An Easy Ring Fin for Small Bottles
Make sure that your launcher can handle this small diameter extended fin before building! There may be very little clearance between the fin and launcher. Don’t attempt to launch this if there is a chance that the fin could snag on any part of the launcher, be especially cautious with moving parts that could suddenly get in the way! There is no problem using this type of fin on the new Rebel Launcher described on this site.
I chose 24 ounce Pepsi products bottles because they have about 5 inches of straight sides and slightly larger tops and bottoms.
You will need besides your rocket bottle:
A 1 3/4 inch section cut from the center of a scrap bottle of the same kind
A 2 1/2 inch section cut from the center of an 18 ounce peanut butter jar
3 pieces of vinyl mini blind cut to 10 inches long for the struts
A “Pringles” chips tube for making an alignment jig
Several wide rubber bands that fit snugly around the bottle
Your favorite suitable plastic cement
Your usual rocket making tools
Cut a neat band about 1 3/4″ long from the center of a scrap bottle. Then split the band vertically along one if its mold lines so that it can be opened up to fit around another bottle.
Cut out the straight center section from an 18 ounce peanut butter jar. If you cut carefully you will get a piece about 2 1/2″ long. Smooth the edges the best you can. ( Turning the jar on a lathe really lets you make smooth even cuts! ) This jar ring will be the fin and it is slightly larger in diameter than the bottle.
Use the Pringles tube to make an alignment jig. Make 3 accurate vertical lines on the tube spaced exactly 1/3 of its circumference apart. Place tape along the right side of each line to help mark the line. Place the 3 struts on the tube aligned with the lines. Hold them in place with several rubber bands. Put some tape along the left edges of the struts to help mark that side. Each strut should be aligned between two pieces of tape. When the struts look straight and perfectly spaced they are ready for gluing.
Cement the outside surface of the ends of the struts to the inside surface of the ring fin. You may have to cement one strut at a time depending on the cement that you use. Let the cement harden with the assembly tightly rubber banded to your alignment jig. When the cement has dried, take it out of the jig and go to the next step.
Take the split bottle section and place it around the center of a bottle. Measure around this slightly expanded section and divide that circumference into thirds. Position one of the struts on the opposite side of the split in the bottle section then align the other two evenly around. Secure them with rubber bands and mark where the struts should go. Cement the struts to the outside surface of the bottle section and hold them tightly in place with rubber bands until the cement dries.
The finished ring fin weighs about 30 grams and can now be placed on any rocket made from 24 ounce Pepsi bottles. Use several wide rubber bands to secure it while testing, add some tape when you find the best position. It can be adjusted about 3 inches along the bottle. At its longest the fin extends about 5 inches beyond the bottle neck. If that does not provide enough stability try making another fin using 12″ struts.
Testing was done on a pair of 24 ounce bottles coupled neck to bottom ( by a method commonly called a Robinson coupling ) and with just a cut-off bottle top for a nose cone. With the fin installed at its longest, it made a straight and stable ascent then stalled, turned and made a straight nosedive to the ground. With it installed at its shortest, the ascent seemed ok but the rocket’s descent was unstable and it was routinely falling sideways by the time it reached the ground. Probably desirable if you are not using a parachute or it often fails to open.
I found this amusing and a little reassuring for the design. During the initial test I apparently did not use enough rubber bands or positioned them poorly. About ten feet in the air the bottle had blasted through and free of the fin. The bottle continued upward pretty much as expected but right behind it was the fin, flying straight!, until its light weight caused it to lose speed faster and gently fall to the ground.
Some additional small ring fins
On the left is a slightly longer fin on a 24 oz. bottle. The dark ring was cut from a large plastic bottle that held some sort of vitamin supplements. The bottle on the right is a 1 liter size with a plastic ring made from a cake frosting container. Both rings are a close match to their bottle diameters but much more rigid than just a section cut from another soda bottle of the same size. Here the fins are only rubber banded onto bottles for display purposes, not secured enough for flying.