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HyperSonic

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Bending stress is highest on the outer limits of a bend plane. We'll assume the neutral (unloaded) fin is the bend plane here.

That's where you want your fibers to be aligned straightest, so the worst tension and compression is running up and down solid fibers instead of acting on glue/fiber interfaces.
Thank you Nytrunner for explaining this to me. I think I get what you are saying. The outer layer would have to bend more than the inner layer. Since I do not know which direction the load will be coming from (different modes of flutter) I can only guess which way to run my fibers on the outer most layer. But my outer most layer (if I ever stop adding CF LOL) will be plain weave with the tows running 0 and 90 degrees. The layer just under that will be running 45 and 135 degrees. These fins will be VERY STRONG! I'm sure they will take anything I can throw at them. Even the 5800!
 

Nytrunner

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Hmmmm, that is a tough question.

Not being a composites expert (professionally I'm used to more isotropic materials), I'd actually point back to model rocket techniques: aligned one of the weaves parallel to the leading edge of your fin!
 

plugger

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Beveling tool 7 degree router bit.
Hi CJ,

As per usual, thanks for the information. I'm in the process of getting my "bedazzling" router table up and running and I was looking into picking up one of the Foredom Typhoon Tungsten Carbide Burs like the one you shared a photo of. This guy.



Please correct me if I'm wrong, but that unit has a 15 degree taper, not 7 degrees like you stated earlier? I'm going to grab the A-KB14179 model as it appears that's what you are using?



Once again, thanks for sharing.

drew
 

blackjack2564

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Ooopps.I was just showing the type of bit to google for....actually it's this one [5 degree], Warning however....it leaves a ruff cut with grooves that need sanding smooth as the teeth are quite aggressive.
Be careful it will be way too sharp, and just a glance will cut ya to the bone. First thing after bedazzling knock off that razor edge with sandpaper.
Hey..I'm older than dirt, ya can't expect me to remember everything accurately...

Screen Shot 2019-05-13 at 2.03.50 AM.png
 

plugger

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Ooopps.I was just showing the type of bit to google for....actually it's this one [5 degree], Warning however....it leaves a ruff cut with grooves that need sanding smooth as the teeth are quite aggressive.
Be careful it will be way too sharp, and just a glance will cut ya to the bone. First thing after bedazzling knock off that razor edge with sandpaper.
Hey..I'm older than dirt, ya can't expect me to remember everything accurately...
Once again thanks for sharing CJ! FYI I've already ordered the 15 degree unit as I'm pretty comfortable to start with that to learn so to speak. Then I'll move to using the 5 degree model. Ideally moving forward I've use the 15 degree for booster fins and the 5 degree for sustainer fins in two stage stacks.
 

OverTheTop

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plugger

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You could then make a jig like I used on my Nike Apache fins:
I'm going down the path of using angled burs instead of an angled jig as imho it's just easier. To make an angled jig I'd need to make accurate cuts in wood at the desired bevel angle. That's not easily accomplished in my shed where most work is done with a dremel or power drill. I personally think it's simpler to use squared cuts of wood to build a right angle jig and then use the bur to handle the bevel angle.
 

phyzzld

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Hi HyperSonic,

I've been following this thread with a lot of interest. I really like your L bracket idea and I'm hoping to use it on my own smaller project. I do have a couple questions. I noticed in your pictures that you applied the fillet on top of the L bracket, but it doesn't look like the fillet extends past the bracket. Is that true (kinda hard to tell from the picture)? It seems that even if it does extend past the bracket that the bracket would have to be really short or the fillet really large in order for the fillet to have much area to bind to the fin and the airframe to help secure it.

I am thinking of doing this in the opposite order, ie, apply the fillet first then the L bracket on top of it. Do you see any problems with changing the order? Should I wait for the fillet to cure first or apply the bracket while the fillet is still wet.

I guess another option would be to apply the bracket and not apply any fillet. Opinions?

Thanks,
Jeff
 

HyperSonic

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Hi HyperSonic,

I've been following this thread with a lot of interest. I really like your L bracket idea and I'm hoping to use it on my own smaller project. I do have a couple questions. I noticed in your pictures that you applied the fillet on top of the L bracket, but it doesn't look like the fillet extends past the bracket. Is that true (kinda hard to tell from the picture)? It seems that even if it does extend past the bracket that the bracket would have to be really short or the fillet really large in order for the fillet to have much area to bind to the fin and the airframe to help secure it.

I am thinking of doing this in the opposite order, ie, apply the fillet first then the L bracket on top of it. Do you see any problems with changing the order? Should I wait for the fillet to cure first or apply the bracket while the fillet is still wet.

I guess another option would be to apply the bracket and not apply any fillet. Opinions?

Thanks,
Jeff
Hi Jeff,

The L bracket idea is only on this rocket to help keep the fins attached to the airframe while traveling at Mach 3 down low (around 6000 feet) If I were staying under Mach 2, I wound not have put them on. There are many on this forum who have gone Mach 3 without L brackets or tip to tip. I'm just adding a little insurance to help hold on at Max Q. The CTI 5800 has been the end of many a MD rocket in the past. My goal is to survive Mach 3 and bring him back home in one piece! And keep it all under 50K feet.

My L brackets on this rocket go beyond my fillets on both the fin and on the airframe. They feather in very nicely to fin and airframe at the edge. There are 3 layers on each side of each fin. The CF used for the bracket is only .006" thick (times 3= .018") When 3 layers of CF (1"x 12") lay on top of each other and there is wet epoxy between them, they tend to move independent of each other when pushed into place. Some rode higher up the fin and less on the airframe, and another layer did just the opposite. In the end, all 3 layers were pressed into the corner between the fin and the airframe on both sides and clamped into place using an aluminum L bracket from Lowes and some hose clamps.

As far as putting the bracket over the fillet, yes that will work too. I call that the Tip to Tip. I dont stop the CF after the fillet, I run it all the way from the tip of one fin, across the airframe to the tip of other fin. I'm in the process of doing that now.

I think the fillet on the fins helps it be less draggy. So you might not want to get rid of them.
 

HyperSonic

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It seems that even if it does extend past the bracket that the bracket would have to be really short or the fillet really large in order for the fillet to have much area to bind to the fin and the airframe to help secure it. Should I wait for the fillet to cure first or apply the bracket while the fillet is still wet.

I forgot to tell you that in my case, the L brackets are whats holding the fin on to the airframe, not the fillet. Thats why they extend past the fillet on both the fin and the airframe. The fillet on top the L brackets make sure that the L brackets stay in a L shape. When traveling at extreme speeds, fin flutter can act in mysteries ways and try to bend things around. The fillet also helps with the aerodynamics. When I lay a few layers of CF on top of the fillets and L brackets from tip to tip, this will stiffen everything up in that section. Like I said before, unless your planning on pushing the envelope as far as speed is concerned, or your fins extend down way below the airframe and keep breaking upon landing, the L brackets are not needed.

The chemical bond (epoxy still green when applying next layer) is stronger than the mechanical bond (epoxy is cured before applying next layer). Once again, if your not pushing things or having fin breaking problems upon landing, the mechanical bond will be fine. Very good read is a sticky thread called fiberglass techniques in the Techniques forum written by G_T.
 

phyzzld

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Thanks Hypersonic for a very helpful explanation about the L brackets.

Jeff
 

HyperSonic

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Here are some pics of the tip to tip process.

20190818_132824.jpg


The pump I have been using for many cycles at 6 hours at a time.

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I always try to put some sun on my epoxy if possible.

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The first layer of CF went just to the edge of bevels on leading and trailing edges. The second and final layer (WOOHOO!!!!!) of CF went from tip to tip.

20190823_113126.jpg


I'll take a pic of the whole booster section when I get home. Just need to finish the leading edges and the fins are finally done, except for finishing.
 

HyperSonic

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Some more pics, everyone likes pics, right.

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The booster section weighs in at 4 pounds 13.7 oz. The exact same CF tube that I put the fins on weighed in at 2 pounds 9 oz. before the fins. This means the fin can portion of this rocket weighs around 2 pounds 4.7 oz. Each fin has 20 layers of CF rotated in so many different directions that I confused myself during construction!


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Nytrunner

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What's your technique for forming ans smoothing leading edge joints at the body tube?
 

HyperSonic

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The fins are between .215 and .220 inches thick. The diameter of the leading edge is around .070 inches. PePe is lean and mean and won't be happy until there is a 10 foot flame shooting out the rear end!

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Are you happy with that style vacuum pump for vacuum bagging? I currently have a piston type Welch pump, but I am thinking about switching to something more portable.
 

HyperSonic

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What's your technique for forming ans smoothing leading edge joints at the body tube?
After the first section has cured and all bagging material removed, any carbon or glue that goes beyond the imaginary line from the center of the fin down the tube, gets sanded/filed away and feathered into the airframe. I use a dremmel for the bigger stuff and finish by hand. Then on the next section I just lay it on top of the first section and after it cures just sand off the excess. Any low spots get filled with epoxy, and any high spots get sanded down. You could do this when it is still green and avoid the sanding, but I choose to let it cure. I was afraid of pulling up the fabric from the fin while trying to cut it with a razor knife. The trailing edge is done in a similar fashion. Just a lot of sanding/filing to get it how you want. I am not an expert by no means so YMMV.
 

HyperSonic

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Are you happy with that style vacuum pump for vacuum bagging? I currently have a piston type Welch pump, but I am thinking about switching to something more portable.
Yes. This pump has well over 100 hours on it and still running fine. I think I have the FJC 5cfm model. They make a 1.5cfm, 3cfm, 5cfm, and 7cfm models. This one has been good to me.:) P.S. I use a lot of slow setting epoxy so the pump tends to run a little longer.
 

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Thanks for the pump info :) (sorry if I sidetracked the thread). I learned that volume is important for bagging, so it makes sense that you got one near the higher end. Awesome looking fins on your rocket, I am really impressed! The attachment looks extremely strong... no easy task.
 

HyperSonic

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I'd like to share with you all (I'm sure some already know this) what I learned when vac bagging my fin can. Once the airframe was in the bag (pics above) and I turned on the pump, if there are no leaks, the bag gets the air sucked out in seconds, leaving very little time to straighten out the wrinkles in the bag. These wrinkles will show up in the fin can when you remove it from the bag. I shut the pump off thinking this will let me pull out the wrinkles, and make it more flat where you want it. Since the bag was pulled tight all around the airframe, it was difficult to remove the wrinkles because the bag would not pull away from the airframe since the air was gone inside the bag. The answer to this (which I didn't think of until I was all done) is to use a bleeder valve like this one https://www.fibreglast.com/product/bleedoff-valve-assembly-00902/Vacuum_Fittings This will allow you to bleed off some of the vacuum when first turned on, giving you more time to pull the wrinkles out. You can adjust it as needed to give you the time you need, then when you got all the wrinkles out the best you can, just close the valve to go back to full vacuum. Every small wrinkle WILL show it's ugly head in your part when removed from the bag. I had to sand them off with a lot of work, and your part won't look so pretty. Some were wrinkles, and some were trapped air pockets under the fabric, mainly because of improper technique. Read G_T's sticky. As far as peel ply is concerned, well that's a story for another time.
 

Thundercloud

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Did you use Peel-Ply and cotton badding, or just the bag directly against the layup? I always used nylon peel ply against the layup, then a generous amount of cotton badding to absorb the excess epoxy that bleeds thru the peel ply, then the bag over that. The badding also provides an air path to the fitting.
 

HyperSonic

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Did you use Peel-Ply and cotton badding, or just the bag directly against the layup? I always used nylon peel ply against the layup, then a generous amount of cotton badding to absorb the excess epoxy that bleeds thru the peel ply, then the bag over that. The badding also provides an air path to the fitting.
Yes, I used peel ply directly on the fabric, then perforated release film (note the small dots of epoxy in post #104) After the perforated release film comes the breather cloth (cotton badding) Then the bag followed by one more non perforated release film so I could reuse the bag again. The perforated release film helps to meter the amount of epoxy being drawn out by the breather cloth, so you don't wind up with a dry layup. This is the breather cloth I used. https://www.fibreglast.com/product/Breather_and_Bleeder_579/Vacuum_Bagging_Films_Peel_Ply_Tapes I know that G_T does not like the use of peel ply, but that ship has already sailed.:D
 

HyperSonic

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Opps, I meant the non perforated release film first, then the bag. I get confused from time to time:confused::confused:
 

watheyak

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The rockets below were done with peel-ply, breather then bag. I've found that the wrinkles caused by wrinkly bag can be minimized a few different ways.

First, I use epoxy compatible spray adhesive to place the tip to tip reinforcement on the rocket. This allows me to put it exactly where I want it without wrinkles. I use Fibertack MT-1 from Composite Envisions for this. There may be better stuff. I also used the spray adhesive to carefully attach .75oz fiberglass to the carbon fiber before applying it to the rocket. An old R/C sailplane trick that gives you a sanding veil for any tiny wrinkles you do get. I wouldn't do this in a super high performance rocket, though. I was going for aesthetics on these.

For the breather I use blue shop towels from home depot. Cotton batting is too impressionable and everything prints through it. It also feel it absorbs too much in the case of T2T. I also use a light dusting of spray adhesive to keep the peel ply attached to the blue towels. Makes adjusting everything in the bag easier if all the layers stay in place. Wrinkles went way down after switching to these.

Then much bag straightening and fussing. As you've already found out. I don't use a valve, but that sounds like a great idea.

IMG-1124.jpg
IMG-1126.JPG
IMG-1127.JPG
 

HyperSonic

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The rockets below were done with peel-ply, breather then bag. I've found that the wrinkles caused by wrinkly bag can be minimized a few different ways.

First, I use epoxy compatible spray adhesive to place the tip to tip reinforcement on the rocket. This allows me to put it exactly where I want it without wrinkles. I use Fibertack MT-1 from Composite Envisions for this. There may be better stuff. I also used the spray adhesive to carefully attach .75oz fiberglass to the carbon fiber before applying it to the rocket. An old R/C sailplane trick that gives you a sanding veil for any tiny wrinkles you do get. I wouldn't do this in a super high performance rocket, though. I was going for aesthetics on these.

For the breather I use blue shop towels from home depot. Cotton batting is too impressionable and everything prints through it. It also feel it absorbs too much in the case of T2T. I also use a light dusting of spray adhesive to keep the peel ply attached to the blue towels. Makes adjusting everything in the bag easier if all the layers stay in place. Wrinkles went way down after switching to these.

Then much bag straightening and fussing. As you've already found out. I don't use a valve, but that sounds like a great idea.

View attachment 400846 View attachment 400847 View attachment 400848
Wow, those really look good! I will certainly remember to try your method in the future. I believe I caused some of my wrinkles by using to many layers of material. The peel ply was wetted out and I don't think that caused any problems, but then there was the perforated release film, the breather, then another non perforated release film, and then the bag. All of these have there purpose, but can also cause wrinkles in the end. By spray glueing these down like you did, and I did not, this would minimize movement, especially when pulling the wrinkles out in the bag after the vacuum was applied.

If you don't mind me asking, why do you think that your technique would not be acceptable in a super high performance rocket? Once again, great looking rockets!!
 

watheyak

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If you don't mind me asking, why do you think that your technique would not be acceptable in a super high performance rocket? Once again, great looking rockets!!
I guess the spray adhesive makes still makes me nervous. I used to just use 3M77, but switched to the fancy Fibertack stuff for this project. Theoretically better, but probably not ideal from a layup integrity standpoint. I have no formal training, I'm not an engineer, this is just a hunch.

Another part of the technique I might leave out for a higher performance rocket would be the light fiberglass sanding veil. One of the things that's handy for is helping regular carbon fiber pieces hold their shape after being cut, and that's not necessary with the spread tow stuff, as you found. That stuff is great. The only issue then is that if there are any wrinkles you're sanding away your reinforcement, not fiberglass.

I have another one in the works with lighter weight, higher modulus cloth. I'm gonna skip the fiberglass and go super sparingly on the spray adhesive. Fingers crossed, this next set of fins are gonna be pretty thin.
 

HyperSonic

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I guess the spray adhesive makes still makes me nervous. I used to just use 3M77, but switched to the fancy Fibertack stuff for this project. Theoretically better, but probably not ideal from a layup integrity standpoint. I have no formal training, I'm not an engineer, this is just a hunch.

Another part of the technique I might leave out for a higher performance rocket would be the light fiberglass sanding veil. One of the things that's handy for is helping regular carbon fiber pieces hold their shape after being cut, and that's not necessary with the spread tow stuff, as you found. That stuff is great. The only issue then is that if there are any wrinkles you're sanding away your reinforcement, not fiberglass.

I have another one in the works with lighter weight, higher modulus cloth. I'm gonna skip the fiberglass and go super sparingly on the spray adhesive. Fingers crossed, this next set of fins are gonna be pretty thin.
I think that if you are only using one or two layers of CF over something that is not as stiff as CF, then you would not want to sand into it if you can help it. But if there are many layers of CF under the first one, I would not be as worried. I certainly didn't intend to sand into mine, but the wrinkles have got to go. I get better each and every time I do it, and by sharing ideas here on the forum, we all can get better at it in a shorter period of time. I will never go back to anything else besides spread tow fabric from Composite Envisions, as this stuff will not fray, and is easier to work with. I can cut any pattern I want very easy. I'm not an engineer either, but I don't always have to prove things by the numbers, as experience will go a long way. And if I don't have very much experience in something, then I play with it to gain the experience that I need.

A very good question is how thin can you go with fins to reach a certain speed without problems. Every rocket is different with the size and shape of fins, so many variables come into play. I will not claim to know the answers. But this unknown is what drives me.

Your fins in the pics look very good to me. This is what PePe looks like now.

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watheyak

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I hadn't seen yours all polished up with the leading edge treatment yet. It looks great! I followed your thread about the ceramic treatment with great interest. I'm excited to see how it holds up.
 
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