38mm Minimum Diameter Rocket

James Kobel

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This is going to be new thing that I am going to try! Well... kinda. I've already built a 54mm MD rocket but with the motors that I would like to fly it on would push past the waiver of the usual rocket club that I attend (ROC). Building a 38mm MD I would be able to launch almost any Aerotech motor without going over the waiver. The new experience for me here is fiberglassing my body tubes and doing tip to tip carbon fiber on the fincan. My dad has some experience with these techniques, but I would like to see what tips and tricks that y'all have that you think may help me out. Thanks!38 mm MD Open Rocket.jpg
 

thzero

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Some of us have had good luck with soller composites fiberglass sleeves. I've not tried their carbon fiber sleeves. I had tried their shrink wrap to give a nice finish and did it without release fabric - I got ok results, but really had an issue with pin holes. I am going to try on another rocket (5.54") but using release fabric and then the shrink wrap and see if I get a better finish without all the pinholes.

On tip to tip

This is one of the videos I watched prior to trying it myself


I only did a single layer of 4 ounce cloth, but wasn't expecting anything from true performance. If I were to do it again, I'd do a few layers more like the video does.
 

James Kobel

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Some of us have had good luck with soller composites fiberglass sleeves. I've not tried their carbon fiber sleeves. I had tried their shrink wrap to give a nice finish and did it without release fabric - I got ok results, but really had an issue with pin holes. I am going to try on another rocket (5.54") but using release fabric and then the shrink wrap and see if I get a better finish without all the pinholes.

On tip to tip

This is one of the videos I watched prior to trying it myself


I only did a single layer of 4 ounce cloth, but wasn't expecting anything from true performance. If I were to do it again, I'd do a few layers more like the video does.


Yea, I have already bought a kevlar and fiberglass socks. Putting the kevlar on first and then the fiber sock which will give it a smoother finish. Then we already have some sheets of carbon fiber available and I was going to use a layer of that for the tip to tip on each fin.
 

manixFan

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That is a great design for going high and fast. I've built a fair number of very similar rockets (from kits and scratch) and have never done tip-to-tip, but that's with carbon fiber fins. I have gone well past Mach 2 on the 54mm version. My 38mm version sims pretty close to what you show - Mach 1.8 on the CTI J530.

If you are practicing new construction techniques, a 38mm rocket may be a bit of a challenge just due to the small size of the fins. I find it easier to work with larger surface areas. The biggest issue with tip-to-tip is keeping the fiberglass in place - there are many techniques, with vacuum bagging probably being the best. I have used the sand method with pretty good success, but definitely practice by doing dry layups several times first, and maybe even fiberglass some scrap pieces.

If you are fiber glassing your body tube for the first time, that again is a bit harder on smaller tubes, at least in my experience. I made a 'rocket rotisserie' using a couple of light stands and an actual rotisserie motor to spin the body tube while it was curing so it would be nice and even. Again, vacuum bagging is the best way to do it, but way more work and expense. If your dad has experience with glassing body tubes, for sure get his assistance the first time around, it can be a challenge by yourself.

Here is another good fiberglass video:



Good luck and have fun, and take notes of what works and doesn't. I've found that sometimes it can be months or more before I do something again and it's hard to keep track of the little lessons learned along the way.


Tony
 

Dipstick

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Yea, I have already bought a kevlar and fiberglass socks. Putting the kevlar on first and then the fiber sock which will give it a smoother finish. Then we already have some sheets of carbon fiber available and I was going to use a layer of that for the tip to tip on each fin.
A kevlar sock and glass is probably overkill. What tube are you using underneath the composite?

I've flown a 38mm min diameter on Js and it was just glassed phenolic.
 

James Kobel

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A kevlar sock and glass is probably overkill. What tube are you using underneath the composite?

I've flown a 38mm min diameter on Js and it was just glassed phenolic.
I bought some cardboard 38mm motor tubes (34" long) from balsa machining. And to respond to the first part, this is more so practice for me with newer building techniques
 

James Kobel

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Dry fitting all the parts. Looking to apply the Kevlar and glass socks on Monday…
 

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James Kobel

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Found out that the easy glass sock was way too big for this body tube and I couldn’t find any other 1.5” diameter 2 oz fiberglass sock. So it’s just going to be this Kevlar sock that I’ve epoxied onto the tube. With a lot of sanding and a thick layer of primer, it should come out just fine 1D9CE1DE-6B7B-4206-A75F-BA93C3C0E6D4.jpeg 18B56DBE-6052-497B-AEDD-8E808A4D58EE.jpeg
 

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Dipstick

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Kevlar doesn't usually sand smooth, it will pull fibers until you have a tube of shag carpet...you will probably have to find some glass as a cover to finish completely.

I have never tried it, but if you prime, then fill, and lightly sand it might work?
 

manixFan

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Kevlar doesn't usually sand smooth, it will pull fibers until you have a tube of shag carpet...you will probably have to find some glass as a cover to finish completely.

I have never tried it, but if you prime, then fill, and lightly sand it might work?
I've used kevlar cloth a number of times. It has to be covered with a regular fiberglass veil for finishing. You can't sand it, not at all, at least not any of the stuff I used. The moment you cut through the epoxy you will raise fibers that are a huge pain to deal with. It also weakens the structure. I first used a super light cloth as a veil, like maybe .5 oz. and that worked ok. Later I used a bit heavier cloth to better fill the underlying weave. I knew I could sand as much of the veil as I needed without worrying about the integrity of the layup. It was a lot of work.

I basically quit using it because it was so much trouble to finish. But that was years ago, hopefully my skill have improved by now.


Tony
 

James Kobel

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I've used kevlar cloth a number of times. It has to be covered with a regular fiberglass veil for finishing. You can't sand it, not at all, at least not any of the stuff I used. The moment you cut through the epoxy you will raise fibers that are a huge pain to deal with. It also weakens the structure. I first used a super light cloth as a veil, like maybe .5 oz. and that worked ok

Yea, I can see that. I have some thick primer that I’m going to put a a couple of layers on and then sand. Then repeat that process a handful of times…
 

James Kobel

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I have finished all my filets and now I will start to sand them a lot… probably next week I will have some time to carbon fiber tip-to-tip over the fins!
 

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rokit

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Yea, I can see that. I have some thick primer that I’m going to put a a couple of layers on and then sand. Then repeat that process a handful of times…

You can also try a filler to save you from several prime/sand cycles. Bondo (and similar products) is readily sourced and sands pretty easily. I've used this epoxy-based filler before:

 

James Kobel

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I have almost completed the tip to tip on each set of fins. Just need to sand them a bit more. Pretty close to done, just got to get the recovery systems set up and stability margin where I want it (about 1.5-2)
 

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James Kobel

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I have it all tapped up and ready for the first few layers of primer!
 

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James Kobel

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Got my traffic come orange on the body tube and ready for a clear glossy coat over the whole thing!
 

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crossfire

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There has been a number of pictures where flyers apply the clear coat and the paint bubbles up. The paint really needs time to dry and vent out before applying clear coat. A guy told me if you can smell paint it's still venting out.
 

James Kobel

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There has been a number of pictures where flyers apply the clear coat and the paint bubbles up. The paint really needs time to dry and vent out before applying clear coat. A guy told me if you can smell paint it's still venting out.
Ahhhhhhhh, i see what you mean. I guess I'll just wait a week and see how it is then... and do the sniff test lol
 

3stoogesrocketry

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Your rocket has a terrible design flaw that replicates a 75mm bird I built in 2000 . You have a separation line where the kevlar sock stopped above the fins and the carbon fiber started . In my case , I flew my rocket 6 times on 75 mm K thru M motors , then put a 54mm J in it for small field flight . My adapter was just a short length of motor tube and two rings . There was zero support above the separation line and the fin can literally sheared clean off due to the sideways momentum on landing. If you do adapt down , make your adapter end well ahead of the joint.
 

James Kobel

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Your rocket has a terrible design flaw that replicates a 75mm bird I built in 2000 . You have a separation line where the kevlar sock stopped above the fins and the carbon fiber started . In my case , I flew my rocket 6 times on 75 mm K thru M motors , then put a 54mm J in it for small field flight . My adapter was just a short length of motor tube and two rings . There was zero support above the separation line and the fin can literally sheared clean off due to the sideways momentum on landing. If you do adapt down , make your adapter end well ahead of the joint.
I see what you are saying here and my intention as of now is to not adapt down to 29 or below. My original goal was to see how fast I would be able to make a 38mm MD rocket go for not to big of a building expense. I think total costs for this rocket is around $120. We’ll see how I do. I think the J570 pushes this rocket to about Mach 2.05 but I’ll fly it a handful of times before that because who knows what’s gonna happen to it when it’s going that fast!
 

manixFan

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Great photo! You know it's going to be a great flight when the flame is as long as the rocket. I love the orange color against the mountain in the background, really stands out. If it flew nice and straight - meaning good fin alignment - it should handle Mach 2 no problem.


Tony
 

James Kobel

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Great photo! You know it's going to be a great flight when the flame is as long as the rocket. I love the orange color against the mountain in the background, really stands out. If it flew nice and straight - meaning good fin alignment - it should handle Mach 2 no problem.


Tony
MACH 2 is the goal for this project! I’ll work my way up that testing a few recovery configurations 👍🏻
 

wsume99

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This is exactly the kind of build I'm working out in my head at the moment. I see a tube coupler in your design but it looks like you are just joining two tubes together? Am I correct in assuming you have single deployment and use a chute release?
 

James Kobel

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This is exactly the kind of build I'm working out in my head at the moment. I see a tube coupler in your design but it looks like you are just joining two tubes together? Am I correct in assuming you have single deployment and use a chute release?
Here’s the final updated version…
725B30FB-1755-47CA-8E8F-571456E67553.jpeg
 

James Kobel

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This is exactly the kind of build I'm working out in my head at the moment. I see a tube coupler in your design but it looks like you are just joining two tubes together? Am I correct in assuming you have single deployment and use a chute release?
Also, I have yet to use any electronics or chute release mechanism. However I have recently acquired a line cutter and will be using an Eggtimer Quark for both apogee and deployment events with the line cutter releasing the main chute around 1000 ft
 
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