[Motor Purchased] 5 inch diameter rocket and a CTI O3400.

I see nothing wrong with that, I say go for it. Now if it were a minimum diameter rocket a lot of thinking and preparation would have to be put into the rocket's design.

andrewlathrop87 said:

What is everyones thoughts about a 11' tall rocket 5" diameter with a 4" motor tube on a CTI O3400?

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If it's that tall, it means you're carrying around a lot of excess weight which will keep your speed in a reasonable regime.

My concern is fin attachment. TTW fins don't really give much benefit when there's such a small diameter difference between the motor tube and the airframe, since cutting through the airframe does nothing but weaken it. On the other hand, surface mounting is less strong than a minimum diameter surface-mount attachment unless you completely fill in the gap, since there's no motor preventing the airframe from flexing.

I'd suggest using a TON of centering rings under the fins and filling the fincan with the highest-density expanding polyurethane foam you can get, before carefully surface mounting the fins on top.

I plan on 2 layers of 5.7 oz 2x2 twill Carbon Fiber tip to tip

andrewlathrop87 said:

I plan on 2 layers of 5.7 oz 2x2 twill Carbon Fiber tip to tip

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Surface mount or TTW?

How fast does it simulate to?

If it's >Mach 3, how do you intend to protect the edges? Do you expect to have to redo the reinforcement after each flight?

Oh no not over Mach 2. Est: Mach 1.7. Yes through the wall fins. I am not going for speed or altitude. Its a personal challenge!!!!!

I recommend internal fillets and also filling it with expanding foam. That should help in not making you worry about losing a fin. Doing the tip to tip would be a great addition as well.

Oh yea process would be like this: Glue in motor tube with centering rings, but leave the after CR off. Then tack in fins. Apply internal fillets where the fins touch motor tube and also where fin tabs touch airframe. There would be 12 total internal fillets. Then glue in aft centering ring. Also, a thrust plate is going to be used. Then the outside fillets. On top of that tip to tip.

Andrew , whats your estimated pad weight ?

Eric

andrewlathrop87 said:

Oh yea process would be like this: Glue in motor tube with centering rings, but leave the after CR off. Then tack in fins. Apply internal fillets where the fins touch motor tube and also where fin tabs touch airframe. There would be 12 total internal fillets. Then glue in aft centering ring. Also, a thrust plate is going to be used. Then the outside fillets. On top of that tip to tip.

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I would additionally fill the spaces between the fins with high-density foam; it can't hurt, it won't add much weight, and it will help redistribute load around and off of the slotted airframe. Drill a pair of small ~1/8 should be more than enough) holes through the airframe in each section, once at the top centering ring, once at the bottom, mix up the foam in a syringe, and inject it (while still mostly liquid) into the bottom hole while the rocket it upright; give it more than you think you need, and use good tough tape to seal off the bottom hole as you pull the syringe out. Watch as the foam moves up, and seal the top hole once all the air bubbles are gone. Repeat for the other sections.

Also, instead of doing two simple full-coverage tip-to-tip layers, I'd be tempted to do three staggered layers; one that is full-coverage (covers the full fin), that one is last; one that is about an inch smaller in every dimension, that one is second to last, and finally one that is an inch smaller everywhere again, that one will be first. Then you have more distributed load, more strength across the fillet where it needs it (BIG fillets are a must to keep the stress-concentration in the carbon to a minimum). It also means the fin profile is constantly changing as you move through the fin, which makes the vibration and resonant mode behavior much more complicated; this should help resist fin flutter. You could vacuum bag all three layers at once. Someone here on TRF came up with that layup schedule, but I don't remember who it was.

Should be a spectacular flight!

Edit; if it wasn't clear, the first steps are the same as what you suggested; slot the tube, glue the MMT in without the aft CR, glue the fins in, make fillets. I don't think there'll be a need for 'tacking' the fins in; just use your structural epoxy (be sure to use a real structural epoxy, Hysol or Aeropoxy ES 6209 A/B (sold as aeropoxy structural from Giant Leap) or some such, structural epoxies are much stronger than laminating epoxies with colloidal sillica or some such) to glue the fins down to the motor mount. Note that this is basically the same method that PML kits use for assembly, if you or a reader wants an illustrated guide to this process; just look up the manual for any PML kit.

IDK what it will weigh on pad. I am guessing 30lbs without motor. Another 45-50lb with motor. S around 70-80lbs loaded!!!

Andrew, This looks like a good project. Last year, I built a 5.5" diameter with a 98mm mount.
This is the build thread FYI.
https://www.rocketryforum.com/showthread.php?43381-L3-Carbon-Fiber-Build

Guy

andrewlathrop87 said:

What is everyones thoughts about a 11' tall rocket 5" diameter with a 4" motor tube on a CTI O3400?

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Saw one shred nicely at Airfest this year.

Oh was it cardboard? LOL....... just kidding. I am trying to use the best materials I kind get for that simple reason. Fiberglass is super strong if the build is though. When you have eliminated the impossible whatever remains however improbable must be the truth.

andrew

I'd suggest using a TON of centering rings under the fins and filling the fincan with the highest-density expanding polyurethane foam you can get, before carefully surface mounting the fins on top.

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Where is the best place to get this foam?

PML's two part foam is adjustable density. Very handy stuff.

US composites has various foam densities available, I believe.

Well my project is underway. Pics shortly!! 66 inch booster, 36 inch payload, FW VK nose cone, 36 inch 98mm motor tube. I have the motor tube with CR glued into airframe and fins tacked on. I used 4500 Proline to attach all CR. I used 5 minute epoxy to tack fins on. Using thrust plate with 3, 1/8 G10 CR and 1, .5 inch wood CR. The thrust plate butts up to the airframe. Fins slots start roughly 1 inch from bottom of airframe so the CR fill the void and butt up right against fin tabs.

Awesome I'll be watching

andrewlathrop87 said:

I used 5 minute epoxy to tack fins on.

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You just gave me the shivers. I have stopped "tacking fins on" with lesser epoxies...

That said, on a rocket this size, the amount of bond length spoiled by the 5-minute epoxy is very small.

Use the two part expanding foam so those fins wont come loose. I like the red airframe .

I plan to foam is on order!!!!

andrewlathrop87 said:

I plan to foam is on order!!!!

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Good, that should do the trick. Keep up the good work.

Not to sound like a jerk or anything, I'd just hate to see you waste your money. O motor rockets aren't built with 5 minute epoxy and expanding foam or the "foam and fly" method. That should be saved for 29 or 38mm kits.

maxvelocity said:

Not to sound like a jerk or anything, I'd just hate to see you waste your money. O motor rockets aren't built with 5 minute epoxy and expanding foam or the "foam and fly" method. That should be saved for 29 or 38mm kits.

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Your not sounding like a jerk at all. Truth is I do not see why this technique should not be used on a rocket of this size?

I'm not entirely certain what the foam brings to the project (unless you are using something like the 8# stuff from US Composites). I would think a better solution is the inject-able fillet process Wildman espouses. Do it once to get the MMT to fin then flip it around and repeat for the fin to inside the BT fillet). The carbon fiber reinforced epoxy is quite durable. Alternatively, just slit to the back end, and assemble the fin can outside the body with glass tape mounting the fins to the MMT.

Considering you are already planning tip to tip, I just think the foam is gilding.

H_Rocket said:

I'm not entirely certain what the foam brings to the project (unless you are using something like the 8# stuff from US Composites). I would think a better solution is the inject-able fillet process Wildman espouses. Do it once to get the MMT to fin then flip it around and repeat for the fin to inside the BT fillet). The carbon fiber reinforced epoxy is quite durable. Alternatively, just slit to the back end, and assemble the fin can outside the body with glass tape mounting the fins to the MMT.

Considering you are already planning tip to tip, I just think the foam is gilding.

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+1 to this. You need to get some real epoxy/composite structure in contact with those fin roots to fly an O.

maxvelocity Not to sound like a jerk or anything, I'd just hate to see you waste your money. O motor rockets aren't built with 5 minute epoxy and expanding foam or the "foam and fly" method. That should be saved for 29 or 38mm kits.

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Sir, I just used 5 minute epoxy to tack fins down in place!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Yes, I will be making fillets where the fin touches the motor tube, fillets where the fin enters the airframe, external fillets using proline 4500. Each fin will have 6 fillets. I am using us compsites epoxy with chop carbon fiber mixed in to make internal fillets. Once that is done I am going to foam the whole fin can!!!!!!!!!!!!!

I suggest using all 12 holes in the motor retainer, not just three. That's a lot of motor mass and the T-nut threads can strip.