Stability Analysis – OpenRocket vs. RasAero vs. Aerolab

I’ve always been distrustful of RasAero’s stability analysis. And when my RasAero designed 54mm Minimum Diameter went unstable at Mach 2.5, this distrust was confirmed.

Keep in mind that I am only testing free programs here, so analyses from programs such as Solidworks CFD and Aero/HyperCFD are not included.

While OpenRocket predicted this instability, it predicted that my rocket was unstable at around Mach 1. In the flight, this proved to be untrue, as it powered through Mach 1 and went unstable right at the end of the burn, at around mach 2.5 – While RasAero claimed I was stable until Mach 4.

OR was far too pessimistic with it’s stability analysis, while RasAero was far too optimistic. RockSim included no Velocity-based CP predictions, which means that it’s essentially useless for this purpose. My next goal was to find some middle ground between RasAero’s extremely optimistic predictions and OpenRocket’s extremely pessimistic ones.

This is the predicted CP for my 75mm MD using three different programs. As you can see, OR is the most pessimistic of the simulations while RasAero is the most optimistic. Both are wrong – RasAero thinks this rocket is stable until Mach ~4.5, no matter the Angle of Attack, while OpenRocket claims the rocket goes unstable at Mach ~2.3. Aerolab is a fairly unknown program but it seems to be the best middle ground between the two. I will do more tests with more data in the future.

 

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One comment

  1. Chuck Rogers says:

    The RASAero Supersonic Center of Pressure (CP) issues identified here were corrected in the new RASAero II software released on Sept 12, 2015. The new RASAero II software is available as a free download from the RASAero web site at http://www.rasaero.com .

    See the RASAero II CP prediction comparisons with Supersonic CP wind tunnel data for the ARCAS sounding rocket in the following Rocketry Forum thread:
    http://www.rocketryforum.com/showthread.php?130843-RASAero-II-Comparisons-with-Supersonic-CP-and-CD-ARCAS-Wind-Tunnel-Data

    and the CP shift with Mach number, including my recommendation for using 2.0 calibers stability margin for all Supersonic Mach numbers, in the following Rocketry Forum thread:
    http://www.rocketryforum.com/showthread.php?130690-Affect-of-mach-on-CP

    Chuck Rogers
    Rogers Aeroscience

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