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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Fin Design Reading

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
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LamdinRockets
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Fin Design Reading

Post by LamdinRockets » Tue Apr 03, 2018 6:09 pm

I am making a rocket as part of a project in my degree as such all design parameters have to be justified. Leading from that does anyone have any knowledge of reading material for the design and implementation of Rocket fins.

I have made a similar post for parachute dynamics so if you can help with that please post in the other topic

Thanks for any help



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anachronist
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Re: Fin Design Reading

Post by anachronist » Fri Apr 06, 2018 12:40 am

Your request is too generic. What are your requirements? What is the most important thing? Durability? Ease of attachment or replacement? Aerodynamics? Weight? Stiffness?

There are fins that attach individually. There are fins attached to a ring that you slip over your rocket. There are box fins (see http://www.uswaterrockets.com/construct ... torial.htm for a tutorial). There are fins that fit onto a bottle cap that you screw onto the end of your bottle. All of these are designed to meet different priorities.

I've been trying to 3D-print some fins based on a NACA airfoil, but I'm finding it's beyond the limit of what 3D printers can do -- it's really hard to print long, thin parts without them warping or bending.



LamdinRockets
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Re: Fin Design Reading

Post by LamdinRockets » Fri Apr 06, 2018 7:12 pm

The main requirement would be weight (and obviously stability), I'm unsure on what material to use and how to calculate the size of the fin needed based on the parameters of the rocket.

I find with 3D printers some are better than others with intricate prints (usually the more expensive ones such as UpBoxs)



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anachronist
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Re: Fin Design Reading

Post by anachronist » Fri Apr 06, 2018 7:47 pm

Stability is determined by making sure the center of pressure is behind the center of mass. A bottle rocket is bottom-heavy when fully loaded, but that load is ejected in about a tenth of a second, so the rocket will spend most of its flight time empty, and nose-heavy, so it's fairly safe to use center of mass when empty (the center of pressure won't change). There are rocket simulators available that help you calculate the distance between center of mass and center of pressure. But water rocketry is all about experimentation. I have gone through half a dozen fin designs myself. My current design uses a removable tail cone with fins attached.

Lightweight materials include balsa and coroplast or corriflute. The current class D record holder uses a woven graphite composite sheet for the fin material.

For 3D printing, the printer you use doesn't really matter. The warping is caused by the PLA shrinking as it cools. Long thin objects will bend because the part isn't made all at once, but in incremental layers that don't all cool at the same time. A heated bed and warm 'greenhouse' around the printer helps somewhat.



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retrotec29
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Re: Fin Design Reading

Post by retrotec29 » Sat Apr 07, 2018 12:01 pm

Hello and welcome to the forum. You have stumbled upon a very big topic in the model rocket world. There is still quite a bit of grey area as anachronist and I have tossed around in occasion. I would suggest familiarizing yourself with Open Rocket. It is a free program and you can create simulations of you project. Once you have the general parameters of the airframe established you can try different shaped fins. I draw a lot of inspiration from NAR low power competition. Flights are subsonic and the rocket design is based on efficiency. What looks fast to most of us may not be efficient at subsonic speeds. A model rocket supplier Apogee has an extensive archive which includes some fin design commentaries and a great article on creating thrust curve models for water rockets. Several simulations may be needed for appreciable average but you will have some real data before you fly.


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