On the official webcast of SpaceX's Iridium-4 mission a "throttle down and throttle back up of the Falcon 9 first stage engines" is mentioned. Why would one throttle down at a crucial time during steep ascent, way before MECO? Is it to weaken the effect of max Q?

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    $\begingroup$ For those that aren't regulars on Space.SE, MECO - Main Engine Cut Off, Max Q - Maximum Aerodynamic Pressure. $\endgroup$ – JPhi1618 Jan 3 '18 at 15:40

Why would one throttle down at a crucial time during steep ascent, way before MECO? Is it to weaken the effect of max Q?

Yes, exactly that. The engines are throttled up to the highest extent possible during all phases of flight to reduce gravity losses; but must be throttled down during at least two points during flight:

  1. When Falcon passes through Maximum Aerodynamic Pressure (Max-Q) which is a combination of atmospheric density and velocity that puts the greatest strain on the vehicle. This effect is more prominent during Dragon launches due to the frontal shape of Dragon.

  2. As Falcon nears MECO to limit acceleration on the payload, if necessary.

These two flight aspects can be seen very clearly in this exceptionally detailed graph produced by /u/veebay on r/spacex:

enter image description here

The graph on the right hand side shows a reduction in net acceleration for Dragon missions around Max-Q, along with a tailoff in net acceleration just before booster cutoff.

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    $\begingroup$ Technically, you don't have to throttle down during Max-Q. You could also build a stronger rocket. But you don't want to lift a stronger (and thus heavier) rocket for the entire 2 minutes just because you need strength for literally a couple of seconds, and that's why you opt to throttle down. $\endgroup$ – Jörg W Mittag Jan 3 '18 at 20:19
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    $\begingroup$ Anything's true if you put it in the right context. Technically you could build a stronger payload too and not need to throttle down prior to MECO, also. But, alas, we live in reality and have technical, physical, and structural constraints, so Falcon does have to throttle down :) $\endgroup$ – ReactingToAngularVues Jan 3 '18 at 21:29
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    $\begingroup$ Technically, you don't have to do either. You simply have to reduce atmospheric density to reduce the stress incurred during Max-Q. Oh, sorry, thought this was World Building SE. $\endgroup$ – corsiKa Jan 4 '18 at 1:33
  • $\begingroup$ /u/veebay made several more interesting graphs. Worth studying! $\endgroup$ – asdfex Apr 3 '18 at 18:12

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