What is the upper limit of standard TTW fins?

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wsume99

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I'm looking at options for doing a L1 cert and I'm wondering what the airspeed limit is for "stock" TTW fins. By stock I mean 1/8" ply with no glass but they would have epoxy fillets all around (BT + MMT). I was running a sim on a possible option with an I600 motor and the max velocity was ~ 1000 mph. What can I say - I like to test the outer bounds of the design space. The sim runs with smaller H engines were in the ~650 mph range for top speed. So in any case I'd be close to or well over Mach 1. Not very familiar with that territory so I'm looking for advice from people with more experience than I have.
 
PM Aksrockets he is into minimum diameter rockets and he should be able to answer your questions.
 
keep in mind that, you don't get your cert until you bring the rocket back for it's post flight inspection :).
Rex
 
hmmm .. my response is simple....

When they fall off....

Kind of hoping to avoid that.

I was thinking I'd get replies like ... "I flew XYZ rocket to a max airspeed of > 1.0 M and had no problems"

I saw that FinSim is no longer for sale. I guess I'll have to do some spreadsheet calculations based on the Info Central article but it doesn't seem like that is a very accurate method.
 
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I've flown a 24 oz. LOC Vulcanite on a J350 without any issues. No glass and cheap epoxy construction.

John
 
"it depends" - can you post the geometry and size of your fins? This is the most important thing next to speed for determining whether you will have flutter. Long trailing edges will flutter, as will large fin heights. the more of a pyramid shape you get, the less flutter you should see.
 
Apogee claims that the Aspire (a min diameter rocket (with surface mounted fins), is capable of going Mach when built in accordance with their instructions (including papered fins).

I'm sure that TTW is quite a bit stronger if done properly.
 
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TTW wont really reduce flex on the fin at all which causes flutter, it does however retain the fin stronger than simple bonding to the outside of the tube where the interface of the tube and fin, or the materials of the tube itself will fail. Lots of Aspire's have shredded going mach lol. The fin shape isnt really ideal for it, and building technique probably has a lot to do with it as well.
 
Here is a snapshot from OR. You can see the fin size and shape.
Fins.png
These fins are pretty tall so I'm thinking they'll experience flutter at lower airspeeds.
 

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At the very least, I'd flip that profile around, it'll be lower drag and less likely to flutter. A profile that leans forward, can rotate around (thats what flutter does on rocket fins), that causes the leading edge tip to go into the airflow and then get pulled into the rotation further. If you flip it around, the trailing edge will be larger and protrude more - the airflow over this forces the fin back. If you do get flutter, you can survive it longer.

Here's a quick, very simplified version of one of the actions of fin flutter .. this isn't all there is to it, but is one that can very quickly rip a fin to shreds or off the rocket.

finrotation.jpg
 
Ive flown 38 mm min dia on a I- 1200 with the same fins I believe it was 7 ply maybe 9 the more ply the better
 
This is an extremely complex question to answer.

It comes down to the stiffness of the fin AND the strength of the bond between the fin and airframe. Most of the issues are going to happen in the transonic envelope, so from ~ 0.83 M to ~ 1.2 M. As I understand it, fin flutter is a function of airspeed, fin stiffness, and fin profile. But it's way more complicated than that. But that is a good starting point to understand what's going on.

IMHO, I think you would be fine with a good grade of 5 layer plywood, 3/16" or 1/4" (but 1/4" would be more draggy, but more robust). I would be wary of anything thinner than 3/16". Use a good epoxy like Aeropoxy, WEST Systems, or RocketPoxy or even a good wood glue. I don't think you need to do tip to tip layup of FG or carbon. Most rockets are not in that envelope that long.

Spend time on getting your fins symetrical and on straight.

So, the basics are:

  • Good wood
  • Symmetrical
  • Good fin/airframe bond
  • On straight

Greg
 
Thanks for your input everyone.

Another question - If I want to thicken the fins could I just glue a thin layer of ply to the outer surfaces or does this need to extend onto the TTW tab as well? It would be easier to not have to enlarge the fin slots.
 
Thanks for your input everyone.

Another question - If I want to thicken the fins could I just glue a thin layer of ply to the outer surfaces or does this need to extend onto the TTW tab as well? It would be easier to not have to enlarge the fin slots.

You can do that, but I don't know why you would want to.

Typically you wouldn't need to add another layer of ply to the surface. But if you want to put a veneer on the surface of the plywood, just use a good wood glue, thinly spread (and by "thinly" I mean pretty thin, as you can see through it) on both sides, then clamped together to dry overnight. I would have a layer of MDF between the veneer surface and the clamp to insure an even distribution of force.

You don't need to modify the tap/slot to do this.

My :2:

Greg
 
When I said thin ply I'm talking 1/64" or 1/32" plywood added to the existing 1/8" fin. Basically a veneer of very thin plywood.
 
When I said thin ply I'm talking 1/64" or 1/32" plywood added to the existing 1/8" fin. Basically a veneer of very thin plywood.


I would go with the 1/32". I'm not sure any substantial stiffness would accrue by adding 1/64".

Greg
 
If you are going to attach a thin veneer why wouldn't you just do a couple layers of glass instead? I think it will serve you better, and will easier to execute well.

As others have said, your fin shape is certainly not optimal for Mach.


Mark Koelsch
Sent from my iPhone using Rocketry Forum
 
If you are going to attach a thin veneer why wouldn't you just do a couple layers of glass instead? I think it will serve you better, and will easier to execute well.
I don't know about everyone else but adding a wood veneer seems a lot simpler than putting layers of glass on the fins. But you have certainly given me something to consider.
 
Doing a tip to tip is pretty easy- really.

If your body tube has the glassine layer over it I would peal it as the wax does not promote quality adhesion.

I would do 2 layers of 6 ounce glass. Cut the pieces to be longer than you need- so that the glass hangs over the fin edge. Mix your epoxy, and apply a thin layer on the fins and body tube. Lay the glass across it starting at one outside fin edge and working it into the epoxy. Once it goes clear make sure there are no bubbles. Then lay the second piece down in the same way. Do not apply more epoxy unless it does not go clear. Ideally, you want the glass clear, but you want to be able to see the cloth fibers versus having it be smooth with pooled/excessive epoxy. Once there leave it sit until the epoxy really starts to setup, and then carefully trim off the excess. Let not sit for a day.

That takes care of one set of fins. You will do this in total three times for a three fun rocket.


Mark Koelsch
Sent from my iPhone using Rocketry Forum
 
Have you considered using 3/16" ply instead of the 1/8"?

This does add a bit of extra weight, and more consideration must be given to rounding of the leading and trailing edges, but it might be the easiest way to make the fins stronger.
 
The fins in the kit are 1/8" ply. I could certainly make new fins from 3/16" ply but then I'd have to enlarge the slots in the BT which I'm not super excited about doing.
 
Glassing fins is *really* easy to do. Get some decent epoxy, a brush or roller, peel ply and some cloth.

DSC_7144-L.jpg

Tape a piece of paper to your fin, tight over the fillets and body tube. Slit it and tape it down so the paper will sit flat. Trace the outline of both fins on the paper, then take the paper off.

DSC_7146-L.jpg

Cut out the paper on the lines you drew then cut some cardboard templates out from that paper. Transfer any slits you made in the paper to get it to sit flat onto the cardboard too, so you can transfer it to the cloth.


Wash your fins and tube with acetone. Once it has evaporated you are ready to work. From the point you have used acetone onwards ensure you are wearing nitrile gloves (NOT latex. Latex is not suitable for use with chemicals).

You can then use the cardboard as a template for cutting your glass out. Draw a line in permanent marker (sharpie) offset by 1-2cm from the cardboard template so you have overhang. You should only need 1 layer for each fin pair, so you will need to cut out 3 pieces of cloth. The weave should run straight down the fuselage, and straight from tip to tip. Cut out peel ply another 1-2cm larger than your fibreglass. Always handle cloth to be used in composites with gloves on, your skin oils will create weak spots. You cannot clean your hands well enough to remove them, they are natural and part of you, and want to be on the cloth.


Mix up your epoxy, then you can either brush or roll some epoxy onto the fins/body tube (only do one fin pair at a time). Settle your cloth onto the fins/body tube, add more epoxy until it becomes completely transparent. Place the peel ply on, and then carefully work over it , making sure it's fully wet out (will go darker) and that there are NO bubbles trapped under it. The ply should be dead flat, and may have some pooled epoxy on its surface. If possible run a piece of flexible plastic like an old credit card over the surface, pushing down as you scrape. This will help remove excess epoxy from the layup and compact the layup. Again make sure you have no bubbles. Pay special attention to your fillet area - you want the glass to have no gaps here.

Alternative, you can wet the cloth out on baking paper (parchement paper) or cling wrap first, then move the wetted out cloth to the rocket, however this can make it much harder to get rid of bubbles later.

After an hour or two, when the epoxy is hard but still a little flexible (known as "green", its like a green tree branch.. hard yet flexible vs a dried branch, hard and brittle) you can tear the ply off and then trim the glass with a knife or scissors. When the epoxy is green, the epoxy and glass (carbon and kevlar too!) can be very easily trimmed. Move on to the next surface. If you want to speed up the epoxy cure, use a cheap fan heater or blow dryer. Keep in mind that when the epoxy goes on at first and gets hot, it will become less viscous and flow - this can cause your epoxy to slide off if you have it vertical... so ensure you only heat when the fins are horizontal.

Any twill or plain weave glass from 60-150gsm would do.
 
If I glass just the fins won't that leave a slight ridge on the BT just above the fins? Would I have to do a lot of filling and fairing to eliminate this?

edit - Just looked at some glass cloth on US Composites and the 4 oz standard weave is 0.006" thick. So I'm guessing a single layer plus the epoxy is maybe 0.010 - 0.015" thick. That pretty much alleviates my concern about having a significant ridge to deal with.
 
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glass is pretty darn thin, If you do it right the laminate will be the same thickness as the glass, however you'll usually end up with a tiny bit more. You can easily sand the leading and trailing edges into the body tube to create a very very smooth transition.
 
I'm looking at options for doing a L1 cert and I'm wondering what the airspeed limit is for "stock" TTW fins. By stock I mean 1/8" ply with no glass but they would have epoxy fillets all around (BT + MMT). I was running a sim on a possible option with an I600 motor and the max velocity was ~ 1000 mph. What can I say - I like to test the outer bounds of the design space. The sim runs with smaller H engines were in the ~650 mph range for top speed. So in any case I'd be close to or well over Mach 1. Not very familiar with that territory so I'm looking for advice from people with more experience than I have.

Glassing, laminating with carbon, pushing the envelope... It's all good.

But for a certification, isn't success and a signature the most important part?

My advice... Use an adapter, put a 29mm H in it, and get the autograph first. Then you can push the limits to your heart's content.

Had a buddy used to like to push it. Sometimes, paper and plywood (alone) will survive Mach. Most of the time, it won't.
 
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