Unnusual fin design question.

[CZ Brat]

My club is having a contest where the goal is to fly a rocket 8 times in one day on each of the following twice: 1/2A6-2, A8-3, B6-4 and C6-5. If your rocket gets damaged, your DQ'd (not Dairy Queen). The other aspect of the contest, is the total cumulative flight time of all 8 flights (assuming your rocket is still alive). So you want it to go low and fall fast.

So I was considering replacing normal fins with a disc. I have attached a OR screen shot to show what I mean (the best rendering I can do in OR). But the CP is much higher than I think it would be for real.

Anyone have experience with this or something similar?

For simulation purposes, the fins are 2.5" high, 3' thick, and root and tip chord of .25". Material is balsa.

 

[Solomoriah]

Rockets using discs or other flat plates for stability are not stabilized in the normal way. Base drag created by pushing the plate through the air is what provides stability. Unfortunately, programs like OpenRocket (and the Barrowman Equations such programs are based on) only figure stability the normal way.

Drag-stabilized rockets like your design become unstable pretty much the moment the engine burns out. They tend to start tumbling immediately. If you build one small enough, you can actually depend on that behavior for recovery (as it is, strictly speaking, a "tumble recovery" design).

 

[TheAviator]

Try a Tasmanian Devil. They're a hoot, and once they get spinning, they don't go much of anywhere.

https://www.rocketryforum.com/showthread.php?9940-Upscale-Tasmanian-Devil
https://georgesrockets.com/grp/CONTEST/Plans-C/Copter/TasmanianDevil.pdf

 

[GlenP]

the disc may get ripped off the tube without some additional glue seams and supports to hold it in place. try adding some conventional fins as supports, or angle brackets, to attach the tube to the disc. alternately, you might consider building a cone or larger diameter saucer for the tail.

 

[Solomoriah]

Glen, I've never seen a failure like that. I suppose it would depend on the disc material, but even a C engine wouldn't hit all that hard.

 

[BABAR]

Monocopters would work as well.
I'm assuming given your
"The other aspect of the contest, is the total cumulative flight time of all 8 flights (assuming your rocket is still alive). So you want it to go low and fall fast."
that this is like golf, the LOWEST score wins?

 

[Charles_McG]

18mm scale of Estes blender. Handles a C nicely.

 

[cbrarick]

one of Art Applewhite's steath's

 

[GlenP]

Glen, I've never seen a failure like that.

I have never seen a rocket with fins like that, so I defer to your experience.

If they are balsa, some thought should be given to the grain direction at least, making the disc from wedges with the grain oriented radially outward, or possibly laminating two sheets together with grain at 90deg, if not too heavy. The extra thickness would help with the body tube to disc glue joint as well for strength.

 

[AlfaBrewer]

one of Art Applewhite's steath's

Or a cone fin like the Deep Surface Probe from Rogue Aerospace.

https://www.oldrocketplans.com/rogue/rog10028/rog10028.htm

 

[GlenP]

I stumbled upon this: when the moon hits your eye, like this rocket
https://www.squirrel-works.com/catalog/pie/pie.html

 

[Solomoriah]

I have never seen a rocket with fins like that, so I defer to your experience.

AH.

You are confused, I'm afraid. He's talking about a disc at the rear (like half a spool rocket). The design he shows is as close as he could figure out in OpenRocket; the OP expected that OpenRocket could give him guidance on the stability of the design, but it can't (as I pointed out) because discs like that are drag stabilized.

 

[GlenP]

What if you put a really large body tube with a short transition at the end of the rocket, could that work in open rocket, since it would not be using a fin model, but a body drag model. There may be some equivalent body transition size for a disc of a given size?

 

[Solomoriah]

You don't understand. OpenRocket does not know how to figure stability when you are using drag. It will consider a cone that flares out as improving the stability only based on the Barrowman effects, but will not take base drag into account. There is no way to simulate your design with any simulator I know of.

 

[Solomoriah]

(In other words, you've got to build it and test it to find out.)

 

[GlenP]

There was an interesting article in an old Peak of Flight newsletter. Not sure how valid this approach is for general disc fins. The example shows how to make a correction for base drag calibrated for a known stable rocket, the FatBoy, to match an essentially known stability margin using RockSim. Not sure if that same scheme would work for an unknown rocket. But a clever idea. Maybe not applicable in OpenRocket.
https://www.apogeerockets.com/education/downloads/Newsletter154.pdf

An older article about base drag in RockSim, not sure if this is taken into account in the stability analysis:
https://www.apogeerockets.com/education/downloads/Newsletter103_sm.pdf
"RockSim does calculate base drag. You can find it by going to the Cd Analysis screen to see what it comes up with."
the base drag might only come into consideration in trajectory/altitude sims, but not for stability, perhaps?

Maybe a good question for the authors of those articles. It does sound like there is some experimentation required in any case.

from: https://www.apogeerockets.com/Newsletter/newsletter_archive

 

[bill_s]

^^-- It's not really base drag, it's a drag object at the base end of the rocket. However simulating it as a cone like that, in this case larger than the airframe diameter, would be one way to do it. In other words, use a cone fin which it can sim. It might even be appropriate to move it back like that, or extend the rocket for sim purposes, depending on what is believed about flat drag surface characteristics, although actually OR's sim of cone fins can be wildly optimistic as it is. Consider also a cone fin, they are also quite draggy, diameter without much weight, a good structural shape too. On my 5.5", 5 oz. rocket you can see every dip and bump in the thrust curve during flight.

I don't think OR can put a nose can in the back, but you can use transitions.

The simplest and roughest calculation would be the old cardboard cutout method, turning the drag plate sideways.

 

[Solomoriah]

As I understand it, OpenRocket uses a version of the Barrowman equations to figure stability. Drag is considered only as it affects altitude, not as it affects stability. I might be wrong, but I don't think either OpenRocket or Rocksim can properly indicate the stability of a rocket stabilized by drag.

 

[bill_s]

^^-- I don't know much about drag plate or disc characteristics or how much is known -- it may be anyone's calculation is going to be rough. Overconservatism and/or test may be needed. It is still more simulatable than when the whole rocket is a disc such as a saucer -- in that case the "virtual" CP of the drag shape being behind the shape itself becomes essential. However you should be able to calculate something. OR requires something visible in the side view to go on for the disc to even try, such as a cone. Be sure to check CP at 90 degree angle of attack as well, since that is often a worst case.

I was told that OR can't simulate a finless rocket and that if you tried it would never show it as stable. So I tried it for a design and the only problem was OR was overoptimistic. The 90 degree AOA CP was usable though. I used it as a tool and in that sense it was helpful, but in the end I didn't let it trump wind testing results and flight seemed to confirm that was a good thing. I have another design I derate to 4 fins for CP calculation due to configuration but use all 6 for altitude and CG. You can't always use a design for sim that actually looks like your rocket.

Just noticed in the original post: 3' thick?!? that would be another way, model the disc as much thicker, but 3 feet would be overkill

 

[Micromeister]

Actually guys Its a combination of Drag and Base Drag that makes this kind of model stable.

There are any number of different combinations using "Flat Plate drag stability on all kinds of models. CZ Brat your design is fine, if it were me I'd close the drag fin/body seam hole and use a decent fillet top and bottom of the seam.

Below are just a few of the various Drag Plate flying models I have currently in my fleet. Hope the variety and different mounting methods help with your project