A 3 x mini cluster scratch build

Haven't decided what I'm going to call it yet, but here's a little bird I put together over the weekend. It will fly on a cluster of 3 mini motors (1/2A3-2T or similar).



Some detailed pictures in the followup posts.

Here's the detail on the motor mount. I used a BT-55 tube coupler, cut into 3 slices as centering rings. The triad of BT-5 motor tubes were a bit loose in the coupler, so I cut some BT-55 body tube slices, removed a segment, and glued them inside the coupler rings. The result was a snug fit of the motor tubes into these makeshift centering rings. The motor tubes are 6" long so that I will (hopefully) get good ejection. I still need to fill in the forward gaps between the tubes so that the ejection gas doesn't leak out the aft end of the rocket and cause a deploy failure. I'll cut something out of stiff fiber board for that. The kevlar thread shock cord is tied to the forward ring, and the whole assembly is removable (held in place with three nylon screws that thread into the forward ring, which you can see in the first post message).

Here's the modular view of the rocket. As I said, the motor mount is removable, held in place with 3 nylon screws. Why didn't I just glue it in? Mostly because I like modular designs, and more practically, so that if the shock cord ever fails, I can easily remove the mount and tie on another length of kevlar thread. Plus, I think the screws will give it a cool look.

I just did one an awful lot like yours except for having 4 motors and swept tapered fins. The SEMROC BT5 tubes do slightly "distort" the bottom of the BT55 but it's acceptable, maybe an Estes BT5 wouldn't, maybe. I put the fins on top of the very slight bulge, you don't notice it unless viewed from the rear. I also just did 3 BT50's in a BT70. They were loose in the BT70 but place them in a coupler and they fit real good. Both are filled and ready for primer...whenever it gets warm enough to do that

dave carver said:

I just did one an awful lot like yours except for having 4 motors and swept tapered fins. The SEMROC BT5 tubes do slightly "distort" the bottom of the BT55 but it's acceptable, maybe an Estes BT5 wouldn't, maybe.

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Cool sounding rocket.

Mine seems undistorted. I used a Fliskits BT-55 body tube and coupler, and a Estes BT-5 - so far a good fit.

dave carver said:

... and swept tapered fins...

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I have so many rockets with either swept fins or trapezoidal fins - love them, but for this design I kept thinking about that 'missile' look and settled on the simple triangular design, aft end of root chord flush with aft end of the body. I'm leaning towards calling the rocket the 'Triagon' - 3 motors, 3 sided fins.

Two of the three wedges (to seal the upper part of the mount from leaking ejection gas aft) installed, the third waiting to be. I made them from some scrap card leftover from my USS Enterprise build. Once all three are in I'll seal the edges with wood glue and call this component complete.

Painting in progress. Won't get completed until the next 50 plus degree day.


Matt Laudato
NAR #91867, L2

Had a warm day yesterday - rocket is finished and ready for flight. Hopefully will fly 3/22 at the first RIMRA launch in Kingston, R.I.

Flew it yesterday at the inaugural RIMRA launch on 3x A3-4T motors. Flew straight and true - and fast. Nominal flight and recovery, very happy with this little rocket.

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matthewdlaudato said:

Flew it yesterday at the inaugural RIMRA launch on 3x A3-4T motors. Flew straight and true - and fast. Nominal flight and recovery, very happy with this little rocket.

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Congrats on the first flight of the little unnamed bird. :clap:

I'm going to fly a 4 A-3-4 mini cluster at the next RIMRA launch Matt. 4 Estes BT-5 tubes fit nicely in a BT-56

Mini clusters rock! I made a mini-Ranger sort of downscale with a triple motor mount. Love that little bird!

rocketjet787 said:

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Congrats on the first flight of the little unnamed bird. :clap:

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Thanks!

BigRiJoe said:

I'm going to fly a 4 A-3-4 mini cluster at the next RIMRA launch Matt. 4 Estes BT-5 tubes fit nicely in a BT-56

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Cool. Did you fly anything yesterday?

Marc_G said:

Mini clusters rock! I made a mini-Ranger sort of downscale with a triple motor mount. Love that little bird!

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That sounds like a nice rocket!

I lost the booster section of this rocket today. Well, I know where it is. If there are any CATO members with a ladder, it is up on the roof of the Sheep, Swine, Rabbits and Poultry building at the Durham, CT fairgrounds, right next to the field...

Nooooo....

dave carver said:

Nooooo....

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Tell me about it! It's just sitting on that roof, taunting me..oh well, I will build another one and fly with a streamer next windy day...

A mini cluster rocket?? BRILLIANT! Gotta build me one......dont have a clue as to where to start.......! VERY nice job you did on your mini cluster missile!!

Feel free to copy the design - I'm already scavenging parts to build a new one, on the assumption that no one at CATO was able to pull the old one off the roof.

Just remember 4 mini motors fit perfectly in a BT-56 tube

I have actually have a rocket that fits 4 BT-5s into a BT-55. Look for "The Spike" in the August 1994 issue of Sport Rocketry. The Spike flies well on a pack of A3-4Ts.

Cool, maybe I will try that for a future build. Sticking with my 3x for now, already built a new motor mount and cut a body tube.

Now you guys are just tempting me to build one!

rocketjet787 said:

Now you guys are just tempting me to build one!

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Do it rocketjet! Mine jumped off the pad and was a straight and true flier before its early demise. Can't wait to have a new one ready.

Okay I will start planning it out.

Just love building and flying 2, 3 and 4 motor 13mm motor cluster models. I do however prefer not trying to enclose the motors in a huge Drag inducing airframe.
I mean it's fine for just sport flying but if you really want to get all you can out of 1/4A to 1/2A3 motors it's best to put them in external pods to reduce the frontal area as much as possible. Over the years I've also found just adding Nosecones to the pods wasn't the most efficient drag reduction either. I'm now using a 7° taper on the pod with flat cardstock closures.
Here are a few fun flying examples you guys are free to use if you'd like. Several of these have set NAR competition cluster alt records along the way. The last three pics are of 1/2A Cluster/Alt competition models intended to fly with Altimeters which did add a touch more complexity to the flights.
Have to admit the majority pictured are just plain a Blast To fly... Fun fun fun!

Micromeister said:

Just love building and flying 2, 3 and 4 motor 13mm motor cluster models.

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Those are some very nice birds John! I really like the pod configurations with tapers. Will have to consider something like that for an upcoming small bore build. Feel like I need to put on some insect repellent though, what with all those Mosquitos!

LOL! you can put the repellent away, the Mosquitos are used in most of my static photo's for Scale. Just about every Rocketeer knows the size of the Old Mosquito so they(the mosquitos) make for a quick and immediate sizing reference.

Cutting the external pod tapers can be a little tricky for some. Below is a series of Photos "without mosquitos" showing the process Once you've done one or two sets it becomes almost second nature

These particular photos for the most part are right out of the camera un-captioned or labeled. Let me point out the 1/4" balsa ejection pod plugs with Stainless tape facing, Aluminum Duct sealing tape will also work but does not last as long. These super lightweight sealing plugs are critical to keeping the flat tapered pod 7° closures in place, even using -0 booster motors the burn-through BP pressure is enough to blow the flat caps off if the seal off plugs are not used. These photos also show an experiment using .005" G10 for the pod covers instead of the usual 67lb cardstock or even Tracing vellum covers either of which sands and smoothes to shape after CA soaking a good bit easier. The method works on any size tube, I've used it on minimum diameter Micro-Maxx T2+(.281"OD) to BT-80 (2.6") size tubes.
Hope these help