Double Configuration 3" Build

I lawn darted my only 38mm MMT rocket a few months ago and have this pile of 38mm reloads I can't use. A really intolerable situation.

I have a 3" nose cone and some tubes and a new 38mm MMT so I put together a plan for a new 3" rocket with 38mm MMT. The short or lower BT is a full length 34" long. I have coupler and everything I'll need to build a electronics bay. I also have an extra 3" tube I can use as a upper payload bay.

This rocket will fly motor ejection with the 34" BT and nosecone, the short version, and also with dual deploy and the electronics bay and upper payload, the long version.

The short version first. It has a split fin design, just because I think it's cool and I hope it whistles on the way up.

Attached is the Aerolab drawing of the rocket.

Fins first. I'm using 6mm (1/4") plywood for the fins. I cut two 3 11/16" wide pieces to get the 3" span on the fins. The MMT to outer BT is 11/16".

I cut and trimed them on the table saw. Since I got two fins out of each piece of wood, I started with 4 fins, although I'm only going to use three.

After they were cut to shape, I used the table saw to cut the 1/2" slot in the fins. This took 5 passes on the table saw to finish.

This part actually went pretty fast.

Finally I had four fins shaped, and they were all pretty close to being the same size.

Next was beveling the fins. I made up a jig a couple of years ago. I hadn't used it since then and was glad I had the extra fin. As it turned out there was one of the four that wasn't quite as good as the rest.

I use the angled jig, clamp a guide to the table of the router, clamp the fin to the jig and run it past a 1/2" straight bit. When I do the second side, I put a 3/32" flat piece under the fin as I clamp it to the jig. This keep it up some and makes for a more even cut on both sides. I also put tape along the area that will be cut to help prevent any splinters from forming.

As you can see, there are now only 3 fins on the router table.

I had to make up a fin guide for assembling the fin can. I used some 1/4" plywood I had laying around. It would have been much easier to make if it was for 4 fins instead of 3. Those 30/60/120 degree angles that have to be dealt with are not as easy as the straight 90 degrees.

Anyway, I got the fin guide made and put the fins on the MMT. I used Titebond II to attach the fins. I'm adding fillets of epoxy with 1/4" fiberglass shavings added. This is probably over building, but a quick sim of the dual deploy version on a J500G shows just over Mach and a mile. Better safe then sorry.

OK, that's all for tonight. I'll see what I can get done tomorrow. The next BattlePark launch is in two weeks.

The jig you made for your router table is brilliant ,I shall be making one of those right away ! It`s more simple then the one I was usuing.


Thanks

Paul

sodmeister said:

The jig you made for your router table is brilliant ,I shall be making one of those right away ! It`s more simple then the one I was usuing.


Thanks

Paul

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Thanks.... Wish I could say I was the brilliant one, but I'm not sure where I got that idea from. I've got several books on routers and jigs and fixtures for them. That's were I got the plans for the router table and a lot of the jigs I use. That jig is very easy to make, providing you have the table saw to cut the angles on the faces of the two pieces of wood with.

Only got a little done today, maybe more tonight.

I added 1/4" chopped fiberglass to the epoxy when making the internal fillets.

Just one pic of that, all three sides are the same, really!

Well, the launch got rained, or mudded out yesterday and I'm still nowhere near done. Guess I'll have to shoot for next month.

Anyway, I needs some centering rings for the fin assembly. The last time I cut CRs for 3" LOC tubing I made an extra half dozen. I pulled out two set up to drill the center hole. A 1 3/8" forsten bit is just right for the 38mm MMT.

The first pic I used my centering head for the combination square to draw a couple of lines through the center of one of the discs. I then marked the forward and aft faces of each disc, taped the two together, and made alignment marks on the edges.

The reason for this is to make sure the MMT stays parallel to the BT, even if it might be slightly off center. As long as the two CRs are aligned on the MMT the same way they were drilled, the MMT will stay parallel to the BT.

The next pick shows the forstner bit and the discs being aligned. I do this with my calibrated eyeball. With a little practice it's amazing how accurate that can be. At least as accurate as the tolerances you have when working with wood.

The last pic is the setup on the drill press. All clamped in place and ready to go.

One of the finished CRs went on the top of the fin can, the other is set up for positive retention. The following is how I do it. It's worked great for me so far and until I have an issue, I'll probably not change.

First I measure and drill the holes for the inserts. I use the centering head on the combination square to get a line through the center and mark that equidistant from the center hole and outer edge.

The first pick show the inserts. I flatten two of the opposite barbs. This way the two remaining can penetrate the 1/4" CR in the center of the wood and avoid splitting or cracking the edges.

I then start the insert in the hole and press it into place. I do th

One of the finished CRs went on the top of the fin can, the other is set up for positive retention. The following is how I do it. It's worked great for me so far and until I have an issue, I'll probably not change.

First I measure and drill the holes for the inserts. I use the centering head on the combination square to get a line through the center and mark that equidistant from the center hole and outer edge.

The first pick show the inserts. I flatten two of the opposite barbs. This way the two remaining can penetrate the 1/4" CR in the center of the wood and avoid splitting or cracking the edges.

I then start the insert in the hole and press it into place. I do this by putting it between two pieces of plywood and clamp it with the vice. See the 2nd pic.

Third and fourth pics are the epoxy to hold the insert in. This is the only place I use JB Weld on the rocket. As I've said, I've had no problems with this method yet.

So far I've been working on the business end of the rocket. Once the fin can is done and installed, I can put a nose cone on it and fly.

But the name of this thread is Double Configuration. I intend to also use an av-bay and upper payload section and fly dual deployment.

So, while the JB Weld was setting up on the CRs, I started on the av-bay. The last av-bay I build worked out quite well and I using the same design this time. I start with a 6" coupler and cut a piece of tubing a 1/2" shorter. This allows room for the two 1/4" end caps to sit inside the coupler.

The first pic shows the couple and the shorter tubing. The second pick shows the tubing inside the coupler. The tubing is split lengthwise and a section removed so it fits inside the coupler. A generous amount of Titebond II is used to glue the tubing inside the coupler.

The third pic shows the small piece of plywood used to mount the power switch for the electronics. This is a homemade switch made from circuit board material and a brass screw and nut. I used brass because is solder easily to the circuit board. One face of the wood is then applied to the sander to give it a shape to match the inside of the av-bay

A hole is drilled in the av-bay to match the one in the wood mount. A small screw is used to mount the switch to the mount and the mount is glued to the av-bay with a little more Titebond II.

Now for the av-bay end caps. I had to break out the Wheel and Circle Cutter for the drill press for these. See the first pic. I use a scrap piece of lauan to make sure I had the size right first.

Once they were done, I drilled the holes for the threaded rod. I'm using 3/16" threaded rod. This has worked great in my 4" 9 lbs rocket so I figure it will work just as well here. The second pic shows the caps with holes drilled for the threaded rod and how I align the u-bolts. The metal strap that comes with the u-bolts goes on the inside and strengthens the cap.

The width of the threaded rod is based on the altimeter sled I have. It was originally built for my 2" ID L1 cert rocket El Juan. There's build thread in the archives on that some where. The av-bay in my 4" was built to hold it too. When El Juan lawn-darted, the sled was rebuilt the same size to fit the 4" rocket. Now this av-bay is being built to fit that same size also. One of these days I'll get another altimeter and get away from this one size fits all, but it's worked so far.

The threaded rod is held onto one cap with nuts and lock nuts. The other end is attached with wing nuts.

Progress is slow!

It seems there is always something else I need to do. Anyway, I installed the recovery anchor on the fin can. I considered using a U-bolt or an eye-bolt. Instead, I used some 1/2" Kevlar strapping. With the epoxy, I don't know if I'll save any weight over the metal connections but I think a little will be saved.

I folded the Kevlar in half and tied a figure 8 knot in the end. This is epoxied to the fin can so the end of the loop extends above the body tube but the knot will be just below the top edge. Hopefully this will provide a little anti-zipper affect.

I also got the slots cut in the BT.

Here's a few pics that show the progress. It's a little hard to see, but the last pic shows an extra piece of wood glued to both CRs to make rail button attachment easier.

Your project is coming along nicely. Do you think that with the fins in that configuration a whistle will be produced?

bradycros said:

Your project is coming along nicely. Do you think that with the fins in that configuration a whistle will be produced?

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I'm certainly hoping so!

Handeman said:

I'm certainly hoping so!

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What do you think causes a whistle? Lack of air foiled edges? Distance between the fin sets following and leading edges? Air velocity? A combination of factors that allows a resonant frequency to be found by dumb luck? Inquireing minds want to know.

You sure are skilled with power tools. Did you think up those jigs and fixtures yourself? Your doing a great job of using them.

I love building rockets with power tools! Great build!

bradycros said:

What do you think causes a whistle? Lack of air foiled edges? Distance between the fin sets following and leading edges? Air velocity? A combination of factors that allows a resonant frequency to be found by dumb luck? Inquireing minds want to know.

You sure are skilled with power tools. Did you think up those jigs and fixtures yourself? Your doing a great job of using them.

Click to expand...

I'm not sure about the whistle. I build a LPR I shot on B motors and tried a lot of fin combinations to try to get a whistle. No luck.

I don't know if this will work, but I'm hoping it will. Why is anybodies guess. I think the air rolls in the slot and vibrates causing a whistle, but like I said, I really have no idea.

As far as the jigs and fixture, I have several books on building jigs and fixtures or routers. They have a lot of amazing things and a lot can be adapted to other power tools. I also tend to build new ones whenever I feel I need one.

To find out for sure, you would need some wind tunnel time and several of those split fin sets.

For what it's worth my Darkstar fins whistle

Marsman said:

For what it's worth my Darkstar fins whistle

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What's the measurement for the gap between the fins? Are the following and leading edges in that gap air foiled or are they squared off?

gdjsky01 said:

Been a great thread so far. Lots to see and think about. Love the Av bay. Ooooh a circle cutter... I bet I could hurt myself with one of THOSE!! :bangpan::bangpan:

I built an Av bay for my late THOY Nighthawk that had a hatch on the side complete with flanges around the circumference and t-nuts to screw the hatch on. All hand cut. Oy! When I lost the Nighthawk to a stupid mistake (aren't they all? :bangpan I kept that Av Bay. I put too much work in it (my 1st at the time) to get rid of it. It awaits a new ride.

Again wonderful build thread! Keep up the great work.
:clap:

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Thanks for the encouragement. I need it. Without it, I get off track on other things like work, school, and life. I need to keep up with the rockets to keep a level of balance!

This weekend has been the Battle of the Rockets at Battlepark so no building, but I bought a bunch of motors that I can fly it on!

Handeman said:

Thanks for the encouragement. I need it.

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No worries my friend. Its a great build! Glad you were able to obtain the flaming bits!

No intention of hijacking the thread but since I am down in the rocket cave, I thought I'd post a pic of the Av Bay from the late and lamented THOY Nighthawk (which was a 7x29mm cluster). Its a hatch cut out of the outer tube, and then the couplers inserted in far enough to make a flange. It housed a BlackSky AltAcc 2. I REALLY liked the Altacc. When you used Konrad's software you got tons of info about yer rockets performance. I still have the AltAcc and it still tests good. Alas the software is long gone.

Rock on dude... get that bad boy ready to whistle! :clap:
Jeff

Handeman said:

The third pic shows the small piece of plywood used to mount the power switch for the electronics. This is a homemade switch made from circuit board material and a brass screw and nut. I used brass because is solder easily to the circuit board. One face of the wood is then applied to the sander to give it a shape to match the inside of the av-bay

A hole is drilled in the av-bay to match the one in the wood mount. A small screw is used to mount the switch to the mount and the mount is glued to the av-bay with a little more Titebond II.

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Very nice build thread, I may use some of these techniques.......Could you elaborate more on this switch, and perhaps provide more pictures.

I have just purchased my first DD Alt. and am kicking around the idea of either a switch or just tying the wires together.....

Marsman said:

For what it's worth my Darkstar fins whistle

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I get a whistle from the fins on my "No, it's not a darkstar". It's the same principle as blowing over the top of a bottle, at much higher velocity.

chadrog said:

I get a whistle from the fins on my "No, it's not a darkstar". It's the same principle as blowing over the top of a bottle, at much higher velocity.

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You raise my hopes!

Very cool rocket BTW!

kelltym88 said:

Very nice build thread, I may use some of these techniques.......Could you elaborate more on this switch, and perhaps provide more pictures.

I have just purchased my first DD Alt. and am kicking around the idea of either a switch or just tying the wires together.....

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I'll try to get some pics together this week. Right now my son is bugging me to get my lunch packed so we can head to the Battlepark launch.

Gotta go!

Quick update of the finished av-bay. Just have to slide the sled into it and I'll be good to go.

I use red wire on one end cap and green on the other to make it harder to mix up the main and drogue ends. I need all the help I can get. :smile:

Just a progress report. Got the internal fillets done, finally! I said this was going to be a long build.
While the wife and daughter watched the final episode of Survivor, I crept down to the Rocket Scientisting cave and went to work. Now it's time for school work.