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  #1  
Old 05-30-2018, 02:30 AM
CuriousAmateur CuriousAmateur is offline
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Default Disposable air foils needed

Hello,
Does anyone have suggestions on good but cheap/disposable air foils? As we are doing test fires, many of our rockets are being lost. A cheap, disposable, and even biodegradable body is pretty easy to make, but the fins are proving a little more difficult. From chunks of cardboard to repurposed dollar store toy parts, we feel like we've tried or at least considered a lot of options. For the most part, they are always either unstable or unsustainable (take too long to make or cost too much to get in quantity).

Does anyone have suggestions on something cheap and easy, yet viable? We've looking for either "DIY" or store-bought options, but preferably not items only bought online, or 3D printed. These are mostly for low-power rocketry, so options of..."questionable" quality may be viable.

Thanks!
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  #2  
Old 05-30-2018, 07:49 AM
astronwolf astronwolf is offline
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Quote:
Originally Posted by CuriousAmateur
Hello,
Does anyone have suggestions on good but cheap/disposable air foils? As we are doing test fires, many of our rockets are being lost. A cheap, disposable, and even biodegradable body is pretty easy to make, but the fins are proving a little more difficult. From chunks of cardboard to repurposed dollar store toy parts, we feel like we've tried or at least considered a lot of options. For the most part, they are always either unstable or unsustainable (take too long to make or cost too much to get in quantity).

Does anyone have suggestions on something cheap and easy, yet viable? We've looking for either "DIY" or store-bought options, but preferably not items only bought online, or 3D printed. These are mostly for low-power rocketry, so options of..."questionable" quality may be viable.


Get a 3"x36" sheet of balsa and cut it into 3x3 squares. That'll give you 12 fins very cheap and easy.

But you don't give us any more detail about what you are doing, so I'm only taking a guess that my suggestion is even in the ballpark of what you are looking for.

What's the hurry? And are you mass producing biodegradeable rockets? Why do they have to be biodegradable?
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  #3  
Old 05-30-2018, 11:34 AM
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tbzep tbzep is offline
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We need to know what kind of performance you are expecting from your motors. That way we know how strong the material and the attachment process needs to be.

1/8" balsa will handle D and E power. 3/16" E and F, 1/4" balsa with a little epoxy coating can handle a full G160 surface mounted!!!!! My old buddy Jim had an FSI Black Brant that we could not destroy with his EX motors, and they were great motors.

Beyond that, 1/8" aircraft plywood will handle up through H and I and 3/16" will handle J and possibly more if shaped correctly and attached through the wall. It will take them a long time to rot, though, especially if coated with epoxy.

As a reminder

A 2.5 n/s
B 5 n/s
C 10 n/s
D 20 n/s
E 40 n/s
F 80 n/s
G 160 n/s
H 320 n/s
I 640 n/s
J 1280 n/s
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Old 05-30-2018, 02:21 PM
CuriousAmateur CuriousAmateur is offline
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Default Reesponses and details

astronwolf: Good suggestion for large builds. Thanks, I'll keep it in mind. There's no particular hurry, it's just that taking several hours to produce a single set of fins which will only be used once is a bit of a waste. They do not necessarily need to be biodegradable. The bodies typically are, since some of these are not recoverable (get lost).

tbzep: Right now we're actually building some rockets for E45 motors, though we also want to do this for small A10 motors. In other words, both high and low stress options are wanted. Surface mounting is obviously easier, so we may try the 1/8" balsa and epoxy you suggested.


We have all the basic parts of home-made rocket motors working, so now we are experimenting putting them together. Things sometimes don't go quite as planned, so when something goes wrong mid-flight, we sometimes lose the rocket. Since these are only tests, we don't much care what they look like, as long as they are aerodynamically suitable. These are not model rockets which are carefully and lovingly assembled with care to details. This is more of the duct tape and cardboard option than the elegant one. The easier and cheaper they are, the nicer it is for us.

Thanks!
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Old 05-30-2018, 02:37 PM
astronwolf astronwolf is offline
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I can conceive of a place somewhere between taking "... several hours to produce a single set of fins..." for "...model rockets which are carefully and lovingly assembled with care..." and the "...duct tape and cardboard option."

You are testing E45 motors. You want the fins to stay on, or not?
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  #6  
Old 05-30-2018, 02:56 PM
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1/8" balsa reinforced with cardstock (or just a thin layer of epoxy) will be plenty strong for an E45 as long as you use something like a long rooted delta, trapezoidal, or clipped delta shape (Asp, Nike, etc.) that doesn't hang way out there and lend itself to flappin in the wind. 3/16" balsa should survive naked, but if you want to reuse the rocket, a coating of something is needed to keep them from absorbing moisture if you land in wet grass, etc. These fin shapes that aren't swept back will also prevent or minimize landing damage and allow you to reuse the rockets without wasting time on repairs, assuming you find them.

Epoxy is fine to attach the fins, but if you are using regular hobby rocketry paper body tubes, plain old yellow wood glue will hold more than the wood or BT. No big deal unless you are epoxy sensitive. Jerry Irvine has noted that he's stress tested some wood glue assembled airframes with some pretty gnarly motors. I can't recall how large, but they went well beyond E range. I don't see a need to mount balsa through the wall. If you are using heavy airframes and fast descent rates you might go to 1/8" plywood TTW, but what's the point if you are losing most of them? lol

If you go the epoxy route, several batches of fins at a time, laminating a bunch of them at once on a table. Then install them on the rocket body at your convenience. Several choices are available, West Systems, Aeropoxy, Z-Poxy, and several others make slow cure hardeners or packages for laminating. West Systems is probably available at nearly every boat/marine shop. Just don't forget to get some quicker curing epoxy for attachment!
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Last edited by tbzep : 05-30-2018 at 03:12 PM.
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  #7  
Old 05-30-2018, 05:23 PM
CuriousAmateur CuriousAmateur is offline
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Errr... Yes, we do want the fins to stay on. Admittedly, the previously mentioned cardboard and duct tape method has sometimes failed at this.

It may be a crime against humanity and rocketry to even mention this, but we've found that hot poly glue even works, as long as your casing stays cool enough. Unfortunately, we're using poly casings right now which reach about 250-275 degrees (F) during the burn, so hot glue does what hot glue should at that temperature. When the shell stays cool though, the poly glue has withstood direct fin to tree branch collisions at high velocity without giving way.

When we want to waterproof wood surfaces, we typically just use polyurethane, but I suppose the epoxy would be faster to do. Interesting.
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