Boat tail fin can

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strud
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Boat tail fin can

Postby strud » Mon Jun 24, 2019 2:06 pm

Hi All

I've started on a 4" vehicle design that includes a boat tail for base drag minimisation. The addition of a boat tail makes significant improvements to the simulated altitude and the motor this vehicle is based around allows for a nice boat tail implementation.

The current overall design of the vehicle is something like this (may still be tweaked a little)

https://drive.google.com/open?id=1P4yEbAvpfeGclLzHoYtsXKskeLw_jJI4

And the fin can component like this

https://drive.google.com/open?id=1DtVFs7hFCS2bpXBoo7hticG5sfj-86o3

So after watching George's great video on using carbon sock for his pressure vessel builds, I realised this is a nice way to build the fincan tube with integrated boat tail.

Firstly a mandrel/plug is required. So I decided to use timber for this since I have plenty and it is MUCH cheaper than buying a solid plastic bar for the job. There are some downsides to using timber (more effort eg filling, release agents etc) but if it works I will likely use it more extensively in the future for things such as custom nose cone plugs etc.

https://drive.google.com/open?id=1PjLzTFV4ZwnDb58_DyheQdoB0BXP3cVz

https://drive.google.com/open?id=1wLfej7kbMMXU-37cl1I7pvk9IIA34d8o

https://drive.google.com/open?id=1YeZm0hXM2d5tMn_L9Qzpm_TBsz1hoyvX

Next step will be to insert a metal centre into the other end so I can get reliable location of this plug when I put it back on the lathe. Then I will turn this down close to size (say 2mm over final) then seal and fill the surface before taking down to final size. Will probably need another round of filling/sealing especially on the tapered section where the end of wood grains will be exposed the most.

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Re: Boat tail fin can

Postby air.command » Mon Jun 24, 2019 2:22 pm

Looking good! What do you fill and seal your wood plug with? What will you use as a final top coat on the plug before using any release agents?
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strud
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Re: Boat tail fin can

Postby strud » Mon Jun 24, 2019 2:32 pm

Hi aircommand

At the moment thinking of using West Systems with the fairing compound added.

I think there will be some of this used raw as well, poured into the cracks to help hold this all together long term as well, since the pine log is very dry (which is good) and hs cracks.

I have some release agent 'wax' which I will probably end up using, since cannot use 'glad bake' etc due to the taper corner etc.

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Re: Boat tail fin can

Postby Aquaman33 » Mon Jun 24, 2019 2:56 pm

Looks like an exciting project.

Your have certainly picked a rough bit of timber for that mandrel.

Best of luck with it.

Look forward to seeing updates.

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Re: Boat tail fin can

Postby strud » Tue Jun 25, 2019 12:17 am

Added a centre on the tailstock end

https://drive.google.com/open?id=1EGpGWczp2QeeVocN5HS-HRXAUM984DEc

Will drop some epoxy in the cracks next and when cured turn to near final shape.

strud
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Re: Boat tail fin can

Postby strud » Tue Jun 25, 2019 10:12 pm

turned to near final shape and then coated with thinned (10%) epoxy (west systems) to seal and provide a bonded surface to turn to the final size.

https://drive.google.com/open?id=15UBML8sUYssWXp1eJ05_Ci5IZqciM9Ll

https://drive.google.com/open?id=1WNUcm2niKiAKdLDaFAfcr0aR3LZr2SlY

left it running at 6RPM and heater on to get it to cure overnight (very cold here....)

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SpaceManMat
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Re: Boat tail fin can

Postby SpaceManMat » Tue Jun 25, 2019 10:22 pm

strud wrote:Hi All

I've started on a 4" vehicle design that includes a boat tail for base drag minimisation. The addition of a boat tail makes significant improvements to the simulated altitude and the motor this vehicle is based around allows for a nice boat tail implementation.

The current overall design of the vehicle is something like this (may still be tweaked a little)

Image

And the fin can component like this

Image

So after watching George's great video on using carbon sock for his pressure vessel builds, I realised this is a nice way to build the fincan tube with integrated boat tail.

Firstly a mandrel/plug is required. So I decided to use timber for this since I have plenty and it is MUCH cheaper than buying a solid plastic bar for the job. There are some downsides to using timber (more effort eg filling, release agents etc) but if it works I will likely use it more extensively in the future for things such as custom nose cone plugs etc.

Image

Image

Image

Next step will be to insert a metal centre into the other end so I can get reliable location of this plug when I put it back on the lathe. Then I will turn this down close to size (say 2mm over final) then seal and fill the surface before taking down to final size. Will probably need another round of filling/sealing especially on the tapered section where the end of wood grains will be exposed the most.


Nice project Strud.

BTW to get the Google Drive images to display properly do the following.
Change:

Code: Select all

[url]https://drive.google.com/open?id=1P4yEbAvpfeGclLzHoYtsXKskeLw_jJI4[/url]

To:

Code: Select all

[img]http://drive.google.com/uc?export=view&id=1P4yEbAvpfeGclLzHoYtsXKskeLw_jJI4[/img]
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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SpaceManMat
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Re: Boat tail fin can

Postby SpaceManMat » Tue Jun 25, 2019 10:28 pm

strud wrote:Added a centre on the tailstock end

Image

Will drop some epoxy in the cracks next and when cured turn to near final shape.

strud wrote:turned to near final shape and then coated with thinned (10%) epoxy (west systems) to seal and provide a bonded surface to turn to the final size.

Image

Image

left it running at 6RPM and heater on to get it to cure overnight (very cold here....)


Nice, starting to look rocket shaped now.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

strud
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Re: Boat tail fin can

Postby strud » Tue Jun 25, 2019 10:32 pm

Thanks, was going to look up how to do that soon....

strud
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Re: Boat tail fin can

Postby strud » Fri Jul 05, 2019 5:57 pm

So after some more work on my CAD model and looking at the thicknesses of the carbon and glass sock, I came to the realisation that the plug diameter was too small and my composite tube wall thickness was going to be around 3mm :shock:

Rather than throw away the plug (maybe I should have) I built it up using west systems and fillers to a target of 97.5mm from 95.2mm... New wall thickness will be 1.85mm (1 layer of light glass + 2 layers of heavy carbon sock + 1 layer of light glass).

The outer layer of glass will be somewhat sacrificial if need be ie I can sand it down to get the outer layer smooth.

One more skim now and we should be on the money

Image

Image

Picking up the sock etc soon so should be able to make up the first prototype in a week or so.

In terms of release layers/materials, I plan to coat the plug with an epoxy rated release wax AND probably use a layer of GLAD bake on the parallel section.

Also thinking about just doing a layer of light glass and one layer of carbon first to ensure that can be released before I lay up the rest.

Anyone have other suggestions?

CS

air.command
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Re: Boat tail fin can

Postby air.command » Fri Jul 05, 2019 6:47 pm

Looking like good progress there strud. When I once made up a wooden plug that was too small, I just wrapped one layer of light fiberglass on it to make it a little wider. That way it gave a nice even layer thickness all around. Then I just filled and sanded it to a smooth finish. I also gave it a couple of coats of Estapol 7008 2 pack polyurethane clear coat. http://lpb-dev.azurewebsites.net/Uploads/TDS/D5.07%20Estapol%207008.pdf to seal the surface and give it a smooth finish.

I have tried using this Carnauba Wax based mould release directly on it https://www.trmoldrelease.com/shop/tr-102-regular-mold-release/ but didn't have much luck trying to remove a small nosecone from the plug. Perhaps I didn't give it enough coats to make it separate easily. I had to end up cutting the fiberglass nosecone along one side in order to get it off the plug. We use this wax to polish our PVC mandrels before applying the baking paper and that works really well.

Perhaps make it in several thin-wall pieces that can be easily released one at a time from the plug. Trim them, and then put them back on the plug join them with a bit of light fiberglass. Once all together and cured then you can put the two layers of sleeves over the top of the whole thing. Here is an example how we made our tail cone out of several pieces except we never applied a sleeve over the top. https://youtu.be/TNecW-RsiCk?t=407 (it starts about 6:50 into the video) We used a number of gores in this example but a fiberglass sleeve for the tapered section is easier.
Crop Circles: ... just a bunch of guys looking for their rockets ....

strud
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Re: Boat tail fin can

Postby strud » Sat Jul 06, 2019 7:55 am

Hi George,

Excellent fedback, many thanks.

I will look for some of that 2 pack estapol, will likely use that in the future instead of the West systems epoxy.

Did you try heating the whole thing to soften the wax? That is something I might try if it gets a bit 'difficult'

I think that I'm going to have a go at a single layer of glass on the mold with wax and a layer of glad bake on the long parallel section.

However, I am tempted to put a small draft of say 0.2mm over the last 400mm of the parallel section since once the thing starts to move at all it would release easily that way. I still need to leave the first 100mm or so parallel to interface with the coupler.

strud
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Re: Boat tail fin can

Postby strud » Sat Jul 06, 2019 8:06 am

Sounds like addition of PVA or even hairspray on top of the wax is a good insurance policy

https://www.epoxyworks.com/index.php/mold-release-pva-and-hairspray/


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