How To Make Fiberglass Airframe Tubes
Karl Upton August 2009
Becoming increasingly interested in building a vertical trajectory system (VTS), I quickly realized that the standard cardboard airframe tube would not support a working servo being mounted to it.--At least not for very long! Rather than take the easy route and fiberglass over a standard cardboard tube, I decided to construct fiberglass airframe tubes.
Most of the information on the Internet concerning this process is for large scale, high power rockets. I could find no information on building tubes for low-power rockets; BT-20, 50, 60 and 80 airframe tubes in particular. The reason for this is that cardboard is the best material for low power rockets. (18 and 24mm power) Cardboard has the greatest strength-to-weight ratio for these airframe sizes—as long as the rocket is using a static fin configuration. My fiberglass tubes turned out to be 1 to 1.5 times as heavy as its cardboard counterpart. I plan on just using the fiberglass tube for the portion that houses the running gear which consists of 2 servos controlling 4 forward active fins/canards and their linkage. The rest of the rocket will be built out of the traditional cardboard and balsa.
Becoming increasingly interested in building a vertical trajectory system (VTS), I quickly realized that the standard cardboard airframe tube would not support a working servo being mounted to it.--At least not for very long! Rather than take the easy route and fiberglass over a standard cardboard tube, I decided to construct fiberglass airframe tubes.
Most of the information on the Internet concerning this process is for large scale, high power rockets. I could find no information on building tubes for low-power rockets; BT-20, 50, 60 and 80 airframe tubes in particular. The reason for this is that cardboard is the best material for low power rockets. (18 and 24mm power) Cardboard has the greatest strength-to-weight ratio for these airframe sizes—as long as the rocket is using a static fin configuration. My fiberglass tubes turned out to be 1 to 1.5 times as heavy as its cardboard counterpart. I plan on just using the fiberglass tube for the portion that houses the running gear which consists of 2 servos controlling 4 forward active fins/canards and their linkage. The rest of the rocket will be built out of the traditional cardboard and balsa.
Exploiting the lost wax technique, I used plaster of paris for construction of the mandrel. After sealing the mandrel with stain and coating it with a mold release, the fiberglass is wrapped around the mandrel and left to dry. After curing, the whole mandrel/fiberglass assembly is soaked in water and the plaster of paris removed from the center of the fiberglass tube.--Sounds easy, don't it?--Making wine sounds easy too! I've put up about 800 bottles of wine in my younger years and making fiberglass tubes—the plaster of paris mandrels especially, is just as tedious of a job.--Not brain surgery mind you, but tedious work. The lost wax technique is fine for a few tubes or to experiment, but if you want to make it a LOT simpler process, procure a permanent mandrel of some sort so that the fiberglass tube slips off the mandrel instead of destroying/losing the mandrel for every tube you make.
I used the 105/206 West system of epoxy and found it to be an excellent product.--Hardly any odor and a real nice epoxy.--Far better than any other epoxy Ive worked with. The mandrels were cast in the cardboard airframe tubes we all know and love. I made BT-20, 50, 60 and 80 mandrels, but any tube smaller than a BT-60 is kinda, sorta a waste of time. The BT-20 and 50 mandrels are VERY fragile to handle and are a major pain to clean out the plaster of paris from due to their small diameter.--Do not be fooled like I was, into thinking you will just soak these mandrel/tube assemblies in water and the plaster of paris will “melt” away.--It doesn't happen like that! I soaked my tubes overnight and I still had to chip/drill out the mandrels from each one.--Awesome fun! I think I read somewhere that a high power hose/pressure washer would get the job done easier, but I do not have one at my disposal right now. Hot water works better than cold to loosen the plaster from the tube.
After you build/prep the mandrels the fiberglass is cut to the appropriate length and number of wraps to be applied. You cannot work with the standard 6 oz. fiberglass cloth readily available everywhere.--It is too stiff and heavy for anything below a BT-60.--Even a BT-60 is pushing it. A second wrap of lighter 1.5 oz. cloth is needed to keep the seam down on the mandrel. This is after I opened up the new package of six ounce cloth and rolled it into a 1.5” tube and stored it that way for a week! I have read somewhere about tacking the heavy cloth down with cheaper polyester resin epoxy and THEN coat it with the good epoxy. ?? I just used the lighter 1.5 oz. Cloth and made multiple wraps for each tube. For example, in order to achieve a 12 oz. weight of cloth on a 24mm tube, I made 8 wraps of the 1.5 cloth. I made all 8 wraps at once with one continuous sheet measured out. Please keep in mind that I did not add 12 oz. of cloth to the 24mm tube, the 12 oz. number is how much that sheet of fiberglass would weigh in a 3' X 3' sheet.
After the epoxy and cloth are wrapped around the mandrel, I wiped the cloth down with paper towels to remove as much epoxy as I could. Wiping the fiberglass is the secret to getting decent quality tubes,--you want to remove as much epoxy as possible, just enough to keep the fiberglass mesh wet. This will keep the weight to a minimum and produce nice looking tubes.
The standards for cloth weight to apply worked out to the following:
9 oz. for a 18mm/BT-20
12 oz. for a 24mm/BT-50
15 oz. for BT-60 and 80.
In future tubes I will try to knock an ounce off of every weight listed, but we will see. Fiberglass tubes in this size are usually made for altitude competition and are therefore EXTREMELY light weight, they only have strength along the flight axis of the rocket, but very little strength torsionally.--It would be easy to accidentally crush them in your hand! These competition tubes make a standard cardboard tube seem heavy duty and would not be able to handle having a servo mounted to them. The weights listed above make for the minimum strength needed to mount working servos onto. Once I have the formula for the fiberglass weights worked out I will try reducing that weight even further by experimenting with Kevlar or carbon fiber wraps over the fiberglass wraps. I think the best method would be to have an innner and outer wrap of Kevlar or carbon fiber surrounding a fiberglass wrap core. Or, quite possibly a complete C.F./Kevlar tube. ?? This would make a nice winter project.
I would appreciate any tips on improving my method or maybe personal experience with the fiberglass weights needed would be appreciated. I have never worked with carbon fiber or Kevlar before so any info on that process would be appreciated also. This whole process was pretty much trial and error.
9 oz. for a 18mm/BT-20
12 oz. for a 24mm/BT-50
15 oz. for BT-60 and 80.
In future tubes I will try to knock an ounce off of every weight listed, but we will see. Fiberglass tubes in this size are usually made for altitude competition and are therefore EXTREMELY light weight, they only have strength along the flight axis of the rocket, but very little strength torsionally.--It would be easy to accidentally crush them in your hand! These competition tubes make a standard cardboard tube seem heavy duty and would not be able to handle having a servo mounted to them. The weights listed above make for the minimum strength needed to mount working servos onto. Once I have the formula for the fiberglass weights worked out I will try reducing that weight even further by experimenting with Kevlar or carbon fiber wraps over the fiberglass wraps. I think the best method would be to have an innner and outer wrap of Kevlar or carbon fiber surrounding a fiberglass wrap core. Or, quite possibly a complete C.F./Kevlar tube. ?? This would make a nice winter project.
I would appreciate any tips on improving my method or maybe personal experience with the fiberglass weights needed would be appreciated. I have never worked with carbon fiber or Kevlar before so any info on that process would be appreciated also. This whole process was pretty much trial and error.