To form a powdered propellant into a sturdy and durable propellant grain, you have to convert it into a solid stick, and you do this with a material called a binder. A binder can be anything that, when added to the propellant. fills in the spaces between the particles, and glues the whole mass together. Ideally, a binder should take part in the combustion reaction, so rocket makers normally choose a binder that doubles as a fuel.
Red Gum
Red gum. also called Accroides gum. or yacca gum. is the burgundy colored resin exuded by the bark of the Australian grass tree of the genus. Xanthorrhoea. The Australians dry it. clean it. crush it into a fine, orange powder: then ship it all over the world, where it’s used in everything from varnishes to fireworks. Red gum is soluble in alcohol and acetone. Its high structural strength and excellent adhesive properties, combined with its ability to take part in a combustion reaction, make it an ideal binder for powdered rocket propellants.
From the 1940s to 1993. red gum was the main binder used by the American fireworks industry. In 1994. under pressure from environmentalists, the Australian government declared the grass trees (also called “yacca trees”) a protected species. They temporarily stopped all exports of red gum: then resumed them in 1995. when the gum industry agreed to a resource management program. As of this writing, the future supply of red gum is secure. However, during the shortage, the American supply ran out. and the dealers who used to sell it deleted it from their catalogues, it is currently being imported again in large quantities, but as of the year. 2001. a few dealers are still not aware of this fact. Hopefully, this topic will help spread the word, and remedy that situation.
Most red gum comes from Kangaroo Island, off the coast of South Australia, about 70 miles from the city of Adelaide. The island is large: about 1.000 square miles. The growing environment is ideal, and there are several exporters shipping it out of the main town of Kingscote. Anyone who wants to import large quantities of red gum should contact the business community on Kangaroo Island. I’ve noticed in the past year that there are several Kangaroo Island websites on the Internet.
To find red gum in the U.S.. look in The Yellow Pages under headings like CHEMICALS, RETAIL, or LABORATORY EQUIPMENT & SUPPLIES. If a nearby dealer doesn’t sell it. you can order it from the Teleflite Corporation at www.teleflite.com and www.amateur-rocketry.com. or the chemical dealers in the classified ads in American Fireworks News ( page 175). If you encounter someone who used to sell red gum. and thinks that it’s unavailable, show them this topic, and tell them to check with their wholesalers. And a final thought. Because the environment on Kangaroo Island is similar to some Southeastern parts of the U.S.. it might be possible to grow the grass trees domestically. They take 15 to 20 years to reach harvestable size, and it might be an interesting business venture for someone already engaged in tree farming.
Goma Laca Shellac
In 1995 one of my readers sent me a half-kilo sample of something called goma laca shellac. She said that it came from Spain, and that during the red gum shortage, the Spaniards developed it as a man made substitute for red gum. Like red gum. it is reddish in color, and soluble in alcohol and acetone. Its adhesive properties are about the same, and so are its effects on propellant performance.
Both red gum and goma laca shellac have an acrid odor when wet with acetone. When dry. the goma laca smells like freshly-sawed pine. Otherwise, the match is so close that if she hadn’t told me it was synthetic. I wouldn’t have guessed. I used it in a batch of the NG6 propellant: then built an E and an H motor, and found no significant difference in performance between these goma laca motors and the ones made with red gum.
Under a regular, light microscope, red gum granules vary in color from black to pale pink, and here and there you’ll see a piece of dirt or yacca bark fiber. The goma laca is a clean, uniform, apricot color, but what’s most interesting is its price. As of this writing, rumor has it that at least two U.S. fireworks companies are importing it in large quantities for their own use at about half the price of red gum. None of my contacts know anything about it. and it’s apparently (at least as of 2001) considered a trade secret by the people importing it. The person who sent me the sample wants to
remain anonymous, and would not divulge the name and address of the manufacturer. This being the case. I have high hopes that the publication of this topic will shake something loose. If enough people ask for it. someone might get motivated enough to locate the source, and start importing it for retail sale. Finally, since it is man made. I see no reason why it couldn’t be produced right here in the U.S. Goma laca shellac is apparently inexpensive to make, and the market for it is already established.
Vinsol Resin
Vinsol resin is a by-product of the logging industry. Pine tree wood is pulverized into a mass of fiber containing up to 4% pine pitch. This crude pitch, called “gum rosin”, is extracted and sent to a refinery, where it is chemically separated into several dozen products, one of which is vinsol resin. Among other things, vinsol resin is used as an additive in asphalt and concrete, as a modifier in phenolic resins, and as a fuel and a binder in fireworks.
Like goma laca shellac, it smells like freshly sawed pine, and it quickly dissolves in alcohol and acetone. Unlike goma laca or red gum. it noticeably retards a propellant’s burn rate. This means that a rocket motor made with vinsol resin requires a smaller nozzle throat, or a longer propellant core than an identical motor made with red gum or goma laca shellac.
Figure 6-41. A bo.x of baking soda. Baking soda is a cheap and effective bum rate modifier.
As of 2001. the main producer of vinsol resin is Hercules Incorporated. 1313 N. Market St.. Wilmington. Delaware 19894. They make it in both flaked and powdered forms, but their minimum purchase is 2.500 lbs. For 50 lb. quantities, contact a company called Chemcentral Corp. Chemcentral is the main distributor for Hercules resins, and they have offices in almost every state. For 1 lb. to 25 lb. quantities look in The Yellow Pages under headings – like CHEMICALS, RETAIL, and LABORATORY EQUIPMENT & SUPPLIES, or contact the Teleflite Corporation at www.teleflite.com and www.amateur-rocketry.com. or the dealers that advertise in American Fireworks News (page 175). If you still can’t find it. call or email Chemcentral. Tell them where you live, and ask them for the name and address of the nearest retailer that sells it.
