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At each SpaceX launch, they always make some big deal of Max Q which most of the time comes at around 13Km and at a speed of 1700Km.h-1

But on the way back, stage 1 is at this same altitude of 13km at around 3000km.h-1, and instead of having aerodynamic fairings to help splitting the air, it shows several nozzles to the airflow.

Here is a plot from FlightClub.io showing a simulation of the aerodynamic pressure (Q) on the latest Starlink launch showing this difference:

Falcon 9 aerodynamic pressure plot

So the aerodynamic pressure on the nozzles is far more impressive than the one while climbing, isn't it?

Then why making such a big deal with this first Max Q which is by far, less important compared to the second one?

Thanks for your ideas and comments,

Lim.

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    $\begingroup$ The cost of failure makes it much more exciting on the way up ;-) $\endgroup$ Jan 19 at 11:43
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    $\begingroup$ One factor is historical reasons. Maximum dynamic pressure was quite an important event in rocket launches back in the days. Announcing it has become somewhat of a tradition. And, as pointed out, the return is just bonus. If something fails on return, the launch was still successful and the mission can happen. Recovering the booster is merely cost savings for SpaceX, that is simply not as exciting. However, all of this is my speculation and impression as an observer, not backed by hard facts. $\endgroup$
    – Polygnome
    Jan 19 at 13:40
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    $\begingroup$ My opinion: During ascent, the rocket has a payload. It's very important to deploy payload in the desired orbit or else. That is exactly what a rocket is for. After the descent, it becomes less important than the previous case. $\endgroup$
    – Auberron
    Jan 19 at 14:07
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    $\begingroup$ In addition to what others have said, look at the nature of the forces on the rocket. During ascent, the entire vehicle is being squeezed between the engines pushing on the bottom and the air pushing on the top of the fairings, which themselves are just hollow ultra-lightweight shells protecting the payload. During descent, most of the mass is in the thrust structure and engines at the base. Internal components like helium tank mounts might see higher forces due to the high accelerations, but much of the structure sees lower loads. $\endgroup$ Jan 19 at 15:18
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    $\begingroup$ Related: Has Max Q historically been a common failure point? $\endgroup$ Jan 19 at 17:07

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Then why making such a big deal with this first Max Q which is by far, less important compared to the second one?

Calling out Max Q is a tradition.

The overwhelming majority of vehicles that launch to space either never come down or we don't care in which state they come down. Therefore, for the overwhelming majority of vehicles and launches, Max Q on descent is completely irrelevant.

Because of that, there was never a tradition established to call out Max Q on descent.

Note that even for vehicles that do return, Max Q on descent is not necessarily relevant:

  • Space Shuttle and Buran had lower Max Q on descent than on ascent.
  • Falcon 9 has higher Max Q on descent, but by that point, the primary mission is over, so a failure would be less costly.

Crew capsules are an important exception: they experience significantly higher Max Q on reentry than on ascent. And mission control most certainly does care about those loads. But there is simply not a tradition of calling this point "Max Q" or calling it out over the press loop.

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