Full fin can vacuum bagging

I was curious if anyone can help me out with a few questions.

I am making a 4" minimum diameter rocket with 4 fins, and i want to vacuum bag the entire fin can in one shot. I am planning on getting this rocket close to mach 4 so the fin can needs to be as strong as I can make it.

So my questions are:
How do you keep the layup on the underside from falling off as you rotate the assembly around?
Do you think it's better to use one piece of bag and pleat it around the fins, or use a seperate sheet for each surface? Obviously the latter would be harder to seal, but might result in a smoother product?
Are there any tips that may be helpful in my endeavors?

I followed Jim Jarvis' layup technique, one side of a set of fins at a time.

In my case I bagged the entire booster and kept it under vacuum for 12 hours, then de-bagged, trimmed and did the next set of fins immediately, repeat until complete. I also built a custom jig for 3 and 4 fin layups, which helps to keep the cloth that is extended beyond the fins flat until I trim it. I can tell you it is a bit tricky keeping one side flat let alone attempting to pleat around all the fins. I cannot see how you could easily accomplish what you are considering doing unless you build a custom fin jig, which is possible but I am not sure the result would be that much better.

I think if you share more information about the rocket and the flight profile it may be helpful in getting some assistance.

Zebedee just did this. Probably three fins, but all at once. He used separate bag pieces I think. Maybe he'll chime in.

Jim

If you have three fins, just use three opposing layups and sheets of high-temp film. Seal off well with your tape. Turn on vacuum. Put in oven go through your appropriate temperature cycling. High performance build with fin can layup shown here: https://indianarocketry.fr.yuku.com/topic/468/Mach-Madness-2013-Build.

Thanks Jim

If you have time and you want a nice neat end result at the fin edges I recommend you don't do all fins at once. I went with it so I only had to do one bagging session and I knew I was going to be coating the edges in high temp epoxy (plus I'm only planning to hit M2.5). I used a lot of the info in Jim's carbon guides - they are excellent and well worth a read (or 10) - and then tailored that info to the process I wanted to follow.

If you do decide to do all three then I would think about the following:

The aeropoxy I used (2032 with the 3660 hardener) will keep the carbon & most of the peel ply in place while you're doing other sections but not the breather.

I used small pieces of tape front and back to keep the main section of breather in place and then when I had both sides of a fin done a small piece over the top from one fin to the other to keep the edges of the breather in place.

I made the bag from three pieces of stretchy bag material - one per section. The 3-way (or 4-way in your case) section at the back needs some support/tape to keep it together through the bagging process. Figure out a way to extend and block off the rear of the main tube so your bag does not get sucked into it (and pull the 3-way join apart). I also sealed the front of the tube (but not perfectly - was losing some air there).

I taped out the cloth with masking tape and cut it so that there was a thin (1/4") piece of tape left holding the edges. Cut carbon cloth frays if you so much as look at it or breath wrong - I have no experience of whether glass cloth is the same.

I then trimmed the front and back of the cloth where it will lay on the body tube before placing it. Then trimmed the fin edges after placing it to minimize fraying. However, trimming as you go lengthens the whole process and makes it more likely you'll end up with stray carbon "bits" where you don't want them. If you only do one section at a time and have a support jig you can trim later for less stress and a cleaner result.

Make sure your peel ply is a bit larger than the carbon otherwise you'll have breather stuck on which has to be sanded after it's all setup.

It took me about an hour from mixing the epoxy to get everything done and under bag. I had the back and sides of the 3 part bag already sealed so I could just slip it over the back and seal it on the body tube ahead of the fins. I also masked out the body tube for a good 4" under where I had the vacuum attachment.

I de-bagged after 4 hours @ 100F (Texas is pretty hot this time of year), removed breather/peel ply and trimmed rough areas with an xacto then left it over night to cure.

Attached a photo.

Zeb

Zebedee said:

If you have time and you want a nice neat end result at the fin edges I recommend you don't do all fins at once.

Click to expand...

I'll second that advice. Composite strength is very process dependent: maintaining cleanliness, handling composites minimally, and repeating a consistent process will all go a long way in helping achieving the most strength possible. All of these are much easier to do when you aren't rushing to get a bag on, or fighting composites that are peeling off the underside of a layup.

On top of the advice Zebedee gave, I'd make sure you have a good plan before mixing the epoxy. Lay out all the stuff you need beforehand preferably in the order you'll need it and make sure you maintain cleanliness This post shows how we prepped for and laid up a similar can.

Good luck!

Agreed - I rehearsed the steps many times in my head beforehand and it went pretty much to plan but I was still approaching the end of the pot life on the 3660 hardener before I got in under vacuum.

The toughest part sometimes is getting a good complete seal on the bag.
I tried doing the full booster (56" long ) and I had problems.
One was due to running out of bagging film.


JD

Thank you Zebedee for that explanation, that will help me out alot.

One reason I want to do the full can at once is because I plan on extending the carbon about half an inch past the "core" of the fin. I want that extra strength from a chemical bond around the perimeter of the fin instead of just the mechanical bond to the fin surface. I am not super concerned with how the cloth looks after I am done since I plan on painting high temp epoxy on the leading edges.

junkyard said:

One reason I want to do the full can at once is because I plan on extending the carbon about half an inch past the "core" of the fin.

Click to expand...

Are you trying to do something like in the pic where the fin edge is in red and the carbon tip to tip is in black? I think that would be kind of difficult.

Jim

Yeah, that's the basic idea. Although I won't leave the entire half an inch of carbon hanging off the fin, so it should look more like this picture

Junkyard
Did you do this all four fins at once? How did it come out?