Anyone build the MadCow 8" DX3?

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grandcross

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Looking into building this kit, but finsim is showing fin flutter at some relatively slow speeds for a 98mm powered rocket. Tip to tip reinforcing is obvious, but I'd expect even that has its limits.

Flutter speeds by method:
U vs G: 166 fps
SQR(X) vs 1/k: 337 fps
Quasi static (main display): 830 fps

It wouldn't take much to exceed these speeds. With a 98mm motor it might be a challenge to stay under these speed.

Who's flown this successfully? What motors/speeds? How did you handle this?
 

madmax

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Those numbers seem incredibly pessimistic. I haven't used Finsim in years but double check your materials. I personally have not flown that rocket but I am hoping CJ will chime in here or one of the other guys from the Midwest as one of their clubs has had multiple drag races in the past with 6" Ultimate Wildman kits on various big N motors without issues. I know for sure they did N10,000s and I think they did N5800s. And that kit is 6" Diameter with the same thickness of fins. Higher velocities than an 8". A friend of mine at BALLS flew a PR Mad Max kit on a research O with no fin issues. That is probably the bird that the 8" DX3 came from anyway.
 

ChrisAttebery

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I ran into the same issue with a Blackhawk 75 kit. Finsim said that the stock fins wouldn't handle the speeds it was capable of. Make sure you use the Barrowman 3-D lift slope prediction technique. It's the least pessimistic setting. The difference was around 3x IIRC.
 

blackjack2564

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I personally have not flown that rocket but I am hoping CJ will chime in here or one of the other guys from the Midwest as one of their clubs has had multiple drag races in the past with 6" Ultimate Wildman kits on various big N motors without issues. I know for sure they did N10,000s and I think they did N5800s. And that kit is 6" Diameter with the same thickness of fins. Higher velocities than an 8". A friend of mine at BALLS flew a PR Mad Max kit on a research O with no fin issues. That is probably the bird that the 8" DX3 came from anyway.
3/16 fins on split fin 6in.Ultimate Endeavor [fiberglass] in N-10,000 race hit Mach 1.6 at 780ft. Coasted to 11,800
3/16 fins on U Wildman 6in. N-5800 race hit M-1.9 There were more issues with nose cone collapse and destruction of 3 rockets than any fin issues. Those 3 had old molded glass cones instead of spiral wound.

7.5in cardboard/carbon "Gone Bananas" used stock 3/16 fin from a 6in rocket did M-1.3 on big AMW N-skid [N-2800?]

It's going to take pretty good size motor to push that 8in heavy...draggy rocket into a mach realm capable of doing any fin damage. I would guess at least something bigger than a ''N". I've been known to brutalize my share of rockets with motors.
 

grandcross

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3/16 fins on split fin 6in.Ultimate Endeavor [fiberglass] in N-10,000 race hit Mach 1.6 at 780ft. Coasted to 11,800
3/16 fins on U Wildman 6in. N-5800 race hit M-1.9 There were more issues with nose cone collapse and destruction of 3 rockets than any fin issues. Those 3 had old molded glass cones instead of spiral wound.

7.5in cardboard/carbon "Gone Bananas" used stock 3/16 fin from a 6in rocket did M-1.3 on big AMW N-skid [N-2800?]

It's going to take pretty good size motor to push that 8in heavy...draggy rocket into a mach realm capable of doing any fin damage. I would guess at least something bigger than a ''N". I've been known to brutalize my share of rockets with motors.
I know this should be giving me the warm fuzzies but really it's having the opposite effect. I realize finsim should be on the pessimistic side, but this is ridiculous. Using the Barrowman 3-D fin slope technique as Chris Attebery suggested does give me what look to be more reasonable numbers, but isn't that just choosing the results I want? FWIW, here are those numbers:

divergence: 1560 fps (1.4 Mach)
flutter: 2123 fps (1.9 Mach)

Rocksim with an ATI N4800 (do they make these anymore? I haven't been able to find any... not really looking just browsing) has this doing 1569 fps. I'll still be doing a tip to tip reinforcement.
 

OverTheTop

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I'll still be doing a tip to tip reinforcement.
Fiberglass or Carbon Fiber? CF might be worth considering. Would stiffen the fins and lift resonant frequency well I think. Worth running through FinSim.
 

grandcross

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Fiberglass or Carbon Fiber? CF might be worth considering. Would stiffen the fins and lift resonant frequency well I think. Worth running through FinSim.
What I've done in the past is 3 layers each of CF/FG in alternate layers, with 1/3 - 2/3 - full layers similar to this:

[video=youtube;PtYYsX3VEmc]https://www.youtube.com/watch?v=PtYYsX3VEmc[/video]

Makes for a flying tank :)
 

jrkennedy2

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FWIW, did my L3 on a extended 8" DX3. The payload tube was 16" longer that std., as that is what RW had available at the time (deep discount). I launched it on a CTI M-1890 Red (98mm, 4gr). 80# AUW, went to 8100'. Don't have the measured speed numbers but she came down fine and I might be able to chop wood with the booster section. She's a beast!

Now maybe a 98mm 6gr vmax might prove interesting... :shock:

The avatar image is that flight.

CP.jpg
 
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PropellantHead

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FWIW, did my L3 on a extended 8" DX3. The payload tube was 16" longer that std., as that is what RW had available at the time (deep discount). I launched it on a CTI M-1890 Red (98mm, 4gr). 80# AUW, went to 8100'. Don't have the measured speed numbers but she came down fine and I might be able to chop wood with the booster section. She's a beast!

Now maybe a 98mm 6gr vmax might prove interesting... :shock:

The avatar image is that flight.

View attachment 323858
Just out of curiousity, how much did yours weigh when fully loaded & prepped?
 

jrkennedy2

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AUW (all up weight) was 80 lbs (79 & 11oz on scale), ready to launch. I reckon the igniter / starter was good for the last few ounces. Although, that was left at the pad at go-time. :)
 

PropellantHead

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AUW (all up weight) was 80 lbs (79 & 11oz on scale), ready to launch. I reckon the igniter / starter was good for the last few ounces. Although, that was left at the pad at go-time. :)
I have been guesstimating that mine, if I go with something like the "baby M" CTI M2250, would be in the 70-75 pound range once I have everything together. Looks like I'm certainly in the ball park. Thanks!
 

grandcross

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FWIW, did my L3 on a extended 8" DX3. The payload tube was 16" longer that std., as that is what RW had available at the time (deep discount). I launched it on a CTI M-1890 Red (98mm, 4gr). 80# AUW, went to 8100'. Don't have the measured speed numbers but she came down fine and I might be able to chop wood with the booster section. She's a beast!

Now maybe a 98mm 6gr vmax might prove interesting... :shock:

The avatar image is that flight.

View attachment 323858
Great! Did you do any tip to tip, or was it straight up fins with fillets? Is there a build thread anywhere?
 

jrkennedy2

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Didn't do a build thread, sorry. I tend to think my builds are nothing special so... Eh.

The fins were full-length thru-BT mounted to the MMT. I used 14" fiberglass strips (I do ugly glass work, sorry there too) to mount them and internal fillets:

aft_glass_img.jpg

I then used fillets on the outside (Rocket-poxy) to pretty it up a bit.

fillets.jpg

I tend to call my paint jobs, "20' jobs". The look real good at 20'.
 

grandcross

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Didn't do a build thread, sorry. I tend to think my builds are nothing special so... Eh.

The fins were full-length thru-BT mounted to the MMT. I used 14" fiberglass strips (I do ugly glass work, sorry there too) to mount them and internal fillets:

View attachment 323873

I then used fillets on the outside (Rocket-poxy) to pretty it up a bit.

View attachment 323874

I tend to call my paint jobs, "20' jobs". The look real good at 20'.
Thanks! Looks like my build plans are unnecessary for an M, but I always like the option of pushing my rockets :)
 

jrkennedy2

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Thanks! Looks like my build plans are unnecessary for an M, but I always like the option of pushing my rockets :)
And keeping options open is a big part of the satisfaction. Build-on and build it the way you dream it! When you talk to the class III folks, be sure to drop me a line. I wanna be there...

Thank YOU!
 
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