I need to spend some time to clean up my RASAero model, but this is what I have from OR.

The rocket is a Madcow 1.6" Tomach modified to minimum diameter

Here is a screen shot of the OR simulation (note - I did start with the file from Madcow - thanks)

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And here is a plot of the CP/CG calibers and Mach number

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Some things I noticed. The first figure on the right states I have around 2 calibers at Mach .30, but the CP/CG curve doesn't show 2 calibers until Mach 1

Not to speak for the Open Rocket developers, but I'm assuming that the 2.0 calibers stability margin at Mach 0.30 on the right-hand side of the rocket drawing is with no wind.

Again, you can apply the standard stability margin for the Barrowman Method (which with no wind I assume is the method used for the CP shown on the rocket drawing) of 1.0 calibers, and then add an additional 1.0 calibers of stability margin for wind induced instability.

The rocket clears the launch rail with the 11 mph wind, and with wind the method used in Open Rocket (as I remember the Galejs What Barrowman Left Out Tech Article Method) moves the CP forward. Actual stability margin when the rocket leaves the launch rail, 0.30 calibers positive stability margin.

Longer rail of course, higher velocity leaving the rail, for the same wind speed a lower angle of attack and therefore less forward movement of the CP.

The same analysis of course can be done on RASAero II. My viscous crossflow model has improvements over the Galejs model (although I did use the Galejs body planform area equations, thank you Galejs), but it is good that the Galejs model is included in Open Rocket.

So using the Barrowman Method for Subsonic CP, I recommend a minimum stability margin of 2.0 calibers for Subsonic. This example illustrates why the extra 1.0 calibers stability margin is there for wind induced instability.

With the Rogers Modified Barrowman Method, since it already includes the forward movement of CP with angle of attack, applying a 2.0 calibers stability margin to the predicted Subsonic CP is probably too conservative. But RASAero II just uses a minimum stability margin of 2.0 caliber for All Mach numbers and All CP methods, and after the warning message is displayed, the User can then go look at the CG and CP details and further assess the stability situation.

Chuck Rogers

Rogers Aeroscience