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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

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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)

 

Guardian 54 Final Paint designs

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My design is a four color fade on the separating upper section. The booster with the fincan on it will have no paint whatsoever.

The fade is based on the fade design for gun/knife skins in CS:GO.

This rendering was done using SolidWorks 3D CAD software, where I have also done the majority of the design work on Guardian

Accidentally gone full Curt Von Delius – Introduction to my 54mm Min Dia, Guardian

Ridiculousness of minimum diameter rockets can of course be measured in units of Curt Von Delius. For example, fins on a minimum diameter rocket that have a span 1.5 calibers are about 45% kurt von delius. Scratchbuilding ups the CVD coefficient, giving you about 75% Curt Von Delius for the aforementioned scenario.

In my case, I accidentally went full CVD. (Well, almost. I have an airframe. That gives me about 95% CVD).

IMG_0524

 

100% Scratchbuilt carbon fiber MD with the motor case acting as the coupler. The fins are 9 layers quasi-isotropic 5.7oz plain weave carbon, at about 0.085″ thickness.The airframe is 4 wraps of 5.7oz plain weave carbon, about 0.045″ thickness.

The motor is a Loki Research 54/2800 case, with either Soylent Green or my new ‘Supernova’ propellant. Expecting Mach ~2.5-3.1