1.   Case too dry when loaded or case rolled too loose.

  1. Case fits too loosely in mold. Make sure the mold must be forced to close on the case. Check especially the nozzle end and wrap two or three extra layers of paper about three diameters (I.O.) wide around the nozzle end of the case.
  2. Flared bore at nozzle end of mold. Often when drilling, sanding, or trimming the bored hole in the mold body, the diameter at the nozzle end will be inadvertently increased more than the rest of the body. This will allow the nozzle end of the case to expand and split while loading even though the rest of the case is snug.
  3. Excessive force used to compress powder. As was explained in the loading section, the impulse force generated by impact of a mallet can be extremely high. Let the weight of the mallet do most of the work. Fairly light blows will do the job quite well. Only experi­ence can give you the feel for how much force to use loading the engines. The nozzle clay and the first two or three powder charges should have less force used to compress them since the following charges will also compress them farther.



  1. Insufficient force used to compress powder.

    2.      Weak case caused by too thin glue, poor quality paper, or loose rolling. Loose rolled cases can cause inside layers to split with no outside evidence of it. Cases should be hard and rigid.

    3.   Voids in powder.  Air pockets in the propellant increase the burn­ ing area and so increase the internal pressure. Air pockets are usually a result of trying to compress too large a charge at one time. Wrinkles or other irregularities on the inside surface of the case can also cause voids by stopping the loading dowel.

4.       Propellant too powerful. Use only charcoal sulfur and potassium nitrate (saltpeter).  Any substitutions will change the effect of the powder. Never use chlorates or perchlorates in the mixture.  Do not use commercial black powder or smokeless powder as these are much too powerful and can explode rather than burn progres­sively.

If experimenting with new engines the nozzle diameter may be too small or the core too long. Either of these conditions will cause excessive internal pressure. If the standard engines continually burst, the cases may be weaker than usual and it may be necessary to decrease the core length, increase the nozzle diameter, or increase the case wall thickness. Increasing the case wall thick­ ness is the least desirable because this requires more wraps of paper and increases the difficulty in rolling tight cases. For the same reason, the larger engines are more difficult to build properly and people more frequently have trouble with bursting cases and ejected nozzles and top clay headings in these engines. There should be little problem with engines up through 5/8 inch I.O.




 1.   Propellant too weak. If the ingredients and methods of prepara­tion explained in this book are followed precisely, there should be no problem. All the components must be a very fine mesh-­-especially the sulfur and charcoal. A minimum of 125 mesh and pre­ferably 325 mesh should be used. Insufficient mixing can reduce the power of the powder also. The powder must be mixed thoroughly with the prescribed amount of water. Mechanical mixing without water will in no way produce acceptable powder. The powder must be loaded moist to compress properly.

2.   Insufficient force used in compressing the powder will decrease the total amount of powder in the engine and so decrease the amount of powder available to produce thrust. This will normally occur only in small engines. In larger engines, the case will usually burst due to increased burn rate.

3.       In experimental engines too large a nozzle diameter or too short a core will result in insufficient thrust.         



1.       Engine and propellant not completely dry.

2.       Too large powder charges can cause irregularities in the propellant which can cause non-uniform burning.

3.       Insufficient force or varied force used in compressing the charges can cause non-uniform propellant.

4.       Impurities, foreign particles (such as sawdust or metal filings) or insufficient propellant mixing can cause non-uniform propellant.



1.       Insufficient force used to compress charge.

2.       Case too dry when loaded preventing proper seating of clay nozzle.

3.       Check glue fillet.  Use epoxy if you are having problems.

4.       Poor quality case. A properly rolled and glued case will be a solid tube and the layers of paper cannot be separated from each other with­out being cut. The nozzle cannot be expelled without taking a chunk of the case with it and the case will usually burst before this happens. If the case is not rolled and glued properly, only one layer of paper must be torn free to expell the nozzle and this will happen every time. The usual problem is the glue was too thin to properly adhere the layers of paper to each other.

5.       Wet clay. If the clay is not thoroughly dry, the clay can be easily sheared and will usually be destroyed. Also the high temperature exhaust gases will vaporize the moisture in the nozzle and cause the nozzle to actually explode.

6.       Nozzle too short. If the nozzle spacer is not tight, clay will be forced out around it and not enough will be left to form the neces­sary length of nozzle. Some will be forced out anyway and this must be cleaned off down to the paper casing so the glue adheres to the case.

7.       If the nozzle is too small or core too long, the nozzle and/or top clay heading may be expelled before the case bursts.



8.       All the problems causing the nozzle to be expelled also apply to the clay heading. Usually the clay heading will go first because it is not com­pressed near as much as the nozzle. It can also shrink when it is dried where the nozzle being compressed more won't. This leaves the entire load on the adhesive strength of the glue fillet which is not sufficient. The clay heading must either be compressed sufficiently to not shrink or else it must be removed and have glue applied around its sides in addition to the glue fillet. One last possibility might be to fold over and glue several of the layers of paper above the clay and then drill through it down to the propellant when the glue is dry, or drill holes as described at the end of the “loading the engine” section.




1.       Hole in the clay heading not extended to propellant. When making the hole with a drill bit, some of the powder should remain on the bit when it is removed if it is actually down into the powder. The powder is very hard in comparison to the soft clay so there should be no problem in telling when you have reached it.

2.       Clay heading hole plugged. Any material such as clay particles in the hole will prevent the flame from traveling to the ejection charge.

3.       Empty clay heading hole. The hole should be filled either with propellant or ejection charge particles. If the later is used, it should be ground or crushed small enough to fit easily into the hole. Tamp it in with a drill bit shank. Do not crush the ejection charge itself. Dump out the excess crushed ejection charge.


first page previous page  next page  last page  ►

home     about manual