Converting Partizon NC to Boat Tail

Oberon · Sep 1, 2015

I'm planning out a mod of a Partizon kit, with an extra nose cone converted to a boat tail. I'm having a couple issues:

1) I'd like to cut off the tip of the nose cone such that a 38-29mm centering ring just fits in the back (I'm going to leave a bit of 29mm motor tube hanging out the back to install a screw on motor retainer).

2) I want to extend the fins onto the cone, and need to know the profile.

So for both of these, I need a precise profile of length vs. diameter for the cone. Is there a good free calculator and/or published profile somewhere? I have OpenRocket but not RockSim.

MCriscione · Sep 1, 2015

I would assume the nosecone cutting would be pretty straight forward. I'm not 11% sure where you mean by 'the back' for the CR. Just take your motor mount tube (29mm) and place it on the front of the NC, draw a line around, and cut the NC there (or just a bit above). From there you will need to sand the ID until it is is large enough to fit the 29mm. Remember that the thickness of the nose cone is only important if you don't plan to sand down the ID to get a smooth transition. Otherwise, you can cut lower (or sand the flat end) until the motor tube fits, but you'll have to fill to achieve a smooth transition due to the thickness of the nose cone. I imagine that a sandpaper wrapped dowel or BT of a slightly smaller size (maybe 24mm or 1") will be the easiest way to sand the ID. Disclaimer, I haven't done this before.

The fins though will be tough. They already have tabs cut for the through wall mounting, and that means that the fin profile is cut to match the OD of your BT. It won't follow the taper of your boat tail if you move it backwards. You'll either need to make new fins that follow the correct profile, or fill the notch/gap that will be left at the back end of each fin with some kind of filler. I've never done this either, but would probably lean towards cutting brand new fins from a sheet of plywood. Cutting the appropriate width/length slots into the boat tail may be difficult to mark and even harder to cut. I don't know what to say about that. Maybe a jig / dremel with appropriate width bit?

Oberon · Sep 1, 2015

MCriscione said:
I would assume the nosecone cutting would be pretty straight forward. I'm not 11% sure where you mean by 'the back' for the CR. Just take your motor mount tube (29mm) and place it on the front of the NC, draw a line around, and cut the NC there (or just a bit above). From there you will need to sand the ID until it is is large enough to fit the 29mm. Remember that the thickness of the nose cone is only important if you don't plan to sand down the ID to get a smooth transition. Otherwise, you can cut lower (or sand the flat end) until the motor tube fits, but you'll have to fill to achieve a smooth transition due to the thickness of the nose cone. I imagine that a sandpaper wrapped dowel or BT of a slightly smaller size (maybe 24mm or 1") will be the easiest way to sand the ID. Disclaimer, I haven't done this before.

The fins though will be tough. They already have tabs cut for the through wall mounting, and that means that the fin profile is cut to match the OD of your BT. It won't follow the taper of your boat tail if you move it backwards. You'll either need to make new fins that follow the correct profile, or fill the notch/gap that will be left at the back end of each fin with some kind of filler. I've never done this either, but would probably lean towards cutting brand new fins from a sheet of plywood. Cutting the appropriate width/length slots into the boat tail may be difficult to mark and even harder to cut. I don't know what to say about that. Maybe a jig / dremel with appropriate width bit?

By the "back of" I just meant that I want to truncate the boat tail at a larger diameter than that of the motor mount tube. That way, the motor retainer won't protrude beyond the minimum diameter of the boat tail. I do have some 38mm tube laying around so I could use the approach you mentioned I think.

I definitely plan plan to cut new fins. The main question is whether to try to fit the fins to the surface profile of the tail, or cut them such that they go all the way to the motor mount tube and then slot the boat tail. Either way having an actual profile of the cone would help so I can measure it out rather than guess and check.

MCriscione · Sep 1, 2015

Got ya, I had disregarded the motor retainer, so what you've stated makes sense. Not sure how to get the nose cone profile, but if you slot the nose cone, it doesn't matter much. Maybe you could use something like this to transfer the curve? https://www.joann.com/flexible-curve-ruler/13702675.html

MikeyDSlagle · Sep 1, 2015

I literally just did such a thing using an Estes Super Alpha. Haven't got to slotting the boat tail yet. Starting a build thread now. MC is right about making the boat tail, slide the appropriate centering ring or body tube over the tip, mark and cut. Some sanding was necessary in my case. I wouldn't worry about following the profile for the fins. Just be sure the tabs are sufficient enough to hold the fins and the "root edge" of the fins is shallow enough to be hid by the boat tail. It's also pretty easy to shorten the boat tail but trimming the other end if necessary.

This profile should be pretty darn close.

View attachment 271117

If not, check out K'Tesh's Open Rocket files under "Plans". I know he has a Partizon in there somewhere. If you don't have Open Rocket, get it.

Mike

Build thread for my project:
https://www.rocketryforum.com/showthread.php?128498-Project-Deep-Space-working-title

Cl(VII) · Sep 2, 2015

I would definitely go through the wall fins. Cutting and slotting the boat tail isn't too difficult, but it is easier if you mark the fin slots before cutting off the tip. I did something similar on a 4" diameter project. See posts 15, 16, 35, 44 and 46 for of this thread for an overly detailed description of how I did it.

K'Tesh · Sep 2, 2015

For more ideas on slotting the tail cone, you might also want to look at my Velociraptor Downscale Thread...

Sooner Boomer · Sep 2, 2015

One way to generate a profile of the cone (or actually, about any surface), is to apply some sort of putty that will harden/set up. It needs to be applied in a straight line, from base to tip, and it needs to form a 90 degree angle. You might need to apply some sort of release agent, if you're using an epoxy putty or something that is really sticky. When it dries (or sets up), pull it off and you will have the exact profile of that surface. This can be scaled up/down to the limits of practicality.

Converting Partizon NC to Boat Tail

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Converting Partizon NC to Boat Tail

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