How to find stress of a rocket using Ansys

[ljproductions]

Hello Fellas,

I'm looking to try to model my rocket in Ansys to find the stress on the fins and rail buttons. I have used CFD to find the drag force which I am now going to use as constraints in the Static Structural analysis of the rocket. I just don't know how to set up a rocket during flight up in a static based solver. I know it has to be possible but I can't figure it out after researching for a day or two.

Please help,

Larry

 

[NTP2]

we don't do much CFD here we have a bit but if Ansys has forums that's the best place for help.

 

[Alan15578]

Ignore the drag force and use the normal force.

 

[lr64]

The drag force is fine for rail buttons unless they have some sort of extended shape. The lift force on the fins, at least at other than 0 angle of attack, is likely to create far more stress on the fin material than any drag force. Particularly since, if you look at the fin as a spar, it must be a hundred times deeper in the drag direction than it is in the lift direction. Furthermore, the drag is likely to be less than the lift at any reasonable angle of attack. I'm not sure what you're sticking point is, but if it was me, I might figure out how many degrees the angle of attack might get to, worst case, during the fastest part of the flight and figure out the stresses for that condition. I'm guessing this might be during coning? Not that I know how to evaluate coning or anything. Most likely, you won't get to higher angles of attack, where vortex lift could muddy up the picture. Something else would probably break first.

In any case, I suspect that, with fins that thin, flutter may be more of an issue than lift forces. At least they have a low aspect ratio and are tapered. Flutter analysis, of course, is far more difficult than stress analysis.

I admit I don't know anything about Ansys.

Caveat: My actual rocketry experience is limited, but I have an engineering background and have been looking into airplane stuff for decades on the hobby side. I own both volumes of Hoerner, though I can't claim to understand most of it. They make pretty good paperweights.

 

[lr64]

P.S. I suggest some back of the envelope estimates to see if the CFD and Ansys are in the ballpark. If they don't at least roughly agree, figure out which one is wrong. Or maybe both are.

 

[robopup]

Looking at your image with the full rocket, and your comment that you have CFD pressure forces, you probably are thinking of applying inertia relief as your boundary condition on the rocket. I'm not an Ansys guy so not sure what the specific Ansys card for this is. That said, it may or may not be what you want, and know that boundary conditions for structural models are very hard to get right.

In general, you're going to be much more successful if you try to solve a very specific question with simulation and start with a very simple model, especially if you can do a qualitative comparison rather than anything quantitative. For example: "I can choose from these two fin layups. If I assume a max normal force on the fin of X lbs at some reasonable location, I can expect layup A to have ~Y times the displacement/natural frequency/max strain etc etc etc of layup B." Then make a design choice based on that information. New FE users tend to throw the kitchen sink at a problem without any clear problem statement and end up with a lot of complex models, fancy pictures and wrong answers.

Also FYI that the mesh on your fins is extremely coarse for anything other than very rough max displacement results and that making that mesh much finer, even by a factor of 100, would have basically no noticeable performance hit for you. Google "mesh convergence" if you're interested in going further.

 

[robopup]

Ignore the drag force and use the normal force.

The lift force on the fins, at least at other than 0 angle of attack, is likely to create far more stress on the fin material than any drag force.

Adding, I'd guess OP has raw CFD pressure data which is resolved in neither drag nor normal force. It's just raw pressure data which, depending on the problem, is what should be used. Who knows what the angle of attack of the rocket/fin/etc was in their CFD simulation, but if they're interested in fin bending, hopefully it wasn't 0.0deg.

 

[rocketguy101]

As others have asked, what are you trying to achieve? If you are looking at stresses and natural frequencies of the fins, you don't need the whole rocket. Your meshing will go a lot easier if you aren't trying to mix solid elements with thin plate elements. From your published image, it does not appear the motor mount is modeled. Your tube stiffness would not analyze correctly with that type of detail.

I suggest you take a look at this thesis from the Air Force Institute of Technology. They analyze fin shapes for a sounding rocket built by the Air Force Academy. There is a lot of useful information there! They used FEMAP rather than ANSYS. They published an excerpt from this thesis as an AIAA paper, but I believe you have to pay to obtain a copy. You might find a copy in a university library or such. The thesis is free and more detailed.

Material properties and boundary conditions are the heart of this. Wood and composite materials can be tricky to model in FEA simply due to the fact the readily published values (MatWeb for example) may not exactly match what you are using...but close enough might work.

Ansys has a ton of great training videos free on YouTube...look for the Ansys Learning Channel.

Years ago, there was a posting about fin flutter and somebody posted some great pictures of a fin that had fluttered without failure. You could see the flutter patterns in the paint cracks. I ran a modal analysis at work and obtained some mode shapes. Some of them were close to the observed fin, but I have no idea if the frequencies were even close.