The fins

The issues

The goal of the fins is to insure the stability of the rocket, by pulling the center of pressure (CP) at about 1 diameter of the rocket under the center of gravity (CG).

To do that, we can act either on the area of the fins or on their position toward the center of gravity. The lower the fins will be, the less the area of the fins will be important.

Minimize the area of the fins is important because, if they enhance the stability of the rocket, the fins have a negative influence on the performances. They increase the weight, the coefficient of drag and the frontal area towards the air penetration. It's necessary to take in account all of these parameters to design good fins.

My first designs

When I began in the water rocket activity, I used some "easy to use" solutions even if they give less performances, so my first fins are made of strong cardboard, cut in old office calendars. The picture below shows clearly for this rocket the use of a 1998 calendar. I use a polyurethane glue for wood in a first time, then PL Premium (thanks to Hervé Bregent who give me a tube that he bought in USA). We do not find PL Premium in France, but I find another Polyuréthane glue named "Sikaflex 11 FC" which seems to be similar. I do not made any comparison beetween them, but I plan to make such work.

To glu the fins on the body of the rocket, we need an assembly to position each of them, then to maintain them during the drying of the glu. The pictures bellow show what I do for that, with fruits packaging.

On the packaging, I cut a circle (dia 33mm) in the middle for the cap, and four grooves for the fins.

I glu two pieces of cardboard for each fin to maintain it verticaly.

Then I glu each fin, I put it in its groove and I put the rocket in the middle.

The paper sheet around the bottle draw the line where the top of the fins reached. (I use also the method to cut the bottles)

Installation before gluing and insertion of the bottle. Simple and cheep

Present achievement

To enhance the performances of my rockets, I don't use anymore cardboard for the fins, but I use PET.

The fins are lighter and stronger, but the achivement is not so easy. They are made with sheet which is folded and glued. The sheet of PET is cut in a sacrified bottle, then it was very curved. By folding and gluing it, the result is a flat sheet (in almost cases :-)).

Bellow you can see the different steps :

I make a template composed of 2 times the fin's profile associated by their leading edge. I have to don't forget the flaps with which I glu the fins on the body of the rocket.
I cut in a bottle, the cylindrical part, then I cut this cylinder, as shown on the right. I have now a rectangular sheet.
I draw with the template, the outline of each fin on the sheet, such as the leading edge (therefore the fold) is on a generatrix of the cylinder, or a perpendicular to this generatrix. This allow to have the 2 parts of the fins with the same curve but with opposite way, thus after gluing, the set is flat.
I draw a line for the fold of the flap.
I fold the sheet of PET along the leading edge.
I have to glu the two parts with cyanoacrilate glu. I must be carefull to don't glu the two flap. For that I fold them before gluing. On the picture, one face is gluing.
Then the two parts are folded and I have to rub on one face to spread the glu on the whole area. (Be careful for your fingers).

Because I use Cyanoacrilate glu, I have to press the two parts. For that I use two boards, pressed together with clamps. The flaps are outside and folded, so they are not glued.

I cut the fin according to the outline drawn previously. Then I have the final fin as shown on the picture on the right.

Finally I have to glu each fin on the body of the rocket.

Bellow, you can see a 2*2liter rocket with this king of fins, with a close-up of this tail and another tail of a 2*1.5 liter rocket which I paint in fluorescent orange to see it better.