by David O’Bryan
(Note: photo illustrations and construction video appear at the end of the article)
This article gives the modeler the basic techniques needed to produce lightweight and strong bodies and components for model rockets used in FAI competition. By utilizing lightweight fiberglass fabric and epoxy over a form or mandrel, the modeler is able to construct shapes that can be very lightweight and strong when compared to ordinary kraft paper tubes or plastic components. The materials needed are available at most hobby shops or mail order houses or can be fabricated with some ingenuity by the modeler.
The basic materials needed are as follows:
- a form or mandrel
- fiberglass fabric
- mold release
- epoxy brush and a mixing cup
- latex gloves or skin protection
In addition it is good to have:
- a cradle for the form or mandrel
- finishing materials (Japanese tissue, mylar, etc.)
- a scale for mixing epoxy accurately
- sandpaper (120 to 600 grit)
- sharp scissors or an X-acto blade or razor blade
- paper towels
THE FORM OR MANDREL
The form or mandrel should be of a material that has a very smooth and solid surface. Typical forms for simple tubes are done utilizing brass tubes, steel drill rod stock, Teflon rods, hard plastic rods such as Delrin, solid aluminum rods (hollow aluminum rods usually bend too easily), and any other material available that will maintain a straight smooth shape. Some people have successfully used wooden dowels with shellac coating, but it must have NO imperfections. This is usually very hard to achieve, so I don’t recommend it. When you remove the finished body from the form, it must slide off the form and any surface irregularity or variation in diameter will not allow it to release.
For more complex shapes, a mandrel with the appropriate shape can be made on a lathe. The materials most used in this application are usually aluminum, stainless steel, or Teflon/Delrin. The material must be solid and very stiff. The use of the plastic materials is not recommended for long or thin mandrels as it is very difficult to work on a lathe due to the flex in the material. Again wood could be used if extreme care is taken that the finish is perfect and sealed with shellac or varnish. I find that aluminum is readily available, machines easily, and gives a very good result. The only drawback is that you must never use anything metal on it that can scratch the surface.
The shape of the mandrel can be of a constant diameter or of a decreasing diameter. The critical thing is that the diameter cannot decrease and then increase as you go down the mandrel. This would result in a choke area that would not allow the body to slide off of it. As an example, for FAI rules for parachute duration (S3) and streamer duration (S6) models the typical model has a main portion of the body at 40mm which then tapers at 5 degrees to a diameter of 10 to 13mm (depending on the engine size). This stepped mandrel is shown in the accompanying photos and is a good illustration of a mandrel that has a changing diameter but will still release the body.
Some people find that access to a mandrel made on a lathe is difficult. The standard size tubes and rod stock are available from hobby shops and metal supply warehouses, but access to a machine shop is sometimes not easy. My suggestion is to look up the nearest machine shop(s) in your area and go talk to them with an illustration in hand. I have been successful at getting my local machinist to make several mandrels by bartering with him. We usually finish off the mandrel with fine sand paper and then a Scotch Brite pad or other polishing material. An alternative is to order a mandrel via internet sources. Typically there is a group order once a year that is arranged by some US flier with good machinist connections and announced on the NAR FAI Yahoo group. Prices are typically $70 to $100 per mandrel. NAR Technical Services now also sells these mandrels.
The thing to remember about your form or mandrel is that the surface must have no protruding imperfections, ripples, choke points, or anything else that may hang up the body from releasing.
The material most commonly used for construction of these type of model rocket bodies is fiberglass fabric. The weights of fabric easily available range from 0.5 oz./sq.yd., 0.75 oz./sq.yd., to 1.4 oz./sq.yd. and 2 oz./sq.yd. and up. Most ultralight building is done using the 0.5 oz./sq.yd or 0.75 oz./sq.yd. fabric. Be sure the fiberglass fabric you are using has been treated to readily accept epoxy. Most fabric is sold this way so it is not usually a problem. You will find that when handling the light weight fabrics they will want to ‘stretch’ along the weave so be gentle and don’t pull on it much.
The most common bonding agent for these bodies is epoxy. A Polyester resin could be used, but it has the undesirable characteristic of setting too quickly, and having a very strong odor (any backyard mechanic will recognize the smell of Bondo). The epoxy that gives the best results is one that has a slow cure or set time and is not very thick. A 30 minute epoxy is about the quickest I would use. Best bet is the ‘finishing epoxy’ type, such as West Systems 105, not a construction epoxy. Finishing epoxies typically have a thinner viscosity, a long pot life (working time), and set in 6 to 24 hours. You need enough time to mix, apply the epoxy and fabric, and apply any finishing touches before the epoxy starts to gel or set. This will take anywhere from 15 to 30 minutes.
The epoxy will come in two containers. The components are mixed in the proper ratio just before application. It is handy to have a small scale or some other method to accurately mix the proper ratio, usually 5:1. The epoxy is usually mixed in a disposable plastic container with a mixing stick and applied with an epoxy brush. The epoxy and mixing accessories are readily available from hobby shops or mail order hobby supply houses.
My favorite epoxy is WEST SYSTEM 105 Epoxy Resin with 206 Hardener (http://www.westsystem.com/ss/epoxy-resins-and-hardener).
Apply mold release to the mandrel to prevent the epoxy from adhering to the mandrel so the body can be removed. There are a variety of materials that can be used. Most all of them utilize a wax base. Some suggestions are:
- Aervoe/Crown Mold Release #3470N- mold release for epoxy, an aerosol wax
- Carnauba wax
- KIWI Neutral Shoe Polish
- DOOR EASE Stainless Stick Lubricant (at automotive stores)
My favorite is the Aervoe/Crown Mold Release. It sprays on like clear paint and gives a very smooth finish. Some of the other mold releases go on lumpy and have proven difficult to work with. Do not use a PVA Mold Release for this application. That type of mold release is not suitable for forms where the body must slide of the form to be removed.
When laying up a body, it is good to have some sort of cradle to lay the mandrel in. The cradle should support the mandrel at the ends and allow the fiberglass layup to be done without getting in the way. See the photos for an example.
You should also protect yourself from the epoxy. It is recommended that you use gloves or possibility PIC Skin Shield (a protective hand cream) to keep from absorbing the epoxy into your system. Adequate ventilation must also be used. It is possible to be sensitized to epoxy so be sure to avoid unnecessary contact. ( I know of one modeler who now must use total skin coverage and a respirator whenever using any epoxy). If you have any questions as to the proper safety procedures for any of the materials please contact the manufacturer and ask for the Material Safety Data Sheet (MSDS) for the product.
The construction methods I am going to outline are not the only way to prepare these models. There are many ways and each modeler develops his/her technique. These methods will work, but do not be afraid to experiment and see if something else works better for you.
PREPARE THE MATERIALS
Before you actually begin the construction process it is a good idea to gather and arrange all the materials you will need. Once you have mixed the epoxy time is a factor and you do not want to have to scramble looking for that piece of material you forgot while the epoxy is setting.
Determine how much fiberglass you will need to cover the mandrel. For a lightweight body about two wraps with one-half inch of overlap is OK. Use paper to make a template that will achieve the coverage you desire. Cut out the fiberglass to this shape using sharp scissors (which eventually get dulled) or an X-acto knife or razor blade.
Gather together the other materials you will need as listed above. I recommend working on newspaper or other disposable surface.
PREPARING THE FORM
The form or mandrel must be absolutely clean and free of imperfections before using. Any problems at this stage can result in difficulty when it comes time to remove the body. I usually clean the mandrel with a little acetone and then rub it lightly with very fine steel wool to remove any residual epoxy or mold release and to smooth down any imperfections. Be sure not to remove part of the mandrel when cleaning it or over time you may inadvertently change its diameter.
Now apply the mold release. If you are using an aerosol mold release you spray it on as if you were applying a light coat of paint. Three or four light/normal coats is better than one or two heavy ones. You do not want any runs or drips as they will reflect onto the final body shape. If you are using a solid wax type of mold release apply a good coat with a towel or your finger. Make sure you have an even coverage without any lumps.
Mix the epoxy according to the products instructions. How much you mix depends on how much fiberglass you are using. I mix about 8 grams of epoxy to do two wraps on a 40mm form about 15 inches long. After some experimentation you will determine the proper amount for different situations. It is better to mix too much at first than too little.
With the mandrel in the cradle apply the epoxy to the mandrel. Use the epoxy brush to brush the epoxy on evenly all over the mandrel where the fiberglass will be applied. (Do not epoxy the mandrel onto the cradle). It is normal for the epoxy to bead up due to the mold release.
Starting with one edge of the fiberglass you cut into the proper pattern, lay the fiberglass onto the top of the mandrel with the remaining fabric laying down one side. Use your finger or the epoxy brush to gently smooth the epoxy into the fabric as you turn the mandrel. You want the fabric to be gradually wrapped around the mandrel and absorb the epoxy. Any tendency to wander or bunch can be corrected by carefully rubbing and smoothing the weave to get the fabric to lay flat and smooth. The lightweight fabric will go around transitions readily if care is taken to work out the irregularities before they get out of hand.
When you have the fabric entirely on the mandrel take a look and see if there are any areas that could use some additional epoxy. Dab some epoxy onto these areas and smooth it in if necessary. You want a uniform amount of epoxy throughout the fabric. It is OK to see the weave of the fabric as long as it is fully saturated. Take this opportunity to check for any final smoothing you might want to do to the fabric. Keep in mind that any imperfections evident now will have to be sanded off later.
An alternate method to applying the epoxy to the mandrel and then laying on the fabric is to lay on the fabric and then add the epoxy. To do this, you put a small amount of epoxy in a line from one end of the mandrel to the other. Use this to help adhere one edge of the fabric to the mandrel and then wrap all the fabric on the mandrel. Now carefully paint on the epoxy and saturate the fabric. I always had trouble with the fiberglass fabric getting fouled by the epoxy brush so this method never went well for me.
Another method is to lay the fiberglass fabric on wax paper. Paint the epoxy onto the fabric and saturate the weave. Now carefully pull the fabric up off the wax paper and wrap it around the mandrel. Again this method did not do well for me but this illustrates the many options available.
The important thing is to get the fiberglass onto the mandrel and have it saturated with epoxy, however you do it.
CURE AND REMOVAL
Once the glass is laid up the mandrel can be set aside to cure. If you want to reuse the epoxy brush it can be soaked in acetone and it will last for 6+ applications. Let the leftover epoxy cure in the mixing cup and you do your next mix right on top of it.
Let the epoxy cure for the entire recommended time. The curing can be accelerated by placing the mandrel in a lightly warmed oven (maybe 125 degrees). Do not melt the wax mold release! If the epoxy is still ‘rubbery’ when you try to remove the body the body will probably fold up on itself.
The body is removed from the mandrel by one of two methods. If it is a metal mandrel then heat is applied to melt the wax and allow the body to be pulled off. If the mandrel is plastic it can sometimes be put into the freezer where it may shrink enough to allow the body to be freed from the mandrel.
The most common ways of applying heat to the mandrel are by immersing it into very hot tap water (not boiling water) or putting it in a warm oven (240 degrees for 8-9 minutes). I find that the hot water in my house works fine for anything that I can get into the sink. Apply heat until the entire mandrel is warmed, it actually might be hot to the touch. You should be able to grab the metal part of the mandrel in one hand and the body in the other and just slightly twist them to see that the wax has melted and the body can move. Now pull the body off the mandrel. This sounds easy but may take some practice to get good results. Try using two hands and pressing the mandrel onto a table. Sometimes twisting slightly while pulling will work. Usually holding the body at the leading part will prevent it from wanting to crumple on itself. Try different methods until you find one that works for you. Do not be discouraged if it didn’t work well the first time. After some practice it will.
After the body is off the mandrel let it cure for another 24 hours before doing any sanding (unless you are in a hurry). Apply baby powder to a clean mandrel and slide the body back onto it. Don’t worry, it should easily slide off. Now you want to use sandpaper in progressively finer grits to achieve a smooth surface and lighten the body (if desired). Be careful not to sand through the body and into the mandrel. When you are finished you will have a body that is light weight and with good strength when a nose cone and engine are added.
There are some other methods you can use to enhance the final product. The above method produces a good body but it does take much finish sanding to really make it light weight and smooth. Here are some “tricks” you can use to do even better with possibly less effort.
Some modelers use an additional material on the body besides fiberglass. This material has the purpose of adding some color or a pattern and of sealing the fabric weave so that less epoxy is needed and a smoother finish results. The two materials I know of are Japanese tissue and 1/2 mil aluminized mylar. Don’t be afraid to experiment.
The Japanese tissue can be applied on the mandrel prior to adding the fabric or it can be added to the outside on top of the fabric. If you apply the Japanese tissue first then just barely wet it (on the dull side) and place one wrap of it on the mandrel, with the wetted side up (away from the mandrel) and the shiny side down. Now add the fiberglass fabric, using the wetted tissue as the source of the epoxy for the fiberglass. Finish sanding as before. If you apply the Japanese tissue to the outside of the fabric you first lay up the fiberglass as normal and then wrap a paper towel around the mandrel and soak up any excess epoxy. Now apply the one dry wrap of Japanese tissue to the mandrel. Help the remaining epoxy on the mandrel to soak into the Japanese tissue and add epoxy if necessary. One advantage of this technique is that there is really no significant sanding to be done as the Japanese tissue creates a very good finish.
The 1/2 mil mylar is used just like the outer wrap of Japanese tissue. It gives an even slicker finish and does not need any epoxy to saturate it. Just run a small bead of epoxy down the small overlap you allow for the one wrap and adhere the final edge of the mylar onto the initial edge. You cannot attach fins or anything else to the mylar so it cannot be used on the very lower sections of the body. As is shown in the photos I use both the Japanese tissue and the mylar on my models.
For those who have the proper equipment you can try vacuum bagging the mandrel to see how that turns out. Usually you get two seams where the bag wrapped onto the mandrel. I have also known of someone who wrapped the mandrel in release film and then applied pressure by wrapping elastic bands all the way up the mandrel.
There is no magic to making these models. The techniques utilized can be modified many ways to suit the need. The premise is the same.
- Peck Polymers – Japanese Tissue
- Aervoe Industries – CROWN mold release (sold in cases of 12 cans)
- Composites Store – WEST System epoxy, fiberglass fabric
PHOTOS (by Dave O’Bryan)
Mandrels and stand
Ready to start a layup
Spray on mold release (fan blows fumes outside)
Brush on epoxy, rotate mandrel to smooth it out
Lay one edge of fiberglass on mandrel
Rotate tube and use finger or epoxy brush to work fiberglass onto mandrel without wrinkles
Mandrel with epoxy impregnated fiberglass
Roll a paper towel over fiberglass to remove excess epoxy
Apply tissue and press onto fiberglass until saturated with epoxy
Roll mylar onto body if desired
Finished body after curing, ready to remove
VIDEO (by Dave O’Bryan, Kevin Johnson)