L1/L2 2.6" Madcow DX3 Build

It's official, I'm going to make an attempt to get my L1 and L2 certs through NAR this year. That's the plan anyway, we'll see what life throws up in the way in the mean time. I'm a 32 year old mechanical engineer, and the father of two young children (8mo and 3yo), so free time is always at a premium. I don't expect to be fully complete until at least April

I'll be making the certification attempts with a 2.6" thin-wall fiberglass Madcow DX3 with dual deployment. I'll be documenting what will likely be an extended build process here, mostly as a record of my work for potential review by the individuals signing my cert, but also as a means to ask questions and leave a record from which others can learn. I probably won't be attempting many revolutionary things in regards to construction techniques, but there are a couple methods that I'm looking forward to employing.

I think I finally got nearly everything together that I'm going to need, except for paint and primer and possibly some other hardware odds and ends. To that end, here is an obligatory introductory photo of what will eventually become my rocket. [Electronics and associated hardware (a whole box full) is not pictured].

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I see three couplers?

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Double check your parts. I bought a MC 2.6" Frenzy XL and got two BT couplers (only needs one) and no switch band. I haven't called to ask for the replacement part. I think I see a switch band taped to the fins, but not sure.

The shorter coupler in your pic is for the nose cone.

Yea, I'm not sure what's up with the extra coupler either. I don't have plans to use it. The short one to make the nose cone shoulder. I'll use one of the longer ones for the Altimeter Bay.

I'm wondering if this is due to the way Madcow was kitting them. They might have had a coupler for the kit, and the Altimeter Bay 'kit' was added, which would have also included a coupler piece.

Now that I think about it, I didn't see a switchband last night either, but thought I remembered one in there when I opened it. Turns out, after going back and looking again, is was wrapped up in some of the paper that was covering the body tube end. So not pictured, but I have one. The things taped to the fins are the two (very thin) centering rings.

Nice choice. Hah, even my double jointed Darkstar only has two couplers!

Nice, looking forward to following. I did my NAR L1/L2 with the Madcow Super DX3 cardboard version. The DX3 is one of my favorite rockets.

Big Madcow fan, will be watching the build!

Funny, I'm building my Madcow paper & plywood 2.6" DX3...I'm sure the fiberglass version is beautiful.

that is a pile of awesome waiting to happen.

Progress will probably be on a 'what I did last night' basis, for at least the steps involving epoxy. Last night all the fiberglass parts got a bath in water with laundry detergent. All subsequent steps involving adhesives will be preceded by a through cleaning with rubbing alcohol. No pictures of the bath, which was actually performed in the bathroom sink.

Next was lightly filing the ID of one of the centering rings which still had some overhanging fiberglass threads that were preventing proper assembly. This ring was also marked at two places 120 degrees apart, and notches were filed into the ID to permit the 1/2" tubular kevlar to be inserted.



The kit came with a large length of 9/16" tubular nylon. I am supplementing the booster end connection with a kevlar loop to alleviate some of the potential for degradation over time. This booster will be particularly difficult to replace the shock cord mount due to the very low profile of the centering rings, and won't allow for the use of an eye bolt. I understand that many (all?) 54mm motors use a sealed forward closure which allows for mounting the shock cord directly to the motor case, but I also plan to fly this rocket with 38mm motors and would very much like to have the ability to use motor ejection on those flights.

I was looking at that one last night and lamenting the fact I didn't buy one during the BF sale.

If it ends up around 4 lbs ready to fly, it'll go a nice comfortable 1300 feet on a small H.

On a baby J it'll go about a mile. If you haven't done a mile yet you can hit two milestones in one with your L2 cert.

I'm not sure how good your eyes are, but thats around the point you'll want a tracker if you don't have one already.

According to Thrustcurve, it'll get up around Mach with a large 38mm J. I had to burn a $100 K1200 in my 3 inch 8 pound rocket to get there.

I'm gonna buy one of these. It's just a matter of when.

I was 7.5!pounds with a K1127 loaded on the way to the pads last summer. I didn't really try to save weight, so 4,4.5 may be possible with thinner all thread, smaller eyebolts, lipos, 1/4" Kevlar instead of 9/16 TN, and smaller fillets.

Honestly, saving weight is not my main priority, although I will be using lipos rather than 9V for the size factor. I suspect that Mach is well within the reach of this rocket with the right motor selection / waiver height. For me, the trick will be keeping it below the Berwick ME 10,000 ft waiver, if Mach is something I decide to push for.

A mile plus is definitely in sight for the L2 attempt, and yes, there will be a tracker. Specifically, I picked up an Eggfinder LCD Starter kit, and built it over the holidays, it's all set to go. I just need to build a sled for it and add a nosecone a-bay.

J510w, K1127 Loki blue, J1026 Loki cocktail. All Mach 1.2 and 8600' or so.

MCriscione said:

Honestly, saving weight is not my main priority, although I will be using lipos rather than 9V for the size factor. I suspect that Mach is well within the reach of this rocket with the right motor selection / waiver height. For me, the trick will be keeping it below the Berwick ME 10,000 ft waiver, if Mach is something I decide to push for.

A mile plus is definitely in sight for the L2 attempt, and yes, there will be a tracker. Specifically, I picked up an Eggfinder LCD Starter kit, and built it over the holidays, it's all set to go. I just need to build a sled for it and add a nosecone a-bay.

Click to expand...

I use an Eggfinder/LCD tracker too. On my (one) mach flight, it lost tracking shortly after launch, which is what I expected after reading the info on the system and posts here. It re-acquired and my LCD started beeping again right as the main deployed at 700 feet. It didn't matter, since I could see the rocket the whole time. Of course, if I hadn't put the tracker in I would have lost sight of it immediately.

I mounted the tracker to a piece of hobby board and cut it so it would fit in the nose cone, then tucked it in with some bubble wrap on each end.

At any rate, you're gonna have a lot of fun.

Last night i got the upper centering ring and kevlar strap epoxied on to the motor tube. I used Loctite 5 minute epoxy to place the ring on the tube, avoiding the area right near the straps. That locked the centering ring in place pretty quickly, and about 30 minutes later, I came back and added fillets and epoxied down the kevlar strap (with a light overlay to 'fillet' or 'feather' the edges of the strap into the motor tube. This part was done with 20 minutes BSI epoxy with System 3 colloidal silica added until it was a thicker peanut-butter-like easily spread consistency that wouldn't drip and run. The binder clips did a pretty good job of holding the shock cord in place without getting in the way of the epoxy.



The next steps was to mount the lower centering ring and the base of the motor retainer. (done tonight, pictures tomorrow)

I ran into a snag tonight laying out the backing board for the Eggfinder. I had intended to 'mount' it in the nosecone such that is was removable using an Estes 29mm motor retainer. However, I quickly realized that the 2S lipo battery that I had purchased won't fit through the available opening. I can't believe I didn't check that dimension closer when ordering. Now I'm stuck with either finding a new battery (either using a 9V or finding a smaller Lipo) or using some other nosecone mount, most likely with t-nuts and bolts of some kind. I really liked the easy accessibility of using a motor retainer, and I'll be a bit disappointed if I have to reject that part of the design, so I think at this point I'm looking for a different battery. Preferably something like a 2S with a cross-section that will fit inside a 1.125" diameter circle. I have two 1S nanotech 300mAh lipos that I could chain in series that could work, but one of those was supposed to run the Eggfinder (along with a secondary deployment channel 2s lipo) and I don't have a third. Arrrg, stupid shipping fees from Hobbyking!

A bit of delay due to some further tracker sled planning, but as I mentioned previously, I did manage to get the rear CR and retainer epoxied in place with JB Weld. The rear ring needed a very light sanding afterwards to remove a touch of excess epoxy, but fits into the airframe just fine. I may have to clean up the JB Weld on the tube that will be near the fin tabs, I don't think it over-runs any of them, but I'll check again tonight. I could always just mount it a touch further back than where I planned instead if it turns out to be a hassle.



The next plan for this part is to create some 'channels' where the fin tabs will end up sitting out of some spare basswood strips. It's been mentioned in a couple other threads, but this will create a dam within the fin can, letting me directly inject epoxy into the fin slots and not have it run all over the motor mount. This should let me create some 'internal fillets' or in this case, just some internal 'fill' without too much hassle.

I did end up going and ordering a new 2S lipo battery (2 actually) that is a bit smaller than the one I had currently. This one should fit through the 29mm retainer without a problem, assuming it matches the size listed on the site. It still provides 260mAh, which is a touch less than recommended, but as I'll be powering it on probably less than 20 minutes before launch, I'm not too worried about it. It will be in the nosecone, with an external switch 'pull pin' switch.

So does anybody bother to seriously airfoil or even begin to taper 1/16" fiberglass fins? I'm considering just rounding the leading and trailing edges. I'm not really considering actually airfoiling the fins, the ratio is such that they would be ridiculously thin on the trailing edge, but I was thinking of perhaps putting a 'chamfer' or angle on them as I've seen others do with thicker fins, but I'm worried that any significant angle will cause the edges to become awful thin. Recommendations requested!

MCriscione said:

So does anybody bother to seriously airfoil or even begin to taper 1/16" fiberglass fins? I'm considering just rounding the leading and trailing edges. I'm not really considering actually airfoiling the fins, the ratio is such that they would be ridiculously thin on the trailing edge, but I was thinking of perhaps putting a 'chamfer' or angle on them as I've seen others do with thicker fins, but I'm worried that any significant angle will cause the edges to become awful thin. Recommendations requested!

Click to expand...

I'm not sure about anyone else, but on my 2.6" Madcow Black Brant II with the same 1/16" fins, I didn't do anything to them. I felt they were too thin as well.

On my screech, I left them. Plan to do the same thing on my 54mm I'm building now. It's hip to be square.

I'm not sure it's too thin... for 38mm it's surely thick enough...and most 54's. only the 2800's are going to challenge this.... who's gonna be first?

MCriscione said:

Last night i got the upper centering ring and kevlar strap epoxied on to the motor tube. I used Loctite 5 minute epoxy to place the ring on the tube, avoiding the area right near the straps. That locked the centering ring in place pretty quickly, and about 30 minutes later, I came back and added fillets and epoxied down the kevlar strap (with a light overlay to 'fillet' or 'feather' the edges of the strap into the motor tube. This part was done with 20 minutes BSI epoxy with System 3 colloidal silica added until it was a thicker peanut-butter-like easily spread consistency that wouldn't drip and run. The binder clips did a pretty good job of holding the shock cord in place without getting in the way of the epoxy.

View attachment 279801 View attachment 279802

The next steps was to mount the lower centering ring and the base of the motor retainer. (done tonight, pictures tomorrow)

I ran into a snag tonight laying out the backing board for the Eggfinder. I had intended to 'mount' it in the nosecone such that is was removable using an Estes 29mm motor retainer. However, I quickly realized that the 2S lipo battery that I had purchased won't fit through the available opening. I can't believe I didn't check that dimension closer when ordering. Now I'm stuck with either finding a new battery (either using a 9V or finding a smaller Lipo) or using some other nosecone mount, most likely with t-nuts and bolts of some kind. I really liked the easy accessibility of using a motor retainer, and I'll be a bit disappointed if I have to reject that part of the design, so I think at this point I'm looking for a different battery. Preferably something like a 2S with a cross-section that will fit inside a 1.125" diameter circle. I have two 1S nanotech 300mAh lipos that I could chain in series that could work, but one of those was supposed to run the Eggfinder (along with a secondary deployment channel 2s lipo) and I don't have a third. Arrrg, stupid shipping fees from Hobbyking!

Click to expand...

Wow. I wish I'd seen this prior to building my Tomach! I really like this approach! best of luck in your L1 and L2 attempts! I also left the fins the way the are. So thin I don't think it matters unless you just want it for appearance.

Sounds like the general concensus is to leave them alone. Seems good to me, although I may still round them a touch.

So I've been somewhat delayed in my build progress by general work/life events, but have been making some headway. I'm extremely delinquent in keeping this thread updated. I find myself spending far to much time reading other threads instead! I have a number of things to talk about, and plenty of pictures, but I only have a limited amount of time to post tonight so I'll keep it short and sweet.

The motor mount is 100% complete and ready for installation in the booster. As you can see in the photographs, I've chosen not to use the removable rear CR method due to the thinness of the CRs. I'd never be able to get epoxy down the gap to fillet anyway. I think pretty much everybody who has built one of these kits (or BT/MM dia. combo) has essentially done the same. Nothing new here.



What was a bit 'new' or at least seems to be talked about a lot recently is the use of balsa 'dams' to hold in the epoxy around the fin mounting area. I went ahead and followed through with this, as it seemed like a very reasonable way to get a bit of extra epoxy into the area without having it run and cover the whole motor mount while it set up. I didn't have any balsa laying around that wasn't either already dedicated to other projects though, but I did have some very thin 1/16 x 1/8 bass wood strips which I used to create the dams. I inserted the motor mount into the booster and used a mechanical pencil to draw a line on the MM through each fin slot. (I had to extend the lead a bit further than usual, but it fit beautifully right in the slot) I made sure to keep the pencil vertical when I did this, so that the line wouldn't be displaced. Close would be good enough though, because the dams are pretty wide. The rest was just cutting, sanding, gluing, and re-sanding. I cut all the basswood strips to length, including the 3 short pieces (the rear CR acts as the forth wall of the rectangle). The short pieces got match-sanded to motor mount to add a curve on the bottom side. Then I used some medium CA to glue them all down. I didn't bother with epoxy because these aren't structural. Plus, when I put the fins in, they will get plenty of epoxy washed over them, so I'm not worried about them ever moving. Finally, the strips were sanded to assure they were flush with the rear CR. This required a few trial fits in the booster to get all the high points, some weren't obvious, but it now fits perfectly.



The only thing left to do before mounting the fins (other than gluing the motor mount INTO the booster) is getting a hold of a drill press. I have a friend who is going to lend me one for a month or so to help me complete this project. I need it for a few things, but in this case I need it to drill a few holes right along the fin slots so that I can fit my syringe tips in them. 1/8" holes should be plenty, and will be totally hidden by the external fillets eventually. I plan to use 3 or 4 of them to get good coverage. Pictures of that when it's done, of course.

I've made good headway with the tracker sled and nose cone bulkhead, but I'll have to save that for another post. That has been a lot of fun so far! More to come, stay tuned. :wink:

Alright! Finally found some more time for a post so you get some pictures of the tracker sled and nosecone bulkhead coming together. I got together the beginnings of the parts that I had prepared, including a 29mm Estes motor retainer which was epoxied into a short length of 29mm tubing. Also in view is the Eggfinder that will eventually call the new sled home.



The circular wood pieces were actually cut using a dremel with a circle cutting attachment, which worked surprisingly well. Those pieces and the wood sled are all 1/8" 'craft' plywood, which should be plenty strong for this location, but it does have some bend. That tendency was corrected with a couple strips of 1/16 x 1/8 basswood, which I used as stiffeners along the bottom side. The smaller split circle fits inside the 29mm tube, and the larger whole circle fits nicely inside of the motor retainer cap. Eventually, this sled was drilled with holes for standoffs for the Eggfinder, wire routing and other access, but at this point, the battery that I ordered still hadn't shown up, so I didn't have final sizes, and I didn't want to make any assumptions. Not shown in this photo is a small triangular key that was later added to align the sled when it's 'installed' in the nosecone.



Also shown in the materials photo is a wood 'centering ring'. The DX3 kit came with a fiberglass bulkhead, but I was worried it might be a bit thin to epoxy material to. In addition to drilling a 1.25" hole in that bulkhead, I made a wooden centering ring to 'back' it. These were epoxied together to form a thicker bulkhead. Next, I took a strip of 1/8 ply and beveled the edges by 30 degrees, cut the strip in half, and glued it together to form a channel for a small piece of aluminum tubing. This was epoxied across the composite bulkhead and the middle was cut away. I got a touch sloppy here sadly, and should have taken the time to clean up a bit better. The tubing was then sized, cut, roughed up with sandpaper and epoxied in place. The 29mm retainer was then epoxied in place. You can see here the cut-out in the retainer that receives the key on the sled. Also shown is the pin that will eventually be used to keep the tracker off when not in use to save battery.



I nearly forgot, but noticed when I uploaded the images, there is also a small electrical connector that I added to the outside of the sled. I'm not certain that I'll end up using it, but the plan was to have an LED on the nosecone that would act as a repeater for the Eggfinder's '1s' connection light, which indicates a connection to the GPS satellite(s). It's not clear yet if this will end up working AT ALL, but it's in progress. One thing that doesn't work well already is the short leads, I really cut them far too short to be practical, and it will be difficult to solder them, wherever they end up going.

Anyway, back to real life. More updates soon, I'm sure.

Had a few other pictures on my phone of the sled progress. All the components were laid out (except for some wire and JST plugs) I started by drilling holes to mount the tracker and zip-tie the battery in place. As it turns out, the battery plus sled meant the zip ties I started with wouldn't reach the 'long way around' so those holes got a bit bigger when I found longer, wider zip-ties. The bottom side of the sled needed some reliefs carved into the stiffeners to allow the tiny little nylon hex nuts to fit. I should have spaced them based on the nuts to start with, but didn't think of it. As it was, a couple minutes with an xacto blade cleaned it up without a problem. The hex nuts were CA glued in place and the standoffs were added.



I attached components to the sled the other night, but forgot to snap pics.

Speaking of pictures, is there an upload limit here on TRF? I haven't reached it yet, but this thread might do it for me. If necessary, I can start hosting photos elsewhere.

Ended up working from home today and finally found a few minutes tonight to snap a few pictures of the assembled tracker sled. It's essentially finished except for drilling the relevant hole in the air-frame/nosecone coupler and actually epoxying the thing in place. That part won't happen untill after I'm a bit further along and have my nose weight installed/figured out. In the last picture you can see the shadow of the pin guide, which I'll use as a guide when drilling the hole.



I'm still not certain what/if I'll get that LED connection that I installed working or not, possibly just as a power indicator at this point. It turns out that the led I'd like to replicate is such a low current that I won't be able to do so without some kind of transistor arrangement, and I'm not certain I have the EE skills to pull that off. If anybody has any advice related to this I'd be glad to hear it.