Custom Cluster Mod

Hi, I'm new to this forum and I'm not sure if this is the correct area for this post but we will see.

I'm working on the design for a LPR removable cluster mod that adds 2 more 18mm motors. It will friction hold on to the single motor in the body tube and have two side mounted motors that will sit externally between the fins of the rocket. The design is to fit my Estes Baby Bertha (BT-60 body) but if I can fit it on other rockets I won't be upset. It will only work on a 4 fin configuration because a 3 fin will not allow a straight line of motors to fit on. I know it will have additional weight and drag from having something sit outside but the boost from two more motors should be able overcome those for a net gain in altitude. The goal is to keep it single stage but give it a boost in altitude because the Baby Bertha is a tad bulky and I am topping out at about 500'.


[o]|x|[o] is a basic sketch of the design. [] represents a motor mount attached to the plywood that sits outside the body tube o is the motor in the mount. || represents the body tube of the rocket itself with x being the main motor in the body tube that the mod fits on to.

I am just getting back in to the hobby slowly after 15 years and getting my step daughter interested. I have never done scratch building and want the opinions of more seasoned builders on this. To see what will and won't work and what I need to change about my plan to make it work. Thank you!

Sounds really cool! Make sure the added mass at the rear end is offset by some added length (or mass) to the nose. Clusters get incredibly tail heavy real fast. You may also want to make sure the ejection charge(s) don't pop too early, resulting in a zipper on the body tube from the parachute coming out early. If you are building a booster to friction fit on the main engine (sustainer now) you may also want to equip it with a parachute fired off by one of the outboards. I can see a C6-0, C-6-0, C6-3 (chute or streamer) feeding a C-6-7 (or whatever) combo for a pretty thrilling flight! Good luck-straight smoke and good chutes!

N3rdM3tal said:

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The goal is to keep it single stage but give it a boost in altitude because the Baby Bertha is a tad bulky and I am topping out at about 500'.
SNIP

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The easy fix for this would be to upgrade the engine mount to a 24mm tube and use D engines. Add enough nose weight to make up for the difference between the 18mm and 24mm engines. Another option is CHAD (cheap and dirty) staging where a booster motor is taped to the end of the sustainer motor. Functions as a 2 stage model without a booster stage to lose. Big Bertha's responded to this, but the Baby Bertha may not be able to handle the extra weight at the rear without adding nose weight. Whatever you end up doing, keep us posted.

Even though fyrwrxz^* and Shield Wolf both already touched on this, I feel the need to emphasize: the added mass in the tail will probably make the rocket unstable if you don't add a pretty substantial nose weight to compensate. Particularly as the Baby Bertha is rather short, you're pretty sure to have a problem on your hands.

Also, be aware that the further cluster motors are from the centerline of the rocket the more trouble may be caused by slight variations in thrust among them. The diameter of a BT-60 plus the diameter of a BT-20 would put the outboard engines about 61mm apart, which is probably not too bad. But think about it.

I have two questions. First, what is the purpose of this? If it's just to get more altitude then there are a number of other ways. Wolf suggested changing over to a 24mm motor mount for D engines. If you're going that route, you could build the new mount for 24 x 95mm E engines. You'll still probably have to add nose weight, but not nearly as much. You could also keep the unmodified rocket and load Aerotech single use 18mm composite engines; they come in D10 and D21. Or, if you go 24mm, there are E15, E30, and E32 in the 70mm length. That's the way I'd go, if it were just for me.

On the other hand, if you want to do it because it's a cool project (which it is) then I'd love to see a more detailed description and pictures; I don't have a clear image of the design.

* Dude, have you got a name?

It was inspired by the fact I want more altitude on my Baby Bertha and I thought it would be fun. So even if it's impractical I'm going to go through with it because it is a cool project. I'm going to the hobby shop to pick up supplies today for that and my low altitude custom build I'm making for my daughter. I will have pictures as the project moves along once I get started.

Trip to the supply store got postponed until later in the week. Trying to decide what material to use for the mod. I got some 1/16" basswood for the centering ring on the scratch build I'm doing. Torn between two ply of the basswood (made with yellow wood glue) or single wall box cardboard. I also have some 1/8" balsa I used to make the fins on the scratch build.

Mod is assembled and glue is drying. Tonight I cut it to a bit less bulky shape and paint.

Finished mod loaded and attached. Bulky as can be but for what it is I'm impressed at how low profile it ended up. I need to add some removable Nose cone weight to balance out this very bottom heavy rocket. Without a balance weight the CG is 4" up the rocket if it's standing on its fins. It is only 12" long. It moves the CG a full inch toward the base of the rocket from where it was with only the one motor.

It's unclear from the pictures, but it looks like the original motor retainer no longer works. Are you planning on your boosters falling away due to drag(and there's plenty of it with that design,) or are you planning on them staying with the rocket? Either way, what retains the center motor on ejection?

If the boosters are supposed to separate, I recommend a bright color for them, to make recovery easier.

If the boosters stay with the rocket, I recommend nosecones on each pod to reduce drag, or you may spend 3 times as much per launch with no increase in altitude.

It's not the way I would have done it, but that's what makes it so interesting to me. Be sure and let us know how it flies.

Very Nice! Looks very cool. Keep up the thread, very interesting. I really like clusters. Yours is very different, good luck with the flight. Pictures please!

The retainer doesn't work now. That's the issue I'm working with now. Brainstorming ideas to make it better. The other idea is, like you said repaint and have it eject like a first stage but fire off simultaneously add a cluster with the sustainer. Should be able to tumble rather safely.

All in all it will probably not even work but is just to see if I can do it. If it even works at all then I call it a victory for science, if it fails then I know what not to do and why. For science!

A few tweaks. Shaved down the sides a bit and cut a notch in it so the retainer from the sustainer holds the whole attachment on. I also cut an engine block out of 1/8 balsa to keep the sustainer in place since the motor could fall out of the apparatus. By preventing the motor from falling out and retaining the mod to the body I should have dealt with the application problems. The flight problems are another story that I will have to deal with later. I don't have a whip to fire clusters right now, so I can't test flight short of unloaded side tubes and a single motor.