Rocketry Planet

Editor's note: This tutorial originally appeared on the Aerosleeves, LLC website, but since the company is no longer in business, the article was published here for use by others. Thanks to Nick and Jeff of Aerosleeves for sharing this with the hobby rocketry community.

Your composite tube may need more than one layer of material to achieve the desired strength and wall thickness. It is possible to design tubing with preference to strength in the axial or transverse axis, by using a sleeve at a diameter smaller or larger than the nominal sleeve diameter.

A. Planning

Your composite tube may need more than one layer of material to achieve the desired strength and wall thickness. It is possible to design tubing with preference to strength in the axial or transverse axis, by using a sleeve at a diameter smaller or larger than the nominal sleeve diameter.

Examples: Examples of projects that will experience axial loading are a rocket airframe or a weight-bearing column. A common project using transverse loading is a pressure vessel. A tube subject to both bending and shear forces would benefit from equal preference to axial and transverse strength.

Sample data: The following data illustrates how the respective amount of axial and transverse strength can be manipulated by changing the sleeve diameter. These examples use the 2.0" and 2.5" sleeving products.

The above data shows that a larger braid angle (2.0" used at 2.5") offers increased transverse strength and stiffness (modulus) compared to the baseline (2.5" used at 2.5") values. Likewise, a smaller braid angle (2.5" used at 2.0") offers increased axial strength and stiffness (modulus) compared to the baseline.

To summarize: Use a sleeve with a smaller diameter than your tube for increased transverse strength; use a sleeve at its nominal diameter for equal amounts of axial and transverse strength; use a sleeve with a larger diameter than your tube for increased axial strength. Using these principles, you should be able to design an tube with the optimal strength properties for your application.

Also, as a rule of thumb, a single layer will create tubing with high axial strength and low transverse strength -- it will hold its shape, but it is easily deformed by squeezing it. A single of layer is generally only appropriate for tube diameters of 3.0" or less. Using two or more layers will ensure strength along both axes, and will be more difficult to deform along the transverse axis. Three or more layers will produce an exceedingly solid tube.

B. Find and prepare a mandrel

A mandrel is the basis of your composite tube project — it will determine the shape and size of your tube. Your mandrel can be as simple as a paper or cardboard tube or phenolic tubing (used in model rocketry). A considerably better choice is a metal mandrel, as it is by far the easiest to work with during the molding process, and will provide a durable and reusable mandrel. We recommend aluminum over steel for most applications due to its lower weight, thermal release characteristics, availability in numerous sizes, easy handling and machining, and lower cost. Applications requiring high finished tube tolerances, or that involve high temperatures, should use steel or other materials.

We suggest Online Metals for purchasing a metal mandrel.

Tip: The size of your mandrel is critical. The outer diameter (OD) of the mandrel should match the desired inner diameter (ID) of your tube. The mandrel should also be at least 10 inches longer than the desired length of the tube for optimal results.

C. Create an effective release

Creating an effective release is often the most difficult part of the molding process. Unfortunately, there is no universal solution that covers every possible application.

The easiest solution is not to use any release. The sleeves can be applied directly to a paper or phenolic tube to reinforce its strength. Thin walled paper tubing, available from various model rocket retailers, also works very well. It is extremely lightweight, available in various sizes, and allows you to create a composite tube in under 10 minutes.

For applications requiring full composite tubing, a release mechanism is needed. This is generally achieved by using a thin, stiff plastic known as Mylar to cover the mandrel. The Mylar needs to cover the full length of the mandrel to be effective. The recommended method of making a "Mylar" mandrel follows.

1. Cut the Mylar so that you'll fully cover the mandrel plus another 1/2 rotation around the mandrel.
2. Scribe or draw a straight line onto the mandrel using a angle iron.
3. Align the Mylar with the straight line, and then tape just the ends of the Mylar to the mandrel with a good quality masking tape. Then take 3M Super 77 spray and carefully spray the last inch of Mylar with it.
4. Now slowly roll up the Mylar onto the mandrel. Because the ends are taped you should be able to get a pretty good roll going. Doing this with another person is also very helpful in keeping the Mylar tight while you roll it up. Carefully roll up the last bit of the mylar with the Super 77 on it.
5. Now take out a knife, and cut off the ends that have been taped. What you are creating is a Mylar sleeve. The mylar sleeve should freely slide up and down your mandrel. This makes it easy to remove the tube after you are done.

We recommend 2 mil mylar, available from Tap Plastics. 5 mil mylar can be useful for very large tubes (8in or larger).

D. Cut a piece of material

Cut a piece of material to the desired length, keeping in mind that the length of the sleeve will change according to what diameter it is used at.

Thus, you should expand the sleeve to the diameter of your mandrel before measuring a piece, otherwise your material will be too short or too long.

The best way to do this is to slide it over the mandrel, place a small piece of tape at the correct place to cut, then remove the sleeve and carefully cut along the tape.

As with any woven fiber product, a quality cutting device is essential — either sharp scissors, a rotary cutter, or ceramic scissors.

E. Slide sleeve over your mandrel

Once cut, slowly "bunch" the braid so that it is much larger than your mandrel, then slip the sleeving onto your mandrel, making sure that the fibers do not get caught. Doing this with another person will help immensely, as will using a nose cone to easily expand the material to the diameter of your mandrel.

F. Add additional layers, if desired

If you need a stronger tube, add a second or third layer of sleeving now. Once epoxy has been applied to the sleeving, it is impossible to apply another layer.

Before applying another layer, carefully tape the radius of the sleeve to one end of the mandrel. Now slowly work sleeving all the way onto the mandrel. The tape will keep the sleeve underneath in place during the application process.

G. Calculate, measure and mix epoxy

We recommend the following process for calculating the amount of epoxy to mix up:

1. Weigh the material that you previous cut, in grams, and round to the nearest multiple of ten. This is how much resin you should use. Example: If the material weight is 76g, you should measure out 80g of resin.
2. Based on the mix ratio of your epoxy, calculate the hardener component. Example: Aeropoxy has a mix ratio of 100:27 (resin:hardener). So, given that you need to mix up 80g of resin, you need 21.6g of hardener ((80*27)/100).

This process will provide adequate epoxy to wet out the material, and enough extra to account for epoxy lost in the cup, brush, drips, etc. Most importantly, it limits your epoxy use to what is needed for the material only, thus minimizing the overall weight of the tube.

H. Apply epoxy to the sleeve(s)

Wearing vinyl gloves, mix up an adequate amount of epoxy.

Using your preferred epoxy applicator (e.g. acid brush, foam brush, stick), spread the epoxy onto the sleeve(s).

Make sure the material is fully saturated, but try to limit epoxy pooling on the surface.

I. Using gloves, squeeze out the extra epoxy

For this step, wearing double gloves on each hand is highly recommend. This is for your safety, to insure epoxy never reaches your hands.

Taking your gloved hands, wrap both hands around the mandrel, starting in the middle.

Grip tightly and pull your hands along the mandrel till they reach the end.

This will help constrict the sleeving to the proper diameter, along with removal of most excess epoxy.

Do this a number of times, until the material is adequately constricted against the mandrel.

Lots of epoxy should be coming out of the sleeving (this is good), so make sure to have proper material underneath to collect epoxy.

J. Apply heat tape, outer wrap, or vacuum bag

To get the best fiber to weight ratio, or a smooth surface, we recommend using polyester heat shrink tape, which constricts when heated. When wrapped around the tube and then heated, it will compress radically, squeeze out more excess epoxy, and leave a smoother surface.

Note that there will be lines left due to the wrapping of the tape, though this effect can be minimized by using the larger heat tape (2.5" wide), or by wrapping the tube in a layer of peel ply or mylar before applying the heat tape.

Peel ply will leave a uniform, slightly textured surface. Mylar will leave a glassy smooth surface, if done correctly, though it is more difficult to achieve compared to the peel ply surface.

If done incorrectly, bubbles and voids will form underneath the mylar, leaving "low" spots in the completed piece. For this reason, we recommend using peel ply if a finished surface texture is desired. The peel ply surface is easy to fill and sand using conventional fillers or Superfil.

Vacuum bagging can be used on the tube, but is difficult and out of the scope of this document.

K. Cure completed tube

Let the epoxy cure according to manufacturer's instructions.

Use of a curing oven is recommended, especially for high performance applications.

This will provide a better overall cure.

More specifically, the more epoxy that fully crosslinks, the stronger and more heat resistant your tube will be.

L. Remove from mandrel

Once fully cured (patience is useful here — trying to remove a half cured tube is very difficult and you may end up folding the tube and ruining the project after all that hard work), remove the tube from the mandrel. Using the mylar sleeve technique described above, this should be very simple.

Simply undo the tape holding the mylar sleeve on, and pull the whole assembly right off of the mandrel.

Then peel the mylar sleeve away from the inside of the tube.

M. Enjoy your finished tube

We hope you've found this tutorial helpful. An Adobe PDF version of the article has been provided for your benefit as well.

This tutorial was supplied by Nick Anderson and Jeff Holman of Aerosleeves, LLC.

Thanks to Nick and Jeff of Aerosleeves for sharing this with the hobby rocketry community.