PVC SPIDER

FOR CLUSTERING BLACK POWDER MOTORS

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BACKGROUND

CLUSTERING DIFFICULTIES

Successful clustering of model rocket motors has been achieved using a select few methods. The standard method is to use Estes igniters (wired in parallel) connected to a sufficient power source. Igniter leads are usually either twisted together in the proper configuration or connected using several "clip whips". Another method employs flashbulbs and short pieces of a quick-burning pyrotechnic fuse (thermalite) inserted into the motors' nozzles. An alternative method uses a more sensitive type of either commercial or homemade igniter (electric matches); however, all these techniques depend on reliability on the part of the individual igniters or availability of certain restricted or even hard-to-find items.

Fortunately, the standard clustering methods are no longer the only techniques available. As of 2004 the Spider device promises to be the most reliable and hassle-free method for igniting large Estes and Quest motor clusters. The main difficulty involved is with the construction of the spider device itself; however, once the device is built, model prep time is drastically reduced, as only one electrical igniter is necessary per flight. This is essential in a scale competition if more than one flight becomes necessary--especially in FAI (international) competition where time is actually restricted to a specified launch "window".

ORIGIN OF SPIDER CONCEPT

I first found out about the spider concept in 1990 from an article on international competition by Bob Biedron. Apparently, the Russians have been using these devices for several years to cluster their many-motored Soyuz scale models. Bob describes the spider as a central chamber containing a charge of black powder, from which metal tubes lead to each motor nozzle of the cluster. Thirteen years after first reading that article I began designing a large Saturn I model to fly on eight 18mm motors. The hope of creating a worry-free cluster method led me to design a cheap and dirty spider that can be easily assembled and used in a safe manner.

PVC SPIDER BASICS

The resulting PVC Spider concept (still in progress) is a re-inventing of the FAI spider design using readily available parts in construction, including schedule 40 PVC pipe, plywood, small diameter metal tubing, and standard fastener hardware. The device is reusable, fairly durable, and is simple enough to redesign for a number of different motor configurations. It is also relatively safe to operate, as long as one follows a few ground rules and common sense.

Unlike the FAI spider, the PVC spider uses Pyrodex instead of black powder. Presently, Pyrodex black powder replacement is not as strictly regulated as BP, so no special permit is required to purchase it. Pyrodex, however, does have certain combustion characteristics that must be considered when using. When tightly confined it burns very rapidly (explosively) and does not produce much flame. Such a condition is not conducive to motor ignition. However, the PVC Spider employs a 0.95"x0.30" powder "cup" that is open to the air inside the device, so the Pyrodex charge is able to "flare-up" properly, producing a brief "whoosh" sound once ignited inside the chamber. (During all spider tests, the cup is filled between 2/3 and 3/4 capacity).

My original designs utilized K&S brass tubing to direct the powder, but this proved to be too soft for the resulting motor exhaust heat and thrust. All but one tube showed signs of severe melting after the initial test flight.

The resulting modification is the implementation of stainless steel tubing (purchased from McMaster-Carr Supply Co.). Although it is much tougher than the thinner-walled brass tubes, the tips of the steel tubes will still melt and contort under the force of the motor exhaust. Fortunately, the melting is nominal and will not pose a problem for several flights. Unfortunately, the steel is much harder to work with and may require professional services to cut parts cleanly. (I paid $15.00 for a local machine shop to cut the tubes for this spider).

The next potential modification may incorporate removable brass tubes that can be changed as needed after every flight. Brass tubes are extremely easy to cut with a K&S cutter, and can be ready en masse for replacement. This may be a preferrable compromise between the permanently mounted brass and steel concepts.

The PVC Spider is easy to mount to a launch rail using an L-bracket with two pairs of carriage bolts and wing nuts (for more info on rails see www.railbuttons.com), but it could probably be adapted for a launch rod by replacing the carriage bolts with eye bolts and building up the rod diameter at the proper mount location.

SPIDER LIVE TESTS

NOTE: Pyrodex RS used for all tests; Saturn I test model used for all tests

SPIDER DRAWINGS & BUILDING TIPS

CLICK ON A THUMBNAIL TO VIEW FULL-SIZE IMAGE
(drawings updated 8-8-04)
SPIDER DRAWING 1 SPIDER DRAWING 2

The internal Spider rings can be made from regular aircraft ply or even lite ply. I chose lite ply for my latest spiders simply because Balsa Machining Service did not have 3/16" ply on their materials list. So far 1/4" lite ply seems to work fine.

INTERNAL REINFORCEMENT

The internal spider parts get a lot of abuse from rocket exhaust during lift-off. I would recommend coating the inside of the Powder Cup, as well as the top of the Anchor Plate with Some sort of epoxy clay. You may even want to coat the underside of the Top Ring; just don't clog the Exhaust Tubes! I also recommend sealing all edges of all the plywood rings with thin CA (super glue).

DON'T SPILL THE PYRODEX!

Unless you want to make things difficult for yourself, follow my example and make the Bottom Plate/Cup Assembly able to mount to the launch pad BEFORE you dispense the Pyrodex. That way you don't have a chance to make a mess of things.

PLUG THE ALIGNMENT PINS

Model Alignment Pins (1/8" O.D.) are easy to plug (from the inside) with tiny #2 screws CA'd in place. You can find a bunch of little screws and nuts at Radio Shack or at online stores such as www.microfasteners.com.

USE A REMOVABLE MOTOR MOUNT

If you want to change motor configurations or test-fit the Spider's model alignment pins, a removable motor mount is easiest way to go. This may not be necessary on smaller models.

BLAST DEFLECTOR?

The epoxy layer that holds the Exhaust Tubes in place serves as an adequate blast deflector of sorts. Since most of the flame is either directed to the inside of the spider or off the tops of the metal tubes, there is very little damage done to the top of the Spider body. If so desired, a simple deflector can be made from 26-gauge ductwork steel with holes punched to fit around the Exhaust Tubes, but this is not necessary--I have used an additional deflector for only one spider test to date.

USING THE PVC SPIDER

Typical Countdown Checklist

  1. Prep the rocket recovery system and motor mount.
  2. Prep the motor nozzles (I lightly scratch the propellant with a scribing tool to make sure there is no excess nozzle clay).
  3. Test fit spider alignment pins into motor mount. Sand and/or use baby powder for a smooth fit.
  4. Insert cluster mount into rocket body and secure in place.
  5. Attach L-bracket mount to launch rail.
  6. Bend Igniter (see Spider drawing 2) and insert into Spider Powder Cup/Bottom Plate. Wrap igniter lead wires around the microclip attachment posts. Use masking tape to insulate the igniter from L-bracket.
  7. Attach Bottom Plate to L-bracket using washer & wingnut (see drawing 1).
  8. Dispense Pyrodex into Powder Cup (approx. 2/3 to 3/4 full).
  9. Slide Spider body onto Bottom Plate and anchor using wingnuts.
  10. Slide rocket model with motors onto launch rail and down onto spider alignment pins until there is about 1/32" to 1/16" gap between motor nozzles and Spider Exhaust Tubes.
  11. Attach micro clips to igniter leads.
  12. Countdown and launch.
  13. Clean motor gunk buildup from inside the spider before flying again. Use white vinegar to clean the spider after flying day. Rinse with water and let dry.

DESIGN MODIFICATIONS

THE "EXTENDED" SPIDER

The current Spider provides a basic vertical path for the flaming Pyrodex particles; however, a more complex setup may be possible for igniting more spread-out cluster arrangements. My original Spider design involved a smaller diameter PVC pipe with brass lamp hardware "legs" extending from the sides of the body to reach "outboard" motors. In order for the Pyrodex flame to reach the outer motors, more powder may be necessary. In an extremely spread out version, perhaps even multiple Powder Cups with "canals" between them could make an Extended Spider feasible. Thorough testing would be necessary to determine if Pyrodex even burns fast enough to work in this manner. Any pre-flight testing should always be done in low-light conditions so that the Pyrodex flare-up can be observed. Testing new designs should always be done at a safe distance just-in-case!

DISCLAIMER:   The information on this page should only be used with adult supervision. The results on this page were determined with limited testing of a specific design, so USE THIS INFORMATION AND/OR FLAMABLE PRODUCTS AT YOUR OWN RISK! We are not responsible for any injuries resulting from use or misuse of information found on this site, and we recommend thorough testing of design modifications or new designs away from bystanders and taking full precautions to insure personal safety.

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PAGE UPDATED 3-16-2006