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JimJarvis50

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Thank you Jim! That is a very informative video.
Thanks Steveh.jae. To the best of my knowledge, no one has built a spin can yet. I hope someone gives it a try.

I really enjoyed the Infinity Squared project. So, I've decided to fly the rocket again at AirFest. It'll be a different flight program requiring different skills, but it should be a fun flight to watch. I'm looking forward to it.

Jim
 

JimJarvis50

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Is a spincan stable on its own or does it still need static fins? I've debated trying to print a fincan for a small rocket (29mm mount).
It's perfectly stable on its own - no static fins needed. The primary value of it is to prevent "control reversal" if using movable canards ahead of the fins. If you don't have canards, there won't be much value for a spin can (other than the fun of trying to build one).

The spin can requies three tubes to work. They can be a coupler tube for the air frame, and airframe tube for the bearings, and then an oversized tube that the fins are attached to. If you could print the oversized tube and fins, it might not be a difficult project at all.

Jim
 

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It's perfectly stable on its own - no static fins needed. The primary value of it is to prevent "control reversal" if using movable canards ahead of the fins. If you don't have canards, there won't be much value for a spin can (other than the fun of trying to build one).

The spin can requies three tubes to work. They can be a coupler tube for the air frame, and airframe tube for the bearings, and then an oversized tube that the fins are attached to. If you could print the oversized tube and fins, it might not be a difficult project at all.

Jim
In that case, I'll give it a go.

Will probably be made for 18mm motors, but I'm gonna challenge myself to get it done before classes start (Aug 23).
 

JimJarvis50

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In that case, I'll give it a go.

Will probably be made for 18mm motors, but I'm gonna challenge myself to get it done before classes start (Aug 23).
You'll need ball bearings that are slightly smaller in diameter than your middle air frame tube. Looks like you can get some pretty small bearings. I predict that you, like me, will discover that ball bearings below a certain size act more like a vapor than a solid. Trust me on this, and buy a few extras!

Jim
 

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Jim,

Since the rocket was MD, forward retention as part of the spincan makes perfect sense. If you were not going with a MD setup, can you see a method of mixing your spincan with "normal" motor retention? I've been trying to see a way of doing it and everything I come up with needs the forward retention to keep the spincan sections in slight compression to allow the ball bearings to work.
 

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You'll need ball bearings that are slightly smaller in diameter than your middle air frame tube. Looks like you can get some pretty small bearings. I predict that you, like me, will discover that ball bearings below a certain size act more like a vapor than a solid. Trust me on this, and buy a few extras!

Jim
Bought an assortment from 1mm to 8mm. 18mm isn't going to work with those low power motors. Gonna upgrade to 29mm so I can use my ex motors.
 

JimJarvis50

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Jim,

Since the rocket was MD, forward retention as part of the spincan makes perfect sense. If you were not going with a MD setup, can you see a method of mixing your spincan with "normal" motor retention? I've been trying to see a way of doing it and everything I come up with needs the forward retention to keep the spincan sections in slight compression to allow the ball bearings to work.
The spin can in the video is built around a minimum diameter motor case. This serves as the inner of the three tubes in the system (with the other two tubes being an air frame tube to contain the bearings above and below, and an oversize tube with the fins. I have a second spin can where the air frame is a coupler tube instead of a MD motor tube. The other two tubes would remain the same (an air frame tube and an oversize tube with the fins). This is how the "Test Rocket" is configured. It's a 4" coupler tube for the air frame and the rocket can fly on 3" motors but not 4" motors.

At the bottom of the rocket, there is the fitting shown in the first picture. This goes at the bottom of the rocket and is screwed to the coupler air frame. The left side of the pic is the lower bearing surface. It's the air frame "middle" tube. This particular fitting also has a larger transition on the right hand side. It actually isn't needed structurally, but it provides a transition from the fin can to the end of the rocket and it reduces the access for dust and dirt to get into the bearings. There is a very slight gap between the bottom of the larger fin tube and this transition when everything is installed. There are a couple of nuts epoxied inside the coupler air frame that hold this fitting in place. There is just enough clearance between those nuts to insert a motor tube and 75mm motor.

In theory, this fitting could be constructed with a plate on the bottom (right hand side) to also include a motor retainer and a motor tube with centering rings. The assembly would just slide onto the bottom of the coupler air frame tube. It would make sense in that configuration to hold the fitting in place with more than just a couple of T nuts since it would have to hold the weight of the motor tube, but forward retention would not be required.

An example of this concept is the fitting I use to fly the Test Rocket in the two-stage configuration. A pic of the rocket with this fitting is included, and there is a blow-up of the components from an earlier post. The "interstage coupler" in the picture is the key component. It is an air frame tube that provides the lower bearing surface (going up the piece) and also provides the interstage couple tube into the booster air frame. The piece has epoxy keys on the outside of the tube (visible as the black ring) that mate with the top of the booster air frame and keys on the inside of the tube (not visible) that mate with the bottom of the coupler air frame. These keys keep the booster from rotating against the sustainer in flight. Inside the interstage coupler is ring that the motor tube piece sits on (these two pieces are separate pieces but are joined together). The motor tube centering rings are constructed so that they can slip past the T nuts for the first fitting. This fitting can't be screwed onto the air frame because that would prevent the fitting from sliding into the booster. So, this design does need to be pulled together from above. The design of this, such that the bearings can be poured into the gap and then everything assembled after that was quite a challenge. However, this is what lets me fly the Test Rocket as a two-stager.

Jim


IMG_2477.JPGJim Jarvis Rocket.jpg1838 text.jpg
 

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After some hours in CAD, I've got a design that I'm going to print little pieces of to make sure they work. It uses three sets of bearings, one using 3mm balls that pushes on the bottom of the fincan (basically a thrust bearing, and two for spin that use 2mm balls. I'm thinking of in the future canting the fins and seeing what that does.
Untitled.png
 

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After some hours in CAD, I've got a design that I'm going to print little pieces of to make sure they work. It uses three sets of bearings, one using 3mm balls that pushes on the bottom of the fincan (basically a thrust bearing, and two for spin that use 2mm balls. I'm thinking of in the future canting the fins and seeing what that does.
My system just has one set of ball bearings on the bottom. I had considered another set at the top of the fin can, but it hasn't been necessary. On the lower surface, I have basically a continuous circle of bearings. I have adjusted the movement of the fin can, up and down, so that the bearings have to stay in a mono layer. One thing to watch out for with multiple bearing locations is contructability. I have found it to be a challenge just to get one layer of bearings installed. If you are going to try multiple layers, you need a plan for how things will be assembled. Specifically, can you assemble everything without dumping bearings on the floor. Bearings have a mind of their own.

Jim
 

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My system just has one set of ball bearings on the bottom. I had considered another set at the top of the fin can, but it hasn't been necessary. On the lower surface, I have basically a continuous circle of bearings. I have adjusted the movement of the fin can, up and down, so that the bearings have to stay in a mono layer. One thing to watch out for with multiple bearing locations is contructability. I have found it to be a challenge just to get one layer of bearings installed. If you are going to try multiple layers, you need a plan for how things will be assembled. Specifically, can you assemble everything without dumping bearings on the floor. Bearings have a mind of their own.

Jim
The purple pieces are just like the plastic pieces in ball bearings. They trap the bearings so that I can hopefully slid the fincan from the front. The fincan has very slight ridges where it should sit on the balls and I hope it works.

Do you worry about air getting inbetween the fincan and the body tube? I'm wondering it I should add some kind of vent holes at the bottom to allow air to flow through and not push everything apart.
 

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Do you worry about air getting inbetween the fincan and the body tube? I'm wondering it I should add some kind of vent holes at the bottom to allow air to flow through and not push everything apart.
My spin can is trapped both above and below. It can only move up and down about 1mm. This is to always keep the bearings in a mono layer. But, air can enter in the gaps at both the top and the bottom, so no vent holes are needed (and it would not be possible to push things apart anyway).

Jim
 

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My spin can is trapped both above and below. It can only move up and down about 1mm. This is to always keep the bearings in a mono layer. But, air can enter in the gaps at both the top and the bottom, so no vent holes are needed (and it would not be possible to push things apart anyway).

Jim
That makes sense. My design is almost completely sealed on the bottom so I may pop a couple holes just in case. Nothing really traps the can from moving upwards. My hope is that the balls lock into the ridge and capture the can so it can't move upwards. If not I will probably print a simple ring to go above the can and glue or screw it on.
 

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That makes sense. My design is almost completely sealed on the bottom so I may pop a couple holes just in case. Nothing really traps the can from moving upwards. My hope is that the balls lock into the ridge and capture the can so it can't move upwards. If not I will probably print a simple ring to go above the can and glue or screw it on.
If you look back at the third pic in Post 788, you can see the upper stop on the air frame. The other spin can has a section that slides over the top of the motor to provide the stop. In flight, the stops might not be needed, even under deceleration, but I would recommend containing the bearings. If you drop 10 grams of bearings on the floor, you get back 5 grams.

Jim
 

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If you look back at the third pic in Post 788, you can see the upper stop on the air frame. The other spin can has a section that slides over the top of the motor to provide the stop. In flight, the stops might not be needed, even under deceleration, but I would recommend containing the bearings. If you drop 10 grams of bearings on the floor, you get back 5 grams.

Jim
I've only lost one of the 2mm bearings somehow. They are just so dang tiny.

I've been looking back through but I must have missed that post (tbf this is a long thread). Honestly I'm not sure how well this is gonna work out. I don't think mine will be as nice as yours, but if I can get it to at least work somewhat then I will post the files.
 
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