innervision Posted July 16, 2006 Posted July 16, 2006 First off, I would like to say that I previously had these rockets working 100%. I have designed these from the ground up three or so years ago. Since then I have made roughly 50-60 of them by this design, and beside the early days of experimenting with various specifications, I have not had any failures. Below is my official design. Up until the fourth of july, I have had these in perfect working condition, but on the fourth everything was a disaster. I couldn't believe it. Not one rocket worked properly. They all went up about 15-20 feet before the payload went off.I use 2 inch shells for the payload which is not a fun thing to see go off 15 feet above you.At first I thought the nozzle was being blown off, but after witnessing a few of them I discovered that what was happening was that the core would ignite, propel the rocket about 10 feet and blow out the fuel from the top and ignite the payload. Now the only thing I changed was the brand of stump remover as the KNO3 source. The only thing I could think of is that the fuel is burning faster than before, although my previous batch was made from recrystallized KNO3 and worked flawlessly, so I don't understand how it could be the problem. I know it's probably not the best idea to not use an endcap on the motor and have the fuel burn up to the payload fuse, but it has worked fine until now. I tested a few tied down to a pole, and it is obvious that it blows out the fuel before it should be anywhere near done burning. I'm stumped about this and not wanting to change a design that has worked beautifully before, does anybody more experienced have an idea why it could be doing this. I have tried making the core shorter and they do work, but they don't have nearly enough power to lift a payload. Here is my rocket design:http://f10.putfile.com/thumb/7/19603152312.jpgto view full size: http://putfile.com/pic.php?pic=main/7/19603152312.jpg&s=f10
d4j0n Posted July 16, 2006 Posted July 16, 2006 Is it rcandy rocket fuel? You should probably post this in the kno3/sugar rockets thread by the way... Maybe you need more delay above the core?
brainfever Posted July 16, 2006 Posted July 16, 2006 You can try slowing the composition down by adding a few % carbonate in order to obtain the same effect you had before the new brand stump remover. Also a possibility is that the old composition had something that made it attach/glue/stick to the casing better so it could withstand a higher pressure. On your rocket design; without an end plug you'll allways have the end blowing out at some point.
innervision Posted July 16, 2006 Author Posted July 16, 2006 I'll try that Co3 slowing down thing, although I don't understand how this new brand could make a faster fuel than recrystallized KNO3 could that I have used previously. Technically, there is an endplug... the shell payload is fastened on tighly to the top of the engine with gummed tape... and when I test an engine with one layer of that gummed tape covering the top as an endcap, the fuel never burns through it - so I never bothered to make a true endcap with a passfire. brainfever, the idea of the propellant not sticking to the casing as well is an interesting idea. Not sure how I could go about testing that though. d4j0n, yes, it is rcandy, and the rocket doesn't nearly burn the fuel all the way like it should, otherwise the delay might have been an issue. The most likely explanation is too hot a fuel, but it puzzles me as to how it could be any stronger than anything i've used in the past?! I'm using the same batch of sucrose btw.
d4j0n Posted July 16, 2006 Posted July 16, 2006 Though your previous designs may have worked, I suggest just using some sort of end plug. Are you rolling your own cases from scratch or do you buy a set size tube or gummed tape?
Mumbles Posted July 16, 2006 Posted July 16, 2006 d4j0n you need to start reading everything. Try looking at the design he has, and your question would be answered. You mentioned you had the fuel burning directly into the payload fuse. It sounds more to me like you are having a problem here if they are going off right away. The fuse is not providing a proper delay. If you mean the shells have an open end to allow fire to pass into, it may provide the problem if indeed my next statement is the fault. Honestly, it sounds like the composition is flashing, which would backup brainfever's comment. The composition is not seated firmly against the wall, and the gases are traveling up the side. Pressing with an arbor press or some sort would help somewhat. The substituting some sort of liquid substance in a few percent for the sugar may also be helpful. Glycerine, or Corn Syrup(called dextrose syrup in some parts I believe) would all be prime candidates. It would give the propellant a bit tackier of a grain with better hopes of adhering to the walls. Perhaps the other brand of KNO3 had more water, or something else in it which kept it a bit tackier and thus better adhered to the walls.
innervision Posted July 16, 2006 Author Posted July 16, 2006 d4j0n, the casings are gummed tape. I do have a fuse on the shells that is about 1/4" or less which is inserted into a shallow hole in the top of the propellant in the rocket. I leave a solid section of propellant above the core that acts as a delay instead of a different comp. It usually worked great - the shell would ignite very high in the sky. I don't have a press so I ram the fuel with a cored Al rod over a nail that's sticking out of a woden block. I used to use a wooden mallet to ram, but now that I am using Al rod instead of a dowel, I am using a regular metal hammer temprarily (I was still using wood when the problems started - so it's not part of the problem). I wouldn't think the other brands of KNO3 had more water because I don't do anything to protect them from sucking moisture out of the air. The fuel is usually lumpy when I get it out to make a rocket, so I shake it up to break it up better (I keep it in either tupperware or a ziplock). I never had any problems with the moisture slowing things down so once again I never bothered to prevent it. I may try the corn syrup idea. Any help on how much to put in? I'm assumong just enough to make a slightly crumbly texture?
Mumbles Posted July 17, 2006 Posted July 17, 2006 I'd say less than 5%. Start with 2 or 3%. I know some people actually incorporate it in their fuels, such as Richard Nakka. It's pretty gooey stuff. Weigh out some of the mix first, and add the syrup. Accordingly add more syrup or rocket mix to get correct ratios and kneed. Adding it to the powder prevents some losses on the weighing container itself.
Boomer Posted July 17, 2006 Posted July 17, 2006 Completely different idea: Did you make a big batch and store them till the 4th? They could have absorbed water, softening the section that is plug and delay in one. Just a thought...
innervision Posted July 18, 2006 Author Posted July 18, 2006 I made a few hundred gram batch on the fourth with freshly dried KNO3 to make sure the ratios were correct... it was rather humid though so I would assume it definately grabbed some water from the air. I never have used thoroughly dried KNO3 though. Btw, I ram the rockets before I launch them, so they shouldn't be any wetter than the powder I ram. Maybe I should try one without the gummed tape - perhaps it is getting slipperyish from the moisture in the fuel. I'll do some tests soon. Probably the paper casing first, then the CO3 and corn syrup.
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