Carbon Prepreg Airframe build for the Loki 54/4000 - Fins Attached

Hi all,
I received my 54/4000 case from Scott at Loki Research a little while back. Something else equally as awesome showed up in the shop today, courtesy of a friend.



On the workbench you can see the Loki case with a seemingly unlimited supply of uni carbon prepreg. I've been toying with using prepreg for a while now, and I've just stumbled upon some. Lots if it. A black, tacky gold mine. It's technically past it's shelf life date, but it has been well cared for and the first bake of the stuff was successful.

So, on with the build. First the airframe tube, then fins. I'm still in the research and optimization phase, but the mandrel for the tube is on it's way. If I like the looks of this mandrel, the tube fabrication will start right away, using the prepreg and Dunstone shrink tape for compression.

More to come...

Definitely subscribed.

Whats your layup schedule for the tubing? No need for a mandrel, just roll 2 shorter tubes on that monster!

op:

Alex

op:

Watching.

Any idea what uni/resin you have? Also where did you get your mandrel?

Oh dear gods

Aksrockets said:

Whats your layup schedule for the tubing? No need for a mandrel, just roll 2 shorter tubes on that monster!

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I'm still working on the exact number of plies, and that might take some trial and error. Currently the plan is to do a hoop ply, a to-be-determined number of axial plies and then another hoop ply. I'm not an engineer, however. So I'm open to suggestions...

I have some research to do. First off, I know almost nothing about this prepreg. The resin is Tencate EX-1522. I have a cure schedule (a couple, actually) but that's about all I know other than it says it's "toughened" and has excellent mechanical properties. Cool, right? As for the fiber, it's Toray M40JB, 145g, and that's all the info I have. I forgot to get a measurement of the thickness before I packaged it back up and put in in the freezer. I couldn't seem to find much info on these after a quick internet search, which I thought was unusual.

As for the the two shorter tubes and no mandrel, this airframe will be a little (extremely) unusual with no airframe breaks, so I need a long (~45") airframe tube. And it just gets weirder from there. If I am able to pull off the fins I would like, they'll be made from prepreg plies of decreasing size, compression molded into a biconvex airfoil, specifically a NACA 65a003 or 4. But we'll see. Typing that just now sounded crazy and over the top.

Off to do more research.

flynfrog said:

Also where did you get your mandrel?

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I ordered two kraft paper mandrels from Coast rocketry. https://www.coastrocketry.com/composites.htm At least I emailed them telling them I would like to order. That was this morning so I probably wont hear anything until at least Monday. For all I know they are out of business. There's a Plan B if that's the case.

I chose paper wrapped in high-temp release film so that I would have a sacrificial mandrel. In the past I have had a maddeningly difficult time getting tubes off mandrels when compression was involved. That way I can soak the tubes and twist and pry the mandrel out of the tubes. I hope. The paper is not without it's potential problems, though.

Kehoes23 said:

op:

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op:

Scott,

Make a test piece using the prepreg an make sure it works and the layers stick together.
I got a LOT of prepreg from ASU, they got it from Hexcel. It would cure OK but the lay ups were no good. I even tried it using a Carver press with pressure from reasonable to ridiculous.
So I junked it, a dozen or so rolls.
I did get some tow that was not pre preged and it was useful.

Mark

watheyak said:

I ordered two kraft paper mandrels from Coast rocketry. https://www.coastrocketry.com/composites.htm At least I emailed them telling them I would like to order. That was this morning so I probably wont hear anything until at least Monday. For all I know they are out of business. There's a Plan B if that's the case.

I chose paper wrapped in high-temp release film so that I would have a sacrificial mandrel. In the past I have had a maddeningly difficult time getting tubes off mandrels when compression was involved. That way I can soak the tubes and twist and pry the mandrel out of the tubes. I hope. The paper is not without it's potential problems, though.

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here is the data sheet for that resin.

https://www.tencate.com/emea/Images/EX1522_DS_Web_01291428-3892.pdf

and the fiber.
https://www.toraycfa.com/pdfs/M40DataSheet.pdf

Im not sure what the resin content is but you may have some issues getting that compacted with just shrink tape as it is an autoclave resin you may have some porosity. If you can you may want to find a way to apply tension to your shrink tape. If you can find away to roll wrap it you would help to avoid wrinkles. If you can work it out adding some angles other than 0 90. Paper mandrels scare me for trying to get a straight tube.

Hi Mark,
I did a test lay up earlier and it seems pretty good. It's deceivingly tacky, and there seems to be good flow under heat. Bond quality seems great, but I haven't gotten it off the mandrel yet. Naturally.

Were yours just peeling apart after cure?

The layers did not stick, I used temps from room to 350f but they peeled apart. it was like the lay up was too dry.

M

flynfrog said:

here is the data sheet for that resin.

https://www.tencate.com/emea/Images/EX1522_DS_Web_01291428-3892.pdf

and the fiber.
https://www.toraycfa.com/pdfs/M40DataSheet.pdf

Im not sure what the resin content is but you may have some issues getting that compacted with just shrink tape as it is an autoclave resin you may have some porosity. If you can you may want to find a way to apply tension to your shrink tape. If you can find away to roll wrap it you would help to avoid wrinkles. If you can work it out adding some angles other than 0 90. Paper mandrels scare me for trying to get a straight tube.

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Thanks for the data sheets. It's 35% resin content. Actually, here's the label from the inside of the roll-


Flynfrog, maybe you give me some expert advice here-

I've had some serious angst about the paper mandrel. I had considered an aluminum tube- Polished, waxed, spray on mold release and then get it cold to slide the tube off. I've never used any spray on release, and especially not at high temperatures. I've read just about everything there is on the internet about making composite tubes, but very few sources are relevant to long, relatively large diameter, non-tapered tubes. The best I've got is an old yahoo composite rockets post from Mick Kelly, but it's pretty dated.

Any advice on how to get a tube this long off the mandrel after it's been compressed would be greatly appreciated. Specifically regarding the preparation an aluminum mandrel.

The last time I did it it took lots of mechanical force and sweat and tears. And about 2 weeks.

When I did my 54mm MD bird I used a steel tube of 2.125 OD then added a half overlapped ply of peel ply to get the ID of the finished tube bigger. There are a ton of releases out there. They don't have to be spray on. Most of what we use are wipe on wipe off. If you are friendly to a distributor you might be able to get a sample. As for pulling it we have a big hydraulic puller at work. Using AL over steel will make it easier as you are doing an elevated cure and AL has a greater CTE than steel. If you can find a way to hold everything it might just come off.

The resin content is pretty normal and I think that resin has pretty good flow. I would be worried about the age. Its rated for 6 months but its 4 years old.

watheyak said:

Thanks for the data sheets. It's 35% resin content.

Flynfrog, maybe you give me some expert advice here-

I've had some serious angst about the paper mandrel. I had considered an aluminum tube- Polished, waxed, spray on mold release and then get it cold to slide the tube off. I've never used any spray on release, and especially not at high temperatures. I've read just about everything there is on the internet about making composite tubes, but very few sources are relevant to long, relatively large diameter, non-tapered tubes. The best I've got is an old yahoo composite rockets post from Mick Kelly, but it's pretty dated.

Any advice on how to get a tube this long off the mandrel after it's been compressed would be greatly appreciated. Specifically regarding the preparation an aluminum mandrel.

The last time I did it it took lots of mechanical force and sweat and tears. And about 2 weeks.

Click to expand...

Liquid nitrogen works.

Another way (Adrian A documented it somewhere) is to use a long piece of all-thread, wingnuts, large washers, and spare airframe tube in order to basically clamp your new tube off of the motor case.


One thing you could try is not using mold release, and no polish: just go with something like parchment paper or release film that slides easily on the tube. Even with compression, you should be able to slide it off because there's no high-viscosity (effectively) wax sticking to the film/parchment paper.

CarVac said:

Liquid nitrogen works.

Another way (Adrian A documented it somewhere) is to use a long piece of all-thread, wingnuts, large washers, and spare airframe tube in order to basically clamp your new tube off of the motor case.

Click to expand...

HA! That was me way back on Rocketry Planet. Maybe Adrian did it, too. I'd like not to repeat that, but now that I think of it, I still have the all-thread...


And Flynfrog, I'm worried about the age, too. I'm going to do more tests parts. But the first one looks great!

One thing that certainly helps is a high quality mandrel. What the group I'm working with uses, is a piston tube. These tubes (and rods) are intended to be used as a piston in a hydraulic cylinder. They are precision ground and chrome plated, so they have a high quality cylindrical surface. I couldn't participate in the lay-up, but I've been told it was quite nice to work with. After curing the tube in the autoclave, putting it into the freezer was enough for it to fall right off.

The drawback, especially for bigger mandrels, are costs and weight. In our case, the list price for a 2m x 100mm x 5mm piston tube was in the neighborhood of 350€ and it weighs about 23kg (51lbs).

Here is an example supplier, if you're interested in this approach:
https://www.teamtubellc.com/en/prod...power-products/chrome-plated-tubing---od.aspx

Reinhard

Another possible mandrel would be press fitting UHMW tubing over an aluminum tube, and machining the outer surface to diameter. You can perform the layup directly on the plastic surface, and then you can put it in the freezer and the plastic will easily fall out.

I don't know if it can withstand enough heat for your prepreg, but for room temperature curable resins you can even heat it slightly to internally stretch the tube, improving compression and eliminating wrinkles.

What a small world. I thought that name sounded familiar. I worked right next door to Tencate for a few years.

So after even more pondering and some suggestions I have decided to do a metal mandrel this time as well. Last time I used a steel mandrel with no heat. I think with the aluminum mandrel and heat, it should hopefully be somewhat easier to remove after cooling it down a bunch. Thanks for all the suggestions. Here's what I ended up getting-

https://www.dxengineering.com/parts/dxe-at1493

I also ordered a 1.5" and now I understand CTI is releasing long burn 38mm motors this month. (I've been waiting for those for a LONG TIME!) They should be here Thursday. I'm currently researching mandrel prep.

I also cured a few more simple layups to test the viability of the prepreg. They look and feel great. They have the "ring" that's really difficult to achieve with wet layups. They're just a different animal than most of the wet layups I've done. But we'll see, I am by no means an expert.

Watheyak,

In a posted you stated you dont know how many plies to use. Take 22.12oz. and multiply it by the thickness you want in millimeters and then divide that by the weight of the cloth in oz. and you will get your number of plies to use.

Note: you will get a decimal answer, but just round it and your all good.

Hope this helps.


Alexander Solis

Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet

SinfulDarkLord said:

In a posted you stated you dont know how many plies to use. Take 22.12oz. and multiply it by the thickness you want in millimeters and then divide that by the weight of the cloth in oz. and you will get your number of plies to use.

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Interesting. 22.12oz came from where? I think I get why this works, but I'm slow...

watheyak said:

Interesting. 22.12oz came from where? I think I get why this works, but I'm slow...

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According to Nic from Australian Rocketry Forums 750gsm or 22.12oz. gives 1mm thickness. Of course in order to get your custom thickness you multiply it by the desired thickness in millimeters and then divide it by the weight of the cloth in oz. since the 22.12 is in oz. it works the same way with gsm.

I have tried this method and it holds true. No more trial and error I'll say.


Alexander Solis

Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet

The mandrels came from DX today. They were pretty carefully packaged and free from any major scratches or other defects. I'll be checking them for straight and round this weekend.

Good luck with the build Scott! Maybe you will break the 35K mark at Eagle Eye, I will definitely be there to watch.

watheyak said:

The mandrels came from DX today. They were pretty carefully packaged and free from any major scratches or other defects. I'll be checking them for straight and round this weekend.

View attachment 174501

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That's one pretty mandrel. It looks like I will be getting mine from them as well. Judging by the way it is right there it looks pretty damn straight.

I didn't get to do any actual work on the rocket this weekend, but I had some productive research gains. As I have mentioned before I'm concerned about getting the tube off the mandrel due to the compression of the tape. I hope I'm over-thinking this, and curing with a hot, expanded mandrel, then freezing it will help a lot. But better safe than sorry.

I took a trip in the way-back machine and did some reading on the old Composite Rockets Yahoo group, looking specifically for mandrel preparation ideas. I came up with a few ideas. First, I think I'm going to put a simple polish on the mandrel with this stuff-
https://www.amazon.com/dp/B0009H519Y/?tag=skimlinks_replacement-20 I think this will be a good way to smooth the thing up without removing much material. It's pretty shiny already.

Then I want to lubricate the mandrel. Not a mold release, just give it something to help with the slide factor...
https://www.amazon.com/dp/B003UTX0R8/?tag=skimlinks_replacement-20 It's high temp, and I don't want anything that will roll or bunch up as I remove the tube. I have never used this stuff before, so I'll have to evaluate after I get it.

Then, a wrap of high temp release film-
https://www.fibreglast.com/product/...film-1782/Vacuum_Bagging_Films_Peel_Ply_Tapes I'm still trying to decide how to secure this. High temp flash tape maybe.

After that, some high temp release wax and I should be golden. Right? I hope.

Any other good advice out there?

why not use 2mil mylar? I'd be afraid that stuff you linked to would bunch up during the mandrel removal. Also do not use any tape, under your layup, to affix your release film to the mandrel. It's amazing how much a 1/2" piece of tape can screw things up.

Tony