Cluster Duck Build

[mtnmanak]

While I have been sitting around (literally) watching the paint dry on my Sirius Refit USS Atlantis build, I grabbed the next rocket in my build pile - happened to be a North Coast Rocketry Cluster Duck.

This is a pretty straight forward build, not sure it will be of much interest to most people, but if you were thinking of building a mid-power rocket with a whole lot of motors, this is a pretty good rocket to introduce a big cluster. It has a core 24mm motor and 6 x 18mm outward motors for a big 7 motor cluster. The nice thing is that it is heavy enough that it won't fly off to the moon with all those motors. At the low end, with a D in the 24mm tube and A's in the outer tubes, the Duck only flies to less than 400 feet. Makes for a nice low flight on a big noisy cluster. Crank it out with an E in the center surrounded by a bunch of C's and she'll hit over 1500 feet.

Initial impressions are great. Really quality materials.

Decision up front - planning to make the body and nose cone purple. Heresy, I know. Black fins and yellow motor tubes. We'll see how it goes. I change my mind about color every 5 minutes or so. Other than that, not many design changes. Planning to stuff the engine blocks in further on the outward motor tubes and not sure if I will stick with the funky steel cable-to-dual kevlar/elastic shock cord recovery setup, but can't see any real changes other than those. Will also add 1010 rail buttons on one side. Only other change I am planning is sequence - I am going to paint all the individual parts prior to assembly, so that may influence the order in which I complete the build.

Parts in the bag are great. Plywood on the fins is very high quality, Very smooth right out of the bag and well cut - all of them are exactly the same size/shape/dimensions. Love the little locking/alignment tabs for the fins into the aft centering rings since this is not a through the BT wall fin installation. Tubes are really high quality. Pretty big seams on the main BT, but I think that makes it easier to fill them, actually. Often those tiny seams are the hardest ones to fill and get smooth. Both seams need to be filled on the BT.

 

[mtnmanak]

So far, just have one of the seams on the BT filled and I sealed the fins with finishing epoxy.

 

[mtnmanak]

Test fit the aft centering ring on the motor tube and it was very loose. Instructions tell you to apply tape to the tube to build it up to get a good friction fit. I prefer to build the centering ring up with some epoxy. Used some quick cure to add a small layer around each of the cogs on the inner wheel. After it cured, was able to sand down each cog so the ring fit perfectly on the motor tube.

 

[mtnmanak]

Filled the large seam in the nose cone and primed it, then painted all the 18mm motor tubes yellow.

 

[mtnmanak]

Glued the aft CR on, painted the tail end flat black (masked the fine lines with micro mask), then used my fin jig to get all 6 fins glued on with slow cure epoxy. Setting assembly aside to get some fillets this evening and curing until tomorrow.

 

[mtnmanak]

Primed the body tube. Need to get the launch lugs on and the holes for the rails buttons in, then some purple paint.

 

[tomsteve]

i finished one up a couple months ago. hopin to get r up in september.

 

[mtnmanak]

Got all the engine blocks in place and the painted outer motor tubes glued in. Got the forward centering ring installed. Filleted everything. Also got the nose cone painted. Main body tube will be same color, but just glued on the 1/4" launch lugs, so need to wait to paint it. I will install the rail buttons after I paint.

So far, this a really fun and easy build, recommended.

 

[MOF410]

The build looks great. I dont have one in my build pile yet but its on my wishlist..


If you get any video please post. Il check in on the progress

 

[afadeev]

Got all the engine blocks in place and the painted outer motor tubes glued in. Got the forward centering ring installed. Filleted everything. Also got the nose cone painted. Main body tube will be same color, but just glued on the 1/4" launch lugs, so need to wait to paint it. I will install the rail buttons after I paint.
So far, this a really fun and easy build, recommended.

What are you doing for motor retention?
Estes metal clips for 24mm and 18mm tubes, or something more positive?

I have one of these puppies in the build pile, and was thinking of adding a proper motor retainer onto the 24mm tube, and slotting the fins around it.
Might also need to move the aft CR forward 1/4", to make room for the fingers to screw the retainer cap on and off.

 

[mtnmanak]

What are you doing for motor retention?
Estes metal clips for 24mm and 18mm tubes, or something more positive?

I have one of these puppies in the build pile, and was thinking of adding a proper motor retainer onto the 24mm tube, and slotting the fins around it.
Might also need to move the aft CR forward 1/4", to make room for the fingers to screw the retainer cap on and off.

I decided to use the metal retainer hook that comes with the kit for the 24mm motor. I thought about it a lot before I committed and did a lot of dry fitting to try and come up with something that would work. The problem with the screw on retainer for the 24mm motor is that you will either have to push the 18mm tubes out (but that will lead to issues with the main BT which fits tightly over the cluster of 18mm tubes) or you will have to extend the 24mm tube lower than the 18mm tubes, which means those 6 motor will fire right on the retainer, which would probably burn through over time (assuming you are using the metal screw on retainers, not the plastic Estes versions).

As far as I understand the intent for the 18mm engines is that they will eject. There is a pretty lengthy thread about how to handle that and an ensuing debate about plugging the booster engines (i.e with epoxy) which, the consensus seems to be, would modify the engine and make it unsuitable for any NAR sanctioned flight. Or you can fill the end of the motor with dog barf and seal it with aluminum tape (or something similar), which seems to be an acceptable method for a NAR sanctioned flight. For a Tripoli event, you could use the plugged motors if the launch event allows research motors.

This is a very relevant conversation for how to handle the retention of the 18mm motors. If you do nothing, their ejection charges will blast against the lower centering ring (by design) and the engines are supposed to eject. If you plug them, there would be no ejection charge to worry about, so the engines will just stay in place. If you go the dog barf and tape method, there will be an ejection charge, but it will be greatly muted so the engines may or may not eject, but the ejection charge should not damage the motor tube.

At the end of the day, I decided to go mostly with the build the way the instructions direct, except I pushed the 18mm motors much farther in the tube. The instructions have you place the motor blocks in the 18mm tubes so about half the motor sticks out. This way they are more likely to eject and not cause over pressure in the tubes and burst them open. I plan to use the dog barf method first to see how it goes and then think about possibly plugging motors later.

 

[kuririn]

Aren't there vents for the booster motor gasses at the end of the motor tubes?
This allows the blowby gasses to re-direct into the valley between the tubes and go out the back of the main airframe. I downloaded the instructions and built a clone some years ago.
Here's a pic showing the vent holes in the instructions:


Then you can use friction fit for motor retention.
Maybe the newer kits don't have the vent holes?

 

[mtnmanak]

Aren't there vents for the booster motor gasses at the end of the motor tubes?
This allows the blowby gasses to re-direct into the valley between the tubes and go out the back of the main airframe. I downloaded the instructions and built a clone some years ago.
Here's a pic showing the vent holes in the instructions:
View attachment 430329

Then you can use friction fit for motor retention.
Maybe the newer kits don't have the vent holes?

If you look at the first pic I posted of the parts, you can see the 18mm tubes - no vent holes.

Too late now for me to add them now, so will have to mitigate using the dog barf method and friction fit. I can always tape them to the tubes for launch, there is still plenty of motor to grab on to at the end of the tube. On the Apogee site, TVM also seems to indicate that motor ejection is the expected result.

I plan to test this design out with the lowest possible motors (A8-0s and a D12-5) to get a very low altitude first flight and see what damage is wrought.

If I ever build it again, vent holes will be added.

 

[tomsteve]

you could still drill vent holes. i think originally this kit came with the holes already in the motor tubes.

 

[neil_w]

Yes, they could absolutely be drilled in now. Seems quite important, to be honest. I would practice the drilling technique on some scrap tubing first.

 

[Buzzard]

Put an used casing or dummy casing in the tube before you drill to prevent crushing the motor tube.

Chas

 

[beeblebrox]

What are you doing for motor retention?
Estes metal clips for 24mm and 18mm tubes, or something more positive?

I have one of these puppies in the build pile, and was thinking of adding a proper motor retainer onto the 24mm tube, and slotting the fins around it.
Might also need to move the aft CR forward 1/4", to make room for the fingers to screw the retainer cap on and off.

Here is the simplest way to do it. Notch three of the fin root 1" from the rear before assembly. (I screwed up and glued it together first...DOH!) So I only used one screw (I use plugged motors for all the 18mm) .Between the motor tubes, glue in with epoxy a 2" piece of 8/32 stainless steel all-thread. Use a small washer and a nut to hold the motors in place. For a seven cluster you need only three at every other space between the mount tubes. You do not need a thrust ring in the center tube, this allows using different length motors in the center tube. D12, E9, E12, various composite motors available. Space the thrust rings in the 18mm mounts so that the motor sticks out 1/2" If using an Aerotech molded case 24mm motor (E or F) I put a 1/4" long piece of motor mount tube on the motor to even it out with the 18mm motors. (The reason: I like adaptability, not all motors have an integral thrust ring, so I use 1/2" masking tape around the center motor for a thrust ring.)

 

[mtnmanak]

Thanks all for the vent hole suggestions. Good points and I will drill the vent holes, although, now that I am really looking at it, I think I will just cut out notches at the top of the 18mm tubes instead of drilling. Easy to do with a hobby knife and easier to get inside the angle to direct the gases along the inner channel along the fin. Probably will notch both sides of the tube and reinforce the notches with a little epoxy.

@beeblebrox - great idea with the threaded steel rod for the retainer. Can still probably achieve that by drilling a hole and epoxying it in. Will give it a try.

 

[neil_w]

I probably shouldn't be chattering too much about a kit I haven't built, but...

I think I will just cut out notches at the top of the 18mm tubes instead of drilling. Easy to do with a hobby knife and easier to get inside the angle to direct the gases along the inner channel along the fin.

This will look better but will direct the exhaust gases onto the fins, if I understand correctly. That is likely to cause a lot of dark staining on the fins. Just something to consider.

 

[mtnmanak]

I probably shouldn't be chattering too much about a kit I haven't built, but...

This will look better but will direct the exhaust gases onto the fins, if I understand correctly. That is likely to cause a lot of dark staining on the fins. Just something to consider.

There is not a whole lot of room. The body tube comes down over the motor tubes quite a ways and covers the end of the engine blocks, so the vents need to be above the bottom of the BT. So, the only place to vent is on to the fins. It is why I painted all the fins black, I already figured they were going to have to deal with a lot of exhaust gases.

Couple pics to illustrate what I mean. The blue tape marks the top of the engine block inside the motor tube. The vent hole needs to be above that. As you can see, when the body tube is installed, it comes down to the tape line, so any vent hole that takes advantage of the space between the fins will have to vent the gases onto the fins.

Open to suggestions if I am missing something.

 

[neil_w]

Ah, I see, a consequence (I guess) of having moved the engine block further up than originally intended. Seems like it should work, and the black fins do mostly solve the staining problem. Making those cuts should be fun.

 

[mtnmanak]

I have a number of Jerry Ivine's (US Rockets) kits buried in the build pile (aka build mountain) somewhere, including a Swarm and Swarm Junior. Need to dig those out and see how he tackles these large clusters.

I readily admit to being a cluster novice. Been building rockets for 40 years and the biggest cluster I have built to date was a Semroc 3-motor Defender. So, learning a lot of lessons here and appreciate all the tips from people that have built bigger clusters. Once I have this one under my belt, will probably try some of Jerry's kits and then branch out on my own to modify some other designs. I have at least 15 Saturn V kits laying around, want to explore multi-stage clustering with one of them.

Also looking to build a cluster box, probably LiPo as per Boris & Paul's box: https://bpasa.com/Cluster-box.htm

They also have a lot of great info on clustering in general, so been looking over their suggestions.

I felt this Cluster Duck was a great way to ease into larger clusters since it is a relatively low flier.

 

[Dugway]

I love my Cluster Duck so much that I built an upscale with a central 29mm surrounded by 6 x 24mm motors. If my calculations are correct, that will give me the equivalent of an H motor using nothing but Estes motors (6 x Es, 1 x F). I hope to launch it for the first time later this month!

 

[mtnmanak]

I love my Cluster Duck so much that I built an upscale with a central 29mm surrounded by 6 x 24mm motors. If my calculations are correct, that will give me the equivalent of an H motor using nothing but Estes motors (6 x Es, 1 x F). I hope to launch it for the first time later this month!

@Dugway - Thanks so much for your helpful Cluster Duck thread - it is where I started. Definitely wish I had your location for launching! Utah is awesome for big rockets and big clusters. Hope to get out to LDRS next year. Looking forward to seeing some video of your scratch built cluster.

Also brings back some fond memories of tromping around Dugway Proving Grounds doing live fires as a very young Soldier about a hundred years ago

 

[mtnmanak]

I did get the vent holes added to the 18mm tubes (and, of course, forgot to take a picture ) and glued the body tube on. Just need to finish the recovery system and get the decals on and I am ready to launch. Hoping to get this in the air sometime this month for an initial test.

Also started looking at the cluster box Boris Katan put together and realized my knowledge of electronics - particularly the relays and how they work, what kind of relays I actually need and how to hook them up, and the crazy amount of battery tech info out there on LiPo's - the amount of info is a bit overwhelming and difficult to know what to choose. For the battery, I can readily choose the same battery Boris used, but for the relay, he used a solenoid from a 1990 Ford LTD Crown Vic. When I googled that part, I was surprised at how many variations came up (the exact one he used from partsgeek does not appear to be available anymore). I clearly have some learning to do about relays before I attempt to put a box like that together.

 

[mtnmanak]

Instructions call for a 36" parachute. Seems like a pretty big parachute for a rocket this size.

Weighed the final config with a D12 and 6 x C6's, came to 653.7 g (23.05 oz). So taking away the propellant weight, the weight after deployment would be about 590 g (20.8 oz).

Not a huge deal since this is a low flier. Will try the initial flight with a 36" dino chute and see how it goes. Can always change chutes out between flights based on motors used and weather.

 

[Dugway]

Boris wrote his Cluster Box instructions quite a few years ago. I've gone back and found what I used to build mine, in case anyone is interested.

I found my Amazon order from Spring 2018 that looks like it was the Crown Vic solenoid and the 12v relay that I used:

Standard Motor Products SS598 Solenoid
E Support Car Relay 12v 40a Spst 4pin Socket Pack of 5

I really think any 1990 Crown Vic solenoid would work (probably along with many others). As Boris explains, the key is that the solenoid have a clamping diode so you don't damage the launch controller. I bought a five pack of relays on the assumption that they wouldn't last long with all that current, but I'm still on my first one.

The battery I use is from Hobby King, it looks like it has been discontinued, but I'm sure you can find something comparable:

Turnigy Bolt V2 1300mAh 4S 65~130C High Voltage Lipo Pack

The only other major components are a 300 Amp two position rotary switch to turn the whole thing on and off, a digital volt meter I had kicking around to monitor the battery voltage, and some very heavy gauge cables (2 gauge maybe?) to get from the LiPo battery to your rocket. The circuit from the launch controller to the 12v relay can be MUCH lighter weight. There are plenty of online calculators to help you figure out wire diameters for voltage/current/length.

I put the light weight clips that go to the launch controller on banana plugs to make it more easy to store/replace them and mounted everything in a cigar box:

 

[mtnmanak]

Boris wrote his Cluster Box instructions quite a few years ago. I've gone back and found what I used to build mine, in case anyone is interested.

I found my Amazon order from Spring 2018 that looks like it was the Crown Vic solenoid and the 12v relay that I used:

Standard Motor Products SS598 Solenoid
E Support Car Relay 12v 40a Spst 4pin Socket Pack of 5

I really think any 1990 Crown Vic solenoid would work (probably along with many others). As Boris explains, the key is that the solenoid have a clamping diode so you don't damage the launch controller. I bought a five pack of relays on the assumption that they wouldn't last long with all that current, but I'm still on my first one.

The battery I use is from Hobby King, it looks like it has been discontinued, but I'm sure you can find something comparable:

Turnigy Bolt V2 1300mAh 4S 65~130C High Voltage Lipo Pack

The only other major components are a 300 Amp two position rotary switch to turn the whole thing on and off, a digital volt meter I had kicking around to monitor the battery voltage, and some very heavy gauge cables (2 gauge maybe?) to get from the LiPo battery to your rocket. The circuit from the launch controller to the 12v relay can be MUCH lighter weight. There are plenty of online calculators to help you figure out wire diameters for voltage/current/length.

I put the light weight clips that go to the launch controller on banana plugs to make it more easy to store/replace them and mounted everything in a cigar box:

View attachment 430610

Thanks for the info, very much appreciated! Going to order some parts and start building when they get here next week

 

[mtnmanak]

@Dugway - got some parts ordered. Newb question here. How do you charge that LiPo battery pack?

 

[Dugway]

I have an old Hobby King balance charger that is no longer available, something like this would do the trick, but is probably overkill if this is the only LiPo battery you've got:

Turnigy Reaktor 250W 10A 1-6S Balance Charger

The important thing is to get a balance charger to keep individual cells evenly charged.

I have a bunch of different batteries that I use to power my altimeters and trackers, as well as various robotics projects, so it's nice to have a single charger for everything. The biggest issue starting out with the Hobby King chargers is that they need 12v inputs but don't come with a power brick. I have a box full of them, so I just soldered up an adapter to go from a barrel plug to the two banana connectors used by the charger. Kind of a hassle, but I was making adapters anyway to go from the XT60 plug that comes with the charger to all the different plugs I've got on my battery packs.