How do they determine dynamic pressure has hit a max?

Question

Max-Q is a key event during launch, and dynamic pressure is closely monitored so they know when a max is hit.

But how exactly do they know when Q hits a max? If you track the rate at which Q is changing, then as that rate drops to zero, you can say you're approach your max---and once you hit zero, you can say "max-Q."

But dynamic pressure data is noisy, and you'll see minute troughs and crests in it which would cause you to incorrectly say "we're approaching a max" and "we've hit a max."

So to use your noisy Q data, you'll have to first remove the high-frequency noise, so the resulting signal is smooth and free of spurious crests and peaks. The only meaningful peak then would be your max Q.

But is this how it's done?

Answer 1

At least for some vehicles, "dynamic pressure is closely monitored" is not correct. You need to have an air data probe to actually monitor it, and not all vehicles do.

Shuttle:

Dynamic pressure was not actually measured¹ during ascent so "Max Q" was not either. The magnitude and time of Max Q was predicted by prelaunch simulations, and the actual values were calculated in post-launch analysis.

Shuttle orbiters had air data probes, but they were only deployed below Mach 5 on entry.

Here's a plot from my logbook showing the dynamic pressure, etc predictions on the day that STS-114 finally launched. Based on data from the L-3:25 hour balloon and the steering commands based on the winds of the day.

Here is the post-flight reconstructed plot of Q (from the L+2 day quicklook presentation). The two lines show the calculations based on 2 different balloons, one pre- and one post- launch.

¹ There may have been an onboard calculated value, but not a measured one.

Source: Worked on day-of-launch trajectory verification for two shuttle missions

Answer 2

There are ways to measure dynamic pressure, and in aerodynamically complicated spacecraft (like the shuttle) if measured on its three four "nosecones" it could conceivably occur at (at least somewhat) different times in different places.

And yet when we watch a launch there is a specific time when the announcer calls out "Max-Q!" at which time the rocket doesn't rapidly and unscheduledly disassemble and everybody breathes a sigh of relief.

A while ago I asked a similar question about the definition of the moment in time that we refer to as "Max-Q": How is max Q for the shuttle actually defined? and @MarkAdler answered

Max Q is simply the maximum of the dynamic pressure of the external flow, ${1\over 2}\rho v^2$. It has nothing to do with the vehicle, except for the vehicle's speed relative to the undisturbed fluid.

I'm not sure if everyone will 100% agree with the exact wording there and lack of caveats, but my interpretation is that when they call out "Max-Q!" on TV, it is the time when the current airspeed of the rocket squared times the expected density of the air at the rocket's current altitude is maximum.