# How do they determine dynamic pressure has hit a max?

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?

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

• Wow! Very nice. That is so cool @OrganicMarble. Hey, I've wondered about the little crests that you see clustered near max-Q---the shoulders to the left and right. I've gotten them in my calculations but I assumed they were caused by kinks in my density-altitude lookup table (I use linear interpolation between data points, and that means slope changes discontinuously, sometimes by a lot at certain points). Anyway, wondering what caused the shoulders in your plot, because it would give me context to understand the shoulders I'm seeing on my side. I don't launch rockets like you did, though :D
– user36480
Jan 9 at 0:52
• I bow, I bow. It's so cool that you've worked on shuttle missions. I didn't know they didn't actually monitor Q. Thanks for sharing!
– user36480
Jan 9 at 0:55
• @Alex for shuttle anyway most of those jaggies are likely due to high-frequency features in the wind. Here's the in-plane measured wind velocity for that balloon i.imgur.com/Ct8l68g.png y-axis is cut off but it's altitude Jan 9 at 1:37
• Interesting! Yeah, wind would seem a logical explanation for the shoulders. Thanks for showing.
– user36480
Jan 9 at 3:21
• "150, okay. 160, okay. 200, errrr disaster for space station." Jan 9 at 19:13

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.

• How about 4 nosecones? ET, 2 SRBs, and the orbiter. Jan 9 at 2:50
• Thanks, @uhoh. I guess my question has to do with determining you've hit a maximum, period. The definition of Q is velocity squared times density, so if they're not measuring dynamic pressure directly, they must be calculating it from other measurements. But whichever way they go, they still have to figure out from their Q data that "here" is a maximum, and do it in real time. They would have to clean up their data to get a smooth curve, it seems, and they would have to look at the rate at which the data changes to spot a peak without having to wait for Q to start to come down...
– user36480
Jan 9 at 3:25
• @OrganicMarble 10 AM Saturday morning after a 48 hour news binge has left me unable to count as well as a laboratory pigeon. Edited, thanks!
– uhoh
Jan 9 at 4:20
• @Alex your question asserts without proof that "Max-Q is a key event during launch, and dynamic pressure is closely monitored so they know when a max is hit." I have a hunch that you're simply wrong about the second part. It's true that the trajectory program reduces thrust for a period of time around Max-Q (both before and after) so that the compressive forces are reduced & perhaps to save a bit of fuel overall in some cases but I don't think that these days there is any actual measurement of dynamic pressure that is used. Instead it all comes from measured velocity and atmospheric models
– uhoh
Jan 9 at 4:30
• "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." For shuttle the PAO officer was reading off a script which had the event listed at the predicted time. Jan 9 at 19:46