Airframe choices (or why you should consider rolling your own airframes)

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joeman
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby joeman » Sun Jul 01, 2018 9:21 pm

Hi,
I had a go at making an air-frame this weekend. It didn't go as well as I had hoped, but considering it was my first one, I guess it wasn't too bad.

I left the Air-frame to cure over night and the next morning I was unable to remove the air-frame from the Mandrel. It did slide a bit...but after a while It wouldn't move. After some gutsy effort I decided to use a heat gun to apply some heat to the tube. This helped greatly and I was ultimately able to get about 1 metre of CF tube off. This was a good lesson in the effects of temperature on the Epoxy.

Question: I'm thinking of removing the air-frame partly cured off the Mandrel....before shrinkage starts to get too bad. Does anyone have any advice on this?

e.g. time to wait?
Perhaps other way to ensure removal of the air-frame off the Mandrel?

I'm using K3600 Epoxy.
Also using two separate layers of Glade Bake and Scotch Tape. The outer layer of Glad Bake slides easily over the inner layer.
BTW...the two layers is "perfect" for the 38mm CTI Motor casing.

Cheers!

Joe
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drew
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby drew » Mon Jul 02, 2018 11:51 am

joeman wrote:Hi,
Question: I'm thinking of removing the air-frame partly cured off the Mandrel....before shrinkage starts to get too bad. Does anyone have any advice on this?

e.g. time to wait?
Perhaps other way to ensure removal of the air-frame off the Mandrel?


My thoughts are that ideally you'd leave your airframe on the mandrel for at least a half day to allow it to harden sufficiently. Otherwise you'll pull it off too early and it'll sag or deform and be largely unusable. Same with a post cure, ideally you want a room temp cure before you move to post cure imo.

As for your specific issue, your comment of "it did slide a bit" makes me think you attempted to remove the airframe from the mandrel the wrong way (for lack of a better term). you should be able to pull and rotate the cured airframe in the direction you rolled the glad bake and get a clean separation. If you push the other way your glad bake will compress, bind, and not let go easily.

For mine historically I've rolled airframes in the afternoon and then pulled them off the mandrel the following morning. If you leave them for a day or more my expectation is that you'd have a much tougher time.
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby joeman » Sat Jul 07, 2018 9:23 pm

Hi Drew,
Thanks for feedback.

Adjusted my setup/procedure a little:-
- Use just one piece of Glad Bake (I had two individual pieces last time)
- Ditched the scotch tape and used a very thin film of glue from glue stick
- Kept the build of the tube close to one end....last time I had it 20cm from one end
- Make sure I had plenty of Glad bake to ensure I couldn't get any glue on the Mandrel...no mishaps
- Decided to do a smaller 'trial' tube build - 35 cm
- Taken a bit more care with the build itself... I did a lot of 'tightening' actions with brush to squeeze out epoxy the first time. This time I was a little more careful with epoxy.

The result - I could remove the tube this time after 10 hrs! Yes!
There are a few spots where there is epoxy missing on inside of tube (air bubble?). But trust me it is a lot better than the first one.

GOPR0097small.jpg
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GOPR0096small.jpg
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Probably need to do a few more practice tubes till I have it fully sorted....and can then start on the real body tube.

Making your own airframes is fun!

Cheers

Joe
L1 - Callisto (H133) - 11-Jan-2016 - LCO
L2 - March Fly (1633K940) - 18-Mar-2018

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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby drew » Mon Jul 09, 2018 12:49 pm

joeman wrote:Hi Drew,
Thanks for feedback.

No worries!

joeman wrote:The result - I could remove the tube this time after 10 hrs! Yes!
There are a few spots where there is epoxy missing on inside of tube (air bubble?). But trust me it is a lot better than the first one.

Fantastic work d00d! As for those spots inside the tube, that's where there wasn't enough epoxy on the mandrel when you did your first lap of cf. It happens to me as well. Normally I take them into account with what I plan to do with the tube. So I'll either trim the tube to remove them if I need to cut it down. If it's part of "what I need" from an airframe perspective i try to locate them on the motor mount end of the airframe as the motor casing itself will provide added structural support. Ideally I try not to have those types of imperfections in the part of the airframe that isn't structurally reinforced.
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby SpaceManMat » Wed Jul 11, 2018 9:53 am

You could probably try filling the dry spots. It would still lack some strength but would be better none the less.

You would probably fill the hole with epoxy, push the mandrel through it to get the excess out. Then you’d have to rotate the tube reasonably quickly to prevent pooling until it had time to set, which would require a rig of some sort.
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby drew » Wed Jul 11, 2018 3:39 pm

SpaceManMat wrote:You could probably try filling the dry spots. It would still lack some strength but would be better none the less.

I suspect it would be roughly identical in strength Matt as logically you'd be filling a gap with the same epoxy that would have been there in the first place.
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby SpaceManMat » Wed Jul 11, 2018 6:02 pm

drew wrote:
SpaceManMat wrote:You could probably try filling the dry spots. It would still lack some strength but would be better none the less.

I suspect it would be roughly identical in strength Matt as logically you'd be filling a gap with the same epoxy that would have been there in the first place.


My understanding (sorry having difficulties finding a reference) is that the epoxy forms into a polymer as it cures (essentially building one large molecule). At the join however intermolecular forces are at play and this is what keeps the 2 or more pieces stuck to each other. The intermolecular force is weaker which is why a break tends to mostly result in epoxy being ripped off the surface of whatever it was stuck to. Once set polymer formation ceases you’d tend to end up with 2 pieces not 1, but probably on a fresh wrap you would still get some arrears of the join forming into a ploymer. It would then be stronger than a join provided it was kept very clean.
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby Oldboy » Wed Jul 11, 2018 9:05 pm

Curious to this,
If the epoxy has cured ,adding more to the surface not having the polymer form long strands and integrating with the C/F .
Would this be cosmetic, like filling a dent on the triton with automotive bog ?
Drew will get the triton reference .
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby Kryten » Sat Jul 14, 2018 2:49 pm

The strength comes from the fibre. The polymer binds the whole lot together. It's essentially a "glue"
Merely adding lots of resin, as in a bog, doesn't add anything to the strength
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby Oldboy » Sat Jul 14, 2018 4:17 pm

That's right .
What I was querying is was filling the imperfections would be merely cosmetic not structural , hence my reference to BOG.
All the structure is in place unless you join the new layer via mechanical bonding ie sand and work in to the existing surface.
Rendering a brick wall does not make it stronger
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby joeman » Sat Jul 14, 2018 10:43 pm

My understanding is that the Epoxy provides the compressive strength of the composite. The CF provides the tensile strength. As the rocket flies it gets bending moments....one side has tensile stresses and on other side has compressive stresses. The air-frame needs to be able to handle both.

I don't think I'd like to be relying upon the "bog" to resist these stresses. I like what Drew said, if it is down near the motor casing, the motor casing provides some structural support. One "may" be able to get away with some small defects at the top, where the bending moments aren't so great.

I think I just need to work out how to build a better tube.

RE: 3rd Tube
I just made a 3rd tube and I only have two smaller defects in this 350 mm length.
I modified my build procedures a little. In addition to applying Epoxy on the Glad Bake, I applied epoxy on the CF fabric immediately before I placed it on the Glad Bake. I didn't apply epoxy on the whole CF fabric, only 130 mm from the start edge.... i.e. enough for one revolution of of the fabric.

Just figuring out what to do next to get an even better build.

Cheers

Joe
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L2 - March Fly (1633K940) - 18-Mar-2018

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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby drew » Sun Jul 15, 2018 12:13 pm

SpaceManMat wrote:My understanding (sorry having difficulties finding a reference) is that the epoxy forms into a polymer as it cures (essentially building one large molecule). At the join however intermolecular forces are at play and this is what keeps the 2 or more pieces stuck to each other. The intermolecular force is weaker which is why a break tends to mostly result in epoxy being ripped off the surface of whatever it was stuck to. Once set polymer formation ceases you’d tend to end up with 2 pieces not 1, but probably on a fresh wrap you would still get some arrears of the join forming into a ploymer. It would then be stronger than a join provided it was kept very clean.

That's how I understand the polymerization of epoxy as well. But what I don't believe you're taking into account is that this epoxy system provides further hardening if you post cure it. So my assumption (and it is an assumption) is that if you add more K3600 epoxy and then post cure it after it initially cures at room temperature is that you would have polymer activation/bonding across the interface of the initial and added epoxy regions due to the elevated post cure temperature. I could be wrong, but anecdotally I do suspect this is how it would work with this specific epoxy system. And from what I've been told the post cure benefits can be realised at any time, ie you can wait months after the initial cure, post cure, and still receive the benefits of said post cure from a hardening perspective.
Andrew Hamilton
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Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby joeman » Wed Jul 18, 2018 6:05 pm

G'day!
Just did my fourth test run.

I decided to reduce the amount of hardener slightly to increase pot-life, so I'm not so rushed. I can take my time and make sure everything is wetted out.
I also cut 10mm off the brush bristles to effectively stiffen it a little...help me push the epoxy through the carbon fibre fabric.

This coupled with very careful application of epoxy for first ~3.14 * 40 (=120mm) of CF led to perfect job on the inside.

The let down is that the external surface is a little deformed. I can "probably" hide most of it by applying some external epoxy and sanding.
I think the cause of the minor deformation is the brush stiffness. I think the stiffer brush while good for the first layer and a bit, is probably too stiff for the last layer and a bit.

So, next time, I'll use two brushes.

Getting there!

Cheers

Joe
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby OverTheTop » Wed Jul 18, 2018 6:30 pm

Good progress Joe. Have you thought about a layer of fiberglass "tissue" as a cosmetic improvement?
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Re: Airframe choices (or why you should consider rolling your own airframes)

Postby joeman » Fri Jul 20, 2018 2:08 pm

Hi OTT,
No I hadn't thought of that. Another idea to add to the pot.

I don't think I'll be able to avoid having to apply some epoxy to deal with little defects to the external surface. So I constructed something to rotate the air-frame while it cures.

Motor/gear-box from Jaycar - 160RPM
Did a 3-D print of "plug" to attach air-frame to motor.

Running off 3 volts...comfortable pace. I can change the voltage (add/remove batteries) to change the speed.


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Cheers

Joe
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