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This is not exactly what I would expect–I would expect that as the canopy changes shape during the filling process, the opening force changes somewhat.
As you can see here, main opening took approximately 11 seconds. We can compare this with a theoretical estimate derived from the following formula (from NASA TM X-1786):
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\frac{t_{f}}{D_{o}} = \frac{0.65\lambda_{g}}{V} |
To determine the shock force, we need to know the upwards acceleration applied by the parachute during opening (subtracting off the effect of gravity). Then we can calculate the opening shock factor using the opening shock force formula.
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