First and foremost, fantastic work Space Opera!! I have indeed been harvesting data from the webcasts manually. Data from your automated tool will be a very useful contribution to those of us that like to plot/analyze flight data.Next, let me jump into the discussion about Semmel's plots (which are great!).1) Being able to see the fairing jettison event in the acceleration data is very cool! I just checked the webcast, and fairing jettison occurs at ~3:40 (220 s), which is a rather good match to what we're seeing!2) I'm with ugordan on the reason the first stage throttling down around the 50 s mark: I think it's for the transonic region before Mach 1. In my estimation, Mach 1 is passed at 62 seconds (there's no "going supersonic" callout in the webcast for this launch, sadly), so the throttling is definitely occurring prior to going supersonic, and well before max Q.3) As for the dynamic pressure issue, I redid Semmel's plots except that I'm using the ISA1976 standard atmosphere for density/temperature calculation (I have a Python implementation of it, which I'll gladly share if anyone wants it). I'm attaching a plot that compares my dynamic pressure, temperature and density curves to Semmel's for the first 185 seconds. Semmel's curves, which I reproduced from the code he pasted upthread, are shown as dashed lines (the temperature curves are nearly identical).Semmel: I think there's a typo in your temperature table. The bold value should be 228.65 (=-44.5°C):std_atm_T = [288.15, 216.65, 216.65, 288.65, 270.65, 270.65, 214.65, 214.65]It doesn't make a huge difference, though (and none whatsoever before the ~95 s mark); the attached plot uses the corrected value.We're using the exact same data for velocity and altitude, so the only difference is in the density calculation. Comparing the plots, it seems that the small difference we have in computed density is enough to noticeably alter our dynamic pressure curves. In my calculation, max Q occurs at 72.2 s, reaching a substantially lower value of 29.6 kPa, mainly because density falls off quicker in the ISA1976 model.Semmel's atmospheric model is a bit weird because he's assuming piecewise linearly-varying temperature yet uses the exponential formula for pressure (which is the solution for constant temperature). IntoTheVoid provided the correct formula in the case of linearly-varying temperature (and that's all the ISA1976 does, really).Still, Semmel's time of max Q is much closer to the callout in the webcast (which occurs at ~83 s) than mine, so I don't really know which to trust more. I think the bottomline is that the dynamic pressure curve is kind of sensitive to small details in the calculation, so we should take it with a grain of salt.
Fantastic meithan, thank you for spotting the typo! And yes, the model I use is quite crude, working basically outside my comfort zone here. I would be happy to adapt your atmosphere model. I want to publish my script once I feel reasonably confident that its correct, is it ok if it contains (full or part) of your code?
I realised that this is much more complex than I thought. I have to fit a physical acceleration profile to the data. But part of the acceleration is eaten by gravity. That part depends on the centrifugal force which in turn depends on the horizontal velocity in orbital reference frame. The data is in ground reference frame and to transform in between the two, my 2D model is not sufficient because I can't model the orbital plane properly. I don't have the time to start over (child and family are more important). Maybe I model the acceleration in the 2D version and we just have to cope with the uncertainty. What do you think?