Modeling complex shapes?

[soopirV]

Hi- I've been tinkering with a LOC Stovi build (29mm option) for the last few months, mostly because I haven't had a ton of time and it's been way too hot lately. I'm modifying it a bit from the factory design- I'm turning the transition section into an AV bay for dd, and, well, that's about it. I'm looking at the aft end of the main airframe, and wondering how I would design and build structures to fill in the transition from the 3" airframe to the narrower "booster" tubes. The centering ring seals everything off nicely, so it's not about successful flight, just a question of style and approach. I don't have easy access to any advanced tools (mill, lathe, 3D printer), just basic wood tools- band saw, table saw, miter saw, drill press, belt/disc sander.
How would you go about fabricating "plugs" that fade into the valleys of the booster tubes, easing the transition? Highly-filled epoxy comes to mind as the easiest option, since I could form and finish in place, but that seems like it would be very heavy. How would you shape a square balsa stock, for example, into this space?
The goal is to achieve something like this, sketched on my phone, so very poor rendition!

 

[soopirV]

*fairings. Couldn't think of the word at the time.

 

[blackbrandt]

I can make you CAD files if someone can 3D print them.

 

[soopirV]

I can make you CAD files if someone can 3D print them.

That's very kind, and I appreciate it, but it's more of an academic exercise. I guess you answered my question tho, computer modeling is the answer. Was wondering how it could be done "old school".

 

[blackjack2564]

I used paper towel/tissue soaked in wood glue packed tight with various tools....flat blade screwdriver, wood mixing stick shaped how ever needed, tooth picks, etc.
Left slightly below large tube level, so I can level/finish flush with epoxy.

When dry, several days....thin coat of epoxy poured over, to hide tissue and give level, flat smooth surface to hold paint.

 

[soopirV]

I used paper towel/tissue soaked in wood glue packed tight with various tools....flat blade screwdriver, wood mixing stick shaped how ever needed, tooth picks, etc.
Left slightly below large tube level, so I can level/finish flush with epoxy.

When dry, several days....thin coat of epoxy poured over, to hide tissue and give level, flat smooth surface to hold paint.

Is that specifically how you did the Stovi, or just how you handle aft fairings in general? Any finished pics either way? I did something similar to seal up a cluster clone I built a decade ago, but didn't give it that much care. Haven't flown it yet, either, come to think of it!

 

[rharshberger]

Another method would be similar to the one used for Saturn IB tank fairings, made from poster board and hardened with CA.

 

[GlenP]

here is an example on an LPR rocket, using cardstock:
https://modelrocketbuilding.blogspot.com/2010/11/dr-zooch-saturn-1-sa-5-build-part-21_17.html

The shape is basically a transition, but with cylinders cut out of it. You could form a flat pattern using a transition tool, then draw ellipses, ie the shape of the cylinder cut at the same angle as the transition. laminated layers of cardstock can get fairly stiff.

 

[Sooner Boomer]

That's very kind, and I appreciate it, but it's more of an academic exercise. I guess you answered my question tho, computer modeling is the answer. Was wondering how it could be done "old school".

It's not too hard if you know a little geometry (conic sections). When you cut across a cylinder at an angle, you form an ellipse (oval). You know the minimum diameter of the ellipse, the diameter of the cylinder (body tube). The maximum diameter is the length of the cut across thr cylinder. You can do a cross-section drawing and measure this. This gives you the shape around the inner cylinders. The shape around the outside edge where the transition meets the outer tube is an ellipse also. Can you figure this out without pictures (I only have Paint to draw with...)?

 

[dhbarr]

You can calc the long axis easily since you know tube diam and height of the cut.

 

[blackjack2564]

Is that specifically how you did the Stovi, or just how you handle aft fairings in general? Any finished pics either way? I did something similar to seal up a cluster clone I built a decade ago, but didn't give it that much care. Haven't flown it yet, either, come to think of it!

How I used to do low power clusters.
Not really what I call fairings, also so long ago I have no pics.
Just how I sealed up gaps in tubes to get nice "finished" look.

 

[new2hpr]

It's not too hard if you know a little geometry (conic sections). When you cut across a cylinder at an angle, you form an ellipse (oval). You know the minimum diameter of the ellipse, the diameter of the cylinder (body tube). The maximum diameter is the length of the cut across thr cylinder. You can do a cross-section drawing and measure this. This gives you the shape around the inner cylinders. The shape around the outside edge where the transition meets the outer tube is an ellipse also. Can you figure this out without pictures (I only have Paint to draw with...)?

That will get you "close enough", even though it's a bit more complicated. The cutout would be an ellipse IF you were cutting a cylinder with an angled plane. Since you are cutting a cylinder with a cone, it gets a little egg-shaped. You could do it "old school" with pencil, compass, ruler and projection methods. The guys who really know how to do this well are in HVAC sheetmetal ductwork. They can whip out a perfectly fitting rectangle to two different size cylinders at 27 degree angles transition out of a single piece of sheetmetal, cut it out, rivet it together, and need only a dab of sealant at the seams. Pretty amazing to watch.
-Ken

 

[kjohnson]

Here is an article that has information on how to draw out a scalloped fairing. It's written with the Saturn I in mind but the technique can be used for your Sovi. https://meatballrocketry.com/saturn-construction-tips/

kj

 

[tomsteve]

that is a very intersting concept and using some geometry a fairing/transition can be cut from cardstock.
however, since i am not that great at geometry, all i can say is make a cone and cut out everything that isnt supposed to be there.

and since im not that good at geometry, i was thinking expanding foam. fill up each section, let it foam up and harden and start sanding/filing to shape.
then some glazing putty to fill in the foam.

or just layers of 2 part glazing putty, but that could be quite time consuming snading and add a bit of weight.

now im thinkin iffen ya had a section of the body tube, you could make the cuts somehow to make a transition.
how the tube fins on the pemberton kraken are laid out and cut comes to mind here,too, for me.

 

[dhbarr]

Anyone have a how-to for intersecting VK NCs with BTs? I want some VK fairings, but I've tried 2-3 times and failed. Might just go back to conical in the interest of time / sanity.

 

[Sooner Boomer]

Anyone have a how-to for intersecting VK NCs with BTs? I want some VK fairings, but I've tried 2-3 times and failed. Might just go back to conical in the interest of time / sanity.

Make a negative mold then recast the shape in something soft or cheap like plaster. Grind/cut away whatever you don't want. This will give you the shape you need. You just have to transfer to to the "working" nosecone or shroud.

 

[RCgothic]

Got a lathe / drill press? Make an oversize cone and drill out tube holes. Then support in middle with PVA or similar and turn down outside diameter until you have the individual sections you need. Soak in water to remove PVA.

 

[kjohnson]

Looking back at this thread, if you are trying to make a part from balsa to fill the gaps and fair them in, why not take two tubes and cover them with sandpaper, glue them together and sand down a balsa block? You could even do it with one tube, and do one side of the fairing at a time, but I would never be able get them to be mirror images.

kj