Finishing options.... Will this be trouble?

[Trenman]

Ok I am building and flying a 98mm mongoose in a L2 capacity as I have little time/ interest (at the moment of course) for a L3. I am flying it on a 75mm motor. Rocksim is projecting minimum 10k feet at Mach 1.7+ depending on which motor I use. I am wrapping the fins tip to tip in 2x2 twill cf (2 or 3 layers of 5.9oz/ fin depending on how much thicker this makes things). I want to finish this rocket as I find finished cf to be a thing of beauty. I am thinking a few light coats of a slightly metallic copper orange from mid fin up to
the avBay tapering the edges to be translucent so one can see the twill underneath. I think this combined with an aeropoxy finish (6 coats with coats 4-6 nearly completely sanded down for gloss and finished with finesse-it) will make this rocket look amazing. I will taper-cure the rocket to ensure that it is heat resistant.

Question: am I kidding myself? Even with high temp resistance, will my rocket bubble up under the heat and forces?

I know there was a thread on this topic 2 weeks ago but I didn't find it to be conclusive. Thanks!!

 

[Brent]

Do you fly with a local club? Personally I think a carbon fiber Mongoose is a little bit of a waste for just a level 2 cert project. How long have you been level one?

 

[Trenman]

So much negativity on this forum.

My question was not "do you think a mongoose is a waste?"
kindly re-read.

And yes I fly with a club. And my motormount is removable allowing 98mm motors should I choose to one day. I doubt I will because when I'm ready for L3 I'll build a new bird.

Why!? Because this is just a damned hobby and I find pleasure in this.

 

[Trenman]

And what exactly is wrong with being "JUST a level 2"? Is that a shortcoming of some sort!? I believe one must reach L2 before reaching L3, at least last time I checked. Perhaps I enjoy challenges: milling custom bulkheads from solid Al, wrapping my OWN CF and airbrushing a beautiful paint scheme instead of settling with something easier just to name a few features of which I am quite proud.

 

[Handeman]

I don't think the paint will be a waste, even if it does burn off. If you're like me, no matter how good it looks, there will always be something you wish you did different. If it burns off, that's your chance to fix those things.

I'm expecting lots and lots of pics.

 

[Trenman]

Believe me I can't wait to post my finished product.

Handeman: If it does burn, won't it create a hard permanent layer of crap on the surface or would one be able to sand through that?

Thank you for your feedback

 

[Handeman]

I've never had a rocket burn the finish, yet But I saw the new paint on one that was burned pretty bad and it was great. It was repainted by an autobody shop so I know there wasn't any structural work done. I would assume that sanding through the burned layers should be pretty straight forward.

 

[troj]

Why!? Because this is just a damned hobby and I find pleasure in this.

That, to me, is the whole point of the hobby.

What one person builds or flies may not trip the trigger of someone else. No biggie -- build and fly what makes you happy!

Now, in answer to your question, I don't think you'll be at high Mach numbers long enough to get any real burning, and even if you do, it'll be on the leading edges of the fins and on the nosecone.

All that said... Remember that composites do degrade under UV light. However, in your case, you're building a 98mm capable rocket, and it sounds like you plan on flying it on motors up through L. I don't think it's going to be an issue.

-Kevin

 

[Trenman]

Thanks for the advice and support. I think I'll go ahead and paint it up!!

That, to me, is the whole point of the hobby.

What one person builds or flies may not trip the trigger of someone else. No biggie -- build and fly what makes you happy!

Now, in answer to your question, I don't think you'll be at high Mach numbers long enough to get any real burning, and even if you do, it'll be on the leading edges of the fins and on the nosecone.

All that said... Remember that composites do degrade under UV light. However, in your case, you're building a 98mm capable rocket, and it sounds like you plan on flying it on motors up through L. I don't think it's going to be an issue.

-Kevin

 

[COrocket]

Trenman,

I like your idea of using a 98-75MM adapter in your mongoose. If you cant use a large motor for reasons such as waiver, recovery area size, or L2 limits, then why not use a 75MM motor? Instead of using a 1,2 or 3 grain 98MM motor, u can fly on a 2,4, or 6 grain 75MM motor respectively. For comparable power, 75 motors are less expensive, and help out the CG/CP relationship by not concentrating mass in the rear of the rocket. Then a Mongoose 98 allows for a sweet 4" motor if you get a L3. Some say that a 75 motor defeats the purpose of a 98 minimum diameter, but for smaller motors not seeking maximum altitude, i don't see a disadvantage.

 

[bobkrech]

Ok I am building and flying a 98mm mongoose in a L2 capacity as I have little time/ interest (at the moment of course) for a L3. I am flying it on a 75mm motor. Rocksim is projecting minimum 10k feet at Mach 1.7+ depending on which motor I use. I am wrapping the fins tip to tip in 2x2 twill cf (2 or 3 layers of 5.9oz/ fin depending on how much thicker this makes things). I want to finish this rocket as I find finished cf to be a thing of beauty. I am thinking a few light coats of a slightly metallic copper orange from mid fin up to
the avBay tapering the edges to be translucent so one can see the twill underneath. I think this combined with an aeropoxy finish (6 coats with coats 4-6 nearly completely sanded down for gloss and finished with finesse-it) will make this rocket look amazing. I will taper-cure the rocket to ensure that it is heat resistant.

Question: am I kidding myself? Even with high temp resistance, will my rocket bubble up under the heat and forces?

I know there was a thread on this topic 2 weeks ago but I didn't find it to be conclusive. Thanks!!

Bubbling is highly dependent on the type of epoxy you use, how you cure it, how you finish it, how you layup your composite and the Mach number of the flight. You probaly won't get bubbling below Mach 2.

A phenolic or epoxy phenolic resin can be cured at hgh temperatures. High temperature curing will get rid of any volitile species in the resin, and some resins are cured at 175C (350 F). Some bisphenol resins I have worked with will not bubble until after several hours at 350 C (660 F).

If you don't cure your epoxy, the water made and trapped in most epoxies during curing can boil and cause bubbling. Additionally, in consumer epoxies that are thinned by solvents, the solvent can come out during aeroheating.

Bob

 

[Trenman]

THANK YOU! That was my reasoning behind the whole thing. Maximum versatility!!

Trenman,

I like your idea of using a 98-75MM adapter in your mongoose. If you cant use a large motor for reasons such as waiver, recovery area size, or L2 limits, then why not use a 75MM motor? Instead of using a 1,2 or 3 grain 98MM motor, u can fly on a 2,4, or 6 grain 75MM motor respectively. For comparable power, 75 motors are less expensive, and help out the CG/CP relationship by not concentrating mass in the rear of the rocket. Then a Mongoose 98 allows for a sweet 4" motor if you get a L3. Some say that a 75 motor defeats the purpose of a 98 minimum diameter, but for smaller motors not seeking maximum altitude, i don't see a disadvantage.

 

[Trenman]

ahh very interesting info thank you. I believe I should be fine! I'm not looking for any records, I just want to fly my rocket once a month and have fun with the boys. I just wanted to make sure a killer paintjob would be safe

Bubbling is highly dependent on the type of epoxy you use, how you cure it, how you finish it, how you layup your composite and the Mach number of the flight. You probaly won't get bubbling below Mach 2.

A phenolic or epoxy phenolic resin can be cured at hgh temperatures. High temperature curing will get rid of any volitile species in the resin, and some resins are cured at 175C (350 F). Some bisphenol resins I have worked with will not bubble until after several hours at 350 C (660 F).

If you don't cure your epoxy, the water made and trapped in most epoxies during curing can boil and cause bubbling. Additionally, in consumer epoxies that are thinned by solvents, the solvent can come out during aeroheating.

Bob

 

[rfjustin]

I think this combined with an aeropoxy finish (6 coats with coats 4-6 nearly completely sanded down for gloss and finished with finesse-it) will make this rocket look amazing.

I can vouch that if you get the CF layups on "cleanly" and then do multiple Aeropoxy filler coats, you will have a slick looking bird.

If you have never done a CF fin-to-fin layup before make sure the Mongoose 98 is NOT your first attempt. Buy a cost effective LOC kit, and give if CF fin-to-fin for practice. You will quickly figure out what do to and what not to do.

Aeropoxy is great stuff for filler work, but do keep in mind that it remains very tacky to the touch for quite some time. Post curing the fin can (~180 degrees) will eliminate any tackiness of the Aeropoxy.



Justin

 

[ben_ullman]

I can vouch that if you get the CF layups on "cleanly" and then do multiple Aeropoxy filler coats, you will have a slick looking bird.

If you have never done a CF fin-to-fin layup before make sure the Mongoose 98 is NOT your first attempt. Buy a cost effective LOC kit, and give if CF fin-to-fin for practice. You will quickly figure out what do to and what not to do.

Aeropoxy is great stuff for filler work, but do keep in mind that it remains very tacky to the touch for quite some time. Post curing the fin can (~180 degrees) will eliminate any tackiness of the Aeropoxy.



Justin

Justin you are 100% correct. I fiberglassed some 7.5" Bt's using aeropoxy. It was like my 3rd time doing glassing and my first with aeropoxy. I had no idea how tacky the sh!t would be!! I let the tubes sit in my shed for one spring and summer and when I got back to them in the fall you could still feel a tiny grab. Not a glass smooth finish.

You may think some comments are negativity but its so you don't throw $1500 down the tube due to n00bness

Ben

 

[ben_ullman]

oh, "p.s."

here is a link to a friends project they did. Mach 1.7 burned off normal automotive paint. it SHOULD NOT burn off aeropoxy. expecially 3-4 layers of it.

https://www.colonialvirginiahpr.org/kryptos/flight.htm

Make yourself a rocket rotater (aka rotissarie) and use that to put it on and cure the tubes. Wait till its warmer out. you can then just set it in the hot______ (insert states name) sun and let it cure itself.

Ben

 

[bobkrech]

Ben

Burning off paint is another issue. The rocket you pointed out was a N motor, and it probably spent a lot of time near it's peak velocity.

The epoxy underneath the paint looks fine. I'm guessing that residual solvent trapped in the paint caused the bubbling, or the paint, which was probably polyurethane, melted and started to flow.

Bob

 

[sandman]

I think it would be cool to have some of the paint burned.

It would let people know just how fast than motha went.

Sort of like a badge of honor war wound.

 

[Trenman]

Alot of valuable info guys thank you. I am looking forward to sharing my completed project with you all.

 

[TheAviator]

I would agree with Ben. I've never done composite lay-ups on a rocket (never had anything that needed it), but I have used them for some other projects. The first time you do a lay-up, you are going to do something you don't like. That doesn't mean that the strength isn't there, just that it might not look entirely wonderful. I may do an FG fin can and tube for F-SRA this year, and I know I'm going to go through at least two or three rockets before I get it perfect. It's just part of the learning curve.

What I'm trying to say in all of this is practice, practice, practice. It's better to mess up a $20 Estes kit than a $1500 CF bird.

 

[ben_ullman]

Ben

Burning off paint is another issue. The rocket you pointed out was a N motor, and it probably spent a lot of time near it's peak velocity.

The epoxy underneath the paint looks fine. I'm guessing that residual solvent trapped in the paint caused the bubbling, or the paint, which was probably polyurethane, melted and started to flow.

Bob

Very true Bob, I was just using that since it was the most recent I knew of. I have no doubt the properly cured epoxy will last and hold up way better than some automotive paint will.

And yes there was no structural damage. They had the rocket repainted and it looks just as great!

Ben

 

[bobkrech]

Very true Bob, I was just using that since it was the most recent I knew of. I have no doubt the properly cured epoxy will last and hold up way better than some automotive paint will.

And yes there was no structural damage. They had the rocket repainted and it looks just as great!

Ben

I'm curious Ben

On the third picture down, there appears to be burn marks on the airframe between the fins and 1/3 of the way forward from the aft end of the airframe. I've seen this before and I'm assuming that where the rail buttons were located.

Plastic buttons don't last too long in a Mach 1.75 airflow and the high temperatures and pressure downstream of the shock waves do a number on the paint. I've seen fins on either side of rail buttons and launch lugs destroyed by enhanced heat transfer due to the turbulent boundary layer tripped by the projection into the free stream airflow.

Bob

 

[ben_ullman]

I'm curious Ben

On the third picture down, there appears to be burn marks on the airframe between the fins and 1/3 of the way forward from the aft end of the airframe. I've seen this before and I'm assuming that where the rail buttons were located.

Plastic buttons don't last too long in a Mach 1.75 airflow and the high temperatures and pressure downstream of the shock waves do a number on the paint. I've seen fins on either side of rail buttons and launch lugs destroyed by enhanced heat transfer due to the turbulent boundary layer tripped by the projection into the free stream airflow.

Bob

Well here is what i do know,

They used 1010 buttons, I am trying to remember if alumium or plastic.

Its very obvious that the stickers they put on OVER tha paint peeled off. I think that helped things along. The heat on the stickers transpired to the paint and with the leading edge slicing through the air it just started melting/peeling away.

They did say they lost about 2k feet due to it not coming off the rail straight. I think they chocked that up to some wind and inadequete pad for the job (not rail size, pad size)

Ben

 

[Diosces]

I would agree with Ben. I've never done composite lay-ups on a rocket (never had anything that needed it), but I have used them for some other projects. The first time you do a lay-up, you are going to do something you don't like. That doesn't mean that the strength isn't there, just that it might not look entirely wonderful. I may do an FG fin can and tube for F-SRA this year, and I know I'm going to go through at least two or three rockets before I get it perfect. It's just part of the learning curve.

What I'm trying to say in all of this is practice, practice, practice. It's better to mess up a $20 Estes kit than a $1500 CF bird.

Excellent advice by Aviator.
You can only go so far learning techniques discussed in a forum. Then put away the analysis portion, roll up sleeves and apply.

Definitely try out some layout on inexpensive structure first. Experience is best teacher, with mistake/errors an invaluable part of process.

For a clear layup, I highly recommend West Systems 105/207 Epoxy kit West Systems at Soller. I get mine from Soller.

Here's an 'old' build thread for my first CF body tube and fin layup CF Jaguar Build at RP