EXP1 KN03/Sugar/Paraffin
EXP2 KNO3/Polyester Resin
This mixture contained 80% KNO3 and 20% Polyester Resin. Open air burn tests were unsuccessful, as the propellant would not ignite. I loaded a 5.5"x1" x 3/8" core grain into a small test motor. The motor throat was .25" diameter, creating a Kn ratio of 511. A small slug of standard 65/35 candy propellant was used at the motor head to help initiate a burn and bring the pressure up. The candy burned quickly on ignition, but the polyester resin grain would not ignite. Upon inspection, the grain was nicely scorched through the core and outer surfaces, but the grain did not burn.
EXP3 KNO3/Sugar/Polyester Resin/Charcoal
This mix used 54.5% KNO3, 18.2% Sugar, 1.8% Charcoal and 25.4% Polyester Resin. When mixing, I started out with a lower amount of resin, but had to add more to get the mix to a play dough like consistency. The next batch I will try to get the resin percent lower and the KNO3 higher. After the propellant was mixed, it was loaded into the same motor as in the above test. An open air burn test was performed again. This mix burned easily with a match, but very slowly. It was about 30 seconds of burn to one inch.
With a little more optimism, I loaded the motor into my old, small test stand. I wasn't sure one of my regular candy igniters would do the trick, but that was all I had. The first test ignition was a failure to ignite. So I added a few small chucks of black powder to the motor, then inserted another igniter. The igniter popped, and the motor sat there and smoked lightly for 5 or 6 seconds, then a couple of chuffs and the motor came to life. The burn lasted for about 11 seconds with no thrust indicated by the test stand.
While you may believe I would have been disappointed by little or no thrust, it was actually better than I had hoped. A full burn, with a long duration, just what I wanted! Now, I need to raise the Kn a bit, and tweak the mixture to burn just a little faster. After firing, I opened the motor and inspected the contents. There were about 12 grams of black, fluffy residue in the case. Hopefully the residue will decrease as I increase the combustion efficiency.
Click Here for a video of the test.
EXP4: KNO3/Sugar/Polyester Resin/Charcoal
This mix uses 58% KNO3, 23.3% Polyester Resin, 16.8% Sugar and 1.9% Charcoal. For this test I built a new test motor. The casing holds 2 grains with a total length of 11", has a 60 degree convergent angle and a 30 degree divergent angle. The casing is 1.5" EMT. Throat is .32" diameter, yielding a Kn of over 900.
This motor was lit with my standard igniter with the addition of a small amount of black powder added. It actually took two tries to get it going, and there was still a long lag between ignition and full burn. Perhaps an ignition aid on the outer surfaces would be in order. As it would turn out, I had one screw not tapped deep enough, and the motor would not fit loosely in the test stand. So no thrust data was available. Once the motor started burning well, the burn lasted about 15 seconds. Considering the much larger grain diameter, it was an improvement in the burn rate but still too slow to develop good thrust. I do believe there was some low level thrust developed however. Upon inspection, the motor retained about 80 grams of residue post firing and the nozzle was reduced in diameter due to deposit build up.
Not a real interesting video, so I'll skip this one.
EXP5: KNO3 55%, Sugar 10%, Potassium Perchlorate10%, Polyester Resin 23%, Charcoal 2%.
With this mix I added some Potassium Perchlorate in hopes of increasing the burn rate. With the added oxidizer, I had to cut down on the sugar, hoping the polyester resin would be an adequate fuel. In open air burn tests, I could not get a strand to burn continuously. I used the new test motor as in the previous test. I also added a 1/2" slice of regular candy propellant above the grain to help with ignition. While it did start the burn better, it also reduced the grain size, as well as the Kn of the motor.
Upon ignition the candy burned in a quick puff, then the grains burned for several seconds before burning well. Or, sort of well. The motor chuffed through the entire burn, picking up in frequency towards the end of the burn. The burn lasted for about 35 seconds. No real thrust was developed, if you look carefully, you can see the needle on the test stand just move perceptibly.
It looks as though I will need to find a burn rate catalyst. Iron Oxide is the logical choice, as others have used it and proven it to be viable. I would like the mix to be formulated from locally available chemicals, but Iron Oxide is not considered a hazardous material and is readily available and inexpensive to mail order. More testing to come when I get in the Iron Oxide.
Click Here for a video of the test gauge. Not a real interesting video, but if you've never heard a motor chuff, here you go.
EXP6: KNO3 65%, Paper 35%.
This was done as a bit of a whim, I know paper has been used for grains in hybrid motors, so I just thought... Well, I know there are a slew of things working against this, but I just had to try. I weighed out the KNO3 and paper, then added about 3 cups of warm water to my blender and added the KNO3. I mixed it for a few seconds until the KNO3 has completely dissolved in the water. Then I added a handful of paper and started blending with a vengeance. As it turned out, I had to add more water to the mix to keep the blender from overheating. When I was done, I think I had about 6 cups of water in the mix.
I then poured the mix into a stainless steel pot and started simmering it, it took about 3 hours to get it to a real thick, but still wet consistency. I then moved the mix to a sheet of parchment paper, and put it in a low oven for several more hours. When done, it was just damp, to the point you could not extract any water from hand squeezing. I then packed it into a couple of 1" grain casings. I used a wooden dowel and hammer to pack it in, adding a little at a time.
The grains are still drying, so I'll post more here when they are dried. I don't expect much. I think it may be a good way to form grains for a hybrid, but that's more than likely the extent of it.
EXP7: KNO3 70%, Fe2O3 7%, Epoxy 23%.
This is one of Richard Nakka's formulas he has developed. I received my Iron Oxide, so it was time to cast some grains. The casting process was very much the same as I had done previously with the polyester resin. This was also my first chance to use my new Ohaus triple beam balance scale, what a joy! The scale makes the weighing process so much easier.
The KNO3 and Fe2O3 were weighed into a plastic container and mixed with the aid of 5 lead slugs in the mix. The epoxy and hardener were than weighed into a glass (Pyrex) bowl and mixed. The dry mix was added about 1/3 at a time and mixed thoroughly. The grains were then cast into casting sleeves made from 1.5 PVC tubes lined with wax paper. After several hours the grains seemed fully cured, and were removed from the casting tubes. The wax paper peeled off easily from the grains.
I test burned a couple of strands of extra propellant. They seemed to burn about twice as fast as the best polyester propellant strands had.
I drilled the cores out to 5/8", and trimmed the grains to 5.5" each. The grains were then loaded into my test motor, the same motor used in EXP4. I made sure this time the motor would fit into the test stand properly. I added about 1/2 teaspoon of the shavings from drilling out the cores, to aid ignition.
Upon ignition, the motor lit very quickly. The burn lasted for about 5 seconds, with a little over 4 seconds where it burned hard. The last second or so seemed weak. My test stand indicated very little thrust, of course, I still had a 400 psi gauge in it, which I knew would be way to big. I still question the readings from the test stand, especially at low thrust levels. After the test I replaced the gauge with a better quality, 0 to 150 psi gauge. The readings from this gauge seemed much better, and much more responsive.
I didn't learn a lot from this test, but it was successful. About 35 grams of residue were in the motor post firing. In addition to the new gauge, I added a pressure gauge to the test motor top closure. I should now be able to read chamber pressure as well as thrust. That should much more informative in future tests. New grains are cast, another test firing of the same propellant formulation coming soon.
EXP8: KNO3 70%, Fe2O3 7%, Polyester Resin 23%
In this test I am duplicating the last test, only using polyester resin rather than epoxy. The propellant was mixed exactly the same as in the previous test.
This is interesting, I tried burning test strands of this propellant, with no success. After 5 or 6 seconds under a lighter flame, the propellant would start to burn, then sputter and go out. I haven't tried test firing the grains in a motor yet, but this is discouraging. I really thought the strands would burn better with the iron oxide. Perhaps lower levels of iron oxide and some charcoal. More to come when I try to fire the grains in a motor.
Update 6.29.03 I made an attempt last week to test fire my test motor with the newly cast grains. After four attempts, including adding black powder and sugar propellant shavings, I gave up on the grains. They simply would not burn. After the failed firing of the test motor, the grains were removed from the motor and lit with a propane torch on the ground. While they did eventually ignite, the burn was slow and sputtering.
The propellant formula obviously will not work.
EXP7b:KNO3 70%, Fe2O3 7%, Epoxy 23%.
This was the exact same formulation as in test EXP7. I used my test stand with the better quality, lower range pressure gauge and a gauge on the motor for chamber pressure. The motor lit quickly again, and started producing measurable thrust after about 1 second into the burn. Here are the numbers I came up with after the burn.
A few thoughts on this propellant. An Isp of 70 is not very impressive. I'm not sure why the Isp was so low. Perhaps better mixing, or a dryer mixture would help. I kept the Kn just a little low also, around 840. It may help to bring it up closer to 1,000.
I'm also not thrilled about the expense of the epoxy. I was using West System Epoxy, and it runs about $40 a quart with hardener. Assuming a quart will make about 10 of the above motors, that's $4 a motor just for the epoxy, the iron oxide isn't a big deal, about $.36 a motor for it. The KNO3 is about $.41 per motor. That makes the total cost excluding the motor itself of $4.77 per launch.
Assuming the same size grains, a KNO3/Sugar motor would cost $.38 for the KNO3 and $.16 for the sugar for a total of $.54 per motor. Now that's cheap!
By the way, if you have a lathe and don't count your labor, these motors cost about $2.62 each. That includes 1.5" EMT tubing, the 1 7/8" steel bar for the nozzle and closure and the 16 screws.
EXP8: KN03 64%, Sugar 32%, Fe2O3 3%, Charcoal 1%
This is another attempt at my quest for a long burn sugar motor. The idea is to accelerate the burn rate of sugar propellant to the point an end burning motor is feasible. The above formulation was mixed in a 150 gram batch, then tumbled with the lead disks for 1.5 hours. The powder was then heated in my casting pot at a setting of 325 degrees F. The mixture melted as expected, and was cast into a 1"x4.5" grain with a paper outside inhibitor. I used a lighter weight paper as an inhibitor, using 3 full turns inside the casting tube, unfortunately, the lighter weight paper pulled up from the bottom by about .5" during compacting.
After casting the grain I formed a couple of strands of unused propellant. The first strand was still warm when it was ignited, and the strand was slightly course in texture. The small strand burned almost instantly, in one quick flash and smoke ball. The next strand was cool when ignited, and was smooth in texture. I measured a 1" mark on the strand and burned it in a horizontal position. The strand lit quickly and burned smoothly. It took 4.22 seconds to burn 1 inch. That's a burn rate of .238 inches per second. That rate is about twice what would be expected from just a KNO3/Sugar strand.
The initial test is encouraging. Next, I need to find out the burn rate under motor pressure. If the burn rate under pressure remains at about twice the usual, it should be fairly easy to design an end burning motor. More to come when I test the grain in a test motor.
Update: I decided to cast a new grain using the same formulation. The small 1" grain would be too small to be practical in an end burning motor. I made a new nozzle, see the details here. The nozzle was made for 1.5" EMT case and had a throat diameter of .2".
The grain was 4.9" long and 1.52" in diameter. I inhibited the outside surface by casting into a cardboard tube, then epoxying a cardboard disk over the top of the grain. The cardboard tube was then lightly coated in epoxy to seal everything. I made the motor casing 9.25" long, so I used a 1.25" wooden dowel 4.25" long to take up the extra space in the forward area of the motor. I have a forward closure with a pressure gauge connection, and the wooden dowel would have sealed the port opening. So I drilled the wooden dowel out from on end about 3/8" x 2" deep. Then drilled through the sides of the dowel with a 1/4" bit in two places at 90 degree angles to each other.
With an end burning surface area of 1.8136 sq. in. and a throat area of .0314 sq. in. I should have a Kn of about 58. That seems pretty conservative. Assuming a burn rate under pressure of 1.2"/sec., and a propellant density of 1.8 grams/c.c., and an Isp of 125. I should have a sustained thrust of 17.25 lb./sec. for just over 4 seconds. Now, there are a lot of assumptions in there.
Click Here for a video of the test.
If you watched the video you noticed a very rapid failure to the nozzle. The motor lit quickly, started thrusting quickly, then over pressured in an instant and blew the nozzle out. What you can't see on the video is the ejected propellant. It went about 100' up, and seemed to be in one solid chunck. It burned out before reaching the ground. The chamber pressure gauge never went up, I'm not sure if the gauge was bad from the start or if the chamber sealed before the pressure made it to the port. The gauge for thrust was actually bent from maxing out. The max pressure on the gauge was 150 psi, relating to a thrust of 195 lbs. Coming from a .2" diameter nozzle it will be interesting to do the math and see what the chamber pressure was! What caused the over pressure? Here are some guesses.
I'm sure if think on it more, I could come up with more options. I think on the next test I will use different inhibitor, and a little larger nozzle.
EXP7c: This is the above propellant EXP7 configured to a new test motor. The motor was 1.5" EMT, and I used the T2 motor nozzle (.42" throat diameter, 30/15 divergent/convergent half angles). The casing was 19.5" long, holding (3) 6" grains. Each grain weighed about 280 grams for a total propellant weight of 840 grams. The grains were totally unrestricted, so assuming a linear burn, the profile should be slightly regressive as the ends burned.
Ignition was quick using one of my standard igniters. From a spectator perspective, the test was impressive. The motor threw out a great cloud of dark gray smoke and the roar was almost deafening. The video of the test does not do justice to the sound, I think the camera must have automatic sound limiting of some sort. I still question my test stand. While the gauge did max out, I extrapolated as best I could and came up with a mere 70.2 pounds of total thrust. Out of 1.85 pounds of propellant, that leaves me with an Isp of around 38! Well, that's not real impressive. But again, I'm not sure about the test stand or the gauge being used. I guess I will have to break down and buy a good gauge, just as a calibration gauge for the cheap ones used on the test stand.
Click Here for a video of the test gauge.