L3 Cert build for AirFest ... DX3 XL

[pendrin2020]

So back in April, I put up a gofundme, and after some really awesome friends and relatives threw in some cash, I picked up a Madcow DX3 XL for my birthday. The intention is to get my L3 at Airfest this year.

Because of my internship this summer, a couple of NASA projects, and some medical issues with my shoulder, I haven’t been able to start the build until the last two weeks. It’s given me lots and lots of time to think about it though.

Construction began with the most detailed Open Rocket model that I could muster. (This was done back in may)



I did several versions of the file for different features and weight considerations. I’ve built enough rockets to know that sometimes the best laid plans don’t always lead to success, there have to be options to deviate towards if something goes wrong, or a part has to be built heavier than expected to fix a mistake.

For a bird like this, where all I’m after is safety and consistency (not trying to break any records), I’m primarily concerned with ensuring that everything is built extremely well, and any weight discrepancies can be corrected with ballast. However, I’m not throwing caution to the wind on weight!

Assembly began by measuring each component of the fin can and dry fitting everything together. The fin slots needed sanding, as usual, but otherwise the slots were nice and straight. The centering rings needed sanding as well. I labeled the fins and slots to match them up for best fit, and then I made markings on the MMT for the positions of the centering rings.

After I had diagrammed the fit, I set everything down for a couple of days and came back to do it again. I am a deep believer in this tactic. I have found MANY of my own errors by embracing the fact that a little bit of distance can breed an incredible amount of clarity and perspective. This time it paid off big time. When I had marked the position of the middle centering ring I had misread the ruler several times. For some reason that day ¾ meant ¼ . It was enough of an error that If I hadn’t caught it when I did, and had installed the MMT, I would never have been able to install the fins and the motor retainer without seriously compromising the design (and possibly my L3). I rubbed the old marks away with acetone, and re-marked to the proper measurements.


With the markings on the MMT fixed, I placed a clamp to support the middle centering ring and epoxied it into place using West Systems 105 resin with 206 Hardener. The upper centering ring was notched in four places to allow ½” Kevlar shock cord to be attached. Two notches held the ends of a long sling, and two notches were used for smaller loops that act as attachment points for the future (after the permanent sling wears out).


After the notches were cut, the Upper ring was secured in place with the WS 105/206 mixture from before. After curing, fillets were applied to the centering rings using WS 105/206 mixed with WS 404 filler to a “catsup” consistency. This was followed fully saturating the shock cord roots with WS105/206, and then four 4”x6” sheets of fiberglass being laid up over them.




Next up, installing the MMT, Building the rear centering ring, and tacking in the fins.

 

[pendrin2020]

I'm sorry, I'm not sure how to remove all of the bad links...

 

[Buckeye]

A GoFundMe campaign to prop up your HOBBY and to buy yourself birthday presents?!??! Gee, how noble of you.

 

[pendrin2020]

I realize crowd funding gets abused for profit constantly, but in my case I was forward and honest about where the money was intended to go, and why I wasn’t able to pay for it myself until I get out of school.

And speaking of school, this isn’t just some ridiculous “wanna play with the big kids” fantasy fulfillment. We’re competing in IREC next year and one of us needs his L3... guess who already had his L2? I’m in a bit of a time crunch as well as a financial one.

I’m not some whiny asshole who just doesn’t FEEL like working for what he wants. I mentioned the campaign purely out of gratitude toward the people who paid for my kit. And the kit is all they paid for.

I dumped about twice that much into this on my own, so I’d appreciate it if you would take your judgement elsewhere so we could keep this thread on track.

If this thread devolves into back and forth over the ethics of how this project was funded, I’ll gladly just stop posting.

 

[Steve Shannon]

Nice build! Please continue the thread. To me it’s much more important that you’re working on this than how you funded it. Experienced team members on IREC teams are vital!

 

[blackjack2564]

I for one applaud your ingenuity for funding. Many projects I've been involved with were fully/partially funded with cash or equipment donations or they never would have started. [Psst.... we just don't mention that to anyone for the reason you found out]

Going for your L-3 at Airfest? Got your T.A.P.s lined up & on board ?
Having a L-3 on board with schools projects, can make the difference between success & failure,
I have seen this many time over the years. I agree with Steve 105%.

Good luck and have fun with your L-3

 

[Andrew_ASC]

Congrats on the L3 kit. You’re not whiny. It’s a considerable expense. My university isn’t even able to compete at IREC. I wish your team the best of luck. They and you certainly have the skill. Do not feel one bit bad about reaching out for help.

 

[pendrin2020]

Thanks guys.

Picking up where I left off...

Having finished the forward centering rings, I needed to assemble the aft rings and Retainer.
Weeks ago, during the 4th of july sale, I'd ordered a 75mm flanged retainer from Mad Cow and decided I'd pick up a couple of extra centering rings to reinforce the installation point. The plan was to epoxy all three rings together, and install the retainer while everything was still wet.
I began with marking and drilling three holes and installing the screws. afterward, I marked the remaining holes, and dry fitted the assembly several times to ensure a smooth installation later.

Next, I drilled the remaining holes. I decided not to remove the retainer as it seemed to be a perfect guide, and there was little chance of any of the rings slipping as I drilled. I made several alignment marks to help put the rings back in the original orientation, then drilled the remaining holes and disassembled the whole unit.

After roughing the surfaces thoroughly with 80 grit sandpaper, I coated the "between" surfaces (and retainer flange) with WS 105/206, then began reassembling the unit. Now the instruction call for threading the fiberglass with specific tap sizes and ignoring the provided wood nuts, but I decided on a different approach.

I had intentionally undersized the holes with the intention of allowing the screws to cut their own threads. Each screw was turned until it pulled the layers of fiberglass flat against eachother (if uneven to begin with), and then turned another 180 degrees. The result was an extremely high clamping force on the rings. Even still wet, I didn't feel like I could have possibly pried any of it apart. As stated earlier, I finished it off with the wood nuts. Each was gripped with a pair of pliers and turned with the maximum force that I could muster. These nuts will add an incredible amount of strength to the aft assembly later, as I will be injecting a heavy mixture of epoxy with filler into the void above the assembly. The epoxy will settle across the top of the assembly in a 1/4" layer; bonding the nuts, ring surface, MMT, and airframe into a single unit. Due to the screws being soaked in excess epoxy at installation, Loctite was not used. Screw torque on the retainer is a part of my preflight checklist. If screws come loose, they will be reinserted with loctite.

Having confirmed the dry-fit of the retainer assembly to the MMT while in the airframe, and confirmed that the wood nuts would not interfere with the fins, I reached inside the airframe and coated a one inch band of the airframe just above the fin slots with WS105/206 Epoxy. I had earlier determined that the leading end of each fin slot was the most consistent from the aft of the tube, and so the MMT assembly was inserted at the forward end and slid down into position. The assembly was rotated and "wiggled" until the dark black of the ring could be seen clearly through the semi-transparent fiberglass on all sides. This indicated the absence of voids. Once the epoxy had cured, I drilled a small hole just above the recently secured centering ring and injected approximately half a syringe of WS105/206 Epoxy into the void. In the photos you can actually see the epoxy flowing from the injection point to the rest of the ring. This process was repeated for the upper ring.

Finally, it was time to load everything up and head to my Dad's house. I'd offered to bring it over so he could help and he jumped at the chance! I started by grabbing an old piece of cardboard and a yard stick.

I don't print my fin guides. It's not an ego thing, it's just part of the fun to me. Makes me feel like I'm more involved and not just slapping glue on pre-cut parts. My tune changes quickly when someone flashes those fancy CNC cut guides *drools* lol.

After plotting it out, i wrapped the yard stick in sandpaper and worked over the MMT as best I could. Finally, I mixed up some WS105/206 and handed my dad an acid brush. We'd secured the airframe upright and all he had to do was push the brush through the slot and coat the MMT while I prepped the fin. He inserted two fins, and I inserted one. Each fin was allowed to settle into the leading edge of the fin slot, and the guide was slid into place. Dad tied some rubber bands together and wrapped them around the fins after I had measured and applied a loop of blue painters tape.

After dinner, the Epoxy had set and it was all packed up to come home to the shop.

Next We have to reinforce the fin roots.

 

[pendrin2020]

Just put in an Order with Chris Short for an Aerotech M1780 Blue Thunder. I think I got the last one!

 

[A-N-D-R-E-W]

Nice progress! Airfest is just around the corner, I'm getting excited myself!

Any plans to paint it?

 

[HandsomeRob]

Love this rocket. Looking forward to watching the build. Good luck!

 

[pendrin2020]

No plans for the paint just yet. I don't name em' or paint em' until I get em' back after the first launches. For this one, I won't name it until the cert flight is successful. Just seems like bad luck to count my chickens like they can't possibly crash. LOL.

Also, on a practical but purely unintentional level, it means I'll be able to shine a flashlight through the glass to let the L3CC inspect the internals.

 

[pendrin2020]

To reinforce the fin roots, I decided to lay a 3/8 wide strip of tubular Kevlar along the length of each side of each fin. Process was fairly simple. I drilled a hole near the middle of each fin side, and used an epoxy syringe to inject enough epoxy to fill the channel formed by the MMT and the airframe against the root. Afterward, I used an old hack saw blade to grip the kevlar strip and maneuver it into position against the root. After that cured a bit, I'd inject another layer of epoxy over top of the strip to seal it in place.


For two of them, I wound up just cutting one long strip and wrapping it around the back of the fin to the other side.

In the above image you can see the actual semi-circular sheen of the fillet.

After that I installed the retainer assembly I put together earlier. It was just a bit of sanding, and then coating the outer MMT, and the inner airframe with Epoxy. I pressed it into place so that the wood nuts were evenly spaced on either side of the fin roots.



After that, I flipped it over and used the gap at the bottom of each fin to inject a full syringe of Epoxy mixed with filler to a "catsup" kind of consistency.

Here's a photo where you can make out the outline of the epoxy settled into the bottom of the fin can and submerging the wood nuts.

 

[Speaknoevil]

Wow, that is some nice work in those tight spaces!

 

[pendrin2020]

Wow, that is some nice work in those tight spaces!

Thank you!

 

[Handeman]

That is some nice work. I'm wondering if it was worth the effort.

Did the epoxy soak all the way through the Kevlar and glue the Kevlar to the fin and tube at the joint? If not, it might not add as much strength as an epoxy only fillet.

My impression of MMT attachment points is that most in flight stress is going to be side to side. As the outer stresses on the fin push it side to side (flutter), the fin will pivot in the BT slot and exert forces pushing the fin sideways on the MMT. IMHO, the strength of the attachment of the epoxy to the MMT is the most important. As long as the epoxy stays attached to the MMT it should keep the fin from pivoting in the BT slot and and moving on the MMT.

My way of thinking may be way off, and just for the record, I didn't use tubular kevlar, I used shredded fiberglass strands to strengthen the MMT fillets in my L3 rocket. They weren't even close to as neat as your's are.

 

[ericm541]

For my L3 i had a 5'' airframe with 98MM motor mount, left the aft centering ring off and did fillets to the motor mount and fillets to the inner airframe. There's no room to work so i just ran epoxy down, as long as the surfaces are scuffed and cleaned just epoxy should be good.

 

[pendrin2020]

Thanks for the compliments!

I injected epoxy down into the space first. So it was already covered in epoxy. Then I put the Kevlar down. And then I did an epoxy fillet over top of that. It’s essentially a very narrow double-layer layup of Kevlar across the root, between the tubes. Fully-saturated.

I haven’t done the outer fillets yet. I might consider doing a 6 ounce tip to tip. just for fun/to be safe.

 

[jamesr]

Do you already have L3CC or TAP members involved with your build? I saw it was asked early on but never saw a reply.

 

[pendrin2020]

Chris short said he'd look over my stuff, but for the time being, I'm relying on the forums to show me things that have been approved before, and just documenting the hell out of everything.

Currently, I'm building the AV bay and working on the write-up. If my can of Hardener gets here today, I'll finish the outer fillets and the nose cone this evening!

 

[Steve Shannon]

Chris short said he'd look over my stuff, but for the time being, I'm relying on the forums to show me things that have been approved before, and just documenting the hell out of everything.

Currently, I'm building the AV bay and working on the write-up. If my can of Hardener gets here today, I'll finish the outer fillets and the nose cone this evening!

Chris is a great resource. Communicate frequently with him rather than racing on ahead following the advice of those of us on TRF, especially if you’re not sure of something.

 

[jamesr]

Yes, TAP/L3CC should be part of this from the beginning, not just "here's my L3, going to fly it today, will you sign off. It's all on TRF for you to review.". Both NAR and TRA have the steps laid out with guidelines and process documented. It's hard to back track of there is a problem that needs corrected.

I see this a lot, person decides to build L3, starts a thread on TRF and goes for it. In many cases it is very clear they have not read the information on the website and does not know the guidelines and has not read all the information they need to know before starting. In some cases this could be a real safety issue. TAP/L3CC involvement is about insuring all steps are made to help fliers ge successful and safe.

One of the first things I see as a TAP/L3CC is if the person follows instructions. That could be the L3 process documents or the instructions on epoxy, electronics information and so on.

People get caught up in the excitement and maybe even get bad guidance from people that are not familiar with the process.

 

[pendrin2020]

I agree with you whole-heartedly. Especially given how some people get into the hobby with little to zero background in science.

Now I just have to find an L3CC within a few hundred miles who answers emails and returns phone calls.

 

[jamesr]

If anyone has a problem contacting or finding an L3CC or TAP in there area, feel free to contact me, my info in on both sites. JamesR 2 at gmail bot com

 

[pendrin2020]

Thank you!

I’ll email you some more in depth stuff tonight. I’ll probably send you the pre-project write-up that I sent to Chris Dondanville before I couldn’t get ahold of him anymore.

404 is a pain to sand but damn if it doesn’t look and feel good. Lol

 

[jamesr]

I will be at Airfest as well. Is this NAR or TRA cert?

 

[pendrin2020]

NAR. I’ll have the build and write up finished in the middle of this week.

 

[jamesr]

Okay sounds good

 

[pendrin2020]

jamesr, I tried that email, but it says it isn't working. can you try mine? I use the same name on gmail.

 

[jamesr]

jamesr, I tried that email, but it says it isn't working. can you try mine? I use the same name on gmail.

You may have missed the 2, jamesr2