Q. How do you figure out the stresses on the the fins whe a rocket is launched

[Oz rocket guy]

Well i made a big silly mistake and built one one of my very first MPR rockets with woold glue ( weldbond) and i need to know how much stress will the fins need to withstand the forces. i have rocksaim with me , is drag force the force that is put on the fins.

 

[jj94]

Building a MPR rocket with only wood glue isn't a bad idea at all. Actually, IMO, it's a smart idea, unless you were using composites like from PML. What went wrong with the wood glue?

To answer your question, no, drag force isn't the force put on just the fins. But if you go to 'Rocket' and go to CD analysis, it might give some useful info to you.

 

[Oz rocket guy]

Nothing its just people sugested that wood glue is not recommended people sugest i use epoxy like JB weld, im using an areotech motor F26fJ-7 for my fist flight.

 

[Oz rocket guy]

Thanks for that i found out that the fins are 247 gm of stress at its max speed and 231 on averagethanks, all i have to fin out know how much weight dose the glue im using hold before breaking.

 

[jj94]

If you do through the wall fins, the rocket should be able to handle much much more than a measly F with wood glue. Epoxy for MPR, with the exception of composites, it way overkill. It'll only add weight and you'll already have all the strength you need with wood glue. I've heard of rockets flying on I motors made with the stuff without a hint of failure. 247 grams equals to about .54 pounds, and I've been able to actually *try* and take a fin off of one of my rockets without it coming off.

 

[astronboy]

This issue has been discussed several times on this forum: tests have shown that when glueing wood to wood, or wood to the paper body tube, good old yellow aliphatic resin glue (wood glue) is stronger than epoxy or super glue.

Of course, if you are gluing plastic, phenolic, or non wood or paper, then other adhesives may be better.

I have built many mid power models, using balsa fins and yeallow glue. For 2.6" birds, I use 1/4" thick balsa. I have flown these models on Es, and Fs with no fin issues whatsoever.

Phred

 

[roadkill]

Thru the wall fin design
with yellow glue can take
practically anything 29mm motors
can dish out and most of 38mm as well...

Even more when designed properly...

...or much less if not !

 

[AndyC2]

I just flew my Big Daddy last weekend on a F24W. Balsa fins, Elmer's yellow glue. No problem. Fins are through the wall, but not necessary for motors this size. Your biggest problem will be loosing the rocket from too much altitude.

 

[Oz rocket guy]

What is a Through wall fin design im using balsa with the grain horizontal no verticle ( ot parallel to the tube) .

here is on desing that has made it through constuction without any troubles.

 

[powderburner]

What is a Through wall fin design im using balsa with the grain horizontal no verticle ( ot parallel to the tube) .

First off, and I'm not trying to bash you with this comment, but it would be helpful if you could take the time to key in correctly spelled words. What you have here makes it necessary to stop and re-read, and sometimes to guess at your intended meaning. Enough of my nitpicking.

I am not sure what calculation you got hold of, but I have to wonder if it is actually a "stress" calculation. Stresses are often expressed in units of pounds per inch (along the edge of a panel) or pounds per square inch (into the cross-sectional area of a structural member). Units of just plain "pounds" makes me wonder if you have some loads data. Whatever the data is, white and yellow glues are plenty for midpower if the other airframe materials are suitable.

Through the wall construction (often abbreviated as TTW) means that the body tube has a slot cut completely through the wall under the root of the fin, and the fin extends through the slot to the inside of the rocket.

What most people really mean when they use this term is through-the-wall-all-the-way-to-the-motor-mount-tube. This design has a fin root tab that extends inside the body tube all the way down to the motor mount(s) and is also glued/epoxied/whatevered to the MMT. The tab might extend along the whole length of the fin root chord, or might only be a portion of the fin root chord.

To make things even stronger, if there is a centering ring between the MMT and the body tube, and you slide it up against the end of the fin tab, this also reinforces the attachment of the fin to the rocket. With an internal structure like this you could easily use white or yellow glues on balsa fins, basswood fins, or plywood fins and still safely use midpower motors.

Hope some of that was helpful. For more info, don't forget that there are a whole bunch of definitions for this kind of terminology over on EMRR;
try https://www.rocketreviews.com/cgi-bin/flattips/glossary.cgi

For more on TTW, try;
https://www.rocketryforum.com/showthread.php?t=1492
https://www.rocketryforum.com/showthread.php?t=6260
https://www.rocketryforum.com/showthread.php?t=4528
https://www.rocketryforum.com/showthread.php?t=1518
and a good picture of what TTW fin tabs look like is at;
https://www.rocketryforum.com/attachment.php?attachmentid=474&d=1036864018

 

[Oz rocket guy]

First off, and I'm not trying to bash you with this comment, but it would be helpful if you could take the time to key in correctly spelled words. What you have here makes it necessary to stop and re-read, and sometimes to guess at your intended meaning. Enough of my nitpicking.

Sorry. Thanks for that .

 

[AndyC2]

Looks a lot like the Dynastar Stonebreaker, though a hair larger diameter tube (2.6 vs 2.2"). The stonebreaker also has surface mounted fins, not TTW as powderburner so clearly described. You might take a look at it for comparison to yours:

https://www.rocketreviews.com/reviews/kits/dyna_stonebreaker.shtml
https://www.apogeerockets.com/stonebreaker.asp

Like you kit, this one was designed for wood/craft glue and up to 24mm F motors.

Have fun!

 

[Handeman]

Personnelly, I would use wood glue to surface mount fins for motors as large as I. The caveat is the fin and BT material. I most cases, the fin or body tube will break or seperate before the glue joint will let loose.

I recently flew one of my rockets built with mailing tubes and 1/4" plywood TTW fins, assembled with wood glue, on an I1299N Warp 9 load, 340 lbs of thrust and 80Gs, no problems.

 

[Oz rocket guy]

Personnelly, I would use wood glue to surface mount fins for motors as large as I. The caveat is the fin and BT material. I most cases, the fin or body tube will break or seperate before the glue joint will let loose.

I recently flew one of my rockets built with mailing tubes and 1/4" plywood TTW fins, assembled with wood glue, on an I1299N Warp 9 load, 340 lbs of thrust and 80Gs, no problems.

holy crap the guy in Aus would probably would be the eqivalent to using super glue only to build an MPR rocket what ever wood glue your using is some seriously strong :surprised: BTW when you recover your rocket dose the glue get all weak by the incredible heat produced by the l1299:surprised: