Super DX3 build

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

dottore

Member
Joined
Jul 5, 2014
Messages
22
Reaction score
0
Some back story: I built model rockets when I was a little kid, but never flew anything bigger than a B. Through a strange turn of events I found my way back to rockets, realized that I'm a grown-up now and I can build big things... so I put together an Aerotech HV Arcas, had fun, and decided to do something bigger with an eye towards certification.

So I thought I'd put up a thread to chronicle the build of my Madcow Super DX3. Here goes! (Comments and criticisms always welcome, I'm quite new at this.)
 
Rockets seem to involve mostly sanding, so I might as well start with some prep: masking off the 9.5mm leading and trailing edges on the fins. As I lack a proper jig (and my CNC mill is in pieces...) I'll freehanding these bevels.

fin-masked.jpg
 
Welcome!!! dx3 is a good looking rocket, I'm making one that looks simular to a dx3
 
Rockets seem to involve mostly sanding, so I might as well start with some prep: masking off the 9.5mm leading and trailing edges on the fins. As I lack a proper jig (and my CNC mill is in pieces...) I'll freehanding these bevels.

View attachment 176855

You're off to a great start. Your intro story sounds vaguely familiar. :) Now about that CNC machine... pieces ?!?
 
Goodluck with the build and your L1 attempt, I built one stock with dual deploy for my L2.
 
You're off to a great start. Your intro story sounds vaguely familiar. :) Now about that CNC machine... pieces ?!?

Yeah – it's nothing impressive, I don't have a VMC in my basement (unfortunately). :wink:

I built a Shapeoko 2 (with some upgrades: DeWalt 660 spindle, NEMA 23 motors, Acme screw for the Z axis, etc). Sadly, it wouldn't really hold any better than a tenth of an inch or so. The problem is that one of the motor mount plates, which holds the X and Y axis together, is slightly warped, so the X and Y axes are out of alignment. No point in putting it back together until the replacement mounting plate appears....

It is really cool to watch a CNC mill work, though!
 
Fastest fin beveling ever! (Okay, I did that part over the last week or two.)

My technique was to clamp the fin to a board and use a Dremel in successive passes over each edge, eyeballing 15 degrees. This... sort of worked. The plywood layers gave me a little visual cue as to where I was in the thickness of the wood, but it still took a lot of frequent pauses to check my progress. The idea was to just take off the barest edge of the masking tape. Once I got close, I switched to 80 grit paper and finished the rest by hand.

The biggest problem I ran into was that it's pretty easy to "notch" and put in a slightly uneven spot on one side, especially near the limits of the fin edge. A little wood filler saves the day!


After I was satisfied with the edges (way too long to contemplate later...) I sealed the fins. I started by sanding down all the surfaces with 150 grit sandpaper, then coated everything except the fin tab with a thin layer of diluted wood filler. After it was dry, I sanded it down with 220 grit, and repeated the process. The third coat of wood filler got sanded down with 400 grit, and I think that's good enough. The fins feel like glass to my fingertips, so hopefully they'll take paint well.

Once they're in place, I'll apply a little CA glue to the leading edges to prevent splintering.

sanded fins.jpg
 
I used some of the downtime on the fins (also because I was getting really tired of sanding) to start assembling the motor mount tube.

I decided to add a third centering ring just forward of the fin roots. I epoxied the center CR on first, carefully setting the distance so that there will be just enough room for the fins, aft CR, and motor retainer. Then I did the forward CR, which I fully assembled first by securing the eyebolt and shock cord. The eyebolt is epoxied on both sides, plus some more in the threads. I thought about using a Rapide link to hold the shock cord, but decided to just tie it instead, so that's anchored with a Palomar knot.

palomar.jpg

Once the epoxy has all set, it's time for a test fitting before securing the motor mount tube to the body tube: looks pretty good so far! (Oh yeah, I obviously filled all the spirals in both body tubes with wood filler and sanded it down with 220 grit. This is really all about sanding, isn't it?)

test.jpg


And everything meets up squarely against the motor mount tube when viewed from aft:

rear.jpg
 
With the fins ready to go and the motor mount tube squared away, it's time to install the motor mount into the lower body tube.

This was a little scary, because the middle centering ring gets epoxied in blindly. But first things first: the appropriate spots in the body tube got roughened up with 60 grit sandpaper and wiped with some isopropyl alcohol. While that was air-drying, I mixed up a small batch of Rocketpoxy and loaded it into a Monoject 412 curved-tip syringe. These are available for about 50 cents each on Amazon, or you could probably get them from a vet or dentist pretty easily. I cut the tip off about halfway up its length, which widens it up nicely since the Rocketpoxy is fairly thick.

I reached into the body tube from the top and applied a thick ring of epoxy an inch or so aft of where the front centering ring would end up, then inserted the motor mount tube from aft. Once the front centering ring was clear of the fin slots by an inch or so, I canted the motor mount tube sideways, then reached in and applied another generous ring of epoxy to the inside of the body tube just forward of the fin slots. The motor tube then slid easily all the way forward, and I rotated it around a time or two to make sure the epoxy was well-distributed along the interfaces. This also seemed like a good time to carefully re-fit those fins, to make sure everything was still spaced properly.

Unfortunately, no good pictures, since (a) I couldn't figure out how to take one and (b) I had a fair bit of epoxy going on. :D

After everything was seated up, I got some isopropyl alcohol on my fingers (gloves on, of course), reached down through the top of the body tube, and tried to fillet the epoxy nicely. It sort of worked –*it was really tricky to get around the eyelet, even with the help of a dowel:

forward cr.jpg


Obviously I want this assembly to dry pointing up, so that gravity is working on our side. Unfortunately, the end of the motor mount tube sticks out slightly from the body tube. So I grabbed some scrap HDPE I had laying around:

drying stand.jpg

The bottom pieces are 1/8" thick, and the top pieces are 1/4". Conveniently, the motor mount tube sticks out almost exactly 1/4"... I needed a thickness or three of masking tape to get the height just right, and I apologize for the bad photo, but:

stand 2.jpg

This is looking end-on at the bottom of the body tube. As you can see, the motor mount tube protrudes slightly, and is resting on the masking tape. The body tube itself is supported by the HDPE spacers, so that the two are (loosely) fixed in space. Since the epoxy had started to set up to tackiness by this point, it wasn't going anywhere.
 
Once the motor mount tube had set up, I marked an extended centerline for the fin slots, to make fin alignment easier later on. This was a good time to put on the launch alignment line, so I just used a piece of paper (the instructions :wink:) squared off to the end of the body tube and marked the fin extended centerlines:

paper-centerline.jpg

This makes it really easy to both find the midpoint and then transfer it back onto the body tube with a straight guide:

centerlines.jpg


I also decided to pour three additional epoxy fillets between the motor mount tube and the body tube. This requires working through the aft end of the body tube, which is a little bit of a pain. I also was very paranoid about getting epoxy near the fin slots, so I masked them off before pouring the epoxy:

fin slot masking.jpg

I left a quarter to half an inch of clearance on either side of the masking tape, which I expect will mostly get filled by epoxy when the fins are bonded in. But that can't happen until those fillets set up!
 
...so while that's happening, I figured I might as well work on the upper half of the rocket. Specifically, the removable avionics bay!

Basic assembly was incredibly easy, and accomplished with the aid of some Loc-Tite Red 271 thread locking compound. The spacer ring was carefully epoxied into place and tacked down with CA glue to keep it in place while the epoxy set. The sled went together with some Titebond wood glue (epoxy seemed like overkill here):

avionics sled.jpg

To mark the location of the mating rivets and static pressure ports, I just used a fin positioning ring against the forward end of the avionics bay:

altbay alignment.jpg

After the spacer ring had set up fully, I drilled three 5/32" holes (as per this table) for the static pressure ports in the coupling band. A little sanding to knock down the rough edges and some CA glue to reinforce the holes finished the job. I'll have to make up some little "STATIC PORT – DO NOT OBSTRUCT" decals for later. :)

The rivet holes went together easily in a similar process. Here's the entire avionics bay test-fitted to the upper body tube:

altbay test fitting.jpg
 
I figured I'd take a minute before I started bonding in the fins to work out the fin fillet dimensions, so I could mask them off before getting epoxy all over everything. ;)

Let me start by quickly reviewing what I think the names for all the dimensions of a fin are:

fin.png

I measured these fins' root chord to be of 30 cm (11.8 inches). The fin fillet radius should be 4-8% of the root chord, so at 6%, that gives me a fillet radius of 1.8 cm (0.7 inches). That looks like this when it's masked off:

fin masked.jpg

That looks really huge! I'll need to find some 1.5 inch PVC pipe to pull that.... Anybody care to double-check my math and/or understanding?
 
I used a half inch PVC pipe to pull mine and they hold up just fine. Those are very large. External fillets are more for looks. The internal fillets are for strength.


Sent from my iPod touch using Rocketry Forum
 
I'm fairly sure the 4-8% guideline is a portion of the semi-span, not the root cord...
 
I think it has to be the root chord length that matters: here's my thinking.

Interference drag occurs because airflow around two different parts of an object (in our case, the body tube and the fin) are forced to occupy the same space. The air flowing around the parts must speed up in order to fit into the restricted space. If you think of the air as being "packetized," it would look kind of like this:

packets.png

OK, so why the root chord instead of the semi-span?

As we move along the semi-span of the fin, away from the body tube, the airflow isn't being compressed into the restricted corner between the body and the fin. This makes sense: the turbulent parts of the airflow behind the fin are at the root (tip vortices come from a different effect). As we move along the length of the fin, though, we have to cram more and more air into that restricted space, and keep it all moving very fast – that corner generates a lot of interference drag, and it does so proportional to its length. So the optimal radius of the fillet has to depend on the length of the root chord. I think.

None of that changes the fact that a 3/4" fillet will probably look huge and kind of stupid, though. :)
 
Just saying, I thought I built fast... You basically built 1/3 of the rocket in one afternoon/evening. Good work though!
 
Bonded in all three fins overnight. The process was pretty simple: mix up a batch of Rocketpoxy, then spread it generously along the bottom of the fin root and fin tang, basically anywhere where the fin comes in contact with anything. Then I carefully inserted the fin through the slot, wiggling it a little to relieve any binding in the body tube and making sure that it was fully seated along its length. I tacked the tip of the fin down with a drop of CA glue to help keep it aligned exactly on the extended centerline, then applied some tension with masking tape:

fin tape.jpg

The most difficult part here was getting the fin to sit exactly vertical. I tried to check this by putting a plumb line on the top of the fin, and verifying that the plumb line split the difference between the other two fins. No photos of that (ran out of hands), and I'm not sure how well it works with a 3-fin design. (I think it could be very effective with 4 fins, though.) Each fin was finished up by applying the interior fin fillets against the motor mount tube.

As I went along, I realized I had the opportunity to apply the other set of interior fin fillets as well – the ones between the fins and the body tube. So I poured those as well. This is from bonding in the last fin:

motor mount.jpg

It took another rotation or two to make sure that all the internal fillets were in place and had a chance to cure in the right orientation, but it was a bit more convenient than doing each fillet separately.
 
Almost forgot! Since I had to mix up a few batches of epoxy anyways, this seemed like a good time to get started on the fiberglass nose cone.

The nose cone, coupler, and bulk plate all got washed down with a ratio of 1 part isopropyl alcohol to 3 parts water to 1 drop soap. Pro tip: I (re)learned that I absolutely have to wear gloves any time I come in contact with fiberglass. I don't know whether it's the release compound or the fiberglass itself, but there's something about it that just makes me itch like mad.

I left the pieces out in the sun to dry, then made a few passes with 60 grit paper on the bonding areas of each part. The first thing to get assembled was the bulkplate eyebolt, which was held together with epoxy and some Loctite 271 on the threads. After that had set up, it got bonded into the coupler:

bulkplate coupler.jpg

The extra hole is a 1/8" air pressure vent, so the nosecone isn't impossible to assemble later on. :)
 
Once the fillets on both sides of the bulkplate in the nosecone coupler had set, I test-fitted the coupler into the nosecone and marked the depth on the coupler at a few different places. I then removed the coupler and verified that the depth of all the marks was the same, at 2.6 inches:

coupler depth.jpg

This way I can verify that the coupler has seated squarely into the nosecone, rather than being off-axis and making a big mess later on. With that being done, the usual surface preparation, a healthy coat of epoxy on the bonding surface inside of the nosecone, and everything just slid together perfectly:

ogive nosecone.jpg


Now if I can just quit being nervous about pouring the external fin fillets....
 
I used a half inch PVC pipe to pull mine and they hold up just fine. Those are very large. External fillets are more for looks. The internal fillets are for strength.


Sent from my iPod touch using Rocketry Forum

Why do you say external fillets are more for looks?


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.
 
Dottore,
Beautiful build! I would suggest forged or welded eye bolts though. I've seen unwelded ones straightened out by hard deployments. U-bolts are even better.


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.
 
...
So I thought I'd put up a thread to chronicle the build of my Madcow Super DX3. Here goes! (Comments and criticisms always welcome, I'm quite new at this.)

But not to building things, evidently. Really nice, clean, detailed build thread. I especially like your choice for an L1 attempt. Rock solid components and you can go low-n-slow for your cert flight and then crank up the impulse afterwards.

fiberglass nosecone???????please explain:)

Madcow offers an FG upgrade: https://www.madcowrocketry.com/4-super-dx3/
 
Great Work!

Steve, Internal Fillets offer more strength than External fillets. The external fillets are more for aerodynamics. You can usually get by with just external but internal offers superior strength.
 
Thanks!

You know, I thought about using a forged eyebolt. Welding it myself was out of the question, because I am an atrocious welder. Seriously, we all have our talents, and that is not mine.

But the part you see is what Madcow supplied in their kit... so that's what I decided to use. Hopefully it holds up – I don't see a way I could possibly replace it without essentially starting over?
 
fiberglass nosecone???????please explain:)

Yes, please explain. I have the all fiberglass DX3, nice kit. Put mine up on a Aerotech K550 to 5,553'
ImageUploadedByRocketry Forum1404693595.182754.jpg
14.4 lbs at the pad.

I'm interested how yours will turn out.
 
I do not like shaping external fin fillets.

As I talked about above, I measured the root chord of these fins to 30 cm. A 6% radius fillet (18 mm) looked ridiculously big to me, so I decided to use a 4% radius fillet (12 mm). So I started out by removing all my existing fillet masks, and re-masking everything off to 12.5mm (1/2"):

fins remasked.jpg

Note that there is a small fillet of structural epoxy against the external joint. I shaped this earlier on, when I bonded the fin into the motor mount tube. My rationale is that cutting a slot in the body tube weakens the tube, so I want a little extra strength at this joint, even though it's reinforced with another structural fillet from the other (internal) side.

For a tool, I dug through my scrap pile until I found a piece of 3/4" ID Schedule 40 PVC pipe that was about 1.5" long. This is (just barely over) 1" OD, so just about perfect. I polished down the ends with some sandpaper to smooth off the burrs.

Then I mixed up a generous batch of Rocketpoxy and started adding microballoons to it. Since this fillet is mostly for aerodynamics rather than strength (see above comment), I didn't see a point in using unadulterated structural epoxy. It would add a ton of unnecessary weight on the wrong side of the rocket. I stopped when the epoxy mixture got to the consistency of airy bread or cookie dough (about the same as wood filler, actually).

microballoon epoxy.jpg

Then I started to pack the fillet joint with epoxy... first mistake.

too much epoxy.jpg

This is way too much epoxy. :eyepop: Something like half that amount would have worked. Instead, I made a mess.... I ended up wetting the entire surface of the fillet and my shaping tool with isopropyl alcohol. Using only one edge of the tool, I scraped away epoxy in successively deeper passes. If the tool felt like it was "catching" I was pulling off too much, and starting to disturb the underlying epoxy – time to stop, clean off the tool, re-wet with more isopropyl alcohol, and start again. I was also not at all shy about getting my (gloved) hands with alcohol and getting in there to smooth things out with my fingers.

After about ten minutes of frantic labor, I had the long sections mostly shaped. I kept at it, making the angle of the tool more and more acute, until I could pass it along the fillet while pressed into the joint (that is, in contact with the body tube and fin masking tapes). That left something like this:

shaping up.jpg

Then I tackled the corners... basically I tried to reverse the direction of the tool around the fin, so that at the tip of the fin the tool was perpendicular. That is a bad explanation, so here is a picture:

fillet shaping.png

Whew. After that was done, and boy did it generate a lot of mess, the epoxy was starting to set up pretty firmly. I pulled the masking tape off and used Q-tips soaked in isopropyl alcohol to clean up the seams (and other stray blobs). I'm reasonably happy with the end result, although it's definitely going to require some sanding and a few coats of wood filler to get the little imperfections smoothed out:

unmasked.jpg

filleted.jpg

One down, two to go....
 
Great Work!

Steve, Internal Fillets offer more strength than External fillets. The external fillets are more for aerodynamics. You can usually get by with just external but internal offers superior strength.

I'm skeptical. What is it that makes an internal fillet stronger?


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.
 
Thanks!

You know, I thought about using a forged eyebolt. Welding it myself was out of the question, because I am an atrocious welder. Seriously, we all have our talents, and that is not mine.

But the part you see is what Madcow supplied in their kit... so that's what I decided to use. Hopefully it holds up – I don't see a way I could possibly replace it without essentially starting over?

Under normal conditions it will be just fine! I would not start over. If you fly as well as you build you should never have to worry!


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.
 
Back
Top