Carbon Types with Sorbitol Propellant



Goal of this experiment is to compare static thrust curves for KNO3/Sorbitol propellant with different types of Carbon added as opacifier.

  • Introduction
  • Method
  • Results
  • Conclusions

  • Introduction

    Earlier motors with carbon in the form of Willow charcoal air float had been unsuccessful due to excessive nozzle erosion. I was not sure if this was due to the willow charcoal or a problem with the nozzle construction (old previously used batch of silicate solution)
    A similar experiment with Carbospheres also had a significant amount of erosion.
    Questions about the cause of the nozzle erosion along with discussion on the SugPro and SS2S lists about opacifiers and the nature of different types of carbons piqued my curiosity about the use of three carbon types I could use in my low-mid power sorbitol motors.
    I had a distant memory from chemistry class about how diamonds and graphite both had carbon to carbon bonds, and it was the orientation of those bonds that made them different in consistency. This was correct. I also knew that willow charcoal makes some of the fastest lift powder. A third carbon I had available was Carbo-spheres from Firefox.
    Carbospheres' original purpose was probably as an ablative material due to their low density insulative properties combined with their very high temperature melting/reacting characteristics.
    So from these facts I thought that willow charcoal would function as a opacifier and fuel while graphite and perhaps carbospheres might make a more inert opacifier.
    However, due to the Sugar lists discussions I went back and re-educated myself on carbon.
    A quick look at Wikipedia for carbon shows that all elemental carbons have carbon to carbon bonds. I should have known that, what do I think the 4 electrons in the outer shell do to make the full complement of 8. Anyway: if you look at the picture of carbon allotropes in Wikipedia you can see that even amorphous carbon has carbon to carbon bonds. DUH!
    This experiment takes a preliminary look at the effect of the three carbon types, Graphite, Willow Charcoal, and Carbo-Spheres on static thrust curves in 24mm sorbitol motors.

    Method

    SAFETY FIRST:
    Propellant batches were made in 20 gram sizes. Untested formulas should never be made in large batches. Should an unexpected reaction or ignition occur small size is an added safety factor. But don't be fooled into complacency. For example: 20 grams of f1ash powder can maim or ki11 a person.
    OK: safety rant off.

    Materials:
    The Potassium nitrate (KNO3) was fine powder (Stock #C170 - OX) from Firefox
    The Sorbitol was Sorbo-Gem food grade powder. This grade of sorbitol is available from PVC ONLY
    The Graphite and Carbospheres are from Firefox
    The Willow charcoal was made from willow branches and anaerobic heating.
    The melting pot is a small triple batch double boiler utilizing paraffin as the heat transfer agent.


    Process:
    Eighty grams of standard 65/35 KN/Sorbitol was prepared in powder form. From this batch four aliquots of 20 grams each were split out. To one aliquot no carbon was added. To the next 0.1gms dry airfloat Willow charcoal powder was added. To the next 0.1 gram fine graphite and to the last aliquot 0.1 grams Carbospheres was added.
    The resulting carbon percentages are 0.5% each
    The formulas were melted and poured into Bates grains. These are slightly progressive grains due to the length. (34mm length, 17mm width, 5 mm core, one end exposed, and the other end inhibited.)


    The grains are placed into motor casings, with 4.5mm nozzle, and static tested with a thrust curve generated.
    To calculate density of the control formula as well as the motor KN values the grain dimensions were plugged into SRM.xls


    The grain and chamber dimensions are input. The actual grain weighed 13.2 grams so I adjusted the density ratio (actual/ideal) till the mass came to 13 grams. The density ratio was 0.97 Nice!
    The static tests showed a small amount of erosion from 4.0mm nozzle opening to between 4.0 and 4.1 (I called it 4.05)
    This data entered can be seen in the image above with the KN of the motor throughout the burn. These motors are somewhat progressive.
    Minimum and starting KN = 59
    Average KN = 92
    Maximum KN = 116


    Results




    The one atmosphere burn rates are as follows:
    0% Carbon: 10 seconds per inch = 2.5 mm/sec
    0.5% Willow C: 9 seconds per inch = 2.8 mm/sec
    0.5% Graphite: 7 seconds per inch = 3.6 mm/sec
    0.5% Carbospheres: 8 seconds per inch = 3.2 mm/sec


    The following is the static curves of the four formulas:



    Conclusions

    It is interesting that at 0.5% carbon none of the carbon types caused any significant nozzle erosion. It looks like the earlier carbon experiment had nozzle failures due to the used silicate solution.
    Another interesting point is that at 0.5% none of the carbons significantly affected the thrust curves. The earlier experiment with carbospheres was similar for 0.5% but at higher %s showed definite overall reduction in performance.
    It should be noted that the KNs of these motors are low with maximum pressures in the low 200 psi range.
    Higher KNs might give very different results.



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