When trying to decide on rocket body tube materials, I wanted something easy to work with, light weight, strong, inexpensive and locally available. PVC met those conditions, but the thin wall material I wanted was not a stock item for most stores. I had to order (5) 10' sections, not a real problem as they were under $6 each. The 3" PVC I use has an outside diameter of about 3.25", and an inside diameter of about 3".
After deciding on a rocket design, I use a power miter box saw to cut the PVC pipe to length. These cuts must be square for the assembled rocket body to be straight. A hand powered miter box saw would be fine as the PVC pipe cuts readily.
Next, I cut out several 1/2" plywood disks. They are used as bulkheads, motor centering rings and as thrust rings. I use a short piece of pipe to mark the inside diameter on the plywood, then sand them round on my 12" disk sander. The bulkhead at the top of the motor section is also the thrust bulkhead, so a centering ring the diameter of your motor should installed on the bottom of this ring to keep the top of the motor centered. The lower centering ring should also be cut out to the diameter of the motor you plan on using.
To properly position the bulkheads, I measure and mark the tube. Use 1/2 of the coupler length. Then push the bulkhead into the body tube using the coupler to keep it square. (see below) Once the bulkhead is in place, I use a light in the end of the tube (as seen above) and mark the exact location of the bulkhead in the tube. The PVC is translucent, so when light is shone inside, the bulkhead can be seen from the outside. With these marks on the outside of the tube, it is easy to drill the screw holes that hold the bulkhead in place. To locate your screw holes equal distance apart, wrap a piece of paper around the body tube, and mark where the paper meets. Lay the paper flat and measure the length (circumference). Divide by the number of screws, I use six, and using a ruler lay out those marks on the paper. Wrap the paper around the tube again, and transfer those marks to the body tube. You may as well mark the paper with three marks for the fin layout as well, if you used six screws, just use every other mark to lay out the fin marks.
The body section couplers are short lengths of the same PVC pipe cut to 4 to 6" lengths, then about 1/2" is cut out lengthwise to allow the coupler to fit inside the body tube. In the photo above, the bulkhead has been installed and the surfaces primed with PVC primer. I use #8 machine screws 5/8" long. Most anything would do here, and perhaps a longer screw would be better. Just don't use drywall screws. I understand they are very brittle and have a low shear strength. Finish the coupler by applying glue to both surfaces and inserting into the body tube. Hold the glued sides together for 30 seconds or so. Originally I also used screws to hold the coupler in place, but recently have been just gluing with no problems.
Fins are the next item. I'll discus fin design at another time. Here I am using a simple trapezoidal fin with base of 5.5" long, with a span length of 4.5" and an angle of 30 degrees at both leading and trailing edges. The material I am using is a sheet plastic of some sort, 1/16" thick. You could use sheet PVC here, and it may be better as the plastic is a little flexible and stiffer would be better. Again, I had (3) 4'x8' sheets on hand, so that's what I used. Mark your body tube with three marks from that paper you used earlier for the screw layout. Extend those lines along the body tube to about 6". Use a piece of angle iron, or some other V shaped object that is square to get the lines perfectly parallel to the body tube. Now get out a pencil tip soldering iron with a pointed tip, not a chisel tip! Once the iron is hot, lay your fin along the line you drew on the body tube. Keep the fin perpendicular to the body tube, and starting in the middle of the fin, weld about 1" of the joint. Keep the soldering iron tip at about a 45 degree angle to the body tube and push the tip into the joint, pull back and push in again, keep doing this to form the weld. You should make a series of little waves in the joint. There should be 10 or 12 waves to the inch. Once the middle is done, do the other side, then both ends on each side. Make sure the whole time you keep the fin perpendicular and parallel to the body tube. The welded joint is not very strong by itself, so be careful and handle it gently. It helps to make a jig to hold the rocket body on while working. Finish between the tacks so the entire length of the fin base is welded on both sides.
In the above photo I am finishing the weld.
The real strength of the joint comes from epoxy. The little waves you formed while welding the plastic to the PVC make great holding points for the epoxy. Use a slow set high strength epoxy and just let it flow onto the joint. Of course, you will have to allow the epoxy to set, then rotate the rocket and do another set of 2 joints. I had a motor cato and blow the lower body tube to pieces, and the fin joints held together. I also had a hard landing one time that bent the fin over to about a 60 degree angle and the joint was intact. So I'm pretty confident of the joint strength.
Update May 2, 2003: Ok, so I was confident of the joint strength. Not so any longer. The launch of the T-2 motor in this frame led to the loss of the fins. The T-2 motor is a high I class to low J class motor, releasing all its energy in about .8 seconds. It was just too much for the fins. See launch test 21 for details. In hindsight, I didn't weld the fins as well as I had in previous rockets. I used a smaller pencil tip soldering iron (18 watt), as the tip on my larger 30 watt iron was getting pitted. This is probably some of the reason for the loss of the fins. Also, I did not rough up the surfaces of the fin and body tube before epoxying them on. Never the less, I reattached the fins using aluminum angle brackets. See below photo.
Above you can see the aluminum brackets holding on the fins. The aluminum angle came in 3' lengths. I cut them to the fin length, then ground the angles on either end and tapered the top edges for a little better aerodynamics. The fins are attached using 6-32x1/2" bolts and nuts. The bottom screws going into the body tube are sheet metal screws and help to hold the motor centering ring in place. I have not yet flight tested this, so I will update when I do. End Update.
Here you can see the lower centering ring installed. I use 3 screws to hold it in place. You can see the 2 small bolts extending through the ring, with wires going up through the upper bulkhead. These are used for a pull wire to start my deployment timer. While very simple, it is a proven way of starting my timer. I have yet to have a deployment failure while using a pull wire.
Here is the upper end of the motor body tube section with the timer installed. I made a 1/2" plywood disk to fit inside the coupler, then cut a slot to hold the circuit board, and screwed a battery holder into the plywood. I use 2 screws from the outside to hold the plywood disk in place. The pull wires are long enough so I can pull the timer module out of the body tube. Of course, you will need to drill small holes through each of the bulkheads to run the pull wire through. I also used a bit of hot glue to help secure the timer into the plywood slot.
The upper body tube is very simple. Just cut some PVC to length, and add a bulkhead just beyond the length the lower tube coupler inserts into the upper body tube. I drill a hole into the plywood bulkhead, run my parachute/recovery line through it and knot it to secure. Also drill another small hole off to one side for the deployment charge wires to run through.
Here you can see the recovery line tied through the bulkhead, and the hole for the deployment charge wires. I also used some epoxy to seal the bulkhead around its perimeter.
Here is a shot of a couple of my nose cones. If you have a wood lathe, you should have no trouble making a nose. I use pine, it's fairly light and easy to work with. Prime it, sand it, prime again, sand again, put on a couple of coats of color, sand, then finish with a clear coat.
Here is the assembled rocket. To keep the two halves together, I use 3 screws through the lower part of the upper body tube into the coupler section of the lower body tube. Paint as desired, you're ready to fly!
This is a little update to show some of the parts in more detail. This rocket was originally designed for the T-2 motor, and has since been had the centering rings changed to hold the 2" motor. The upper body tube was lengthened to hold an electronics bay module.
Above is a diagram with dimensions.
Above is a close up of how the fins attach to the body tube.
Update: I've had some discussions with other rocketeers using the PVC pipe body tubes. There is a couple of things to note when using the aluminum angle brackets. The flat edge of the bracket leaves the fin at a slight angle from perpendicular to the body tube. In itself that really isn't a problem. I suppose it may lead to a slight rotation of the rocket in a cross wind, but that is negligible, all my rockets flew just fine with the fins at this slight offset angle. If the fins are all symmetrical and parallel to the body tube you'll be in good shape.
The real problem is with the bracket not contouring to the round surface of the body tube leading to flexing of the PVC at the bolt holes. All of my flights were fine using the aluminum angle brackets through "J" class motors, but at the "K" and "L" class motor level the bolts and PVC weren't strong enough, resulting in either a fin breaking off by tearing out the bolts, or the PVC itself breaking away. This is due to fin flutter from the weak mounting of the fins. I considered casting aluminum brackets to mount the fins, the cast brackets would contour the profile of the body tube, coupled with plywood centering rings inside the body tube I think the result would be strong fin attachment. A putty type of epoxy could be used as well, but keep in mind epoxy doesn't bond well to PVC, so don't count on the epoxy holding much, but it would give the aluminum bracket a better contoured "foot".
The fin material too should be stronger for high impulse rockets. PVC and plastic don't make the grade. My new rockets using fiberglass can be held horizontally by the fin tips with no flexing at all. Fiberglassed plywood is what I now use for fins, I have also used .065" aluminum sheet with success. The point is: Make everything a lot stronger than you think you need to. Especially the fins, a shred at mach 1 is kinda cool to see, but not what we're after!
Above is a picture of the coupler, in this case I didn't glue it in place but used three screws to secure it. The two wires coming out go to the timer and then to pull wires for timer initiation.
Above is the body upper body tube and nose cone. It's a little washed out and hard to see, but the parachute is in there.
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