leica:
well I can tell you how I make them..
I take a delrin rod that has a .002 (if using .001) or .004(if using .002) less diameter....
for example since I want a .736 OD tube ( so i can use pratt/asp pre-vacuum formed nose cones) the OD of the delrin rod would be .732 if using .002" thick kapton plastic...
In addition to this delrin rod, I also have a piece of delrin tube that is 1/8" thick that I had cut into half so that I have 2 halves or 2 "cradles" that the delrin can sit in...
For a 12" long tube I cut off a piece of 12" x (pi x dia) or 12" length x (3.14 x .736) = 2.311" width .. now I will add a specific "tab or overlap" to this say .25 or 1/4".. for a glue seam
I then will wrap this around the delrin rod to get the kapton plastic sheet to have a 'curvature"........
My delrin rod has a "register line" drawn lengthwise along the rod....
I line up on side of the kapton plastic on thsi line.... I use tape the kapton sheet to each end of the delrin rod..
I place this into the cradle and rotate the rod with the kapton sheet attached until I get to the point where it will overlap....
I rotate and tape to hold in place and then you have several options;
1. double sided tape
2. CA glues then or thick
3. 5 min epoxy..
SO using your preferred adhesion method you lay down a fine line of glue along the length of the seam line..... or if using double sided tape( regular 3m cellophane or better yet is kapton double sided tape
and then rotate to make sure the glue is spread out more or less evenly along the seam line.....rotate and press down hard here......
I then take the 3nd piece of the cradle and lay it one top and then use rubber bands along the length to apply pressure...
cavets....
you need to wax or spray the delrin tube with release spray so the tube will not stick to the delrin tube mandrel....vaseline will also work..... I use a teflon spray available from mcmaster carr....
the amount of seam overlap and thickness of the kapton is still to be determined for 13 and 18mm model rocket engines.... I am still experimenting with this material...
initial experiments seem to indicate that 5 min epoxy is better than the CA or doubled sided tape.....
I have not used less than a .25" width seam.....
The thicker the kapton plastic, the more the seam overwrap, etc will determine the final tubes rigidity.........
consider a 1" length inner wrap of kapton right above where the engine ends internally to help protect from hot ejection charge gases....
to glue to the surface, rough up the area to be glued, like where you will attach fins for example....
and remeber that the FAI guys use a ejection foam plug towards the back....then the recovery stsem and then a smaller foam plug which applies a great deal of the needed rigidity to the tubes during flight........
good luck
shockie B)
