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I noticed this initially when learning to fly Robinson Helicopters but fail to remember the reason my instructor gave me; this again came up when watching a video about the ISS.

  • In helicopters (Robinson R22/44), the main rotor typically operates at 104% RPM.
  • On the space shuttle, after max q, they throttle back up to 104%.

What is the significance of 104%? If this is the maximum, why is this not called 100%, and everything scaled back to fit? This seems the most fit place to ask this, as I have only seen this in aviation / aerospace.

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This happens when the maximum output was changed after design.

100% is simply a reference to a certain value. In the space shuttle's case, the engine output was increased after the initial design. Instead of updating the engine value everywhere, they decided to simply keep the initial value constant.

The same goes for N1 in turbo engines. 100% N1 is simply referencing a certain datum defined by the manufacturer. Going over 100% is not necessarily unsafe.

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    $\begingroup$ You're on ten here, all the way up, all the way up. You're on ten-where can you go from there? Where? I don't know - nowhere. Exactly! If we need that extra push over the cliff, you know what we do? Put it up to 11. Exactly. One louder. Why don't you make ten louder, make ten the top number, and make that a little louder? These go to 11. $\endgroup$
    – ivanivan
    Sep 2, 2019 at 15:01
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    $\begingroup$ For gas turbines, there is an absolute maximum rotor speed, beyond which the stress levels are unsafe. But the power from the engine depends on the atmospheric pressure and temperature as well as the RPM. For a low and cold takeoff, the safe maximum N1 speed may be less than 100% N1. "100% N1" is typically the design speed at the base point for the International Standard Atmosphere tables and formulas, i.e. sea level altitude and 15C temperature. $\endgroup$
    – alephzero
    Sep 2, 2019 at 18:28
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    $\begingroup$ These go to 11 (clip referenced in @ivanivan's comment). $\endgroup$
    – Spencer D
    Sep 2, 2019 at 21:42
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    $\begingroup$ I was counting down the minutes to a Spinal Tap reference lol. $\endgroup$
    – John K
    Sep 2, 2019 at 21:55
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    $\begingroup$ Obligatory xkcd reference. $\endgroup$
    – reirab
    Sep 3, 2019 at 18:10
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@kevin has given the general answer, so I will only give the answer to the very specific question you asked:

What is the significance of 104%

None.

As was explained by Kevin, 100% is the design reference point. Engineers are generally very good at predicting performance, so they will typically only be off by a tiny amount. They are also typically conservative, so they will be off in the lower direction.

This means that the actual performance will typically be just a tiny bit above 100%, and there are not that many numbers that are a tiny bit above 100%. So, it is not surprising that out of the vast number of rockets, airplanes, motors, turbines, engines, etc. you have found two which happen to have the same rated performance figure of 104%.

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  • $\begingroup$ There is often actually very significant difference between rated performance (what the 100% is based on) and possible performance, though, due to significant safety margins in the designs. For example, the GE9X is only supposed to be rated for 105,000 lbf thrust, but produced 134,300 lbf in testing, breaking the previous world record of 127,900 lbf set by its predecessor, the GE90-115b (which was itself 'only' rated for 115,200 lbf.) $\endgroup$
    – reirab
    Sep 3, 2019 at 18:15

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