Designing with grid fins

[neil_w]

Let's say I wanted to use grid fins on an LPR model. Not a scale model, just a sport design for which grid fins would be well-suited. For LPR, they'd be comfortably in the subsonic region. Finally, they'd be 3D-printed, so there is flexibility with regard to geometry.

How does one go about designing them, and figuring out their contribution to CP? Are there any rules of thumb one can apply?

There are some grid fin designs on Thingiverse but none have the geometry I'd be looking for, and I wouldn't know how to evaluate them anyway.

 

[Nytrunner]

Calling the expert @TurbulentSphere

 

[Daddyisabar]

Lpr kits have them from Agressor Aerospace. Scott Haskel would know. He is designing new Chinese and Israeli scale rocket kits.

 

[Daddyisabar]

Let's say I wanted to use grid fins on an LPR model. Not a scale model, just a sport design for which grid fins would be well-suited. For LPR, they'd be comfortably in the subsonic region. Finally, they'd be 3D-printed, so there is flexibility with regard to geometry.

How does one go about designing them, and figuring out their contribution to CP? Are there any rules of thumb one can apply?

There are some grid fin designs on Thingiverse but none have the geometry I'd be looking for, and I wouldn't know how to evaluate them anyway.

Just mindsim!

 

[neil_w]

Just mindsim!

My mindsim doesn't currently support grid fins (it's on the TODO list).

 

[BABAR]

Experimentation.

Use your best SWAG (acronym, not the word)

Build a small 13mm version of it and fly it with no one around to laugh at you. You can consider swing testing, although some claim significant false positive and false negative, my preference is to fly it in a controlled environment, no spectators and no dry grass.

Adjust accordingly.

 

[SDramstad]

Could you use tube fins as a stand in for grid fins? Bunch of small tube fins?

 

[JoePfeiffer]

I've only glanced at this, but it might be a starting point. Could be an OR feature request for a future release....
https://www.icas.org/ICAS_ARCHIVE/ICAS2010/PAPERS/261.PDF

 

[neil_w]

Experimentation.

Sounds like great fun but not practical for me, unfortunately.

Could you use tube fins as a stand in for grid fins? Bunch of small tube fins?

I don't know whether tube fins could stand in for grids or not, but I wouldn't trust whatever OR is telling me at that point anyway.

I've only glanced at this, but it might be a starting point. OR feature request for a future release....
https://www.icas.org/ICAS_ARCHIVE/ICAS2010/PAPERS/261.PDF

Thanks, I will look through that.

 

[mbeels]

Now I'm very curious to see what is coming next. First hints of a 3d printed nose cone, now this. Interesting.

 

[mbeels]

If it were me, and I had time to experiment with the simulation software, I would do the following. (I don't actually know anything about grid fins, but this is what I'd want to try).

Assuming that grid fins add to stability with both a drag component and a fin-like component, I'd run two sims. One with 3 (or 4) normal fins in the same location as the grid fins, with the same side area. Perhaps that gives an estimate of the fin-like contribution from the grid-fins. Then I would do some base drag estimations using an area similar to the frontal area of the grid fins. I'd guess that the real CP of grid fins would be somewhere behind both of those two results.

 

[neil_w]

Now I'm very curious to see what is coming next. First hints of a 3d printed nose cone, now this. Interesting.

Probably nothing will come of this, just an exploration. I have a specific application in mind but I won't be revealing details because it's based on someone else's extremely clever idea, and I don't want to steal their thunder since they will possibly be posting it all to TRF at some point.

Assuming that grid fins add to stability with both a drag component and a fin-like component, I'd run two sims. One with 3 (or 4) normal fins in the same location as the grid fins, with the same side area. Perhaps that gives an estimate of the fin-like contribution from the grid-fins. Then I would do some base drag estimations using an area similar to the frontal area of the grid fins. I'd guess that the real CP of grid fins would be somewhere behind both of those two results.

Not a bad idea. The way this could fall down is if all the interactions within the grid create effects that are not anticipated by the other models. Quite possibly, it'll be a decent approximation in the subsonic region. Need to read through the paper than Joe attached and see what I can glean from it.

 

[kuririn]

As pointed out in the paper Joe linked to, the Vympel R-77 (Adder) missile has grid fins.


Looks like an interesting project for someone. Any takers?

 

[neil_w]

Erockets came out with a Semroc kit a couple of years ago using grid fins made of laser-cut wood. I applauded the effort, but the fins looked way to chunky to me. I don't see that kit anymore, and don't recall anyone posting about building one. I think it was of a missile somewhat like the one pictured above, but I don't remember details.

At small sizes I think 3D-printing is the only way to go (or injection molding, I suppose). They'll probably always look chunky compared to the real thing, but could maybe be "good enough".

 

[BABAR]

Third pic is Semroc MOAB from

https://jonrocket.com/store/products/moab-model-rocket
Not being a Sim Dude, I was wondering however if from a model rocket standpoint, these could be simulated simply as solid flat plates?

 

[neil_w]

Thanks, that’s the one I was thinking of.

 

[dhbarr]

Modeling-wise, flat plates would dramatically overestimate total drag.
A collection of tube fins will likely underestimate interference drag, but in most other respects should be okayish.
Combining angled polyhedral finsets and tube fins could likely approach a reasonable approximation.

 

[BABAR]

Modeling-wise, flat plates would dramatically overestimate total drag.

You may be right. Do you have a source?
My understanding of tube fins is that once you exceed a certain diameter to length ratio they act like solid cylinders rather than fins. In any case, LONGER tube fins are not necessary more effective than shorter ones, after a certain point, a situation that is not true (or at least much less true) with standard fins.

Reference

https://www.rocketryforum.com/threads/another-tube-fin-thread.59741/
Post 26
@gpoehlein

I think this may be acccentuated with smaller diameter tubes. Seems like for model rocket grid fins, the spaces (basically the holes) would be very small indeed, though still far larger obviously than individual air molecules, I suspect the air resistance would be very high, and the probability of laminar or indeed ANY directional flow would be low.

https://www.apogeerockets.com/education/downloads/Newsletter347.pdf
So while simulating grid fins as flat plates MAY overestimate the drag, I doubt it would be very much. I think even low power subsonic rocket velocities are going to have a hard time generating relatively high velocity flow through those little holes.

 

[boatgeek]

Another option is to model the grid bars as individual fin sets. It would be pretty tedious and you miss the added drag of the interactions, but that only adds to the drag and moves CP aft. I would definitely take the delay down a step to account for added drag.

Option 2 as a subset of the one above is to model grid bars and stop when the CP is far enough back, and hopefully before insanity sets in.

 

[Alan15578]

Let's say I wanted to use grid fins on an LPR model. Not a scale model, just a sport design for which grid fins would be well-suited. For LPR, they'd be comfortably in the subsonic region. Finally, they'd be 3D-printed, so there is flexibility with regard to geometry.

How does one go about designing them, and figuring out their contribution to CP? Are there any rules of thumb one can apply?

There are some grid fin designs on Thingiverse but none have the geometry I'd be looking for, and I wouldn't know how to evaluate them anyway.

I would put your grid fin into a wind tunnel or other suitable measurement rig and measure its Cn(alpha). You can then put this devices and data into your simulation. If you are using canned simulation program that does not allow this, simply determine a fin that has the same Cn(alpha) and CP as your grid fin and enter that instead.