mkn Posted October 1, 2014 Posted October 1, 2014 Hi All, I wanted to get some general discussion going on pressures used for rocket fuels, BP and Whistle, hand rolled tubes and waxing the tubes. Thinking about the loss of quality NEPT tubes and the time before a replacement supplier is found and hand rolling tubes. I do not have a lot of experience with rocket motors, and maybe I am unaware of why such high pressures are needed. I am successfully launching motors with pressures much less than recommended and I am using waxed home rolled tubes. I am wondering if waxing the tubes can lead to less pressure needed on the fuel grain ? The higher pressures were developed before the use of waxed tubes, so maybe that is one more benefit of waxing, less pressure needed resulting in tubes of less strength needed which are easier for home rolling. If I understand correctly the higher pressures are needed to consolidate the fuel with no cracks, and to give it longevity in storage so that it will not "relax" during storage. Can we press them using the benefits of waxed tubes and use a lower pressure and still have flight stability and the longevity for storage? With my limited experience it would seem fine to use less pressure as long as the fuel does not contain hard grains. Last night I launched a 4oz hybrid of 50/50 benzo and hot BP with a long delay of Sali and Ti (1.12 " delay I am doing a test as schroedinger has described to me in other posts) The ball shell was made with left over stars that were too large , too long of a burn for the 3.5 " shell. Besides the terrible break, the interesting thing is I pressed them with 1800 psi on the comp and no tube support in a hand rolled tube , waxed and bottom lit. The motor was made and launched within a hour. I did not see any difference compared to similar one made with 6,000 psi on the comp. here is the video: I have also been launching 4 oz Sali rockets with 4,000 psi on the comp and waxed tubes, no catos. I have not used a lower pressure yet because all the literature indicates that higher pressures are needed ( above 6,000 psi ) I would guess the big concern would be that I am not storing these motors and that maybe where the higher pressures are needed? Also the whistle fuels have only been in 4 oz motors, nothing larger. If we find that lower pressures are indeed sufficient with waxed tubes, I believe that we can easily roll quality tubes at home that are strong enough , possibly even with recycled Kraft, or use commercial tubes that were once thought insufficient. Matt
dagabu Posted October 1, 2014 Posted October 1, 2014 That kind of experimentation is what we are after. If the waxing proves to reduce the need for high pressures, lighter weight tubes may be able to be used as well since the fuel grain may act as it's own support. Keep on working on that and post video of your findings. I am sure you will inspire others to start working on low pressure pressing as well.
Maserface Posted October 1, 2014 Posted October 1, 2014 I am working on similar things, but I want to push the pressure down to or below 1500psi. I have some garbage tubes that I would like to use. My end goal is to produce a 1lb bp rocket with 100 pounds of thrust using my crap tubes. Problem is, my tubes split with a single wack on clay 1
Mumbles Posted October 1, 2014 Posted October 1, 2014 When you're calculating pressure on the composition, are you using the entire cross sectional area of the rocket (1/2"), or just the cross section of the tooling (ie subtracting the core area)?
mkn Posted October 1, 2014 Author Posted October 1, 2014 I am using the Dan's calculations from Whichibuggywhip, as that is the pressure to force gauge that I built. So I read 150psi on my gauge x hydraulic factor 2.3 / .1964 = 1756 psi The .1964 not sure if that represents tooling with the core area removed, good question, just checked , it would seem that the core area has not been accounted for in the chart on whichbuggywhip. the core area does represent a significant amount of area, I would have to check my tooling, I would guess my ram has apx 3/16" or so hole, So that would increase the pressure on the comp quite a bit ?
mkn Posted October 1, 2014 Author Posted October 1, 2014 using 3/16" area = .027 square inches subtract from .1964 = .1694 that = 2036 psi on comp
Maserface Posted October 1, 2014 Posted October 1, 2014 there was an excel sheet somewhere, that you could input your tooling specs and it would output the actual force applied, varying from drift to drift, ill find out if the author is ok with it being posted here. even still, 2000~ on the comp is quite low!
Zmuro Posted October 1, 2014 Posted October 1, 2014 (edited) I posted an excel sheet in another thread, but I don't know in which one http://tinyurl.com/RocketPressingCalculator Edited October 1, 2014 by Zmuro
mkn Posted October 2, 2014 Author Posted October 2, 2014 That kind of experimentation is what we are after. If the waxing proves to reduce the need for high pressures, lighter weight tubes may be able to be used as well since the fuel grain may act as it's own support. Keep on working on that and post video of your findings. I am sure you will inspire others to start working on low pressure pressing as well. You bet, I've got the 4 OZ tubes covered, who's going to pony up on the 1# and up ? I posted an excel sheet in another thread, but I don't know in which one http://tinyurl.com/RocketPressingCalculator Awesome xl sheet ! Thanks for posting , I wish we had switched to the metric system as they were teaching us in the 70's it makes so much more sense........
dagabu Posted October 2, 2014 Posted October 2, 2014 You bet, I've got the 4 OZ tubes covered, who's going to pony up on the 1# and up ? Awesome xl sheet ! Thanks for posting , I wish we had switched to the metric system as they were teaching us in the 70's it makes so much more sense........ Give me a month to gather the wood for the roller frame, I will make up some 0.75" tubes and see what i can get out of them.
Blackthumb Posted October 2, 2014 Posted October 2, 2014 I have been building/firing small rockets (1# or less) since I was 15....looong time ago. Recently, I have stepped up the hobby but I still find no need to press my BP comp under the high pressures I see here on this forum. I have been trying 3/4 ID spiral tubes that are very hard from my supplier and I've have no problem ramming the nozzle mix (litter/oil mix) no tube split as well as while ramming the fuel. I do recognize that higher quality convolute tubes would be best, I use some of 5/8" ID but get no better results. I recover most tubes and have "reloaded" them 3 or 4 times before they fail. I don't have nozzle failure or catos.I agree on a better tube for whistle comps, but just don't understand the real need for the high pressure pressing of BP fuels...enlighten please...I like/enjoy doing things at a low cost while maintaining consistent, safe performance.
schroedinger Posted October 2, 2014 Posted October 2, 2014 Well some time ago, (when i first tried rockets) i used 25 mm tubes i got at the local supermarket (they wound the platic bags for vegetables on them), they where spiral wound but really god quality (3mm wall thickness hard as rock). About 1 year later they swapped over much less quality tubes well it is easy to build a rocket out of them but if you fly 'em they always burst in mid air (about 10 m) as the tube can't stand the peessure generated and burst open on the side wall. (But at least they still make god cake tubes).
mkn Posted October 2, 2014 Author Posted October 2, 2014 (edited) Give me a month to gather the wood for the roller frame, I will make up some 0.75" tubes and see what i can get out of them. Dagabu that would be great ! your rocket experience will be a huge addition to the effort. I think you'll like the frame roller, for me it has taken away the troublesome task of finger rolling the mandrel and trying to keep things tight and lined up. and the consolidation with a board after is gone as well, just keep turning the mandrel till your satisfied. I have been building/firing small rockets (1# or less) since I was 15....looong time ago. Recently, I have stepped up the hobby but I still find no need to press my BP comp under the high pressures I see here on this forum. I have been trying 3/4 ID spiral tubes that are very hard from my supplier and I've have no problem ramming the nozzle mix (litter/oil mix) no tube split as well as while ramming the fuel. I do recognize that higher quality convolute tubes would be best, I use some of 5/8" ID but get no better results. I recover most tubes and have "reloaded" them 3 or 4 times before they fail. I don't have nozzle failure or catos. I agree on a better tube for whistle comps, but just don't understand the real need for the high pressure pressing of BP fuels...enlighten please... I like/enjoy doing things at a low cost while maintaining consistent, safe performance. I agree with the BP rockets, I hand ram and bottom light. I too recapture and reload the tubes at least once for BP. But some of these guys are getting incredible performance from BP, 100 lbs thrust on a 1# motor, This maybe where some additional pressure is needed, It would be interesting to see how low the pressure could be and still be a reliable motor, and then combine that with tubes from you or home rolled. I have pressed sali whistle in a waxed spiral wound tube 4oz tube 1/8 " wall and had no issues. Well some time ago, (when i first tried rockets) i used 25 mm tubes i got at the local supermarket (they wound the platic bags for vegetables on them), they where spiral wound but really god quality (3mm wall thickness hard as rock). About 1 year later they swapped over much less quality tubes well it is easy to build a rocket out of them but if you fly 'em they always burst in mid air (about 10 m) as the tube can't stand the peessure generated and burst open on the side wall. (But at least they still make god cake tubes). The pressure generated inside the tube is one area we really need to understand, If a high performance BP motor can generate 100 lbs of thrust, what is the pressure inside the tube ? The tube is pushing the header on end which is the strongest orientation of the tube, like you can't squish and egg by squeezing the ends, but turn it sideways and it pops with little effort. So if a waxed tube can take say 2,000 or 3,000 psi unsupported,( 20 -30) times the thrust, is that enough to keep the side walls from flexing and the grain cracking. That is possibly one cause of a mid air cato. So far those psi seem relatively easy to obtain with a home rolled tube even with recycled Kraft that I am using. The wax is doing the lubricating to keep all the psi focused on the fuel grain consolidation, the psi is not fighting the friction of the tube walls, which in turn would cause wall weakness and reduced psi on the grain. Just like trying to drive a pin into steel, when dry it takes a tremendous amount of pressure to over come the friction, add a touch of oil and much less effort is needed to accomplish the same task, and the walls of the steel hole have not been strained as much as the dry fit. A little off your post, but just thinking out loud Edited October 2, 2014 by mkn
oldspark Posted October 2, 2014 Posted October 2, 2014 Have not done any side by side testing but pretty sure I can tell the difference, would love to do a drag race with high pressure vs low.
dagabu Posted October 2, 2014 Posted October 2, 2014 Have not done any side by side testing but pretty sure I can tell the difference, would love to do a drag race with high pressure vs low. With E-matches and top-lighting, you can ensure identical lighting -or- you can video them both and play them back side by side (much more complicated).
mkn Posted October 2, 2014 Author Posted October 2, 2014 Have not done any side by side testing but pretty sure I can tell the difference, would love to do a drag race with high pressure vs low. Hi Oldspark, are the tubes for both waxed ? I'm wondering if a low pressure waxed maybe similar to a high pressure un waxed. maybe one of the guys with a load tester would be up for a test of the two? Matt
Maserface Posted October 2, 2014 Posted October 2, 2014 If it is being tested, make sure to use a fuel that wont cato in an unwaxed tube. green mixed 60:30:10 with commercial airfloat charcoal perhaps. If it were me, I would alcohol granulate it as well. What pressures are we talking? 2000 waxed and 6000 unwaxed maybe?
mkn Posted October 2, 2014 Author Posted October 2, 2014 that would be a good place to start, I wonder what the grain pressure on the high performance 1# BP motor with 100 lbs thrust was? I forget who posted it, and I can't seem to google the post back. The other point is that I have only done the 4oz motors with this lower pressure waxed, As we know that size is the least likely to cato. I have not done 1# motors with the whistle or hybrid.
Mumbles Posted October 2, 2014 Posted October 2, 2014 These are all good points. I think it would be hard to measure accurate pressure being applied to the composition, versus the walls and everything else. It might be worthwhile to measure the density of the fuel in a low and high pressure situation.
oldspark Posted October 2, 2014 Posted October 2, 2014 Both the low pressure and high pressure rockets had waxed tubes, I need to try a few more so I am sure I am not spewing crap, I do think you can make a better low pressure rocket with the waxed tubes then with out.
Arthur Posted October 2, 2014 Posted October 2, 2014 You will need a neighbour irritating number of tests to get a good spread of test results. IMO and only that, a large increment of powder will need a higher pressure than tiny increments, but then there is the work time taken.
DavidF Posted October 3, 2014 Posted October 3, 2014 The grain cracking is not so big an issue in a waxed tube. Since the outer surface of the fuel grain is inhibited by the wax, a crack or two in the grain only leads to a small increase in the surface area burning at once. I pressed a cored 1# grain of Lloyd's popcorn glitter in a brass mold. It broke into several pieces removing it from the mold, along the increments. I glued the grain back together with crazy glue, lowered it into a 3# tube, and back-filled it with melted wax. It flew! https://www.youtube.com/watch?v=gd4MWxEHpe0&list=UUqu8DxSdN4KWh73BVQ7EKjg I don't know if the link will 'take' or not. It might need to be copied and pasted. I'd like to experiment with lower pressing forces too, but I haven't had the guts yet.
mkn Posted October 3, 2014 Author Posted October 3, 2014 Hi David ! Great post, after seeing your name I remembered it was your method, tests, and BP rocket at 100 lbs thrust ! Outstanding......oh I think I see a sumac outside the window...... The video came thru, and that is great information, it starts to reveal the character of internal pressures. With apx 1/8 inch layer of wax around the grain, it held / was able to contain the pressures wanting to go horizontal to the side walls of the tube ( the most difficult strength for home tube rollers to try and duplicate of NEPT tubes )It also held the glued together increments of the grain, it would seem that a low pressure compaction of fuel should at least equal the integrity of a glued grain, and most likely surpasses it. I don't know how much wax can be compressed, and that property is important to the true character of the pressure and gasses, but for your test, the gases and pressure went the path of least resistance. Re-reading the wax tube topic, the wax keeps the flame from propagating up between the outside of the grain and the tube wall, and prevents that type of cato. I started to explore the low pressure compaction first because it seems that I can easily roll tubes from recycled Kraft ( available everywhere (virgin not so much )) Then a second benefit became clear , a tube support was no longer needed, which adds to complexity and build time. Less pressure should hopefully mean less friction removing spindles, possibly improving safety. A distant fourth would be less wear and tear on tooling at lower pressures. It is looking promising that low pressure compaction has solid legs for some tube diameters so far, and that sufficient home rolled tubes are relatively easy for amateurs to build able to contain 2,000 to 3,000 psi for 4 oz and 1# tubes, based on a few tests of tubes that I have recently made. And for those that don't want to roll, commercial tubes that once thought insufficient will probably work fine if they have decent rigidity. One area that low pressure will not be able to compete is the amount of fuel loaded in the motor, high pressure should allow some percentage more producing a longer and possibly a higher thrust period. I hear you on the low pressure testing, I vividly recall my first hand rammed 3# BP cato, glad I had my shed between us, and I'm sure it was heard for some distance, I was preforming a weight lifting test with my new hotter BP so no explosive header to add to the cato...... I hope to try a 4oz low pressure (2,000 lbs) sali whistle soon. Matt
mkn Posted October 3, 2014 Author Posted October 3, 2014 (edited) This morning tested a 4oz Sali with a sand bag 360gram head + 32 gram sticks, Pressed my tame whistle to 150 on my gauge = 2000 on the comp accounting for the ram hole, waxed home rolled tube with recycled Kraft paper, no tube support. The 2,000 psi was fine, no cato, the overall launch was uneventful, big old rainbow trajectory, my tame whistle, and home made tooling is not able to lift that weight to a safe height, ( I am working the infamous "4 on a 4" ) 350g is my heaviest 4" ball I have. The sand bag did break open so that was great, as I am just starting to use this type of break open and dump weight. Up till now what I sent up.....came back down with a thud ! So that is one more successful low pressure test, before I had used 4,000 psi as the lowest for this fuel. My tame whistle is made with red iron oxide with mineral oil, which is on the mid to lower end of thrust on Dan's whistle test chart. - small steps for me in the whistle territory....... Matt edit: to add tube made with recycled kraft paper, and I'm pretty sure Elmer's glue ( 70 /30) not sure on the glue as I have quite a few tubes that I have made recently .... .... Edited October 3, 2014 by mkn
ddewees Posted October 5, 2014 Posted October 5, 2014 I made a motor with sali whistle yesterday, using 3500 psi on the comp... and the the fuel pulled free from the waxed tube during extraction.Cracks everywhere... Wouldn't have been so disappointing, except that I was using my new 3" ID tooling. I'll try again tomorrow... hopefully with better results. I'll likely try and remove any wax near the end of the tube.
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