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Effects of Caramelization on Sugar Propellant

The longer you keep sugar propellant hot, the more it caramelizes or turns brown until it eventually turns very dark.  I wanted to know how many motors I could make with one batch and how much, if any, the propellant was degraded by caramelizing.  I decided to make a large batch over a period of time and let it get darker and darker, making both propellant grains to burn in motors and test strands for burn time tests.  This was done in August 2006 using potassium nitrate - sugar (sucrose) propellant prepared by dissolving in water and heating with the results below.  The batch size was 400 grams.

All the strands were burned an hour after the last sample so the last ones didn't have a lot of time but were cool and brittle when tested.  Below are the test strands and test grains.  The first picture is the same grains as in the second picture but just larger.  The second picture was laid out in reverse order.

Here are the times for the eight burn tests and the motor tests using the corresponding test grains.  The motors used in the tests had a theoretical design of 16.4 lb max thrust, 30.5 N-Sec total impulse, and a thrust duration of 0.48 seconds.

TEST RESULTS

The results were not very consistent although there were some conclusions that could be drawn from this set.

1. The first test, there was still water in the propellant and it had not reached a temperature to melt the sugar but there was likely no air in the mix.  The specific impulse was low, the thrust duration was longer and the total thrust was low.  Even though the burn test showed a similar burn rate as what would be expected from a good propellant batch, it is obvious that it did not have a normal increase of burn rate at operating pressure which is observed in the longer burn time.

2. The second test was done when the water was probably almost completely gone but had not reached the proper temperature to melt the sugar.  The propellant at that stage was crumbly which means there was a lot of air voids that could not be eliminated from either the test strands or test grains.  The test strand burned twice as fast indicating air pockets where the burning jumped across and accelerated the burn rate.  The expected results were observed in the test motor.  The burn rate due to the air pockets dramatically increased the burn rate and so the pressure as well and the excessive pressure blew out the header and blew the motor out of the test fixture.

3. The third test was done at the ideal consistency with the sugar melted and so the propellant was creamy.  The motor actually performed better than the software predicted, all parameters still being within an expected range of what was predicted but all on the high end.

4. The next two test points were after the time gap and resulted in fast strand burn tests but the motor tests didn’t correlate.  Of these two, the first seemed to have a normal burn characteristic but the second blew out the header and had a considerably higher than expected amount of thrust though a normal burn time.  Could be a calibration problem.

5. All the rest showed slower strand test burning, normal motor burn time, and high thrust.  Tests 5 through 7 should have had good calibration but still showed higher than expected thrust in the motor tests.  So it appears that the more caramelized grains have a different pressure vs burn rate curve and burn faster at higher pressures.  According to FPRED, the thrust for motor 6 of 24 lb would have a corresponding pressure 1240 psi.

   Looking at the motors that failed:  The ID is .804.  Total area = .508.   Anchor holes 7/32 = .219 dia.  Total area of anchor holes = 8*(.219/2)²*p = .30067 in².  Concrete estimated strength 750 psi (actually for Durham’s water putty).  Failure force = 225 lb.  Failure pressure = 443 psi.  Obviously the observed thrust is far past the calculated failure.  There is certainly more holding force than just the shear strength of the plugs.  The plug itself against the wall provides an unknown strength factor.  The anchor cement used actually expands as it sets so would provide a lot of resistance.

   SECOND CARAMELIZATION TEST

A second set of tests were conducted about a month after the first set but this time it was done continuously without stopping in the middle like the previous set of tests.  This time there were no pictures taken and no test strands done, only 14 two-grain motors were made (same design as the last set of tests).  Test strands are only marginally useful because they only give you the burn rate at atmospheric pressure.  Operating pressure in a motor is many atmospheres and so can only give a "ball park" idea of how a motor may perform.

Fri, Sept 8, 06 

Tested 14 caramelization motors, one end-burner, a two-grain and three-grain motor.  Most of the plots were clipped because the 9v batteries in the amplifier were drained.  The caramelization and end burner tests all used internal retainer rings.  Three of the car. Tests leaked out the top through the retainer ring split.  The nozzle and header were adequate length and there were no apparent bubbles.  The thrusts were higher than designed and so the pressure would have been higher than designed.  I suspect that the inhibitor sleeve is not thick enough and is burning through and burning is taking place on the outside surface.  These tests used pre-made sleeves using sodium silicate and they fit a little bit loose also.  Nevertheless, enough data was taken to get a significant trend showing less total impulse and less specific impulse as the caramelization proceeds to get darker.  These were about two weeks old and consistently had low specific impulse.  No pictures taken and no strand tests were done.

 (designed max thrust 16.4 lb, tot impulse 30.5 N-Sec, Thrust dur. 48 sec)  Note the smaller Specific thrusts on these tests.  There was probably a problem with the calibration of the test equipment because the thrust was lower than the design on all of these.  Still, the relative values can give an indication of the performance at different levels of caramelization compared to a non-caramelized propellant (test engine #1).  The trend is very clear.

The results are quite clear that the more the propellant is caramelized (the darker it gets) the lower the specific impulse and because the grains were nearly the same size, the lower the total impulse.  The effect is fairly dramatic with a drop of.  The total efficiency from the ideal condition to very dark drops 23%.  It also appears that there are were kind of two steps: the first was after the first test which was the ideal, and the second about half way through. 

The tests still need to be performed again.  I had a calibration issue.  I calibrated the equipment to use 75% of the available range of my amplifier using the expected maximum thrust of the motor.  Doing that means that when the thrust exceeds 133% of the expected thrust, it exceeds the gain of the amplifier.  The amplifier has a maximum total output voltage that is related to the input voltage and if it is overdriven, it clips the top of the plot so the maximum value becomes unknown.  I have learned an important lesson.  It is better to calibrate the equipment so that the maximum output corresponds to something more in the neighborhood of maybe around three to five times the expected maximum thrust.  This reduces the resolution but for this kind of work, the resolution is fine even at that setting.

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