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  #81  
Old 02-08-2019, 10:14 PM
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tbzep tbzep is offline
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Originally Posted by Les
I am open to suggestions or inputs from others who can help figure this out.

I have to use my brain at work. When I come home, I like to put my brain on a soft pillow to rest while I do mindless things like feeding the cows, watching TV, looking at pictures of rockets, etc.

BTW, I'm enjoying the thread.



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  #82  
Old 02-09-2019, 07:19 AM
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Originally Posted by tbzep
I have to use my brain at work. When I come home, I like to put my brain on a soft pillow to rest while I do mindless things like feeding the cows, watching TV, looking at pictures of rockets, etc.

BTW, I'm enjoying the thread.
And sanding.
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  #83  
Old 02-09-2019, 07:23 AM
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If you noticed in the pix, for a 1/8" dowel the 0.75" cut extends past the dowel. In this case the port size would have to be cut back to less than 0.5". In reality given slop in measurement or cutting probably closer to 0.4" max.
With a 1" port lengths we would include twelve 1/8 x 1.5" dowels. I will do a prototype. It would simplify gap filing and keep the full flow rate. The front and rear of the dowel could be tapered for appearance and aerodynamics.

I think 8-10 psi is fine because the only pressure relief is the payload section coming off. That said I have no objection to the higher psi either, since BT-11 are our strongest tubes and the same story applies. The only pressure relief is the payload section coming off.

Due to the very long ejection tubes and 2 right angle turns, even an oversize charge like 1.2g is very unlikely to expose the parachute compartment to flame or even very hot gasses.

Jerry

Last edited by Jerry Irvine : 02-09-2019 at 08:23 AM.
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  #84  
Old 02-09-2019, 08:03 AM
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I was going to suggest we build up a prototype. We could make a simplified motor pod to replace mine (since I built the kit to allow me to separate it). I played some games with my pix. I think the best bet is a 3/8" wide slot, but lengthen it from 1" to 2". I don't see a reason to limit the length to only 1"

If you recall from my previous post, I already used 1/8" dowels with the ends sanded to a taper to help strengthen the joint.
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  #85  
Old 02-09-2019, 08:29 AM
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Jerry Irvine Jerry Irvine is offline
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Quote:
Originally Posted by Les
I was going to suggest we build up a prototype. We could make a simplified motor pod to replace mine (since I built the kit to allow me to separate it). I played some games with my pix. I think the best bet is a 3/8" wide slot, but lengthen it from 1" to 2". I don't see a reason to limit the length to only 1"

If you recall from my previous post, I already used 1/8" dowels with the ends sanded to a taper to help strengthen the joint.
I will put a version of your suggestion into the instructions an an alternate build option.
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  #86  
Old 02-09-2019, 12:10 PM
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I "may" have a potential solution (unfortunately too late to incorporate into my rocket)

The big concern is the build up of the pressure in the motor pod vs the strength of cardboard.

Especially since a cylinder will fail on its length if there is a weakness, such as a cut port...

In addition, sharp corners create stress points - curves are better

Based on the 3 points above I would suggest we do the following:

1) Install a coupler in the motor pod between the Motor tube centering ring and the nose cone. Ideally, we may actually take a third layer (cut a section of coupler or body tube length wise to make a "coupler" for inside the coupler.

2) Make 2 ports per tube, each 0.5 x 1" long separated by 0.375". This strip in between the two ports should provide some strength. Dowels will be bonded (I show 1/4" diameter) along the pod to close the gaps and add strength.

3) Using a dremel or a dowel with sandpaper wrapped around it, round the ends of the ports.
This would only need to be done for the actual 2.25 motor tube. I am not really concerned with the 1.25 pod tubes as there is the big opening heading up to the passenger pod for the pressure to relieve itself. The two ports with rounded corners is shown at the bottom of the pix below.

Will this work? I'm not sure - I would feel better with 3 layers of cardboard. And even that solution is a definite "maybe".
Of course, one could fiberglass this section to add strength as well (Again - too late for my rocket as the motor and nosecone have already been glued in)

Fortunately, Jerry has offered to send me a replacement kit!!
I will try this idea on that one. But for now on this one I am going to abandon the ducted ejection.
Actually, since I have not glued the pod cones on yet I did try to go in and hack the ports bigger - not a lot of success trying to work with epoxy hardened cardboard from inside a 1.2" tube and having to work to 2.5 ~ 3" down. And since I don't have a way to add a coupler inside the motor pod to increase its strength I would still be concerned with it failing
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  #87  
Old 02-09-2019, 12:25 PM
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Quote:
Originally Posted by Les
2) Make 2 ports per tube, each 0.5 x 1" long separated by 0.375". This strip in between the two ports should provide some strength. Dowels will be bonded (I show 1/4" diameter) along the pod to close the gaps and add strength.

Six 0.5 x 1" ports is about the equivalent area of a 2" dia. tube. That should allow a lot of air to move.
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  #88  
Old 02-09-2019, 06:24 PM
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With rounded corners it would be a bit less.

Assuming the curved corners are a circle, total area would be 2.68 sq in. About a 1.85" diameter.

But maybe rather than looking at the amount of port area as part of the tube circumference, I need to look at it in terms of the equivalent diameter.

Still too late to mod my rocket - the ports are way smaller than the 0.5 x 1 (by 6 ports)
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  #89  
Old 02-09-2019, 07:18 PM
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Quote:
Originally Posted by Les
With rounded corners it would be a bit less.

Or a bit more according to how you go about doing it. If you start with the 0.5" x 1" square opening and then round the ends you get more area. With six holes, that's about 1.5^2" more area. That runs to about the equivalent of a 2.4" tube. How much tube length are you working with at the vents?

Quote:
Still too late to mod my rocket - the ports are way smaller than the 0.5 x 1 (by 6 ports)

I can't remember the size you used. Seems like it was single slots of 1/8" x 3" on the three tubes? That's the equivalent area of a single 1.2" dia. tube.



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Last edited by tbzep : 02-09-2019 at 07:33 PM.
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  #90  
Old 02-09-2019, 08:41 PM
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The total length is limited to about 2.5". With 2 ports 1" long plus the .375" strap, there is not a lot of growth area.

My ports are about 1/8" x 2.25, so with 2 ports presently cut I only have .563 sq in of port
or about a 0.423 diameter tube...

I could make the third port bigger but I don't think it will be enough

SO two choices - keep this one with electronic deployment and try my suggested mods for the next one

OR

Try motor ejection here and hope it doesn't blow up. If not, then we are good. If it blows, then I can try my ideas on the next one (which I am still not certain will work) or wimp out on that one (once burned twice shy....)
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