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Alignment problem when assembling a 3-engine cluster mount

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Dane Ronnow

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When test fitting tubes in the centering rings prior to cementing, there is a significant amount of roll in the tubes when I twist the centering rings in opposite directions. I can eyeball the alignment between tubes, but it's not precise. I can get things reasonably lined up by placing the assembly on a flat surface with the tubes vertical, then placing a small sheet of balsa on top of the tubes just to square the ends, but still it doesn't seem precise. Is there a trick to this? I don't want to end up with spin in the rocket in flight.
 

kuririn

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Sounds like each tube has its own hole in the centering rings?
And the tubes do not touch each other?
if so line the tubes up vertically on a flat surface.
Position and glue one centering ring keeping the ends of the tube square to the surface, and let dry.
An end weight on the other end might help. And a room with no wind.
Glue the second ring on. The glued first ring will keep the tubes square and prevent torsion..
Of course, if the tubes touch each other then just glue two tubes together on a flat surface and let dry.
Glue the third tube on, then the centering rings. Tubes will not twist.
You could consider an angle tool marking straight lines and a laser level, but this simpler approach works for me.
 
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Dane Ronnow

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That's what I was thinking—tubes vertical on flat surface to keep the ends square, with a 3x3 piece of 1/8" balsa on top, also to keep the ends square. Thanks for the input!
 

heada

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I have a set of machinist squares that range in size from 2 inch up to 6 inch


You can use something like a small square to make sure each tube is perpendicular to the centering ring when you glue it. Just added insurance to help the tubes and rings line up.
 

kuririn

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Actually, a three engine cluster with a big twist would be kinda cool.
Hmmm.
 

Greg Furtman

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If I ever do this I would use a large enough V-shaped device, like angle iron or a piece of wood with a V sawn into it to lay the whole assemble on its side when gluing. I would also use a small square to make sure the centering rings are perpendicular to the motor tubes.
 

kuririn

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I was thinking of canted motor tubes and fins canted by the same degree.
Might give a straighter flight than Twisted.
And an interesting exhaust trail.
 

MadRocketer

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I think I would set up a jig in the rocket’s cradle with some guide-lines for the centering rings. Clamp in place and insert the motor tubes.
 

Dane Ronnow

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I think this is one of those instances when a jig is just going to complicate things. Assuming the tube ends are square with the sides, placing them vertically on a flat surface with the centering rings in place squares everything. The flat surface is the jig.

My design uses 4.25" motor tubes, with the forward CR .125" from the front of the tubes. The aft CR is 3.5" behind it (length of the TTW fin tab), .375" from the back end of the tubes.

With rings on the tubes, I place the tubes vertically on my work surface (parchment paper taped flat on a cutting mat, on the table), with the front end of the tubes down, forward CR flat against the paper.

Next, I apply a fillet of Elmer's wood glue around each tube where they meet the forward CR (still flat on the work surface.) Then I carefully lift the assembly and place three small scraps of .125" balsa under the CR, positioning them so the tubes will drop between them. Then I place a 2"x2" piece of flat balsa on the tops of the tubes and gently push down until they are flat against the work surface, which brings the aft CR up .125" (the height of the scraps), smearing the glue nicely between CR and tubes.

Now the flat-surface 'jig' comes into play. With one hand holding the now-glued CR flat against the balsa scraps, and the other hand applying downward pressure with the flat 2x2 on top of the tubes, I rock the tubes ever-so-slightly until I feel the tubes bottom out squarely on the work surface.

Then I place a smallish weight (D-cell battery) on the 2x2 square to keep everything steady, and let the glue dry. The table keeps the tubes square with the bottom CR (which is actually the forward CR), and the top CR (aft), which has been loosely placed a quarter-inch or so from the top of the tubes during this process, keeps them properly spaced from each other.

Two or three hours later, I can glue the aft CR in place, knowing that everything is square with the world.

Having said all that, I realize the only thing more complicated than a jig can sometimes be, is my explanation of how I will do this without using a jig.

Anyway . . . yeah, it works in practice. Now to glue it up.
 

kuririn

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Only thing that could muck that up is if the ends of the tubes are not cut quite square.
And I've seen that happen in kitted parts.
Just double check, then go for it.
 

Dane Ronnow

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Only thing that could muck that up is if the ends of the tubes are not cut quite square.
I know. Have you ever wondered if Murphy's Law has it in for you? I spend almost as much time figuring out what can possibly go wrong, as I do actually building rockets. And this is the first thing that popped into my mind once I figured out how to do this. So I'll be using a square to make that determination. But thanks for pointing that out. (Great minds . . . no matter how much time they spend pulling their hair out . . . think alike.)
 

MadRocketer

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Having said all that, I realize the only thing more complicated than a jig can sometimes be, is my explanation of how I will do this without using a jig.
Maybe “jig” would be giving it too much credit. It’s there to hold it still while I work. Scribe reference marks to make sure it's all lined up. Walk away and come back later to find it just how it is supposed to be!
Dane Ronnow, I do like how you think it all through!
 

Sandy H.

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Sorry for the lame sketch, but I don't happen to have a cluster motor mount in the garage to take a real picture.

Assumption: The Centering rings are pretty much identical (i.e. laser cut or made with a jig, not cut by hand. Not sure how it would turn out with poorly cut centering rings. . . ).

Get a block of wood cut to fit between the centering rings. Put it on a table and put 2 of the 3 motor tubes on it. Feel free to mark the centering ring locations on the tubes if you want it to look perfect, but eyeball alignment works fine for me.

I prefer to do this with spent motor casings in the tubes, but you might not need to. The goal is to make sure that the tubes don't get crushed when you put weight on the assembly. Run glue around the tubes at the centering ring locations on both tubes, slide the centering rings on and then put a weight (maybe another block of wood or anything flat you have that is somewhat heavy.)

Let it sit until set. If doing a big rocket with epoxy, 5-10 min - if an LPR with Elmers, maybe an hour or two. Once the 2 tubes are glued, install the third tube, fillet as desired and you're good to go.

This is the method I've used for many clusters. Get the first 2 tubes right and the other 1, 2, 3, 4 or 7 work fine due to the CR's being well made.

Sandy.
 

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Dane Ronnow

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This is the method I've used for many clusters.
I assembled the cluster mount yesterday using the method I described above, and I've got to say if I were doing it again, I'd use your method. It's better for keeping the tubes absolutely parallel. Looking at mine from the side and rolling it slowly so you can see the top of one tube 'rise' in front of the top of a tube behind it, I see it is about 1/32" off parallel. It will impart a slow spin, which is not the end of the world, of course, but not the way I wanted it to go.

Now I'm trying to decide if I want to order new parts and redo it using your idea, or just live with it.
 

Sandy H.

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What diameter are the CR's and what diameter are the motors? Do you plan to fly a video camera on the rocket at some point?

Sandy.
 

Sandy H.

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I thought I might have some spare cluster mounts I could send you, but apparently I already gave them away. . .

I usually try to be double careful if I plan to fly a camera, as it is kind of lame when the video gives you vertigo (I have plenty that would. . . ) but if it is just a fun flier, a bit of rotation doesn't bother me much.

Sandy.
 

krusty

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When test fitting tubes in the centering rings prior to cementing, there is a significant amount of roll in the tubes when I twist the centering rings in opposite directions. I can eyeball the alignment between tubes, but it's not precise. I can get things reasonably lined up by placing the assembly on a flat surface with the tubes vertical, then placing a small sheet of balsa on top of the tubes just to square the ends, but still it doesn't seem precise. Is there a trick to this? I don't want to end up with spin in the rocket in flight.
If you line up your two centering rings and then cut a small notch in each that show the alignment - once you've spaced them out and put the engine tubes in, use a piece of angle aluminium (or door jam) and get the two notches lined up on the straight edge.

Happy to draw something up if that's not clear?!

Krusty
 

Dane Ronnow

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If you line up your two centering rings and then cut a small notch in each that show the alignment - once you've spaced them out and put the engine tubes in, use a piece of angle aluminium (or door jam) and get the two notches lined up on the straight edge.

Happy to draw something up if that's not clear?!
It's perfectly clear. And it makes perfect sense. If the rings are lined up, then so too will be the tubes.
 
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