Fins

This section will be about various tail fins.

If you are using bottles smaller than 1 liter to build your rockets, check out this newer addition to the FINS section:

https://rebelrockets.wordpress.com/fins/ring-fins-for-smaller-bottles/

Ring Fins for 2 Liter Bottles

So far the only type of fins that I’ve used are ring fins. The bad news is that your launcher must have an upright part that is narrow enough to pass through them safely and tall enough to keep them above any other moving parts. They also can’t be used on any other than a single or first stage rocket. The good news is they seem to be very effective, they can be made to survive a crash, and they can be attached without any special glues.

Materials used:

  • 3 pieces of vinyl mini blind slats of equal length (about 16 inches)
  • the straight section cut from a 2 liter bottle that can be force fit over your pressurized bottle without kinking it
  • a 1 inch ring cut from 4 inch PVC drain pipe
  • thin rubber bands about 1 inch in diameter
  • thin rubber bands about 2 inches in diameter
  • a heavy rubber band that fits snug around the bottle

Making the parts

Cut the very bottom off of the bottle where the straight section begins. Test the fit over your pressurized bottle. You should be able to push it over without creasing the inner bottle. If not, find a slightly larger bottle to cut or a slightly smaller one to pressurize.

Cut a 3 inch section from the bottom of the bottle that you’ve just cut. Be as straight and smooth in cutting as you can. This will be the ring fin.

Now you can either cut the top off the remaining bottle piece leaving only the straight section, or you can just cut a part of the top off leaving a neat round hole big enough for another bottle’s flange to pass through. That way this piece will slide over the nozzle end of your bottle until it fits like a second layer of plastic.  I’ve noticed that just a straight section will really slide toward the nose on impact, needing a lot of adjustment to fly again. This section with a round shoulder left on it can’t slide forward. This type of section would also be good for attaching the more common straight fins, giving you a larger area for gluing. Which ever style section you’ve chosen will be used to attach the blind slats to your rocket.

Carefully cut a 1 inch ring from a piece of 4 inch PVC pipe using a miter box and saw if possible. This size pipe is a tight fit inside most 2 liter bottles, at least in the USA it is. File and sand the edges and make the ring as straight and even as you can. This PVC ring will strengthen and shape the leading edge of the ring fin cut from bottle. Take the PVC ring and divide its circumference into equal thirds by using a cloth tape measure, a protractor, or wrap a paper around it and then fold the paper into thirds. Mark the edge of the ring with a fine point marker at the three points. Now carefully mark the other edge exactly across from the first three marks using a square or something with a 90 degree right angle. These marks will align the struts for your ring fin and must be accurate. Now take a piece of blind slat and use it to make a second set of marks to the right of the original marks as you go around the ring. The marks should be on both sides of the vinyl slat right at its edges. Make sure you always make your new marks on the same side of the originals so that your struts will be even and balanced. Finally, take a small file and make shallow notches in the edges of the ring where all the marks have been made. Sand paper the notches to remove any sharp places.

You will need to measure and divide the bottle mounting section into equal thirds like you did the PVC ring. Begin by marking the top edge like you did the PVC. You can even temporarily push the finished PVC ring inside the bottle section to place your marks at the notches. If you are using just the straight bottle section go on ahead to the assembly instructions. If you are using the rounded end section you will need to make slits across the bottle where the straight side ends and the curved part begins. The vinyl blind struts will need to pass through them. Make the slits aligned with the marks at the top edge. Making a small round hole at the ends of the cut slit may help keep it from tearing. As an alternative to cutting slits you could make three notches the width of the blind slats, cutting from the neck opening toward the straight section.

Assembly

Push the mounting section over the bottom of your rocket bottle. If it is just a straight section align it with the lower straight part of your bottle. Push an end of each blind slat up between the mounting section and the bottle, being very carefull not to bend or kink it. Make the curve of the slat match the curve of the bottle, don’t force it the other way. Align these three struts with the marks made previously. Stop when the struts poke out the other side about an inch, making sure they are equal.  Place the heavy rubber band around the bottle and over the ends of the struts that are sticking out.

Place one 1 inch and one 2 inch rubber band over each strut. Take the PVC ring and position it within the three struts near the free ends. Align the notches in the PVC ring with the struts. Take each of the smaller rubber bands and pull it down inside the ring then over the end of the strut and let it snap into place. It should be in the shallow notches, forming a square loop that joins the ring to a strut. I have made marks at one inch intervals on the inner side of the struts for easy alignment. Carefully work the ring up the struts until it is four inches from the ends. Smooth out the rubber bands if they have twisted or come out of the notches.

Take the ring fin cut from bottle and force it over the struts and PVC ring until its top edge aligns with that of the PVC. Pull each of the 2 inch rubber bands down over the ring fin and hook them over the ends of the struts.

Do the best you can to make the struts even and parallel to each other and the center line through your rocket. You can make up a jig to help set the alignment if you wish.

Testing

Begin testing with the ring fin mounted as far down the struts as possible. If that goes well, try moving the ring upward an inch and try again. Repeat until your rocket performs worse. When you are satisfied that you are getting the best performance any extra length can be removed from the struts or they can be pushed up farther into the mounting section. You may also try a shorter or longer section of bottle for the ring fin to see if that helps. Depending on what kind of rocket you are trying to launch, you may need longer or stronger struts. A second piece of vinyl blind may be taped inside the struts to strengthen weak spots.

After Crashes

The first several times that I used this fin were on rockets without parachutes and then it was on ones that had habitual parachute failures. It was amazing how much this fin collapsed on itself at impact. It reminded me of pictures of pieces of straw driven into trees by tornado or hurricane winds. The struts were driven forward several inches through the mounting section. Sometimes even the mounting section had moved forward a lot. Before the PVC ring was added, even the light weight ring fin was forced down to the bottle. The struts would twist and bend but did not break, at least not at the low altitudes it had flown at. Its collapsibility and flexibility saved it from total destruction.

Rebuilding was usually a matter of pulling the struts back in place and lining them up again, and then repositioning the ring. Sometimes a small rubber band would break and need replacement. Spare parts could easily be brought along to the launch site. No need to glue parts back together and wait for the glue to cure!

Will this design hold up under the most extreme pressure, maximum acceleration launches? Possibly not, maybe with more reinforcement it could. So far it has not broken apart at launch under my limited stress conditions.

ring_fin

This photo shows the ring fin used with my older, first version of the launcher. For a much improved newer version check out the NEW “Rebel” Launcher in the LAUNCHERS section.

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