Dextrose Alloys with Sorbitol Propellant:
Static Tests



Goal of this experiment is to compare static thrust curves for KNO3/Sorbitol propellant with different amounts of Dextrose fuel alloys in low KN/low pressure motors.

  • Introduction
  • Method
  • Results
  • Conclusions

  • Introduction

    Previously I did propellant characterization PDT 5a and 5c (5a: melt slurry characteristics and 5c: cure rates) to help with the Sugar Shot to Space project.
    Well,... I ended up with small "Bates" grains of these alloy formulas. I was curious how these would perform in my 24mm motors. I'm not sure the data has much application to the Sugar Shot project as they will be using much higher KNs / pressures.

    The % of KN03 is maintained at 65% The amounts of sorbitol and the alloy sugar are varied with the total sugar (fuel) content maintained at 35%.
    A control grain of 65/35 KN/Sorbitol was also made.

    Designation Alloying sugar % Ratio 1: KNO3/Sorbitol/Dextrose % Ratio 2 % Ratio 3 % Ratio 4
    KNSBDX Dextrose 65/30/5 65/25/10 65/20/15 65/15/20

    Method



    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 Dextrose was anhydrous dextrose reagent grade
    The melting pot is a small triple batch double boiler utilizing paraffin as the heat transfer agent.


    Process:

    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 4.0mm 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 control grain weighed 13.2 grams so I adjusted the density ratio (actual/ideal) till the mass came to 13 grams. The density ratio for the control grain 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 following is the static curves of the five formulas:





    Conclusions

    At these lower KNs (motor pressures in the low 200 psi range) the different formulas did not show much variation. I do not see any clear trends. Any slight differences in the curves could well be just statistical variation.
    For me this is actually good news. Swapping to a different alloy formula should be easy and predictable / consistent with the plain 65/35.
    The advantage of using a dextrose alloy then would be just the faster cure time.
    Should I ever be in a situation where I have to pour my grains at the launch site I can get much more rapid curing with the dextrose alloys.
    As mentioned earlier these motors were run at low KNs / low pressures: Higher KN motors might give very different results.



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