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My L3 Project
#1
I am building a 3x Upscale version of an Estes Der V-3...a kit that flew on D motors back in the early 80's. I am going to bring the fin can in for the next winter meeting. If anyone has a 75mm casing, please bring it along with the aft closure so I can see if that fits.

Here are a couple images of the fincan. Nat set up an interlocking fin and ring set. This was glued to the motor tube using West Systems G-Flex epoxy. Instead of using fillets, I reinforced the fins at the root, using fiberglass strips instead of fillets. I have had very good success with this approach.

The 1x1 rods between the CRs give me a mount for my rail buttons. There are two of them to also help distribute the thrust, and also better anchor the shock cord mount.

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Evan Brown
NAR# 92851
Level 2
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#2
Couple more photos taken after the Feb meetings. I added threaded rods to better secure the forward CR. Also added epoxy to the top side of the forward CR. Added T-nuts to the back side of the wooden struts. Did this by using JB Weld to hold them in place and then tightened them into place by using an eyebolt and a screwdriver. I do not want them coming out. I cut the airframe into a 10" forward section and a 38" rear section. The coupler tube is 12" long, but the e-bay will only take up about 4.5" of that length. Switches are in place. I need to work on the nose cone and add weight and also work on the AV bay. I'm pretty close to being done. The lower airframe is installed.

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Evan Brown
NAR# 92851
Level 2
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#3
Make sure you put epoxy on the U bolts to ensure they don't loosen.  Looks good.
Jim Goggins NAR  L3
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#4
Nice build Evan!  8)
I can't imagine you need the threaded rod between the forward and second MM centering ring, but one thing for sure, you are building this like a brick you know what!  Wink
Got to get going on mine..
Greg
Greg Young - L3
NAR #42065
TRA #00234
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#5
At an estimated weight of 35lbs, actually Evan's project is surprisingly light weight, especially for an M powered flight.

And I think the threaded rods are a necessity. If you look at the forward centering ring, which is the sole anchoring point for the shock chord, there is literally nothing holding it back from being pulled out. Which MAY not be needed under ideal apogee deployment especially given the relative light weight. But considering that low weight and an "M" power plant can bring excessive speeds which if deployment occurs while still under power (pre or post apogee), starts to multiply the energy applied to those anchor points.
The threaded rod ties back to the greatest structural assembly of the rocket, transferring the deployment energy all the way back aft  bulk plate/centering ring. It is a cheap, light weight and integral insurance policy for your build.
Shock chord anchor points are not an area to skimp on. And consider there is no easy retro fix for not building it right the first time!
Chris Prinzi
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#6
I got thinking about that, and I came up with the same conclusion - it's a relatively light weight insurance policy so I purchased 1/4 inch all thread, along with 12 nuts and washers earlier this week, to go through 3 of my 4 centering rings (all but the aft ring) of the DarkStar.
I plan to use 2 u bolts, mounted 180 degrees apart on the forward CR to balance the load on the forward ring during deployment also.
Greg Young - L3
NAR #42065
TRA #00234
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#7
Argg....!@
John Derimiggio
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#8
What's it take to pull a motor forward closure out?  Figuring they restrain something like 800psi, if a 3" motor has a 2.75 diameter bulkhead, the area of the bulkhead is approx 6 sq inches.  6 * 800 is 4800lbs so they'll hold at least that.  I plan to use an eye bolt in the forward closure of the motor, with a slack loop on a u-bolt in the top ring for a backup. 

Which would fail first, an OD snap-ring acting as a thrust ring or the ID snap ring holding the forward closure in?  Or the case?

N
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#9
John, I take it you have a different opinion?
Greg Young - L3
NAR #42065
TRA #00234
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#10
I do not argue against the recommendation of threaded rod for Evans scheme.  Connecting the centering rings with rebar and light concrete will make it stronger yet.  I just do not prefer the scheme.

Mount the recovery attachment to the strongest part of the rocket, which is the motor casing.  Manufacturers even provide a nice threaded hole in the forward closure for this.

Nat:  You cannot pull out the snap ring (if the groove is properly cut) without shearing the motor case at the groove.  Other than that the force to do that likely exceeds the strength of the shock cord or bolt etc.
John Derimiggio
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