Madcow DX3 build

Finally the workbench and area in the garage are clear enough to start building. Hoping to get a Mid (erm maybe High) Power bird ready for the April LUNAR launch at Snow Ranch. I figured I'd tackle the Madcow DX3 sitting in the build pile. I purchased this on the Black Friday sale so am excited to finally start building. Started last night and took some photos along the way.

For the build, it is mostly stock components with a few additions. I will use an aeropack retainer and rail buttons. I have a Madcow 2.6" diameter altimeter bay so I will be using rivets for the payload bay coupler should I later want to swap in the altimeter and fly this kit with dual deployment.



To save some time in places, I won't be building things in the steps shown in the building instructions that come with the kit. Like other mid-power and high-power kits of this type, the instructions provide insights on some details, but assume that the builder is comfortable with the overall process.

First off is the motor mount, I am going to use JB Weld for key adhesion (fore and aft centering ring, ttw fin roots, and retainer) of any elements that may be exposed to elevated temperatures. I began by scuffing the glassine on the tube and then found that the forward centering ring was a bit loose on the motor mount tube so my idea of tacking with CA and then doing fillets of JB Weld wasn't going to work.

So I opted for a good dose of JB Weld, followed by positioning things so they wouldn't move much overnight



While that dries... time to deal with the fins. I was originally planning to seal these with elmers wood putty but after inspecting them from all sides, they appear to be very nice plywood, flat and with no major defects. I suspect that priming and sanding will render them sufficient smooth for my purposes. I removed the masking tape on the root edge, keeping the other tape on and sanded them against some 220 grit to open up a good bonding edge, being careful to move in only one direction.

I did a test fit of the parts while the motor mount was curing and found I only needed a little expansion of the body tube slots for the fins. Otherwise, the fit of the parts of the kit was superb... very happy with that. Overnight, the forward centering ring epoxy cured and I tied the shock cord together (it has a kevlar section and a nylon section - total length 8') and tacked it to the forward centering ring with titebond II.

Then I put masking tape on the inside edges of the rear centering ring ( this will be removed once the forward centering ring is glued into the airframe ) so it had a good pressure fit on the motor mount tube and also pull tabs for easy removal.


I put a thick ring of titebond II about 1" above the fin slots in the lower airframe and slid in the motor mount leaving the bottom of the mount nearly flush with the airframe tube. Then I inserted the fins (no glue here yet), and move the rear centering ring against them, to make sure there's sufficient space to attach the aeropack adapter later.


Turning the airframe tube over and looking down into the tube, there's a thick bead of wood glue ready to set up and hold the mount in place for the range (up to H) of motors I expect to fly.


Giving that time to cure, I assembled the payload bay bulkhead eyebolt and covered the nut and threads with 5-minute epoxy. Then I used wood glue to make fillets on both sides of the bulkhead to keep it in place in the coupler. Bottom half:

For the rail buttons, I am using the Apogee Components standard rail buttons that have a flat nut that is epoxied to the body tube with the button held in place with a machine screw. At some later point, that will all be assembled and secured with thread lock, but for now some holes are needed and I'll show how I insert these nuts on airframe tubes that are too small for me to fit my hand into.

I marked the lower button hole by eye, and then used an angle to make a straight extension up the tube to near where the planned lowest CG will be located (approx 13.5" from the bottom of the lower airframe tube).


I drilled 11/64" holes and then sanded out the burr on the inside of the body tube and did a few test press fits. Then used a magnet taped to the end of a bike spoke to position and push out the nut from the inside.

I didn't build my DX 3 for dual deployment but it's a reasonably easy build. Quite simple compared to many of the low powered designs. I can lend you 1 bit of info though. Mark at Stickershock now has the correct vinyls for this bird. What he used to have was scaled down from the Super DX 3 and they were not correct. LW and I got him fixed on that note should you decide to use his vinyls for your rocket. Mine were perfect. The fin laminates were my issue. Mark made the text to my preference.

Madcow kits are great--thanks for sharing.

@Gary - thanks for the tip about vinyl options and great looking rocket. Did you use any top coat over the kit? I'm still experimenting with techniques there.

timro said:

@Gary - thanks for the tip about vinyl options and great looking rocket. Did you use any top coat over the kit? I'm still experimenting with techniques there.

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The auto paint I used didn't require any top coat, but not knowing that, I used some anyway. It didn't improve the already nice finish it had but it will help to keep the vinyls in place.

Like Gary, I built my single deploy. The only issue I discovered, late in my build, was the texture of the plywood. It was not until after 2 coats of filler and a coat of primer that I could see the milling lines in the surface of the fins. I did get it corrected, but it would have taken less effort had I been able to see them before I started filling. I don't know how common this is, but I recommend you give the fins a thorough surface sanding before you start any filling/priming.

Ahh... just took a look over this set and I dont see any milling lines. I sanded the sides with a 300 grit block before airfoiling the leading edge. We'll see after a coat of primer

LW Bercini said:

Like Gary, I built my single deploy. The only issue I discovered, late in my build, was the texture of the plywood. It was not until after 2 coats of filler and a coat of primer that I could see the milling lines in the surface of the fins. I did get it corrected, but it would have taken less effort had I been able to see them before I started filling. I don't know how common this is, but I recommend you give the fins a thorough surface sanding before you start any filling/priming.

Click to expand...

I don't recall having any milling lines on mine but with all fins, I always surface sand no mater what they're made of.

timro said:

Ahh... just took a look over this set and I dont see any milling lines. I sanded the sides with a 300 grit block before airfoiling the leading edge. We'll see after a coat of primer

Click to expand...

I did what I thought was a good surface sanding, first with 220 then with 400. Even still, once I hit it with a coat of grey primer, I could clearly see the ridges and valleys

Progressing now on gluing the fin roots to the motor mount. After giving the forward centering ring about 8 hours to cure, pulled back out the rear centering ring and did jb weld on the first fin root. Started with a strip of the epoxy on both sides, then pushed together and smoothed/pressed the blobs on both sides against the fin and tube. Just before sliding the fin against the motor tube, I placed a small amount of titebond II on the airframe exterior for the outer portion of the fin root. This cures quicker than the jb weld and I hope should keep things together while curing of the epoxy completes.

Pictures of second fin going in. Titebond II fillet applied to outer fin joint with body tube from first fin at the same time (blue tape added to exterior fin to prevent any seepage through fin joint).

TTW adhesion with JB Weld is certainly a hurry-up-and-wait process. But I hope to be worth the trouble. At last the interior fillets to motor mount and body tube are complete and the aft centering ring is bonded to the motor with jb weld and to the body tube with titebond II. After sliding on the aft centering ring, I put jb weld on the inside of the aeropack retainer and slid it into place to let the whole assembly cure together.



Once cured, I applied titebond II fillets smoothed with a wooden coffee stir stick (this was also used for the interior titebond fillets). These shrink down a lot, so they will not produce any sort of aerodynamic fillet good for reducing interference drag.



At this point, I am planning not to create thicker fillets, as I'd rather avoid the additional weight being added to the rocket and especially on the end south of the CP. I have previously used fix-it clay for fillets, which can be built up and shaped well, but also seems very dense. Also, I plan to fly this kit sometimes at a field with a very hard surface, so keeping weight down, and thereby impact momentum lower is a plus. The typically recommended fillet size (4-8% of fin root) for these fins is around 1/2". I wonder if any readers here have an estimate of the density of an epoxy mixed with microballoons vs. other options.

To prep for priming, I cleaned the nose cone with warm soapy water and then scuffed with 150 grit sandpaper. The aft body tube lip and the lower edges of the fins were soaked in CA, and the aeropack retainer body masked with tape. Priming to happen once weather cooperates, with first coats using Rustoleum 2x gray primer.

Fancy smancy jig you built there. Nice rocket too!

I can't take any credit for the jig - it's from Apogee Components: https://www.apogeerockets.com/Building_Supplies/Tools/Guillotine_Fin_Jig . This is the biggest body tube that works with that jig. Looks like they now have a similar one for even larger diameter models.

Weather cooperates. Primed with gray rustoleum 2x primer then sanded with an old 3M foam pad that says "fine" and then smoothed with 600. Masked the bottom of the rocket off so the aft centering ring stays gray and then primed with white rustoleum 2x. Using a white primer coat as the main color coat will be Rustoleum fluorescent orange and I'm hoping it will be a lot brighter that way.

Orange going on, followed by masking and painting with rustoleum 2x gloss black on one fin and the nose cone and upper part of payload bay.


Waited a day and then wet sanded the orange with 800, it was bright, but also did not go on very smooth. I also noticed that where I had sanded the white primer a bit too much at the edge of the body tube that the underlying gray primer made the orange a lot darker. This makes the joint in the airframe where the rocket separates a lot more pronounced than I'd hoped. But really not enough in my book to go back over and redo it. Next time...

Finishing touches - the rail button machine screws were set with blue threadlock and allowed to cure overnight. I like rail buttons that spin a little, although I've never seen evidence that allowing them to spin reduces wear.


I drilled three equally spaced 5/32" holes through the bottom of the payload bay and the coupler, and then inserted black plastic rivets to hold the coupler in place. This way, at a later time, I can swap out this coupler for a 2.6" av bay I plan to build along with a madcow striker. Spaced down from the base of the nose cone lower edge, I drilled 3 more 5/32" holes for altimeter vent holes for flying in this configuration with an altimeter two attached to the nose cone. To keep the nose cone in place, two 6-32 machine bolt holes were drilled and tapped and nylon screws put in. (dual deploy setup would replace these with 2-56 shear pins or masking tape)


Removed all the masking tape and screwed on the Aeropack retainer


Final weight without motor comes in near 552g, with a smooth and not polished finish (just coated with car wax and buffed out). Hopefully with this color scheme it should be visible over 2000'. If the weather cooperates, I might find out this weekend at Snow Ranch

Nice work on the rocket!! Love the paint job! I never used JB Weld as a main component to attach everything. How well does it hold up compared to Titebond II??

thanks This is my first build using JB Weld for attachment of fins and centering rings so I really can't say about the strength long term. Once mixed, it is pretty viscous and not particularly cooperative to how I was trying to spread it with a bamboo skewer stuck up into inaccessible places. So I had to settle with squishing and spreading it as best I could.

I used Titebond II throughout on my L1 rocket, over 6 flights, there's no evidence that the motor mount contact point is getting loose or being damaged by the heat, but it's not in a very accessible spot for inspection!

timro said:

thanks This is my first build using JB Weld for attachment of fins and centering rings so I really can't say about the strength long term. Once mixed, it is pretty viscous and not particularly cooperative to how I was trying to spread it with a bamboo skewer stuck up into inaccessible places. So I had to settle with squishing and spreading it as best I could.

I used Titebond II throughout on my L1 rocket, over 6 flights, there's no evidence that the motor mount contact point is getting loose or being damaged by the heat, but it's not in a very accessible spot for inspection!

Click to expand...

I've used epoxy clay for fillets and such and seems to work well. It can also withstand heat up to 500 degrees. Anyway you did an exceptional job on the build. The colors really pop!

I flew the DX3 with Lunar at Snow Ranch on Saturday on and F50. It didn't rotate a bit during the boost phase until it became so small on the video it was tough to tell... about 1000' but no data from the altimeter. It appeared that the ejection charge was early enough in the flight so that the force compressed the payload bay (converting the coupler rivet holes into 1/2" slots) and confused the heck out of the altimeter 2 on board. I repaired with a generous amount of CA on the coupler rivet holes and airframe bottom payload bay holes for next time.

I had a little extra help out there on the range finding the rocket

Tim,

Very nice paint job! The Madcow DX3 2.6" was my first MPR, it's a great kit and fly's straight and true.

If your going to secure the coupler tube to the cargo bay with plastic rivets, I suggest you epoxy a 1" switch band in the middle of the coupler to relive the deployment charge pressure from being directly on the rivets. Another building design I use when I intend on flying SD, then modifying to DD, is to build two cargo bays. The coupler tube is epoxied to the SD cargo bay and when I build the av-bay for the DD cargo bay I buy a BT 3-4" longer, so there is enough room for the main chute, shock cord an nomex blanket. Then you have a lot of flexibility for a little more than the coast of one kit.

I think I get it, the switch band epoxied to the coupler would push on the cargo bay tube and reduce the dependency of the rivets to move the cargo bay and nose cone off at deployment. Now why didn't I think of that?!

Definitely why I appreciate TRF and all the great experience here. I think I need a switch band for the a/v bay anyway, might as well order and extra one or two or cut from the cargo bay of my Madcow Striker (in the DD build pile for the summer).

The AV/DD update is now complete. This is my first venture into electronic deployment so I ended up taking a long time to get there. I had purchased the Madcow 2.6" electronic bay from Apogee Components, this turns out to be a version that has only a single allthread through the bay, which seemed to have some implications for assembly (at least with my techniques!). Namely, the bay closed by screwing one of the bulkhead ends (with eyebolt) onto the bay, instead of putting the bulkhead in place and tightening nuts on two allthread bolts that go through the bay.

This worried me very little as all along, I had intended to use the electronics for main deployment only with motor deployment at apogee to release a drogue (or just separate the airframe for drogueless). After reading some comments in the forum about corrosion of terminal blocks, I opted for some banana plug style connectors for the deployment charge. This had the added benefit of providing something I could align the sled with inside the bay to make the on/off connection viable through a vent hole.

I am using an Adept DDC22 for the deployment controller, I lucked out and got one of the last ones after it was announced that they would no longer be for sale. To power the controller, I have 9v battery (taped in for ground tests, will be tie-wrapped for flights) and on/off switch is handled with a 1/8" phono jack from radio shack. When assembled, a standard phono plug wasn't long enough when inserted to open the circuit, so I cut a 1/8" steel rod and glued it to my RBF sticker using epoxy clay.

Here's my plan to assemble the bay and pre-flight (better make up a checklist card):

• attach 9v battery and connect terminal
• wait for power-up of ddc22 and beep sequence for all open outputs
• insert 1/8" plug w/ RBF flag (all silent)
• slide altimeter sled into bay, attach molex connector to output terminal
• screw aft bulkhead of bay on, secure with masking tape to prevent in air disassembly by spinning airframe
• tape assembled charge container (cardboard tube) to payload eyebolt, leads shunted
• attach quicklink to aft eyebolt, pack drogue and main and assemble airframe minus payload rivets
• at prep area for launch, remove shunts from charge assembly and connect to terminals
• close payload/main chute bay and secure with rivets
• if not already, load rocket on launch rail
• connect motor igniter leads, check continuity, remove RBF flag
• wait for ddc22 power up and beep sequence confirming continuity on main output

Some pictures, sled with temporary battery attachment, bay and sled before assembly, assembled bay with LED "test" charge:

As the DX3 is a cardboard airframe (pretty beefy tho'), I thought I might have better longevity using blue tape shear pins. I experimented a little in the backyard on the 4th of July to come up with an effective combination. Here, I'm using 3 strips of blue tape on the nose and 0.6g of BP in a cardboard tube with igniter hot melt glued at the end. The remainder of the tube is filled with dog barf and the end sealed with black electrical tape. In the second frame, it's possible to see a blue streak below the nose (that's a piece of flyin' tape!!)

Happy with my first go at dual deployment over the weekend. The flip camera was distorting things a bit (some frames seemed a little warped), or maybe it was just my hand shaking from being a little nervous about the flight :blush:

[video=youtube_share;GekS1f6JTWM]https://youtu.be/GekS1f6JTWM[/video]

RIP... it was fun while it lasted
Today my madcow bought the farm (at least the booster did) as a result of a G53 reload cato. Still have to step through the video, but it exploded right at ignition shooting the forward closure and one unburned grain up into the airframe, blew out the nozzle, sheared the aeropack retainer cap and spit the motor out the bottom. Seems a lot like bp from the ejection charge made it past the delay grain. I had over 10 flights with this 29/40-120 casing and many single and dual deployment flights of this rocket. The a/v bay, payload and nose as well as recovery train will be recycled into a stretch configuration for a madcow piranha nearing completion. Somewhere down the way, I may get another of these kits... It was a great midpower rocket!