From September 2014

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.

 

 Capture

Computational Fluid Dynamics flow simulation of a Von Karman Nose Cone

ss+(2014-09-01+at+12.27.38)

 

I’ve been working on getting SolidWorks CFD to work for future projects. The first test was getting a simulation of the flow of a Von Karman Nose Cone. At Mach 4, the air is forced away from the cone at the tip and never rebounds. This means that there is a relatively low pressure zone about 2/3rds of the length of the cone from the tip. It is therefore probably okay to put Baro relief holes in that part of the cone, since according to SW, the pressure there is about what it would be in a conventional spot (about 5-6″ from the base of the cone, or in a vent band directly below the cone)