Fins are important to your rocket to increase stability and accuracy. Why does a rocket need to be accurate? You don't want to be explaining to your neighbor how much fun water rockets are after its blown through his bedroom window, do you?
Fin Making Guide - 4/23/2006 - A step by step guide on how to create water rocket fins that are aerodynamic and tough.
Fin Jig Guide - 4/4/2006 - A step by step guide on how to create a jig that will help you accurately and reliably glue your fins to your rockets.
Fins can be all shapes, sizes, and made out of almost any stiff, light weight, durable material. Because these are WATER rockets, waterproof material can be key. Also, due to gravity, I also like to emphasize the durability and replaceability of the fins as well. This section will be split up into two main sections: (1) materials and (2) designs.
- Cardboard - Cheap, available, and easy to cut but not very waterproof. I've never used cardboard myself but plenty of people have with great success. Did I mention the price was right?
- Cereal Box Cardboard - I mention this separately because of it's normally plastic coated covering. I've seen reports of people folding this over, glueing together and forming a fin. Again, never tried it but the price is right.
- Coroplast - corrugated plastic, similar to corrugated cardboard, only lighter, stronger, and waterproof. I've used this extensively and it is virtually indestructable. My only issue with it is that it is hard to glue. I've been using super glue with some success. As the rocket is returning to Earth via a tree, the glue bond breaks sometimes leaving the fin in the tree. On even "hard" landings, the fins suffer no damage. I call my local sign shop to see if they have scrap which they oblige me with. Comes in colors too!
- Bottle Plastic - Family and friends save me their 2 liter bottles once they found out about my new hobby. This keeps a steady supply of lightweight, formable, durable plastic coming in. This is going to become my main fin material.
- Carbon Fiber - Light weight, strong and water proof. If it's good enough for the Air Force and the F-22, it's good enough for a water rocket. Unfortunately, my calls to Lockheed haven't been returned.
- Direct attachment - Many people will glue their fins directly to the bottle. I never have and there are two main reasons: (1) The glue may weaken or alter the bottle. This in turn may lead to some sort of "failure" including rupture. (2) With hard landings, the bottle may be compromised meaning that the bottle isn't fit for another launch. It's nice to be able to remove the fin set and re-attach to another rocket.
- Extended Tail Piece - At one type this was my preferred method of fins.
The extension helps put the Center of Gravity (CG) closer to the center, between the Barrowman Center of Pressure (BCP) and the Center of Lateral Area (CLA). Basically, this means that you get a stable flight and more importantly, you get some backgliding behaviors and it saves your rocket from hard landings.
One problem that I've had with these extensions aren't the fins at all, but the extension itself. After much labor and effort, invariably a crack develops in the JB Weld epoxy holding the PVC to the bottle cap. I attribute this to the torque applied to this connection at the time of landing. I suspect a more flexible epoxy would help but I haven't had the time to experiment with it.
- Sleeved fin sets - In keeping with my "modular" philosophy, I like this design. If the bottle fails, a quick detachment of the sleeve allows me to move my valuable fins onto another. Likewise, if the fins are damaged, I can slip another sleeve onto the bottle that I've got ready for launch. As you can see from this picture, the bottle is toast after hitting the driveway after a 300' launch and only a half tennis ball to protect it. Fins are in good shape though!
- Extended Cylender - This is a design in my future. I'm thinking this might give me good back gliding behavior with a rugged, modular design.