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Punisher 4 Kit Bash - Minimum Diameter Build
MarkH, maybe you can review the background on this thread again as I missed it (or forgot it). It looks like you have a red tube with fin slots so you did originally purchase a 4" Punisher, is that right? But because of wanting to build it minimum diameter you discarded the original fins, MMT and CR's and are constructing your own fins, is that right?
Tim, you pretty much summed it up. I am building a minimum diameter (98 mm) Punisher 4.  The  kit comes with a 75mm motor mount. I thought about changing the thread title to Punisher 4 kit bash, but it looks like we don't have the ability to edit thread titles. [Edit: never mind I figured this out and changed the title,  I also clarified the words in the first post a little bit, to indicate the intent of this build]

I would have used the original fins and mounted them 'surfaced' in the slots, but the rearward CG shift using a 98 mm motor compared to 75 mm decreases stability margin, so I am using 4 larger fins instead of the 3 original fins to improve CG/CP. Also the considerable weight savings in the 4 fins I have made compared to the three originals helps the CG/CP relationship as well.
Here is a pic of the red tube again. I sanded the filled-in slots. Need to do a little more work smoothing them.


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Cut some fin guides today after work.  Took measurements on the fin thickness and tube diameter, drew it in Solidworks, and sent it to the laser cutter. It fit perfectly on the first try.

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.pdf   Punisher 4 fin guide IS.pdf (Size: 1.53 KB / Downloads: 0)
Nice. I always hack mine out of faom board with a matte knife. What do you consider the advantages of wood?
John S.
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TRA #15253
Level 1, 2014-Mar-15 -- Aerotech Sumo, H133BS
Level 2, 2014-Jun-21 -- Giant Leap Vertical Assault, J240RL
Level 3, 2016-03-12 -- MAC Performance Radial Flyer, M1101WH, 13,028 feet
I've cut them out by hand with foam board as well. Since I have access to a laser cutter it is just easier, faster and more accurate than I can do by hand. Wood is more sturdy. I suppose I could use the laser on foam board, but a 1/4" plywood sheet isn't all that expensive. I think I paid $5 for a 2' by 4' sheet at Home Depot.
Tacked fins on and applied baby fillets.

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Received an e-mail from Wildman the other day notifying customers of a recall on 4" Polycarb nose cones because the coupler doesn't fit.  My coupler goes in about 0.25 inches as can be seen from scuffs inside the nosecone in this photo.   I thought I was going to have to do some major sanding on the coupler to get this to fit, so I'm pleasantly surprised to see a recall notice and pending replacement of the NC.

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Fillets.  Here are the fillets.  38 mm propellant grain as fillet tool.  Proline 4500 epoxy.   I made one pass in each direction with the 38 mm grain, then cleaned the ends up with my finger, with gloves on.

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A little interlude...

I test fit the nose cone on a  Rocketry Ware House 4 inch coupler and it fits perfectly.    The stock red Punisher coupler diameter is a tad too big to fit in a Rocketry Warehouse 4 inch tube.  Maybe Wildman should send the bad cones to Rocketry Warehouse.Smile

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Well instead of sending the nosecone back to CTI (CTI made these nose cones for Wikdman) and waiting for a replacement, I decided to take matters in my own hands.   I made use of a  lathe today at work with a piece of 80 grit sandpaper.  I put tiger stripes on the coupler then wet sanded (to control the dust) till they were gone.  Rinse, dry, repeat.
It  took three passes total.  Original OD was 3.900, final was 3.885, give or take a couple thousandths on both measurements. It fits the nose cone pretty easily now. 
This was rather easy with the aide of a lathe, but if I had to sand  to fit by hand it would have been a lot of work.

The CTI cone is a pretty well engineered piece by the way.

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