Rocksim stability

[Steven88]

Do any of you trust the Rocksim stability equation for your sims or do you use only Barrowman or cardboard cut out method? I have a rocket that I am getting a stability margin of 1.25 on Rocksim stability equation, but on Barrowman it’s only .47 and Cardboard cutout its -1.41. Would you trust it to fly it on an I motor with those equations?

 

[UhClem]

I trust the Rocksim version: ATACMS

Barrowman: 0.19, Rocksim: 1.16 (with K550)

Run a simulation with your chosen motor and expected winds then plot the CP location and see how much it moves. It can move a lot on rockets with a high length to diameter ratio. The CP location should always be behind the CG. With more or less margin depending on how much it moves.

 

[Rob702Martinez]

Plotting, --time---cp location or caliber---mach or velocity-- is what shows that mach jump during flight. From what I understand if you start sims loaded at 3 calibers and end around 1.2 you're missing the cp bouncing in and out of those ranges approaching mach transition speeds and above. The calibers don't, usually, decrease in linear fashion. Also getting the flight ready weights and dimensions are critical to the sims. If you are able, override the total mass and individual mass by weighing each part and entering it.

 

[Nytrunner]

Do any of you trust the Rocksim stability equation for your sims or do you use only Barrowman or cardboard cut out method?

There's an article from Apogee describing how they took the basic Barrowman equations and reworked them to remove some of the simplifying assumptions and allow analysis of more complex rocket geometries. I'd say the Rocsim equations give a less conservative/more accurate representation of the CP loction