into the combustion chamber.
In aeronautical terms, the word nominal simply means the component is operating within acceptable tolerances/parameters. In other words, it means that the engine is operating within a permissible deviation from a specified value. More often than not, these are expressed as a percentage of the nominal value. Later on in the flight, you'll likely hear them reduce the thrust to (for example) 50% of its nominal value to reduce acceleration loads as the propellant tanks empty.
There's nothing sacrosanct about operating at "100%" of that nominal value. It's just a reference point to calculate trajectories and other operational parameters.
The throttling capability of an engine (Thrust Magnitude Control (TMC)) allows the mission controllers to change the thrust of an individual rocket engine as needed. When a rocket is sitting on the launch pad, there's a big change in momentum needed to get it going. If the engine is designed to operate beyond the stated nominal value (in this case to 108%), you can push the engine in such a manner to get it going where it needs to be. Such capabilities are vital to achieve and maintain the trajectories needed for that specific load and mission.
