Titanium Fine Sponge with Sorbitol Propellant
Fuel Rich Formulas



Goal of this experiment is to compare static thrust curves for KNO3/Sorbitol 65/35 propellant with varied amounts of very fine sponge titanium added

  • Introduction
  • Method
  • Results
  • Conclusions

  • Introduction

    Earlier motors with coarse Titanium needles have worked well with KNO3/Sorbitol to make sparky motors. These are useful for night flights to track the thrust portion of the flight. The large Ti particles are ejected from the motor and burn with atmospheric oxygen giving brilliant branching sparks.
    The purpose of this experiment is to see if very fine Ti can burn internally as part of the propellant. There are several reasons this would be helpful.
    One is that the increased burn temp could help increase pressure. This might result in a boosted ISP value. Powdered metals are used in AP formulas to boost ISP.
    Aluminum has been used with AP but it can be dangerous with nitrates due to it's tendency in the presence of any moisture to react with nitrates.
    Auto ignition is even possible with Aluminum and nitrates,..... NOT GOOD.
    ?Due to titanium's presumed stability at lower temps with nitrates makes it seem a less dangerous powder metal to use? Notice the ? question marks; I didn't say it WAS safer, I didn't test this combinations safety, so it is only a questionable guess.
    Another possibility is the fine Ti might give a strong white flame. Sugar's version of White lightning?
    This experiment takes a preliminary look at the effect of fine Ti on static thrust curves in some 24mm 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 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. 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 Ti was added. To the next 0.4gms dry Ti powder was added. To the next 1 gram Ti and to the last aliquot 2 grams Ti was added.
    The resulting formulas had weight ratios for KNO3/Sorbitol/Ti of:
    65/35
    65/35/2
    65/35/5
    65/35/10
    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 EngEdit. This provides, Average Thrust, and Total Impulse values.


    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 = 2.5 mm/sec
    2% Ti: 8 seconds per inch = 3.2 mm/sec
    5% Ti: 6 seconds per inch = 4.2 mm/sec
    10% Ti: 5 seconds per inch = 5.1 mm/sec


    The following is the static curves of the four formulas:


    After converting the pounds thrust into newtons and entering data into EngEdit the following results were obtained:
     
     0% Ti
      2% Ti
      5%  Ti
      10% Ti
    Avg Thrust  N
      9.6
      10.8
      11.3
      10.8
    Total Impulse NS
      7.98
      8.63
      9.38
      8.97
    Peak Thrust N
      16.0
      20.0
      17.4
      18.69


    The data in the table above is graphed below:


    Conclusions

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

    Since the differences in thrust were not that great it would be good to repeat with three of each to take into account statistical variation.
    It is interesting that the burn rate at one atmosphere increased with increasing Ti content, yet under pressure in the motors the burn times were all about the same.
    It would be nice to get a rough comparison of ISPs for the four formulas but I neglected to weigh the grains. DOH!
    For future testing it would be nice to compare the coarse Ti to the fine Ti side by side for thrust curves and resulting ISP.
    While doing this it would be good to get photographic documentation of the flame fronts. The coarse we know is branching sparks. I recall the fine static burns as having a bright white flame and some sparks. But at 0.8 seconds my recollection could be skewed.
    Notice in the 10% Ti how the performance drops some (though not down to plain 65/35 levels) This is probably due to the way I set up the formulas. As more and more Ti is added it becomes fuel rich. To optimize the 10% Ti formula will probably require removing some of the sorbitol fuel. The higher the %Ti gets the more important this becomes.
    This hypothesis has now been tested at the "Fuel Balanced" page

    This is one avenue I think I will pursue further, especially since I paid an "arm and a leg" for the Ti with it's hazardous shipping costs. :-)



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