Intentionally Breakable fins?

I need to design intentionally breakable fins on Necessities of Life. They are basically really big fiberglassed foam fins that are an upscale of an Alpha. I have to design them to intentially break off on landing, because they are going to break off anyway. Any ideas?

Use plastic cement to hold them in or a glue that can be peeled off of the fiberglass. Something that is storm enough to stay on during the flight but will break on landing and can them be easily taken off.


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blackbrandt said:

I need to design intentionally breakable fins on Necessities of Life. They are basically really big fiberglassed foam fins that are an upscale of an Alpha. I have to design them to intentially break off on landing, because they are going to break off anyway. Any ideas?

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You can bolt them to the airframe, like I did for the polycarbonate fins on my Light Show night launch rocket.

How would those break off? Those seem permanently mounted...

I can't go to the link that Johnshared but I'm guessing you could use nylon screws that would sheer on impact just like sheer pins.


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blackbrandt said:

How would those break off? Those seem permanently mounted...

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The fins are bolted between the rails, so if they break, new fins can be installed.

Remember if they are breakable on landing, they are breakable in flight. If you make them to come off on landing, you might need to consider watching the thrust of the motor or you might shear them off in flight.

cwbullet said:

Remember if they are breakable on landing, they are breakable in flight.

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Good point. I think of it more as making them replaceable than break-away.

I used Nylon screws in this same sort of arrangement on my OTRAG, and in that case the fins survived (at least the first flight).

I would only think of designing or discussing breakable fins on landing in a low power setting. Any mention to the big boys opens you up to the Bulletproofing today, Bulletproofing tomorrow, Bulletproofing forever speech. Be safe.

Matt as stated above by others , you dont want the fins to break away in flight . You want to be able to remove them after they ( may ) break on landing . I should have been more clear . Think of your fins like a crumple zone in a car . The fins will act like the shocks on a car bumper and will give way before the airframe gets damaged . Again as stated above , just a couple 1/4 inch bolts will hold the fin to the airframe . You can glue / screw aluminum angle to your airframe pre drilled so all you have to do is slide a new fin in , two bolts and your flying .

Ahh!! I see. That makes more sense. Thank you.

It could work to have some kind of hook, plus a slot or something, where they only break off in response to pressure from the rear. From the front, and plus side pressure, they aren't breakaway at all except for replacability. Some friction to avoid them just falling off if the rocket tailstands at apogee.

And dont tell the LCO they are break away fins.

g zilla said:

And dont tell the LCO they are break away fins.

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I am sure most of the LCOs are reading this.

Check out this link. It's a Vatsaas rocketry technique.
https://www.vatsaas.org/rtv/construction/removable/fins.aspx
I think it's cool.
-Ken

kenstarr said:

Check out this link. It's a Vatsaas rocketry technique.
https://www.vatsaas.org/rtv/construction/removable/fins.aspx
I think it's cool.
-Ken

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I thought this was interesting:
Another advantage is that damaged fins are easily replaceable. Certification rules allow for frangible parts, so you can still certify if with a broken fin if you can demonstrate that it was designed to be replaced.

I want to make a rocket that drops its fins after apogee, hanging onto them with thin Kevlar cord or something. The problem is that if the fin needs the detaching for landing damage, then it might not be able to hang on in flight stiffly enough.

What about rare earth magnets? Yes, there is weight, but I bet it will hold on during flight and break at landing. Then, no need to clean off old glue and re-glue it. Just stick it back. Also makes transport easy as you can drive to the launch with the fins off, and put them back on in the same time it takes to load a CTI motor (and way quicker than an AT).

I've built a few rockets with removable fins (up scale 7 inch diameter der Red Max) for ease of transport. They were Tab/lock like Vatsaas link above.
I have also done a few with shear pins with the fin sandwiched between two layers of fiberglass (G10.)
The question becomes How much harder is the PLANNED landing than the flight and acceleration on the way up? If the parachute is the right size and the descent rate normal, I would think it's impact would normally be less than the launch "impact."
But then again I busted a 1/2 thick plywood fin on my Citation Patriot (Another 7 inch up-scale) when it came down at a normal speed in a cow pasture....

Something that occurs to me is that the fins need not have the same amount of strength in all directions. It would be easy to design a fin system that holds the fins rigidly between two flanges. The fins/flanges would be resistant to bending forces (for example fin flutter or side loads), but could easily shear with forces along the same direction as the fins/flanges. Would make CarVac's idea of "discarding", or latched on fins easy to implement. Here's a crude drawing, showing a pivot (or possibly a latch) at the top of the fin. An impact, for example on landing, would shear the retaining pins, allowing the fin to pivot upwards.