Madcow 2.6" lightweight fiberglass DX3-how hard have you pushed it ?

Have started assembling my Xmas present, a 2.6" lightweight glass DX3.

Went ahead and got it with a 54mm mount and and am building a motor mount adapter for 38mm motor options.

Have not yet started simming it, but was curious how hard folks have pushed it with the stock .060 G10 fins, both with and without tip to tip composite reinforcements.

I have a 2.6" screech with a 38mm mount. It's flown 4 times on 38/1200 or 38/1320 hardware. K1127,J510,J1026 and another K1127.

I'm building a 54mm booster to replace the 38. A 54/2800 should get around M1.6. We'll see how that goes.

DizWolf said:

I have a 2.6" screech with a 38mm mount. It's flown 4 times on 38/1200 or 38/1320 hardware. K1127,J510,J1026 and another K1127.

I'm building a 54mm booster to replace the 38. A 54/2800 should get around M1.6. We'll see how that goes.

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Madcow's website shows that the Screech and the 2.6" FG Black Brant have the same thickness G10 fins. I'm building my BBII now. How will you reinforce your fins on the Screech to get M1.6? The BBII comes with a strip of fiberglass to reinforce the area between the fins and the boat tail, and I used RocketPoxy for my fillets. Of course, mine isn't DD (until I get the Chute Release) and probably won't be doing anything too extreme with the 38mm mount. Just curious as this is my first FG kit.

I did the 38 with aeropoxy inside, and proline 4500 fillets outside. Big ones.

I'm planning the same style of building on the 54mm. I have no evidence they won't blow off. but I'm fairly confident. If I planned to hit those speed all the time I'd probably reinforce. I'm betting I'll do it once on this airframe and then fly it mostly on smaller motors.

Those are some nice and big fillets for sure. I'm pleased with my first RP fillets, I'm sure my technique will improve. Thanks for the info. Since the BBII is my first FG kit and it's not DD, I'm going to keep those flights relatively low and slow. I have a Madcow 4" PAC-3 to build and that will be my first DD build and my L2 bird. Once I'm comfortable with that, it'll be time for a DD FG kit.

One issue with the 54mm mount in the 2.6" glass DX3 is that the fin tabs are only about 3/16 of an inch deep. Not much room for internal fillets, but I will get some in there.

Worth doing some larger external fillets with this situation.

I have some high performance laminating resin that I will use with milled glass fibers for my internal and external fillets.

Still interested in knowing how much of a 54 mm motor has been flown in this DX3 without doing external tip to tip glassing on these thin .060" G10 fins. The fin planform is good for avoiding flutter issues.

I'm thinking of just injecting into the slots then smushing the fins in. Should effectively make internal fillets.

The DD dx3, and 54mm mounts are a recent thing. It's likely the specific setup has not been tested much in practice.

I was putting some finishing touches on my BB tonight, checking CG, etc. I put a loaded AT H128 in it, put my Aeropack retainer on and noticed when I went to put the nose cone on, that rocket is pretty much airtight. Is that going to cause an issue? Should I drill a couple of holes in the FG body tube to equalize the air pressure inside? That's probably a stupid question, but I definitely don't want any premature ejection! Lol

What do you mean by BB? Cause what you're describing is definitely not a Baby Bertha... lol.

Anyhow, I am assuming when you pushed the nosecone into the body tube it made a whoosing/flush sound. That is normal because air inside is leaving to make room for new air. Try pushing the nosecone halfway or a little after halfway in, if it vacuums and sucks or pulls the nosecone in then the rocket is airtight and you got a problem. But don't drill any holes now until you know what's wrong.

Hope this helps,

Bill

RocketryBill said:

What do you mean by BB? Cause what you're describing is definitely not a Baby Bertha... lol.

Anyhow, I am assuming when you pushed the nosecone into the body tube it made a whoosing/flush sound. That is normal because air inside is leaving to make room for new air. Try pushing the nosecone halfway or a little after halfway in, if it vacuums and sucks or pulls the nosecone in then the rocket is airtight and you got a problem. But don't drill any holes now until you know what's wrong.

Hope this helps,

Bill

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Why would airflow in the interior of the airframe be normal? If the plans are right, if the internal air stays put and all the external air stays external that'd be better, no?

This is my Black Brant II. When I pushed the nose cone in, it wanted to pop back out actually, not get sucked in. Only after I slowly kept pushing would it go in all the way.

I drill a single vent in the booster, and one in the payload.

DizWolf said:

I drill a single vent in the booster, and one in the payload.

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This has a single body tube. How big of a hole should I drill?

Unscientifically? 1/8 or 3/16 should do the trick. Just enough to give the air pressure inside a place to escape on the way up. Doesn't take much.

DizWolf said:

Unscientifically? 1/8 or 3/16 should do the trick. Just enough to give the air pressure inside a place to escape on the way up. Doesn't take much.

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Awesome. Thank you

I received this same kit/setup for Christmas as well. Any thoughts on what you plan to use to mount your shock cord? Right now I'm planning to file away a very small section of the forward CR for a kevlar cord epoxied to the MM, but I'm curious what others are doing.

If I had one I would build it Wildman style and I would not hesitate to pack it full of AP. That is, centering rings sandwiching the fin tabs, injected internals, and recovery with kevlar straps on the motor mount. It looks comparable to my Punisher and we all know what abuse that one can take.

54mm6XL if it fits!

watermelonman said:

It looks comparable to my Punisher and we all know what abuse that one can take.

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The Punisher has 1/8" fins. I'd be nervous about 1/16" G10 even with the DX3's fairly small fin span, but the only way to be sure is to try.

PML uses 1/16" G10 fins and with the fin span on many of their designs, a shred is practically guaranteed on a supersonic flight.

mikec said:

The Punisher has 1/8" fins. I'd be nervous about 1/16" G10 even with the DX3's fairly small fin span, but the only way to be sure is to try.

PML uses 1/16" G10 fins and with the fin span on many of their designs, a shred is practically guaranteed on a supersonic flight.

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Fin thickness is a great point. I did take a look at the shape and thought it fine, but the thickness is important too. Perhaps approach it a little less eagerly than I initially conveyed, but yes like you say, the only way to know is to try!

I tend to ramp up the flights of all my rockets over their lifetime. My Darkstar first flew on an I470 a while back; now I have an L640 waiting. If they shred on the later flights, at least we had our fun together while it lasted!

watermelonman said:

If I had one I would build it Wildman style and I would not hesitate to pack it full of AP. That is, centering rings sandwiching the fin tabs, injected internals, and recovery with kevlar straps on the motor mount. It looks comparable to my Punisher and we all know what abuse that one can take.

54mm6XL if it fits!

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The thin fins are an issue, but they do M1.2 just fine, how far that goes I don't know. The CRs show how little space there is for injecting. Madcow also uses short fin tabs. A good amount of epoxy into the slot should cover it.

DizWolf said:

The thin fins are an issue, but they do M1.2 just fine, how far that goes I don't know. The CRs show how little space there is for injecting. Madcow also uses short fin tabs. A good amount of epoxy into the slot should cover it.

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A good roughening up of the fin tabs and surrounding tube surfaces and a good amount of structural filled epoxy around the tabs and the inside and outside of the tubes in that area and some generous external fillets is my current plan for those .060 fins.

I do have some .090 g10, but want to keep the build as stock as possible.

Most of the time when I am setting fins I use CJs double dip method. I put epoxy on the fin root set it to MMT and remove it and apply epoxy again and set the fin in the second time. It has always worked well for me.

Does anyone have RockSim/OpenRocket files for this rocket? Neither Madcow or RocketReviews.com have any.

copy pasted the DX3 fins from the cardboard version, converted them to G10, and pasted them on my 2.6" screech file.

Should be a decent ballpark.

View attachment fgdx326.rkt

It's been a little over a year now. I've launched mine a handful of times but on nothing larger than a J. I just acquired an L730 for it, which should push it to somewhere around Mach 1.5.

Has anyone pushed theirs that hard yet? Did the fins go flappy-flappy/snappy-snappy? Or did it come back unscathed?

tab28682 said:

A good roughening up of the fin tabs and surrounding tube surfaces and a good amount of structural filled epoxy around the tabs and the inside and outside of the tubes in that area and some generous external fillets is my current plan for those .060 fins.

I do have some .090 g10, but want to keep the build as stock as possible.

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I did a scratch build about a yr ago that was essentially a 2.5" DX but single break head end deploy. If you have the thinker fin stock, USE IT! I used .094 on my fins and Ive been to 11000'+ at mach 2.1. Trust me if you want to really get the most out of it and fly the biggest motors possible, use the thicker fin stock.
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