Mindphreak Posted January 18, 2009 Posted January 18, 2009 Alright, so I've had a wide variation with results in bottle rockets, some go 6 feet others sail. However, recently I've gotten a lot that tend to pop when lit and fly a grand total of 10 feet before nosediving and setting the surrounding grass on fire as the bp runs out. I've got the hole centered, good sized bit, but do I need to add more or less clay to the nozzle? Should I drill a deeper hole?
Swede Posted January 18, 2009 Posted January 18, 2009 In my bottle rocket experiments (look in the tutorials section for u-rocket fun, or something like that) I was using 1/4" OD X 3.5" tubes... a bit bigger than a Chinese paper-rolled bottle rocket, but not by much. I had plenty of CATO's and they were LOUD. But I never had a nozzle fail... the tube gave way long before the nozzle would blow out. The secret is the "bite" of the nozzle into the paper sidewalls. Pressure goes a long way towards making good nozzles, as does a bit of added refractory ("grog") to the clay. Most here would say adding grog or kyanite to a bottle rocket nozzle is overkill, and they are probably right, but it does work. More info needed - how are you making your nozzles? From kitty litter? How are you ramming them?
Mindphreak Posted January 18, 2009 Author Posted January 18, 2009 In my bottle rocket experiments (look in the tutorials section for u-rocket fun, or something like that) I was using 1/4" OD X 3.5" tubes... a bit bigger than a Chinese paper-rolled bottle rocket, but not by much. I had plenty of CATO's and they were LOUD. But I never had a nozzle fail... the tube gave way long before the nozzle would blow out. The secret is the "bite" of the nozzle into the paper sidewalls. Pressure goes a long way towards making good nozzles, as does a bit of added refractory ("grog") to the clay. Most here would say adding grog or kyanite to a bottle rocket nozzle is overkill, and they are probably right, but it does work. More info needed - how are you making your nozzles? From kitty litter? How are you ramming them? I'm using ground up kitty litter, but I think the batch Im using right now isnt fine enough so Im going to grind it again to see if that fixes the problem. I'm ramming them in a drilled woodent tube with a dowel and hammer. I'm asking because I'm planning on making some tooling but I want to get the dimensions right before I start.
Swede Posted January 19, 2009 Posted January 19, 2009 IIRC I used both 0.093" holes and 0.125" holes for nozzles. Most visco falls in that range and sometimes I couldn't use visco for the rocket, had to figure out other ways to ignite the BP. I agree, a finer clay (or greater/longer grinding of the cat litter) will make a better nozzle for these tiny rockets. If you add grog, kyanite, or powdered aluminum oxide to the nozzle mix, it will definitely give better bite to the tube walls, but be aware any refractory in the mix will also play hell with your tooling. Nothing horrible, but it will eventually wear the tooling and score it longitudinally. If you do use refractory, use particles as fine as you can. I wonder of moistening the base of the tube in water momentarily will also help the clay to bite and adhere? A bit of graphite powder in the mix might also help. Ultimately as much pressure as you can exert will make the nozzle successfully; OR, it will rupture the tube. One or the other. If your system doesn't support the tube during nozzle (and fuel) ramming, it's hard to make good rockets.
tentacles Posted January 19, 2009 Posted January 19, 2009 (edited) I'm using ground up kitty litter, but I think the batch Im using right now isnt fine enough so Im going to grind it again to see if that fixes the problem. I'm ramming them in a drilled woodent tube with a dowel and hammer. I'm asking because I'm planning on making some tooling but I want to get the dimensions right before I start. More likely, the problem is that you ground it too much. If you leave it ground only about halfway, with some grit in there still - or using the finer clumping grade of litter - the clay will get a nice bite into your tubes without needing grog. I suggest adding a squirt of graphite to the mix to help release the nozzle from your tooling. It may be as Swede said, though, for the smaller rockets. I've never made anything small, but FrankRizzo's 1lb rocket nozzles never fail - and those MnO2 rockets surely test them to the limit. His mix is just kitty litter, milled until about a third to half is powder, with a dash of graphite. I say a dash because he just squirts some in from his bottle, it doesn't take much. You just want it medium/dark grey. Edit, do you know if it's your nozzles failing, or the tubes? Describe your rocket making procedure and tools.. Edited January 19, 2009 by tentacles
Swede Posted January 20, 2009 Posted January 20, 2009 More likely, the problem is that you ground it too much. If you leave it ground only about halfway, with some grit in there still - or using the finer clumping grade of litter - the clay will get a nice bite into your tubes without needing grog. I suggest adding a squirt of graphite to the mix to help release the nozzle from your tooling. I agree that does make some sense with pure litter nozzles. I've always used grog. Without a refractory, a slightly coarser grind might help with the bite. Of course another option is to make the nozzle longer. Give the exit cone a steeper angle, and the transition from the combustion area to the exit cone remains the same, but the nozzle itself is longer. That will give it more grip. You'll loose some volume for fuel, but a well-designed nozzle geometry might more than make up for that. A good source for cheap but relatively coarse refractory is sand-blast media, of the alox variety.
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