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Problems relating to the casting of sleeve-bonded propellant segments
for the Kappa-DX rocket motor Static Test KDX-001

  • Introduction
  • Casting the segments
  • Preliminary Analysis
  • Possible solutions
  • Other problems
  • Comparison with others' work

  • Introduction

    The kAPPA-DX rocket motor was designed to accommodate a KN-Dextrose (anhydrous) grain consisting of four stacked propellant segments. This arrangement, referred to as a BATES configuration, requires that the outer diametrical surfaces of the segments be inhibited such that only the ends and core of each segment are exposed to initial burning. Inhibiting of the outer surface of the segments was to be accomplished by casting the propellant directly into a tubular inhibitor sleeve, fabricated from concentrically rolled paper forming one or more layers. The apparatus used in the casting process is shown in Figure 1. The inhibitor sleeve fits within a spacer sleeve, also made of concentrically rolled paper (the spacer sleeve is needed to fill the space required by the motor casing thermal liner). This assembly is then fitted snuggly within the casting mould body, which consists of a length of aluminum tubing identical to that of the motor casing. A snug fit of the sleeves is assured by a longitudinal slit along the side of the mould. Another component is the core rod, which is inserted into the propellant immediately after casting. The four small radial rods near the top of the core rod serve to locate the rod in the exact centre of the mould. The casting mould base is an aluminum disc with a hole which accepts the apex of the core rod (the hole is initially covered by a piece of aluminum tape).
    The design dimensions of the segments were 2.23" (57mm) O.D, 0.75" (19mm) I.D. and a length of 3.75" (95mm). The segment design mass was 380 grams.

    casting apparatus

    Figure 1-- Casting apparatus

    Casting the segments

    The paper used to fabricate the inhibitor sleeve for the first segment (KDX-001-01) was bristol board of 0.015" (0.38mm) thickness. When producing the first segment, the melting and casting procedure went smoothly without any unexpected difficulties. The core rod had been preheated in an oven to a temperature of 120C (the mould was not preheated). Initial melting temperature of the slurry was recorded as 120C, and actual casting occurred at a temperature of 135C (note that these temperatures are both significantly lower than those recorded earlier during development and characterization of the KN-Anhydrous Dextrose propellant). Colour of the cast material was a very light ivory, indicative of slight caramelization.

    It was observed some time after casting, when the propellant had cooled down to a large extent, that a gap was forming between the propellant and the liner. This gap (later measured to be about 0.001-0.002" at its widest) extended approximately over one-half the circumference. When the segment was removed from the casting mould, and the core rod extracted (1.5 hours after casting), it was apparent that disbonding had occurred over roughly one-third the segment surface. A "tap" test (using a metal rod) confirmed this assertion. The paper liner was later removed to examine more closely both the paper and the propellant surface. The paper had a coating perhaps best described as being very smooth and having a "rubbery" feel at the region that was disbonded. The disbonded propellant surface was somewhat damp and sticky. The region where the paper had successfully bonded held tenaciously, and could only be removed by a combination of peeling and scraping with a sharp blade. It was felt at this time that the disbonding was a result of overspray of the "3M Super 77" adhesive that was used to bond the sleeve seam.

    For the second segment (-02), the sleeve seam was bonded using a solid "glue stick", taking care not to allow any glue to come into contact with the propellant bonding surface. Also, it was decided to preheat the core rod to a higher temperature of 175C. After casting the segment, it was discovered that disbonding once again occurred, but even more extensively. Exactly half the circumferential surface was disbonded. Again, the disbonded propellant surface was somewhat damp and sticky, and the paper had the same shiny appearance and "rubbery" feel. It was felt at this point that the primary factor leading to disbonding was thermal shrinkage of the propellant during cooling. Although it was clear from the outset that shrinkage would occur, it was felt that this would not create a problem, as the cohesive (bonding) strength between the paper liner and the propellant was considered to be sufficient to maintain the bond. In order for a successful bond to occur, it is necessary for the liner to "shrink" to the same extent. In other words, hoop loading (compression) of the liner sleeve would result. This is analogous to internal pressure loading of a cylinder which results in radial expansion, but in reverse, with radial contraction occurring due to a "negative pressure" corresponding to the bonding force of the propellant to the sleeve. The bond strength (force per unit area) required is a direct function of the thickness of the cylinder (sleeve) wall thickness, as well as of the elastic modulus of the paper. Since neither the propellant bond strength nor elastic modulus of the paper was known, it was not possible to determine with certainty whether this was indeed the source of the disbonding problem.

    In an attempt to alleviate this situation, three modifications were made to the casting process for the next segment (-03): the paper sleeve (as well as the core rod) was preheated to 175C, thinner paper of 0.010" (0.25mm) was used , and the sleeve was not glued at the seam to allow it to contract radially without developing "hoop" loading. The segment was then cast as usual, and the core rod extracted after 1.5 hours. Examination of the segment (using the tap test) showed excellent bonding, with only one minor area of disbonding near the middle of the segment, which was attributed to an air bubble. The only other deficiency was that the segment was not perfectly circular, due to the "free edge" of the sleeve seam which developed a slight gap, as it was not glued (see "other problems").

    The next segment (-04) was produced in exactly the same manner, with the exception being the preheating of the core rod and sleeve mould. For this casting, these were preheated to 150C rather than 175C. Examination of the segment after removal from the mould revealed, to our dismay, that disbonding had occurred over one-half of the surface. The propellant surface was again sticky and the paper had the same type of coating as previously noted. Clearly, there was another mechanism besides thermal shrinkage that was leading to disbonding. Somehow the cohesive properties of the propellant were being affected by conditions that were present during the cooldown phase after casting. It was felt that the problem may be related to temperature and rate of cooldown. As such, it was decided that for the next segment, the cooldown period would be greatly extended.

    Immediately after casting the next segment (-05), it was placed in an oven that had been preheated to 120C then turned off. As such, the cooldown period was extended to 3 hours, double the previous time period. After removal of the segment from the mould, it was discovered that disbonding had occurred over much of the surface, far more so than for any previous segment. The paper sleeve peeled off with essentially no effort. The surface of the propellant at the disbonded region was significantly more damp and sticky, as well. It was now becoming clear that residual moisture was a significant factor. Although the dextrose had been dried, and was presumably anhydrous, doubts about the effectiveness of the drying process were surfacing. It was felt, however, that perhaps the percentage of residual moisture was small, and that the problem simply was that the type of paper that was being used for the sleeve (high quality poster paper with a glossy surface finish) acted as a barrier to the moisture that was trying to escape from the propellant due to elevated vapour pressure from the hot propellant.

    The sleeve for the next segment cast (-06) was made from a single layer of rather porous "postal wrap" paper, of 0.0062" (0.157mm) thickness. This thin sleeve was not considered to be sufficient to act as an effective inhibitor, but rather was meant to act as a "bonding sleeve" that the propellant would effectively adhere to. Additional layers of poster paper would be glued around the bonding sleeve to form the inhibitor lining, subsequent ot the casting of the segment.

    Casting of the segment was done in a similar manner to the preceding segment, except that the cooldown was reverted to the normal (room temperature) method. As the propellant was cooling, it was soon apparent that the paper sleeve was getting quite wet, with moisture being visibly absorbed by the paper. After removal from the mould after 1.5 hours, the sleeve was found to be totally damp, and quite wrinkled as a result. Bonding was spotty, owing to the wrinkled nature of the paper.

    Preliminary Analysis

    It now appears that the disbonding problem resulted from a combination of two factors:

    1. Thermal shrinkage of the propellant when cooling
    2. Residual moisture in the cast propellant
    Thermal shrinkage of the grain requires that the sleeve "shrink" to the final diameter, of the cooled grain. In order for this to occur, the bond strength between the liner and the propellant be of sufficient strength such that cohesive failure (disbonding) does not occur. Compressive hoop loading is induced in the liner from this shrinking process. Since the hoop loading is a function of the thickness of the sleeve wall, it may be desirable to have a thin wall sleeve. The thicker the sleeve wall, the greater the hoop loading, and consequently, the greater the disbonding force. If the disbonding force is greater than the cohesive force between the liner and the propellant, disbonding will occur. It is possible to calculate the disbonding force (Appendix A), but it would be necessary to first determine experimentally the "elastic modulus" of the paper material. Preheating the sleeve and mould to the casting temperature (» 150° ) would certainly help to alleviate (to some degree) the disbonding force by initially "expanding" the sleeve. As well, freeze-up of the hot propellant when contacting the sleeve would occur less quickly, possibly leading to better cohesion.

    It would appear that there was "significant" residual moisture present in the propellant mixture. Although it is not clear, quantitatively speaking, what amount can be deemed significant, an estimate can be made. The dextrose monohydrate used for making these batches of propellant was dried in an oven at » 110C° , in batches of 250 grams, for 100 minutes. Each batch was weighed prior to and after drying to confirm that the mass change was about 9% (representing the difference in mass between the monohydrate and anhydrous form). The potassium nitrate was not dried, but used "as obtained". Earlier testing done on samples of KN indicated that typical moisture content was less than one percent. This implies that the total moisture content of the propellant mixture was likely to be small, probably less than 1%. However, there were two indications that moisture definitely was present. For one thing, it had been noticed that the blended "dry" mixture (prior to casting) was rather cohesive, much like icing sugar, rather than a dry powder. This was even more apparent after the blended mixture has been sitting for a few days--perhaps the dextrose absorbed moisture that was initially present in the KN granules. A second indicator was that the melting and casting temperatures of the slurry were significantly lower than that previously characterized for a KN-anhydrous dextrose combination: 120C and 135C versus 145C and 147C, respectively. For comparison, the initial melting and casting temperatures of the KN-Dextrose monohydrate was characterized as 98C and 123C, respectively. This would certainly imply that some degree of residual moisture was present.

    The residual moisture that remains in the propellant immediately after casting apparently attempts to "escape" via all grain surfaces. The moisture that escapes the top surface (open to the air) immediately evaporates, and was found to not be "sticky" whatsoever after cooling. The bottom surface (which contacts the mould base), however, had been found to be sticky, as well as any surfaces that had disbonded. It would appear that in escaping to the surface adjacent to the liner, the moisture gets trapped, and forms a sticky layer in contact with the liner, if the paper is nonporous, or absorbs into the paper, as was found with the 6th segment. This sticky material is clearly detrimental to the cohesive bonding strength. Thus, when the propellant shrinks as it cools, the sleeve begins to separate as the disbonding force overcomes the weak cohesive bond. It would be expected then, that disbonding would occur over no more than half the circumference, and indeed this would found to be the case. Since the bond was found to be excellent over the remaining half, it would seem that the weak cohesive bond is a transient phenomenon. The bond strength of the material that maintained contact with the propellant throughout (until complete cooling) is that hoped for, quite tenacious, and not detrimentally affected by the temporary presence of moisture.

    Possible solutions

    It would appear that in order to "sleeve bond" this propellant to an inhibitor by casting directly into a paper tube, it is important that the mixture be as dry as possible, with little or no residual moisture present. Quantitatively, just how much moisture can be tolerated is unknown. It is also unknown whether all the moisture present in the KN and the dextrose monohydrate can be eradicated. It is known, however, that nearly all moisture can be driven off -- this was proven during the characterization of this propellant, where the melting and casting temperatures were measured and found to be consistent with the known melting point of anhydrous dextrose (147C). The KN should as be as dry as possible, as well, as it was observed that the dextrose appeared to draw moisture from the KN over a period of some days. It is recommended that the dextrose be dried in an oven at 100C for at least six hours. The KN should be dried in an oven set to 150C for at least an hour.

    As an alternative to complete drying of the constituents prior to casting, the propellant slurry should be heated during the casting procedure to a temperature of 147C, the melting point of anhydrous dextrose. This should ensure that no more moisture is present. Note that heating the propellant to this temperature would result in a certain degree of caramelization, giving a distinct ivory or light tan colour to the final product. This should not be detrimental in any way to the performance of the propellant. However, the propellant may be slightly hygroscopic, even at moderate humidity levels. This assertion is based on tentative observations recorded during the characterization of KN-Dextrose, although further experimentation is required to confirm whether this is true or not.

    The inhibitor sleeve should be fabricated from a paper that is reasonably porous, or at the least, not smooth and shiny (such as poster paper). It would also be prudent to use a relatively thin walled sleeve to minimize the cohesive bonding stress that develops between the propellant and liner surfaces during cooling. Alternatively, the sleeve could be made in the manner that was used for segment #3 onward, with the seam left unglued. This method, however, may result in a segment that is not completely circular in cross-section, which could conceivably reduce the effectiveness of the inhibitor if a gap consequently exists (see "Other problems").

    Other problems

    Another problem existed, not directly related to disbonding but associated with the sleeve design that was devised in an attempt to alleviate disbonding. The sleeve was made such that it was rolled concentrically but not bonded at the seam. This resulted in a small gap at the free edge, as the paper tried to remain straight rather than follow the curvature of the sleeve. After casting, a small gap remained which resulted in the segment being not perfectly circular in cross section. This is illustrated in Figure 2.

    gap at sleeve seam

    Figure 2-- A gap resulted at the sleeve seam (gap is exaggerated)

    Another problem occurred at the sleeve seam which was a result of the paper overlap, as illustrated in Figure 3. Since the propellant is quite viscous and does not flow well (little surface tension effect), a slight gap existed at the joint, even in the instances of good bonding to the liner. The presence of such a gap, or rather a "tunnel, could lead to combustion gases (which are under very high pressure) piping through, igniting the propellant along the length of the tunnel. This would lead to an unexpected increase in Kn. Such a breech on only one of multiple segments would probably not be serious, but if all segments were breeched, overpressurization could well be the result (note that it was not possible to tell how deep into the segment the "tunnel" actually extended). Obviously, the thicker the sleeve material, the more potentially significant the concern. The solution to overcome this problem would entail the use of fairly thin paper material for the sleeve, probably less than 0.010" (0.25mm) thickness. As well, it would seem that preheating the sleeve to about 150C would help by avoiding rapid freeze-up of the propellant as it comes into contact with the sleeve (clearly, this may pose a problem with the adhesive with sleeves that are glued, as the adhesion may fail).

    casting apparatus

    Figure 3-- Problem relating to paper seam

    Comparison with others work

    During investigation of the disbonding problem, two other persons who have worked with the KN-Dextrose propellant were contacted. One worker, who had fairly extensive experience with sleeve-bonding of this propellant, reported no disbonding problems whatsoever. He worked exclusively with casting the monohydrate form of dextrose, using the method of heating until slight caramelization indicated elimination of residual moisture in the slurry. The KN and dextrose were blended just prior to casting, which occurred in an oil bath set at 375F (190C). Note that this temperature is significantly higher than the casting temperature used for the KDX-001 segments. The sleeves consisted of fairly heavy walled (1/8") "upholstery" tubing, for the larger segments, and 24mm "motor mount" tubes for the 29mm motors.

    The other worker reported disbonding that was essentially identical to that encountered by me. He was apparently able to resolve the problem by using slightly heavier, more porous paper (similar to common writing paper), and by preheating the casting sleeve to 70C. In all instances, the dextrose was dried for one hour at 80C, with blending and casting of the propellant occurring soon afterward.

    Last updated

    Last updated June 3, 2000

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