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Projecting Cluster Holes Through Tailcone

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DynaSoar

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I'm working on a design for a 1950s B-grade sci-fi type bird, with only enough tube to hold the very long nose and tail fins (using the Apogee BT-70 fitting blown polystyrene cones). I want the 3-cluster of 18mm engine tubes to project out the bottom of the tail, sort of a Flash Gordon style business end. Any ides for how to project the cross-section of the 3-cluster onto the very long, tapered cone so I can get a reasonably accurate cut to fit the clustered tubes?
 

hokkyokusei

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I'm afraid I don't know the answer, but have a related, perhaps similar, question. I need to cut some holes in a flat bulkhead to make a centering ring for angled motor mounts. Anyone got any ideas?
 

scadaman29325

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Originally posted by DynaSoar
Any ides for how to project the cross-section of the 3-cluster onto the very long, tapered cone so I can get a reasonably accurate cut to fit the clustered tubes?

Do you think you could take a '3 engine motor-mount-ring' and slide it over the tail cone. Then whittle away where the 'inside points' touch the mounting ring. That could give you an idea where to start anyway.
 

rbeckey

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Put the engine tubes in an appropriate motor mount and slide both rings toward the end of the MMT tubes. Put the tailcone on the BT and slide the MMT into the BT. Shine a bright light straight through the MMT tubes and trace the shadow line. Cut near the lines and sand to final shape, test fitting often. It may help to cut the MMT tubes at somewhat of an angle, so they will be close to the contour of the tailcone. This works very well for a central single MMT tube. It will be a little trickier for a cluster, but should work.
 

DynaSoar

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Originally posted by scadaman29325
Do you think you could take a '3 engine motor-mount-ring' and slide it over the tail cone. Then whittle away where the 'inside points' touch the mounting ring. That could give you an idea where to start anyway.
Yes! And keep testing it with a level to make sure it's perpendicular. Make it out of balsa so it's easy to trim step by step. Excellent hack. Thanks!
 

sandman

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I have been thinking about how I would do this and I think I came up with an idea.

This is what I would do.

First set up the tail cone to be drilled on the drill press.
I use a 3/4" (19mm) Forstner drill bit wil the speed set to it's lowest setting.

Tack a centering ring onto the inside of the cone pefectly square and centered.

Set the cone pointy end first into a scrap section of tube.

Slowly bring the bit down through the centering ring and check the alignment.

Be sure the cone is secured.

Now start the drill and sloooooowwwly cut your holes from the inside out.

Or send me the cone and centering ring and I'll do it for you.

sandman
 

DynaSoar

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Originally posted by sandman
I have been thinking about how I would do this and I think I came up with an idea.

This is what I would do.

First set up the tail cone to be drilled on the drill press.
I use a 3/4" (19mm) Forstner drill bit wil the speed set to it's lowest setting.

Tack a centering ring onto the inside of the cone pefectly square and centered.

Set the cone pointy end first into a scrap section of tube.

Slowly bring the bit down through the centering ring and check the alignment.

Be sure the cone is secured.

Now start the drill and sloooooowwwly cut your holes from the inside out.

Or send me the cone and centering ring and I'll do it for you.

sandman
I think I may just take advantage of that. I cut the tip of the cone off and found out the plastic is THICK. Way too thick to cut with a knife. It's going to take power tools. I went to a custom woodworking place and got put in touch with someone they know who could do it, but he was busy. I founf another place right by my job and I'll check with them today. If they turn me down, I'll be in touch.

By the way, my project with the fancy transition is done. I'm having some pictures digitized. They'll be up here soon.
 

powderburner

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Dynasoar, I am not sure that I understand your question. Does the configuration of your fairing/motor tube look something like the crude picture I have attached? (the computer trimmed off the top edge). Are your three motor mount tubes stacked together tightly?
Can you give the dimensions (diam, length, etc) of the tapered fairing and the motor tubes?
 

Jersey Devil

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I've attached a picture of my Jersey Devil rocket that I've just finished. I decided to have the engines (3 of them) positioned along the curve of the tailcone. To attach them I punched several pinholes in the cone where each was to be attached, sanded the surface of the cone and the engine mount tubes, then used some epoxy to adhere them. As I had hoped the epoxy went through the holes to hold the tubes on with better reliability. Its not a big rocket but I wasn't confident in using anything less than epoxy to hold these on. I didn't think anything like white glue would have a good enough bond between the paper tubes and the plastic.

I haven't had good enough weather to fly it yet but if anyones interested I'll let ya know how it goes.:)
 

DynaSoar

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Originally posted by Jersey Devil
Its not a big rocket but I wasn't confident in using anything less than epoxy to hold these on. I didn't think anything like white glue would have a good enough bond between the paper tubes and the plastic.
You're right, it wouldn't have. Plastic cement probably would have worked, but you'd run the risk of using so much for a good hold that it'd make the plastic warp. This is especially likely when you use it in a place where it can't evaporate freely. The fumes get trapped and eat away at the plastic. I found pits in the nose of a Jupiter C two weeks after I used plastic cement to help hold in the body putty nose weight.

Other than such a case, plastic cement works fine for paper/plastic bonding. I built a 12 ounce model out of all styrene except the body tube, nose cone and motor hook (very fancy multiple layer wing/fin Stealth thing with scramjet scoop, hence the weight). Plastic cement worked very well holding it all together. Plus it dries clear and doesn't bubble or pit in fillets.
 

powderburner

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Dynasoar, I thought you wanted those motor tubes to project through the rear end of the boattail? If that's right, do you still want to work on figuring out that pattern?

Added later: Oops, I screwed up-----I was looking at the picture that Jersey Devil posted, and got it confused with yours. Sorry about that.
 

DynaSoar

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Originally posted by powderburner
Dynasoar, I thought you wanted those motor tubes to project through the rear end of the boattail? If that's right, do you still want to work on figuring out that pattern?
Thanks, but I've accepted an offer from Sandman to drill them out. I knew what I needed but couldn't do it myself. There were a couple ways to do the projection, but the very thick walls of the cone meant it'd take more power tool than I could muster to do it right. He had a plan for getting both pattern and drilling donme at once.

Thanks, though.
 
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