Usually it's easiest to buy the tubing you need, but sometimes it just isn't available. For example, a particular size for a scale model. Telescoping casting/liner tubes. A long coupler that slides over, not inside, another tube. And so on.
Convolute paper tubing---the kind used for BP motors---looks simple to make, but when it comes to model rocketry the devil is in the details. Wrinkle-free tubing of precise dimensions can be a challenge and takes a fair bit of practice. (I still have trouble with it.)
By contrast, spiral-wound tubing may appear to be challenging but it can be quicker to master than convolute winding, especially for long tubes. Industrially it’s made on a machine that winds all of the layers of paper simultaneously and continuously. Tubes are cut from the mandrel as they reach the proper length. This process is much faster and less expensive than convolute winding, and has virtually no limit to tube length.
But we don't need a machine for just a few tubes. We can duplicate, by hand, what the machine does. It will be slower…but after all, what is a hobby but a way to waste time?
Materials
1. Decide on the length and wall thickness of the tube. This will be a 12" tube with 0.04" wall, so about eight layers of 0.005" tape.
2. Approximate strip length. You can calculate it: 0.875" x 12" x pi = 32.97 sq. in., divide this by 1.5" tape width to get 22" long strips. What I do is to simply spiral the tape around the mandrel to give the approximate length. It’s a good idea to cut the strips a bit longer so the rough ends can be removed. For this tube I’ve cut seven strips. I also cut the ends at roughly the angle needed to give a square end; about 45° will do.
An eighth strip is cut to the same length as the others, but is a mirror image of them. That is, the cut angle will be in the opposite direction of the angles cut on the other strips. This is the starter strip, #1, and will be wrapped gummed-side OUT. The image shows one of the regular strips and Strip #1 side by side; both are gummed-side up. (From here on I'll just say Strip #1 or just #1, instead of the word 'tape'.)
3. Two pieces of scotch tape, long enough to go around the mandrel plus a little more, are cut. A piece of release paper is cut a few inches longer than the finished tube, wide enough to go around the mandrel a little more than once. Or twice, if it's thin release paper. Experiment.
4. The release paper is wrapped tightly and smoothly around the mandrel (silicone side OUT). Scotch tape sticks to itself much better than it sticks to release paper, so the tape is wound around each end such that the ends overlap. If possible the tape should be positioned so that it adheres to the mandrel as well, as seen here.
If you have trouble getting the release tight on the mandrel before taping, lay it on a large flat surface and roll (not back-and-forth) with the palms of the hands while pressing down on paper+mandrel. If the paper loosens you may be rolling the wrong direction! For very long tubes it may be easier to cut long strips of wax paper and spiral-wind them around the mandrel. Leave about 1/16” gap between the edges of the waxed paper. Do not allow the edges to overlap as that will leave a ridge. Two layers of wax paper strips are recommended in this case, with the second layer covering the edges of the first. This will prevent the gummed adhesive from leaking onto the mandrel, which could make it impossible to remove the finished tube.
5. Cut two more pieces of scotch tape long enough to go around the mandrel. Now spiral Strip #1 around the mandrel, gummed side OUT, covering the release tightly and smoothly. Leave about a 1/16” gap between the edges. Tape each end of Strip #1 to itself and to the release, and to the mandrel if possible.
6. Now comes the tricky part. Wind Strip #2, dry and gummed side IN, a little more than one turn around the mandrel. (These images were re-shot with blue tape for #2, for clarity.) The center of #2 should be over, or close to, the seam of #1. Again, leave about 1/16” between the edges of #2.
7. With one hand, hold #2 in place a few inches back from the end of the tape, allowing the end to come free. Don't let #2 slip out of position! Brush water on the exposed gummed surface. It should be wet enough to activate the gum of both strips.
Now with the other hand, wrap the wetted end of #2 back down onto #1 and smooth it down.
8. If this has been done correctly, #2 is now in the proper orientation, and the 1/16" gap should remain nearly constant as you wind. Wet about a foot of #2 at a time and smooth it down snugly over #1. I usually rotate the mandrel with the right hand and smooth the strip down with my left. Continue until you reach the end of #2. Have the scissors handy in case #2 needs to be trimmed shorter.
Sometimes the edges of the tape move together or apart a little as you wind. To fix this, apply a little more pressure, fingers against the mandrel, to one edge or the other of the strip as it is wound. Not enough pressure to tear the tape, but enough to slowly shift it back to where it’s supposed to be. Again, avoid the tape overlapping itself as that will leave a ridge.
9. Repeat Steps 6-8 for the remaining strips. Less wetting will be needed since only one surface has adhesive.
The tube will shrink a bit as it dries. I stopped this tube with Strip #7 to ensure that it would fit into a 24 mm MMT. If it shrinks too much on drying, another strip can be added later. Trim the ends, primer or filler, sand lightly w/fine grit.
When making tubes more than 2-3 feet long there’s a tendency for the seam to “drift” a bit. That is, you might start with the middle of a new strip over the previous seam, but by the time you get to the end, the new seam is on top of the old one. That’s just a consequence of the uneven pressure of human hands. Probably better to make shorter tubes and couple them together, if that works for you. Still, I’ve made 1” tubes five feet long after some practice.
10. The scotch tape is removed while the tube is still damp—you’ll probably tear the ends a bit in doing so, but that's ok, it's why you make it oversize—and the tube is gently slid off the mandrel. The release paper is removed from inside the tube by pushing a dowel down the tube. The mandrel is rubbed with a little talcum powder and the tube is slid back on the mandrel. This helps it keep its shape and diameter as it dries and shrinks.
The tube should be finished as you would papered fins.
A mandrel that is a bit undersized can be built up somewhat, if you expect to need more than a few tubes. Wet and wrap the undersized mandrel with gummed tape as was done for the tube. Each wrap of 50 lb tape will increase the OD of the mandrel by about 0.01”. After enough layers have been applied the assembly is allowed to dry thoroughly and the grooves are filled, sanded, and sealed as though you’re finishing a rocket. I've made several such mandrels from galvanized EMT wrapped and sealed with tape. The one below was the right size for casting tubes for 29 mm motors.
Convolute paper tubing---the kind used for BP motors---looks simple to make, but when it comes to model rocketry the devil is in the details. Wrinkle-free tubing of precise dimensions can be a challenge and takes a fair bit of practice. (I still have trouble with it.)
By contrast, spiral-wound tubing may appear to be challenging but it can be quicker to master than convolute winding, especially for long tubes. Industrially it’s made on a machine that winds all of the layers of paper simultaneously and continuously. Tubes are cut from the mandrel as they reach the proper length. This process is much faster and less expensive than convolute winding, and has virtually no limit to tube length.
But we don't need a machine for just a few tubes. We can duplicate, by hand, what the machine does. It will be slower…but after all, what is a hobby but a way to waste time?
Materials
- A mandrel with o.d. the same as (or very slightly smaller than) the tube i.d. Metal, PVC, wooden dowel, almost anything that is round, very smooth, and rigid. It must be at least a few inches longer than the longest tube to be made. Here I'm making an adapter to fit 22mm research motors in 24mm motor mounts, so the mandrel is 7/8" o.d aluminum tubing. File or sand off any burrs on the ends.
- Gummed paper tape, water activated. Not fiberglass-reinforced tape unless you prefer lumpy tubes. A convenient tape width is about half again the diameter of the tube. Tape thickness is roughly the “pound” rating with the decimal moved four places to the left. The roll shown here is 1½" wide and rated as "50 lb" so about 0.005" thick (it's actually a bit thicker). Thin tape, 40 lb or thinner, tends to wrinkle and tangle once wetted. I prefer 70 or even 90 lb tape if available.
- Wax paper or other release paper. I like thin silicone paper backing from Con-Tac or from self-stick floor tiles.
- Scotch tape, scissors and a sharp hobby knife or box cutter.
- Foam paintbrush and a cup of warm water.
- Talcum powder. Real talc is still available as a cheap filler for epoxy. Cornstarch might work but I haven't tried it. I don't know how sticky damp cornstarch would be.
1. Decide on the length and wall thickness of the tube. This will be a 12" tube with 0.04" wall, so about eight layers of 0.005" tape.
2. Approximate strip length. You can calculate it: 0.875" x 12" x pi = 32.97 sq. in., divide this by 1.5" tape width to get 22" long strips. What I do is to simply spiral the tape around the mandrel to give the approximate length. It’s a good idea to cut the strips a bit longer so the rough ends can be removed. For this tube I’ve cut seven strips. I also cut the ends at roughly the angle needed to give a square end; about 45° will do.
An eighth strip is cut to the same length as the others, but is a mirror image of them. That is, the cut angle will be in the opposite direction of the angles cut on the other strips. This is the starter strip, #1, and will be wrapped gummed-side OUT. The image shows one of the regular strips and Strip #1 side by side; both are gummed-side up. (From here on I'll just say Strip #1 or just #1, instead of the word 'tape'.)
3. Two pieces of scotch tape, long enough to go around the mandrel plus a little more, are cut. A piece of release paper is cut a few inches longer than the finished tube, wide enough to go around the mandrel a little more than once. Or twice, if it's thin release paper. Experiment.
4. The release paper is wrapped tightly and smoothly around the mandrel (silicone side OUT). Scotch tape sticks to itself much better than it sticks to release paper, so the tape is wound around each end such that the ends overlap. If possible the tape should be positioned so that it adheres to the mandrel as well, as seen here.
If you have trouble getting the release tight on the mandrel before taping, lay it on a large flat surface and roll (not back-and-forth) with the palms of the hands while pressing down on paper+mandrel. If the paper loosens you may be rolling the wrong direction! For very long tubes it may be easier to cut long strips of wax paper and spiral-wind them around the mandrel. Leave about 1/16” gap between the edges of the waxed paper. Do not allow the edges to overlap as that will leave a ridge. Two layers of wax paper strips are recommended in this case, with the second layer covering the edges of the first. This will prevent the gummed adhesive from leaking onto the mandrel, which could make it impossible to remove the finished tube.
5. Cut two more pieces of scotch tape long enough to go around the mandrel. Now spiral Strip #1 around the mandrel, gummed side OUT, covering the release tightly and smoothly. Leave about a 1/16” gap between the edges. Tape each end of Strip #1 to itself and to the release, and to the mandrel if possible.
6. Now comes the tricky part. Wind Strip #2, dry and gummed side IN, a little more than one turn around the mandrel. (These images were re-shot with blue tape for #2, for clarity.) The center of #2 should be over, or close to, the seam of #1. Again, leave about 1/16” between the edges of #2.
7. With one hand, hold #2 in place a few inches back from the end of the tape, allowing the end to come free. Don't let #2 slip out of position! Brush water on the exposed gummed surface. It should be wet enough to activate the gum of both strips.
Now with the other hand, wrap the wetted end of #2 back down onto #1 and smooth it down.
8. If this has been done correctly, #2 is now in the proper orientation, and the 1/16" gap should remain nearly constant as you wind. Wet about a foot of #2 at a time and smooth it down snugly over #1. I usually rotate the mandrel with the right hand and smooth the strip down with my left. Continue until you reach the end of #2. Have the scissors handy in case #2 needs to be trimmed shorter.
Sometimes the edges of the tape move together or apart a little as you wind. To fix this, apply a little more pressure, fingers against the mandrel, to one edge or the other of the strip as it is wound. Not enough pressure to tear the tape, but enough to slowly shift it back to where it’s supposed to be. Again, avoid the tape overlapping itself as that will leave a ridge.
9. Repeat Steps 6-8 for the remaining strips. Less wetting will be needed since only one surface has adhesive.
The tube will shrink a bit as it dries. I stopped this tube with Strip #7 to ensure that it would fit into a 24 mm MMT. If it shrinks too much on drying, another strip can be added later. Trim the ends, primer or filler, sand lightly w/fine grit.
When making tubes more than 2-3 feet long there’s a tendency for the seam to “drift” a bit. That is, you might start with the middle of a new strip over the previous seam, but by the time you get to the end, the new seam is on top of the old one. That’s just a consequence of the uneven pressure of human hands. Probably better to make shorter tubes and couple them together, if that works for you. Still, I’ve made 1” tubes five feet long after some practice.
10. The scotch tape is removed while the tube is still damp—you’ll probably tear the ends a bit in doing so, but that's ok, it's why you make it oversize—and the tube is gently slid off the mandrel. The release paper is removed from inside the tube by pushing a dowel down the tube. The mandrel is rubbed with a little talcum powder and the tube is slid back on the mandrel. This helps it keep its shape and diameter as it dries and shrinks.
The tube should be finished as you would papered fins.
A mandrel that is a bit undersized can be built up somewhat, if you expect to need more than a few tubes. Wet and wrap the undersized mandrel with gummed tape as was done for the tube. Each wrap of 50 lb tape will increase the OD of the mandrel by about 0.01”. After enough layers have been applied the assembly is allowed to dry thoroughly and the grooves are filled, sanded, and sealed as though you’re finishing a rocket. I've made several such mandrels from galvanized EMT wrapped and sealed with tape. The one below was the right size for casting tubes for 29 mm motors.
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