Removable Fin Assembly

I like the idea of making the rockets modular so that I can swap parts from rocket to rocket. This will hopefully reduce rebuild time when there is a crash, as well as letting me mix and match to get the config I want. Part of this modular approach is to build a Removable Fin Assembly (RFA), which is what I have been doing this week.

I decided to go with a trapezoidal fin design to see how they perform. Here is a diagram of the design I came up with, this was based on other diagrams I came across on rocket websites.

Trapezoidal Fin Design

Trapezoidal Fin Design

I am using the P&N 1.25L bottles and the diameter (D) of the bottle is 87mm. The size of the fin is proportional to the diameter of the  rocket body. The trapezoid is D wide, 1/2D at its furthest point from the rocket body and D high where it sits against the rocket body.

I used my leftover corrugated plastic to cut out three (3) fins. I decided to run the corrugations up/down so that the air would channel down through the fins in the normal upright position. My assumptions are

  • it will reduce drag in the vertical position as there is less surface area cutting through the air
  • it will increase the surface area when the air is pushing the fin (and therefor rocket) back to the vertical

we will see if this assumption is right I guess.

Firt cut the fins out of the corrigated plastic

First cut the fins out of the corrugated plastic

Here is a close up looking down from the top of the fin, you can see down the corrugations

looking down corrigations

looking down corrugations

I also cut out six (6) sections of corrugated plastic to use to glue the fins to the RFA body. These were eight (8) corrugations wide and approx 2/3D in length. I used a sharp knife / blade to slit down one side so it would bend back at approx 90 degrees.

fin attachments

fin attachments

The RFA body is the centre section of a 1.25L bottle. This is the same diameter as the rocket body. I cut it to allow for a lip of a few mm to make it easier to attach to the main pressure bottles of the rocket. This lip was already in the bottle where is widens a little near the base of the bottle. You can make it out in the below pic (click the pic for a higher resolution picture)

removable fin body section

removable fin body section

So what I have to work with is this, three (3) fins, six (6) fin attachments and 1 body section.

Removable Fin parts

Removable Fin parts

Next thing to do is measure the exact positions for the fins. I used a piece of paper to wrap tight around the bottle, temporarily tape it and marked where it overlaps. It is then taken off the bottle, unrolled and the distance from the start of the paper to the mark I made (external circumference of the bottle) is measured. Its then divided accurately in three and a ruler used to mark the 1/3 and 2/3 (fin) locations.

The start of the paper and the end mark is the other (3rd) fin location. The paper is put back on the bottle 1cm up from the bottom and the locations marked on the bottle with a permanent pen. Lines are then drawn vertically up the body section. This gives the exact locations to mount the fins.

fin locations marked on body section

fin locations marked on body section

Next the fin attachments are glued to the fins. In order to get these flush, when the glue was setting I held the attachment and fin together against a hard surface. This will ensure when they are attached to the fin body the spine of the fin as well as the fin attachment will fit snugly against the body surface.

glueing fin attachment

gluing fin attachment

I did one side at a time and let it dry before doing the other side. So now I had three (3) fins with both the fin attachments connected. Here are some pics.

both fin attachments connected

both fin attachments connected

fin attachments looking from the bottom

fin attachments looking from the bottom

As the body section of the RFA is not flat, but instead round, the movable fin attachments allow the angle to be more like 100 degrees rather than 90 degrees to follow the curve of the bottle.

Next step is to glue the fins + fin attachments to the RFA body, this was done one at a time and the line drawn previously was used to line up the spine of the fin. These were held in place while the glue dried. I have them 1cm from the bottom so that when the rocket is standing up while not being used, the weight of the rocket is not on the fins.

fins attached to RFA body (you can see the RFA body lip more clearly in this pic)

fins attached to RFA body (you can see the RFA body lip more clearly in this pic)

Due to the type of glue I’m using, I’m not very confident in the strength of it under load, so I decided to strengthen the connection points to the RFA body with wire. The wire is 1.5mm tin wire. I used a drill to drill 2 holes (1.5mm) in each fin attachment and through the RFA body. I then bent a section of tin wire and secured the fins to the RFA body. I used 6 wire fixings in all, one for each fin assembly. This pic shows one already done as well as the next one ready to go it.

fin assembly reinforcement

fin assembly reinforcement

Once all six fin attachments are done it looked like this, ready to be fitted to the main rocket

finished RFA

finished RFA

finished RFA - top view

finished RFA - top view

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This entry was posted on Saturday, August 23rd, 2008 at 3:13 am and is filed under Development. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

 
 

2 Responses to “Removable Fin Assembly”

  1. ishra 123567gre Says:

    whereto get that plastic

  2. todd Says:

    from sign makers or real estate agents. The sign makers will usually have some offcuts they will give you