Titanium Fine Sponge with Sorbitol Propellant
Fuel "Balanced" Formulas



Goal of this experiment is to compare static thrust curves for KNO3/Sorbitol propellant with varied amounts of very fine sponge titanium added. This experiment attempts to balance the fuel to oxidizer ratio. As the percent Ti increases the percent sorbitol is reduced.

  • Introduction
  • Method
  • Results
  • Conclusions

  • Introduction

    An earlier experiment was performed with 65/35 KN03/Sorbitol where increasing amounts of fine Ti sponge was added and static thrust curves generated. At the highest percent Ti the performance decreased due to the formula being fuel rich.
    This experiment repeats the earlier one except as more Ti is added the sorbitol is reduced accordingly. I did not do a stoichiometric determination of how much sorbitol to remove. I simply went gram for gram. This rough approach should be good enough to see if the performance improves for the higher % Ti by avoiding the fuel rich situation.
    Data from the earlier experiment will be included in this write up for comparison purposes.

    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.

    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 Titanium powder is -325 mesh sponge from Firefox (Stock #C198)
    The melting pot is a small triple batch double boiler utilizing paraffin as the heat transfer agent.


    Process:
    The Ti sponge is extremely fine. It is shipped wet for safety reasons. An Email to Firefox confirmed that it is best to keep it stored wet. This prevents an air float suspension that is a health hazard and an explosion hazard. Each formula batch was weighed seperately and mixed well
    The resulting formulas had weight ratios for KNO3/Sorbitol/Ti of:
    65/35
    65/33/2
    65/30/5
    65/25/10
    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.

    The thrust curve is measured in pounds so it is converted to newtons and entered into Excel. From there Average Thrust, and Total Impulse values are calculated.


    Results

    Below is a photomicrograph of the Ti sponge:

    The difference in size from this sponge and the coarse Ti I use for sparkies is dramatic. One needle of the coarse Ti would take up the full width of the above image and be 1/3rd the image height.


    The one atmosphere burn rates are as follows:
    0% Ti: 10 seconds per inch
    2% Ti: 8 seconds per inch
    5% Ti: 7 seconds per inch
    10% Ti: 8 seconds per inch


    The following is the static curves of the four formulas:


    After converting the pounds thrust into newtons and entering data into Excel the following results were obtained:
    NOTE: The data in the first four columns for %Ti (with the -FR notation) is the fuel rich formula data from the previous experiment. The right hand columns are the current "fuel balanced" experimental data.


    The data in the table above is graphed below:


    Conclusions

    While this data is preliminary it does show that Ti sponge can affect the thrust curve for sorbitol propellants.

    Looking at the last graph we can see that the higher percent Ti give the higher peak thrust when the formula is fuel/oxidizer balanced.
    Total impulse is the highest for the highest Ti content when fuel to oxidizer ratio is balanced.
    It looks like the fuel balanced 10% Ti has a lower average thrust but this is due to a slow build up of pressure/thrust at the beginning of the burn. This gave it a longer official burn time which pulled down the average thrust value.
    Giving this motor a faster startup would bring up the average thrust value. Perhaps it needs a bit more primer, or a smaller nozzle.
    Fine Ti as an additive does seem to boost Total impulse for the same size of grain. I did not weigh the grains to determine ISP. While this would be interesting my main interest was performance within the same volume of propellant.

    Fine Ti sponge may have value for increasing ISP while possibly avoiding the risks of auto-ignition found in some metals with nitrates.
    Due to it's cost and hazardous shipping fees it may not be such a great idea though.







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