Nomex honeycomb Fin skins on AMW Der Red Max build

I was reluctant to post another 'Red Max' thread, as it seems like there are plenty of others.

The kit, as delivered, comes with some large, but thin G10 fins that feel pretty flimsy considering the size. Robert of AMW told me about the nomex honeycomb skins, and I decided to give it a go. Glad I did.

The following is a photo documentation of the process I came up with for applying the skins. I have never used this material before, and do not claim to know much about it. I did not easily find much information about this material, or in using it, especially for rocket fins, so I decided to share.

These skins add a huge amount of rigidity to the fins, moving well away from the feeling of the Red Max kit being mostly a novelty rocket.

There are a lot of photos, so I will initially just post them, and come in to further edit this thread when I have more time, detailing some of the steps.


Cheers!


This is what the honeycomb material looks like as delivered. It is 0.075" thick, including a fiberglass 'skin' that is 0.008" thick. The bonding end of the honeycomb structure has a small 'flat' at the top of each cell to maximize gluing surface.




Sanding the G10 fins. One of the cool things about pneumatic sanders, is the ability to feather the speed down to almost stopped, like maybe 50 opm's? or less? Total control.

Sanding the surface of the honeycomb seemed like a good idea. It was slightly glossy. Probably would have glued fine without, but I chose to sand it, very lightly. 220 grit was my choice. Very little pressure required here.

Blowing out and cleaning up with compressed air following the sanding. I have excellent filtration for my compressor output, and the lack thereof could be a source for oil contamination, so heads up.

Cleaning the bonding surfaces with acetone after sanding, to remove any skin oil, or other potential contaminants.

Mixing Raka epoxy into 1oz mixing cups. This was a perfect amount for the small foam roller shown in the photos. This roller setup came from Home Depot, along with a small roller pan. After pouring the very well mixed epoxy into the roller pan, the roller cover is moved back and forth in the epoxy until it is almost all worked in to the cover. If it is worked too long, the roller cover may end up too dry. This seemed like a perfect amount of epoxy. I started rolling the honeycomb structure after almost all of the epoxy had entered the cover, working carefully to be sure that on the first contact with the honeycomb, the epoxy was not in excess, and thus filling the cells instead of just applying sufficient epoxy to bond the cells to the fin surface.

After rolling the honeycomb, the cover was pretty dry, and seemed perfect for applying a very thin layer of epoxy over the surface of the fin. I am not sure if this is necessary, but it is good insurance for bonding every bit of every cell wall.

This is what the honeycomb looks like with the epoxy rolled on. I used a magnifying visor on the first one to be sure that the quantity of epoxy looked sufficient.

Cleaning excess epoxy from the airframe with acetone.

I have a vacuum press/ clamping system, but chose to use a manual clamping setup, as I could imagine all of the cells in the honeycomb being evacuated of air, and the epoxy being migrated as a result. Maybe this is not an issue? Regardless, with the pink foam, constant and significant clamping pressure is applied to the honeycomb.

The clamping surface is a granite measuring plate, flat to within 0.001", with 2 mil mylar coated in PVA mold release. Prior to laminating the honeycomb, I could set the airframe on the granite plate, check the airframe for being square with the plate, and then check the fins for straight and in plane with the airframe. With the G10 material alone, very slight 'wavyness' was detected in the large fins. After both laminations of the honeycomb, the 'planes' of the fins are dead true with no wavyness, aside from the minor dimples in the material itself, another potential benefit of using the honeycomb.

The clamps are slowly, carefully, and evenly tightened down, taking care to watch the edge of the honeycomb skin where it meets the airframe. Any slippage would be a bad thing. In 6 skin glue-ups, I saw zero slippage. With the Raka epoxy I used, with my shop temperatures, I could get 2 glue-ups per day.
Every lamination looked 'perfect'. A new foam 'caul' is enough to clamp both sides of one fin, replacing it after each surface is used. 3 foam cauls, total. The pink foam is pretty dense, and is a good clamping media for 'uneven' ground. Also, if any epoxy on the surface of the glass skin on the honeycomb, the pink foam has no problem releasing from it, providing that there is not much there. Cleanliness is important.

Notice the compression of the clamped pink foam cauls. Where the full thickness of the fin was visible at the edges, prior to clamping, after clamping, the pressure has caused the unsupported foam to be compressed all the way to the granite plate, giving an glue up. In the first photo, where the honeycomb is visible, the pink foam caul is slightly small, covering only the fin, with no overlap. It is enough to compress the eitire fin, but not large enough to compress to the granite plate, where it comes close to the airframe.

to be continued....

Nice, the thickness definitely makes it look more like the original DRM! I'm building same kit, in primer now, the 'thin fin' version and really looking forward to flying it.....I had a DRM back 40 years ago.
Anyway, I'm curious about why the motor tube is extending so far out the bottom. I set mine flush with the main tube and recessed the bulkhead 1/2" for the motor retainer. It looks like I'll be limited on motor size....is that the reason?
Tim

Neat!

It is important to remember that fiberglass dust is bad for the lungs. Wearing a respirator when sanding is not necessarily sufficient protection if the dust is allowed to travel throughout your shop. The most dangerous size of fine dust particles will stay in suspension for days, and are easily stirred up again in the future.

In the photos above, the sanding is done on a 1700cfm HEPA filtered downdraft table. In addition, the sander has a duct collection port that is attached by a hose to a HEPA filtered Festool vacuum dust collector. Very little dust escapes entrapment in this setup. I have a couple of 1700 cfm industrial HEPA filter units in my shop to protect my lungs. I consider these to be my favorite, and most important tools, above all the rest.

Internal fillets next.

Following

Those skins look great. Thanks for posting this thread.

Redwagon said:

Nice, the thickness definitely makes it look more like the original DRM! I'm building same kit, in primer now, the 'thin fin' version and really looking forward to flying it.....I had a DRM back 40 years ago.
Anyway, I'm curious about why the motor tube is extending so far out the bottom. I set mine flush with the main tube and recessed the bulkhead 1/2" for the motor retainer. It looks like I'll be limited on motor size....is that the reason?
Tim

Click to expand...

Yes, There is an Aeropac tailcone retainer. This is what it looks like with a 6GXL:

Keisling said:

Those skins look great. Thanks for posting this thread.

Click to expand...

Thank you.


The honeycomb has completely transformed this rocket. The hardest part with the fins is yet to come, but not too bad, I think. Some precise prep will be done to the edges of the fins, and some carbon fiber will be bagged over the honeycomb and anchored on the G10 core. It will make for a cool rocket, and it is perfect practice for the next one. This one is probably getting painted, likely a candy apply red with a big flake for the airframe to keep with the DRM spirit.

Chris, what is your source for that particular honeycomb material? Thanks.

Paul B.

PBic said:

Chris, what is your source for that particular honeycomb material? Thanks.

Paul B.

Click to expand...

AMW sells it with their DRM kit. It is one of the options.

This is new and cool. Thanks for sharing.

I missed the photo op for the trimming of the excess honeycomb. It trims very easily with a razor knife when 'green'. When it is cured, because of the small amount of resin, it still trims okay. It sands (bevels) to a nice clean line with 80grit on a wood block. I will post more pics when it is time to prep the leading edge for cloth.

After the Honeycomb skins went on, it was obvious that they are much, much stiffer. This stiffness seemed disproportionate to the strength of the thin wall body tube provided in the kit, and calls for good reinforcement. I did internal fillets made with epoxy and milled fibers, followed by 2 layers of 5.7 oz. plain weave carbon cloth. This process added 1.3oz to each of the 3 internal sections, a total of 3.9 oz. additional weight for the fillets and carbon fiber reinforcement. The air frame feels noticeably stiffer from the reinforcement. I am pleased with how it turned out, especially considering the tight quarters in there. There is only minimal excess resin. Peel ply and foam brushes were used to remove excess resin, and also to keep things cleaner inside.

The internal fin tabs will be sandwiched between 2 centering rings, structurally.





I have been thinking for the past few weeks about how exactly I want to finish the fins, how deep to sand the bevel, etc. I ordered some milled kevlar to mix an epoxy based filler for the fin edges. They will be skinned in 2 layers of Carbon that is cleanly ended at the airframe. External 'fillets' will be minimal, and will serve only to give a clean paint line and a slight bit of reinforcement with the honeycomb. The fin edges will be trimmed after the carbon cloth, and probably given a built up epoxy treatment. It might be overkill, but it will be good practice.

All of the extras added considerable weight to the original design, thicker fins, the honeycomb, carbon reinforcement, and the tailcone. It is now a much more robust rocket than the original huge floppy fins provided. I made other changes to the design, also. A 5.5:1 nosecone is added, and the AV bay will have a motor tube through it. I am designing this to fly on an L935, but with a modification to the AV bay internals, it will take a Loki L1040. I will fly it close to the ground first, to ensure that the drogue is sized properly, and if any changes are necessary for a clean descent, and then gradually fly increasingly larger motors. It is a standard dual deploy design. 2 GPS trackers will be in the nosecone, as well as an RDF tracker. With the L935 it sims to over 11,000', max velocity mach 1.4. Stability is 2.2 cal, approximately.

The nosecone is a standard wall thickness, the airfame of the AMW DRM is thin wall. I used a 2.75" long standard wall switchband to act as a tapered transition. The transition was carefully sanded using a flat wood block with 80 grit paper. It mates equally well at both ends now, and aesthetically, it is great, works fine. This took about 10 minutes of sanding.

Good thing I checked this out. There are only about 2.5 threads in contact with the aluminum tip, not enough for a good anchor in soft aluminum.

I will be anchoring the main shock cord here, with a longer screw.

Subscribed to the thread!
Cool build log
Thanks for sharing!

In several easy sessions with a sanding block covered in 80grit, the honeycomb was tapered down into the G10 fin to create a beveled edge. This is after the first coat of kevlar fiber filler. Happy with these results, though I may mix up one more batch of kevlar filler to smooth things out even more. The kevlar filler does not sand easily, as you would imagine, but it does sand, and the fuzzy surface remaining will be great for bonding the cloth over the top of this. The angle in one of he photos makes the airfoil shape look uneven, that is photo angle. In the photos there has only been minimal sanding done to the filler. The fins feel like they will be very stiff, and pretty tough.

This filling coat added .8oz to the total.

Those look amazing. subbed fo sho'

The second kevlar fill was 'money'.. Looking much better than the first coat. Time to let it cure, sand it, and then laminate 2 layers of CF cloth over the fins.

The fills are all faired, looking good, and about to get the final fill coat which will fill the few remaining tiny voids and imperfections, and get the fin surfaces ready to lay CF cloth onto. CF cloth will start being laminated within the week.

Looking at the pile of unused parts from the original DRM kit I purchased, I decided to order another airframe tube from AMW, and build a second one.

I have been considering how to assemble the the fins on a through the wall fin assembly in the cleanest possible manner. Gluing the centering rings to the motor tube as the initial and separate steps implies a messy solution for gluing the assembled motor tube into the airframe, almost certainly with excess adhesive. I tried a glue up sequence I have been thinking of, and it worked great, so I thought I would share. I am sure this has been done before, so nothing new.

Also, as I am using an Aeropack tailcone retainer, some method must be used to properly locate the motor tube in the airframe during the glue up. This method works great.

For this process, a fin alignment jig is necessary.

First step is to dry fit everything, do any 'clean up' necessary to get a good fit.

Masking tape is applied in 2 places on each fin to give a solid interference fit with the fin jig.

Then, all surfaces being glued are sanded and wiped with acetone, and not touched with skin again.

Epoxy is mixed, with a long slender and stiff 'epoxy applicator stick prepared and ready.

The fins are assembled into the airframe with the fin assembly jig.

Each fin is carefully removed from the setup, and epoxy carefully applied to only the mating surface where the fin touches the outside of the airframe. The mating surface of the fin tabs is left for later. The minimum amount of epoxy used to get a good joint.

As the glued fins are placed back into the airframe fin slots, and fin alignment jig, the glue is not seated against the airframe, yet, in order to allow some working room to apply adhesive to where the fin tabs meet the motor tube.

After all three fins have been removed, epoxy applied, and replaced loosely in the jig, the surfaces of the fin tabs which mate with the motor tube are carefully dabbed with epoxy using an applicator stick. Just enough epoxy so as to not make a mess.

A motor case is placed into the Aeropack tailcone assembly, and the tailcone assembly is friction fit onto the motor tube, maybe using a layer of tape if necessary, to get a tight fit. This ensures that the motor tube is installed at a depth which is pretty close to where it needs to be when the retainer is finally glued on.

After the motor tube is 'dry assembled with a motor case, the assembly is carefully lowered into the airframe, and the fins carefully moved into position where the fin tabs mate with the motor tube, and the fin alignment jig is secured.

Then, I used a band clamp, as shown in the photo, to hold everything together while I installed the upper centering ring.

I used a long skinny stick to apply just enough epoxy to the inside of the airframe, and the od of the motor mount tube, about 1/2 inch above the final resting place of the upper centering ring. This is a delicate step in order to not make a mess. The amount of epoxy necessary is minimal, as when the centering ring is pressed into the bore of the airframe, the epoxy is smeared into a nice little fillet. Dry fitting of the centering ring beforehand is necessary to know how things fit, and to know that there is enough clearance for things to fit smoothly, and allow some epoxy to migrate in, but not a loose fit.

This was all one sitting, one step and the fins are in, the motor tube is in, upper centering ring in, and the depth is set for the tailcone retainer.

The band clamp should only be left on for long enough for the assembly to happen, and removed just prior to when the epoxy starts to harden. If anything moves when the band clap is removed, it is likely that the dry fit was not ideal. Nothing should move when the band clamp comes off, and it is never tightened much anyway, jut enough.

This process yielded a very clean result. I had a couple of minor clean ups inside the tube where I accidentally touched it with the applicator stick, but there was not really any 'excess squeezeout', only a minor amount. This was fast, as there was no waiting for epoxy to cure before progressing to the next step. Only a minimal amount of epoxy is left at the glue joints. I did not need to clean up any of the joints, no excess epoxy. No squeezeout was cleaned from any joint in this glue up, as there was no real excess. No smearing glue, only clean.

This one will also get the reinforced internal fillets and nomex skins/ cf, but later on.

The best part of this method is that it creates perfect alignment of the tail cone retainer. Well done.

BDB said:

The best part of this method is that it creates perfect alignment of the tail cone retainer. Well done.

Click to expand...

Thank you, yes, that is one of the beauties. There are other ways to achieve the same result with the tailcone, but this is easy. The clean glue up that this sequence allows is what is really cool. And not having to wait for the motor mount / centering rings to cure before installation. Plus, if the centering rings are on first, how does one do clean internal fillets without injecting? Not that injecting is bad, but with that much room, it can be done cleaner.