dagabu Posted July 14, 2011 Posted July 14, 2011 if your rocket CATO's, it needs a smaller core, or a larger nozzle throat. your efficiency is dramatically increased however. then when your 4" shell breaks at 800', you can reduce the size of your motor and save yourself a considerable amount of fuel. If this were high power rocketry, that would be true across the board but this is pyrotechnics with the art of fireworks in mind as the focus. That said, I have NEVER had a single person tell me that they really liked my "efficient burning rocket motor". It is the beauty of the charcoal tail, the twinkle of the Titanium in the delay, the whooshing sound of the motor working its way slowly up to display height that makes a rocket art. If all I was after was a fast heavy lift, I would pound shells from mortars. If I want to save fuel I will use whistle, charcoal rockets are not made because of efficiency, they are not efficient at all, pick one of a hundred more efficient fuels, the charcoal is for looks. Slow fuels and thus inefficient fuels are lit from the top. I do like these fuels when I have horrible BP rocket fuel that I want to get lit, a good fuel will pop the tube every time when top lit. -dag
nater Posted July 15, 2011 Posted July 15, 2011 R1dermon, I see what you are saying about motor efficiency, but I happen to enjoy a slow graceful take-off with lots of sparks and a long tail, even at the expense of lifting power and travel. There's probably better fuels for sheer power, but I love the traditional BP tails.
donperry Posted July 15, 2011 Posted July 15, 2011 A simple method i've used is to use a small straw to cover the fuse. The straw hides the sparks until it reaches the top where the exposed fuse does the job.
THEONE Posted July 15, 2011 Posted July 15, 2011 (edited) i wont get into expansion ratio's, since they're kind of impractical for fireworks use... Can you explain to me that is the expansion ratio where it helps etc... Edited July 15, 2011 by THEONE
donperry Posted July 15, 2011 Posted July 15, 2011 Thats a whole different topic of physics. Google de laval nozzle
Verge Posted July 15, 2011 Author Posted July 15, 2011 (edited) The white "strike anywhere ones -dag No, they come with a book.Couldn't I also just use a wire dipped in a BP mixture? Edited July 15, 2011 by Vergeltungswaffe
r1dermon Posted July 15, 2011 Posted July 15, 2011 Can you explain to me that is the expansion ratio where it helps etc... the expansion ratio is what will determine the efficiency of your nozzle. its your throat diameter's relationship in area, to your exit area. it's important to note that this is a measure of AREA, so pi r^2 is used to calculate. here's a picture of varying expansion ratio's... in order from top to bottom, you have Underexpanded Optimally Expanded Over Expanded Over Expanded with flow separation. optimal expansion is where the exit pressure and ambient pressure are in equillibrium. overexpanding will produce more thrust briefly, but at an efficiency loss due to suction. and underexpanding will increase your exit pressure too much...it will be higher than ambient pressure but your mass velocity will suffer (since the design failed to optimally accellerate the gasses being produced). it's more efficient to have less exit pressure, but accellerate your gasses as efficiently as possible.
dagabu Posted July 15, 2011 Posted July 15, 2011 No, they come with a book.Couldn't I also just use a wire dipped in a BP mixture? You need a bridge wire, I use a 50 gauge nichrome wire and 6 volts. I have seen many other ideas as well but I thing Christmas light bulbs are probably the widest used by those that need a quick homemade fix. -dag
THEONE Posted July 16, 2011 Posted July 16, 2011 the expansion ratio is what will determine the efficiency of your nozzle. its your throat diameter's relationship in area, to your exit area. it's important to note that this is a measure of AREA, so pi r^2 is used to calculate. here's a picture of varying expansion ratio's... in order from top to bottom, you have Underexpanded Optimally Expanded Over Expanded Over Expanded with flow separation. optimal expansion is where the exit pressure and ambient pressure are in equillibrium. overexpanding will produce more thrust briefly, but at an efficiency loss due to suction. and underexpanding will increase your exit pressure too much...it will be higher than ambient pressure but your mass velocity will suffer (since the design failed to optimally accellerate the gasses being produced). it's more efficient to have less exit pressure, but accellerate your gasses as efficiently as possible. Thank you very much... r1dermonSo the goal is to make an Optimally Expanded nozzle correct ?
THEONE Posted July 16, 2011 Posted July 16, 2011 (edited) Ok guys here i have 2 tests with the same engine, with the one ignites near to the nozzle and another one at the top... :P 1.avi 2.avi Edited July 16, 2011 by THEONE
TigerTail Posted July 16, 2011 Posted July 16, 2011 OMG! Never did I know it made that much of a difference. Is your scale fine?
dagabu Posted July 16, 2011 Posted July 16, 2011 LOL! Yes, that is what happens with top ignition at times, be warned and be careful. Thanks One, it was good to see the difference on your rockets. -dag
THEONE Posted July 16, 2011 Posted July 16, 2011 LOL! Yes, that is what happens with top ignition at times, be warned and be careful. Thanks One, it was good to see the difference on your rockets. -dag My pleasure Dagabu
r1dermon Posted July 17, 2011 Posted July 17, 2011 I spoke with a grandmaster rocket builder on Friday and actually got to see some of his rockets....he uses head end ignition 100% of the time. I also fired an E20 composite motor with a 1.75" salute on top. Judging by how long it took for the noise to reach us, we think it went 3000+ft.
r1dermon Posted July 17, 2011 Posted July 17, 2011 The one. Your motor overpressurized. Did it blow the nozzle as it popped? Increase your throat diameter and retest. You might have to go as much as 20% larger. Look at all the thrust you produced. And dagabu, a good fuel will still show you a sweet tail, it'd probably be a more uniform tail to be honest, and if you can make it more efficient, you can put more titanium in your fuel for an even bushier tail. Less fuel required for more thrust=more room for effects comp by weight. See what Im getting at?
THEONE Posted July 17, 2011 Posted July 17, 2011 The one. Your motor overpressurized. Did it blow the nozzle as it popped? Increase your throat diameter and retest. You might have to go as much as 20% larger. Look at all the thrust you produced. Both Nozzle and end plug blows out...
dagabu Posted July 17, 2011 Posted July 17, 2011 The one. Your motor overpressurized. Did it blow the nozzle as it popped? Increase your throat diameter and retest. You might have to go as much as 20% larger. Look at all the thrust you produced. And dagabu, a good fuel will still show you a sweet tail, it'd probably be a more uniform tail to be honest, and if you can make it more efficient, you can put more titanium in your fuel for an even bushier tail. Less fuel required for more thrust=more room for effects comp by weight. See what Im getting at? Yup, I do understand completely, I used to cast APCP motors that had to be top-lit. There is a place for top lighting but my push back comes from the place where we as artisans are also looking for slow rising motors as well as fast rising motors. Top lighting spikes the thrust at the ignition and then flattens, this is great for heavy lifting and speed but kills soft tubes, slow lift offs and such. I am every bit for the art aspect as I am for the immediacy but dont wish to push aside one for thew other. http://www.youtube.com/watch?v=CpPZk39bJeQ -dag
dan999ification Posted July 17, 2011 Posted July 17, 2011 Ok guys here i have 2 tests with the same engine, with the one ignites near to the nozzle and another one at the top... :P im suprised more than double the thrust. great testing, its also nice to see your results improving.i honestly didnt think you would last long.now build some headers. dan.
TigerTail Posted July 18, 2011 Posted July 18, 2011 Yup, I do understand completely, I used to cast APCP motors that had to be top-lit. There is a place for top lighting but my push back comes from the place where we as artisans are also looking for slow rising motors as well as fast rising motors. Top lighting spikes the thrust at the ignition and then flattens, this is great for heavy lifting and speed but kills soft tubes, slow lift offs and such. I am every bit for the art aspect as I am for the immediacy but dont wish to push aside one for thew other. -dag Adding to that, the total impulse would be nearly the same for both kinds of ignition. So for pyrotechnic purposes, a graceful jet of sparks all the way up to the apogee, marking the path of flight, and holding the audience in anticipation would be much more pleasing than a sudden burst of sparks near ground level, and the rocket reaching the apogee in no time. Still, I must add I learnt something very interesting from rocketry point of view.
dagabu Posted July 18, 2011 Posted July 18, 2011 As long as the rocket motor does not CATO, the total impulse has to be the same, it is the thrust curve that changes drastically. -dag
r1dermon Posted July 18, 2011 Posted July 18, 2011 Assuming you burn the same amount of propellent within the casing then the total impulse would be the same, however some of that impulse would go unused, as your rocket wouldn't go as high based on one method over the other. That's where you'll notice your efficiency loss.
dagabu Posted July 18, 2011 Posted July 18, 2011 Assuming you burn the same amount of propellent within the casing then the total impulse would be the same, however some of that impulse would go unused, as your rocket wouldn't go as high based on one method over the other. That's where you'll notice your efficiency loss. True that.... -dag
allrocketspsl Posted July 21, 2011 Posted July 21, 2011 Yep, if the fuel is too hot this may not work. I've had a fuel so slow before, the rocket lifted about two feet and hovered for a second or two before taking off. Right now I have 8 different motors with different fuel mixtures ready to test using both the J hooked fuse and the full core fuse. There should be a CATO or two in the bunch. I top lite the top of core on every rocket havent had a CATO in a long time and my engines are maxed out right on the edge its the only way as far as Im concerned makes a great swish when they go up
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