Fin can length - using variable lengths for different motor cases

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ckamila

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I am looking into making a fin can for a 75 MD project and want the flexibility to mix and match different cases, probably a 4 and 6 grain case. Fin can would have a minimum length to support the smallest case using the motor as coupler, so for 4 grain case at 24" the fin can would measure about 20". If i want to use a 6 or 6XL case, is it better to use a removable body tube section equal to the added length (10.6" for the 6 grain case), or extend the upper body tube section to cover the added length?

Here is a shot of my 54mm MD setup: Images not uploading.....will attempt later.



thanks,

chris

20150508_102307 edit.jpg
 
Last edited:
OK. Image uploaded.....now back to our regularly scheduled questions....
 
if you're using the motor as "coupler" be very careful (as in don't use masking tape) using masking tape to snug up the fit. The motor tube gets hot (masking tape gets really stick when it gets hot) and the motors grow a few thousand in diameter when they get hot. It a fine line between to loose and to tight a fit.

Tony
 
tfish said:
if you're using the motor as "coupler" be very careful (as in don't use masking tape) using masking tape to snug up the fit. The motor tube gets hot (masking tape gets really stick when it gets hot) and the motors grow a few thousand in diameter when they get hot. It a fine line between to loose and to tight a fit.

Tony
Click to expand...

Exactly. My experience doing this is that the fit when cold has to be uncomfortably loose so that it is not too tight when hot. Can't recommend doing this.

Stu Barrett designs his rockets to allow different motor lengths. It's a bit hard to explain, but imagine a conventional fin can with a zipperless coupler on top (and as a key detail, the coupler does not serve as a forward push point for the motor). Then, imagine that you cut the fin can in half at some point above the fins. Then, you could pull everything together with a piece of all thread from the forward closure of the motor to the top of the zipperless coupler. That is, the motor thrust rings pulls up and the zipperless coupler pulls down to squeeze together the airframe at the point where it was cut. Clear as mud? For different motor lengths, you use a zipperless coupler section with a longer/shorter airframe section, or just insert a piece of airframe between the original pieces. I can't say this is something that I would do, but it works fine for him.

Jim
 
tfish said:
if you're using the motor as "coupler" be very careful (as in don't use masking tape) using masking tape to snug up the fit. The motor tube gets hot (masking tape gets really stick when it gets hot) and the motors grow a few thousand in diameter when they get hot. It a fine line between to loose and to tight a fit.

Tony
Click to expand...

I have watched your videos and marvel at your tight tolerances between body tube and coupler when hand rolling your own fiberglass. So you suggest leaving it loose and just shear pin the coupler to motor or? Maybe i need to re-watch your videos.
 
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JimJarvis50 said:
Exactly. My experience doing this is that the fit when cold has to be uncomfortably loose so that it is not too tight when hot. Can't recommend doing this.

Stu Barrett designs his rockets to allow different motor lengths. It's a bit hard to explain, but imagine a conventional fin can with a zipperless coupler on top (and as a key detail, the coupler does not serve as a forward push point for the motor). Then, imagine that you cut the fin can in half at some point above the fins. Then, you could pull everything together with a piece of all thread from the forward closure of the motor to the top of the zipperless coupler. That is, the motor thrust rings pulls up and the zipperless coupler pulls down to squeeze together the airframe at the point where it was cut. Clear as mud? For different motor lengths, you use a zipperless coupler section with a longer/shorter airframe section, or just insert a piece of airframe between the original pieces. I can't say this is something that I would do, but it works fine for him.

Jim
Click to expand...

OK. Point taken. Obviously i respect both of your opinions so will take a closer look at your suggestions and take an alternative route.

thanks for your input,

chris
 
Jim and tfish - just a quick note. The tape is use just to keep the body tube in line with motor casing, not to keep sections together. I have shear pins (2) that are tapped into BT and into black doughnut around forward closure. I can see your point where the tape becomes a problem and prevents the sections from separating.

thanks again,

chris
 
ckamila said:
Jim and tfish - just a quick note. The tape is use just to keep the body tube in line with motor casing, not to keep sections together. I have shear pins (2) that are tapped into BT and into black doughnut around forward closure. I can see your point where the tape becomes a problem and prevents the sections from separating.

thanks again,

chris
Click to expand...

The attached is my version of your rocket. The amber tape is some heat resistant tape, and in my design, I put some epoxy on top of the motor and shear pin into that.

On the first flight, I kept the tape joint very loose. The rocket flew fine though (set the K record actually). On the second flight, or maybe the third - I don't remember - I decided to tighten up that joint just a tiny bit. Still loose though. No apogee event and never found the rocket. Can't prove it was the joint, but that's what I think happened.

Jim

KesTreLite.jpg

DSC03067.jpg
 
Anybody thought of using metal foil tape to shim the motor portion or even plain foil (like sections of a pop can, small strips would just flutter down after ejection event). Just a thought as I have yet to do a HPR MD rocket.
 
Unless you are shooting at records I would just design it around the longest motor, and then fly the shorter motors within it.
 
rharshberger said:
Anybody thought of using metal foil tape to shim the motor portion or even plain foil (like sections of a pop can, small strips would just flutter down after ejection event). Just a thought as I have yet to do a HPR MD rocket.
Click to expand...

I am in planning stages of a 38 mm- the second one of mine. The first achieved warp and was not seen again. The second one I am planning on using a wrap of aluminum tape as motor retention.
 
Chris, One thing that I found that worked, to take up the slop was in the body tube was "super glue" smeared inside the body tube. It may take several applications to get it to a 'good' fit. It seems to take the heat well. You still have the 'swelling' motor case to deal with. The best I can describe as a "good" fit is...falls apart fairly easily when the rocket and motor are together. I see you have a way to use shear pins which is a very good thing.

Reading up through the thread...a couple of times I used plain old binder paper wrapped around the motor tube. Not all motor tubes are the same diameter. Nor are they the same diameter at different times of the day..temperature.

54mm CTI motors seems to be slightly larger at the flared end.

Tony
 
Last edited:
markkoelsch said:
Unless you are shooting at records I would just design it around the longest motor, and then fly the shorter motors within it.
Click to expand...

Sounds like a plan. Especially if i am going with 4, 5 or 6 grain motors. Design for 6 and use motor gain spacers to reduce.
 

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