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Was playing with some supersonic sims in OpenRocket at lunch today. Just plugging in the CTI 29mm 4-grain loads. Saw some weirdness. Max speeds ranged from ~Mach 1.1 to ~Mach 1.6.
With the H255, the stability factor reported in the main display panel was dramatically less than with any of the other motors. Looked them up on Pro38.com and the loaded weights are all within a few grams. I exported the sim and looked at the file in detail. With nose weight that was giving me 1.49 calibers in the display panel, it came off the rod with more stability factor than that and went up, approaching 3 calibers at max velocity. It only dipped anywhere close to the 1.49 a couple of tenths of a second before apogee, which I'm guessing was caused by AOA change as it nosed over. Haven't exported data for the lower-average-thrust motors, but I suspect the much lower value of stability reported on the display panel is an anomaly related to that weird row, rather than something actually systemic.
My overall concern I was investigating was with the forward shift and the standard rule of thumb to ensure at least 2.0 calibers of stability so that when the supersonic forward shift of CP is in full effect, you're left with one. What I saw in the detailed sim output was the reported stability factor increasing dramatically with speed, as speed went both up and down.
I took a quick look at the tech doc white paper discussing the physics behind the sim, and it discussed an interpolation method to account for the supersonic forward shift of CP. So it's logical to assume that that is included in the sim and when it says something like 2.8 calibers at Mach 1.6, that really is 2.8 calibers at Mach 1.6 (leaving wiggle room for precision in that it's not a perfect sim, of course). Is that a correct assumption? Or could it be closer to, say, 2.2 calibers in reality?
@neil_w
With the H255, the stability factor reported in the main display panel was dramatically less than with any of the other motors. Looked them up on Pro38.com and the loaded weights are all within a few grams. I exported the sim and looked at the file in detail. With nose weight that was giving me 1.49 calibers in the display panel, it came off the rod with more stability factor than that and went up, approaching 3 calibers at max velocity. It only dipped anywhere close to the 1.49 a couple of tenths of a second before apogee, which I'm guessing was caused by AOA change as it nosed over. Haven't exported data for the lower-average-thrust motors, but I suspect the much lower value of stability reported on the display panel is an anomaly related to that weird row, rather than something actually systemic.
My overall concern I was investigating was with the forward shift and the standard rule of thumb to ensure at least 2.0 calibers of stability so that when the supersonic forward shift of CP is in full effect, you're left with one. What I saw in the detailed sim output was the reported stability factor increasing dramatically with speed, as speed went both up and down.
I took a quick look at the tech doc white paper discussing the physics behind the sim, and it discussed an interpolation method to account for the supersonic forward shift of CP. So it's logical to assume that that is included in the sim and when it says something like 2.8 calibers at Mach 1.6, that really is 2.8 calibers at Mach 1.6 (leaving wiggle room for precision in that it's not a perfect sim, of course). Is that a correct assumption? Or could it be closer to, say, 2.2 calibers in reality?
@neil_w