Time after time, at club meetings, on the rocket field, in e-mails, and on the various rocketry internet forums, people fret over the chore of increasing the size of existing body tubes in order to fit some scale project. The prospect of "building up" a body tube seems to be perplexing and daunting for almost everyone. And I would have to agree. Increasing the size of an existing body tube is time consuming (usually more so than the effort is worth) and always unnecessarily increases the weight of any given model (though most methods of "building up" body tubes also add strength).
One thing about this that is so obvious yet totally escapes almost everyone is the prospect of taking a body tube that is slightly larger than what you need and DECREASING its diameter. WHAT??!!! Yup...select the next larger body tube and make it smaller!
For years, I would make tubing couplers by simply cutting a length of tube of the same diameter as the tubes that I wanted to join, slitting it along its length, rolling it so it would slip inside the tube sections, and then glue the overlapping seam by wicking CA along the seam. Once I started doing this with larger tubes, rather than overlapping the edges of a split tube, I would actually remove a section from the tube (for short tube couplers this was easy enough), slip inside one of the joined tubes so that it half protruded, and again gave it the CA treatment.
It will almost take longer to tell how to reduce the diameter of a body tube than it takes to perform the actual work. But let me lay a little groundwork and cover some generalizations about reducing the diameter of an existing manufactured spiral-wound body tube...well, this actually works on thin-wall plastic tubes and would probably work with other tube materials as well.
Through experience, I have discovered that you can easily reduce the diameter of a body tube by as much as 10 percent. Furthermore, it is easier to reduce large tubes than small tubes. The same holds true of thick-walled tubes (up to a point) versus thin-walled tubes.
Realize that the tube will no longer be perfectly round of its own accord. Be prepared to add internal rings to keep the tube round. However, again, the process is easier with large diameter tubes than small but thin-wall tubes are easier (logically enough) to get back to a round shape than thick-walled tubes.
Consider your own skills...because if you have trouble filling and hiding the spirals of a body tube you stand no chance of hiding the end-to-end seam that you will create when reducing a body tube diameter. Your skills should also extend to and exhibit some proficiency in making absolutely straight cuts in body tubes.
Many scale prototypes actually make the prospect of slitting and reducing the diameter of a body tube a desirable prospect because many real vehicles often have external tunnels, fuel lines, or welds that run the length of the body...a perfect hiding place for the seam you will create in the model's body tube.
Cutting It (i.e. The Tube)
First, calculate just how much you need to remove from a tube in order to reduce its diameter by the desired amount. This is quite easy. Let's use a BT-101, like the one in the picture, and reduce its diameter to 3.75". All we have to do is subtract 3.75 from 3.93 (the diameter of a BT-101) to get .18. Next, simply multiply this by the value of pi (3.14159...if you can't remember this number [you should if you intend to stay in scale modeling] then most calculators have a pi function...for example simply hitting the letter P when entering numbers in the Windows calculator inserts the value of pi) to get just a tad more than .56". This is how much you will need to remove from the BT-101.
Accuracy is key. Very accurately mark a line along the entire length of the tube. It must be straight and parallel to the tube centerline. A piece of wood corner trim makes a great cutting guide (corner trim is "L" shaped and will self-align on the body tube). Draw a second line parallel and .56" away from the first. I have found it helpful to put a mark for the second line at each end of the tube.
Now comes the scary part. Don't count on the steadiness of your hand to free-hand cut long these guidelines. I usually tape a long thin metal ruler (any thin, straight, rigid bar or strip will do) so that one edge perfectly aligns with one of the marks. Now, using a new #11 knife blade, make several passes along the line, using the ruler (or bar) as a guide. Don't try to cut through the tube in one pass and don't cut through the tape holding the ruler in place (you will come back and cut through the tube where the tape was later).
Once you have cut through the tube along the first line, check to make sure the cut is straight and true. You CAN fix a bad cut at this point by simply drawing a new line to the outside of the "bad" cut and trying again (just remember to make a new mark for the second cut line or you will end up taking too much out of the tube).
Once the first cut has satisfied you, reposition the the cutting guide and tape it in place. This is the critical cut...you have to get it right the first time as you can't "undo" this one like the first. Again, don't try to cut through in one pass and don't cut through where the tape crosses the cut line. Remove the cutting guide.
Now, using a smaller straight-edge or ruler, finish the cutting job by carefully cutting through the last remaining attachments where the tape passed over the cut lines. Do not discard the "scrap" piece. Use a sanding block with coarse sandpaper to "dress" up the cut edges.
Take the scrap and make a mark along its entire length and down its center. This will be your "lap joiner."
If you will be able to hide the seam with some kind of substantial external detail like a systems tunnel, you might want to put the lap joiner on the outside of the tube and hide it under the tunnel detail. Alternatively, you might want to use the tunnel itself as a joiner. On the other hand, if the seam is to be hidden by a weld line or small detail (or worse, there is NOTHING to hide the seam...you are depending on your skills to fill the seam by conventional methods) then you will want to put the lap joiner on the inside of the tube. For the purposes of this discussion, we will assume the joiner is going on the inside of the tube. The larger the tube, the easier it will be to put the joiner on the inside.
Apply white glue to one half of the lap joiner strip (that is, along the length of the strip, to one side of the centerline). If the tube is of some other material, use a slow drying glue suitable for that material. Once the glue is applied, carefully position the strip inside one edge of the slit in the body tube. You can spread the cut body tube by a surprising amount to do this. Carefully squeeze the strip into position and wipe away the excess glue. Allow this to dry. Once the strip has dried, apply glue to the protruding portion of the lap joiner, roll the tube closed so that the seam is closed. You may have to use a stick or broom handle to press the lap joiner securely from the inside. Use strips of masking tape to hold the tube together until the glue dries.
Voila! Your new "reduced" tube is ready for use!
Alternatives and Variations
Heavy walled tubes can be butt-joined without the use of a lap joiner strip. In this case, medium viscosity CA glue and accelerator are valuable.
Plastic tubes, such as Plastruct tubing, makes good body tube material (if you don't mind the weight) though it is (understandably) more difficult to cut. Since Plastruct tube is generally available in larger sizes in 1/4" diameter increments you can almost always find a tube that is very near the diameter you need (but a bit larger) and reduce it down. Use the same lap joiner technique as with the BT-101 example but use a solvent cement intended for ABS plastic.
E-mail with suggestions or questions