Dual Motor Big Daddy for LDRS

[Neutronium95]

Last year myself and a friend came up with the idea of building some 2x29mm clustered Big Daddy rockets for LDRS. I ultimately ran out of time and motivation for mine, while his ended up having an eventful life, culminating in it's annihilation at Rocstock when one of the motors cato'd.


I decided to scale down my ambitions slightly, and instead of wrapping everything in composites and flying it on a pair of H233 motors, I decided to stick with cardboard and plywood, and just use a pair of punchy G motors. I got some custom fins and centering rings cut by James at Rocketry Works, and they came out great.

This is going to stay a relatively simple build, with motor eject and no composite reinforcement. I might build a version wrapped in carbon fiber in the future, but that's a problem for future me. Just a pair of G motors can get this scary fast, and I would love to fly a version of this on a pair of full H or baby I motors.

 

[StreuB1]

I like where this is going.....

A pair of G138T's would be a very cheap way to test the waters on some seriously punchy G motors. And they take the cheap 29/40-120 hobbyline reload hardware.

What are you doing for motor retention? Look into ordering up some motor hooks from Aerotech, cutting the top bend nearly off so there is no block in motor length. This at least gives you a starting point for motor retention that take up nearly zero room.

 

[Neutronium95]

I like where this is going.....

A pair of G138T's would be a very cheap way to test the waters on some seriously punchy G motors. And they take the cheap 29/40-120 hobbyline reload hardware.

What are you doing for motor retention? Look into ordering up some motor hooks from Aerotech, cutting the top bend nearly off so there is no block in motor length. This at least gives you a starting point for motor retention that take up nearly zero room.

I'm going to go with friction fit to keep things simple. For motors, I'm probably going to go with Cesaroni to start with, since I already have the hardware. The G138 is a really cool motor, and is on the list for future flight possibilities. The one problem with the G138 is that it is a blue motor. My friend's version flew three times, the two flights on blue motors were failures, and the one flight with red motors worked fine, so I'm going to use the G125 for the first flight.

 

[Neutronium95]

This morning, I learned that I am an idiot. I forgot to account for the thickness of the body tube when designing the fins. I could probably fix that with a bunch of sanding, but there are a few other tweaks that I'd like to make, so new fins it is.

 

[DigBaddy]

This morning, I learned that I am an idiot. I forgot to account for the thickness of the body tube when designing the fins. I could probably fix that with a bunch of sanding, but there are a few other tweaks that I'd like to make, so new fins it is.

I wouldn't call it a bunch, just sand some away where the fins contact the outer surface of the tube and you should be good to go. Maybe just a few minutes work. (or I'm missing something else and can't think straight today)

 

[Neutronium95]

The revised fins arrived today.

I also started playing around with some silly motors in Openrocket.

I am slightly concerned that 1/8" plywood and Estes cardboard tubes won't hold up to the gentle breeze of Mach 1.5, so I decided to reinforce both of those with carbon fiber. Here's hoping that I can get this all built in time for the launch. I hit the road on Wednesday evening after work.

 

[StreuB1]

What motors?

 

[Neutronium95]

What motors?

Cesaroni H399s. As I said, silly motors.

 

[Neutronium95]

I got started on the actual construction today.

First I did a dry fit of all of the parts, just to make sure that the motors would fit. I was pretty sure that the 6 grain cases would, but was doubtful that the 6XL cases would fit. Fortunately, that bit of intuition was wrong, and the 6XL cases just barely fit in, if the motor mount tubes are flush with the aft end of the rocket.

Then I laminated a layer of carbon fiber twill onto each side of the fins. I wetted out the carbon fiber between two layers of parchment paper, and squished the whole stack between two tiles, and put a big bucket of water on top to get a bit of pressure. The fins came out alright, with a few dry spots, but no major delamination.

Next up for today is assembling the motor mount, and reinforcing the tube. Surprisingly enough, this will be my first time doing a traditional wrapped reinforcement on a tube. I've used sleeves in the past, and for my current projects, I tend to use tubes that don't require composite reinforcement.

 

[Hobie1dog]

I thought you said you were going with G Motors?

 

[Neutronium95]

I thought you said you were going with G Motors?

I changed my mind. The thought of an Estes kit pulling 100 G and going Mach 1.5 was too cool not to do.

 

[jbr]

it screams with a J motor
I used carbon fiber fins

 

[Neutronium95]

I ran into some issues with wrapping the tube, so I'm going to put this project on hold for a few weeks. The one takeaway that I can provide, is that it might be advisable to not do your first roll wrapped tube late at night on your knees and with bad light.

 

[FlyBy01]

I ran into some issues with wrapping the tube, so I'm going to put this project on hold for a few weeks. The one takeaway that I can provide, is that it might be advisable to not do your first roll wrapped tube late at night on your knees and with bad light.

ouch; sometimes learning is painful on the wallet

 

[ep29030]

This is a definite project for reinforced tube and tip to tip fin reinforcement. Otherwise it will be a one-flight wonder. I had a Lil' Ivan (3 inch) FG kit that flew on 54mm 2-grain J (IIRC, J240) that showed FG fin flutter damage after 3 flights. You are flying a paper/wood kit with more average thrust.

 

[StreuB1]

One thing to point out is that material type is a huge contributor in fin flutter and the resulting damage from said flutter. The resonant frequency of the assembly and the modulus of the material sets the probability for failure. At times, light, rigid yet supple materials will survive where dense, rigid, and less supple materials will rip apart. The flutter frequency of the fin relative to its overall width from root to tip will partially help determine how destructive the flutter could be but not what will be. You have lots at play like construction method, adhesive to substrate bond strength, adhesive peel strength (this is a huge factor), etc. More flexible epoxies that are bonded to fiberous materials will have very high peel strength due to their ability to flex and transmit that flex into the adjacent materials, thus deforming them as well. The more flex you have and less permanent deformation you have, means you have an assembly that is far more resilient than a very rigid structure with high bond strength adhesives. A rigid structure places more loading on the adhesive, the bond line and the peel factor which can cause them to suffer delamination instead of "riding out the storm" like something more flexible.

Some cardboard and plywood construction will be more resilient to flutter than FG or even CF and vise versa.