deer Posted June 2, 2015 Posted June 2, 2015 Hello! Since I'm trying to get video off rocket, I need to stabilize it's axial rotation with fins (so no stick). The thing is most guides talk about pressure point, that in my opinion is valid only for straight body rockets. If my camera compartment is much wider than rocket, it can obstruct most of fins when flying straight. So I'm assuming that cardboard cut out method for determining pressure point would yield fin design that straightens rocket only when it's already severely swerved or am I wrong? OTOH it's not like there is no airflow behind the nose-cone, but it's certainly much less than without one. How do I correctly determine fin size in this configuration?Any more tricks to decrease spin? Previous try with stick:https://www.youtube.com/watch?v=qSMOdLdhatc
deer Posted June 2, 2015 Author Posted June 2, 2015 I just found this calculator: http://physics.gallaudet.edu/tools/rocketcop.htmlLet's hope it helps.. 1
pyrokid Posted June 2, 2015 Posted June 2, 2015 (edited) That calculator looks great! I will have to play around with it! Further reading: http://www2.estesrockets.com/pdf/TIR-33_Center_of_Pressure.pdf Of special interest to you may be the section on page 5 regarding static margin. Edited June 2, 2015 by pyrokid
FlaMtnBkr Posted June 4, 2015 Posted June 4, 2015 Could you put some small fins on the nose cone? Similar to some military missile designs?
deer Posted June 5, 2015 Author Posted June 5, 2015 No, that would have the opposite effect. Mil missiles usually have them for steering.
FlaMtnBkr Posted June 5, 2015 Posted June 5, 2015 Guess I was thinking you would still have the rear fins to keep it flying nose first, and the ones in front would keep it going more straight. But my rockets explode at the end so I'm no expert.
deer Posted June 11, 2015 Author Posted June 11, 2015 (edited) While I was waiting for windless day, the rcandy fuel had already accumulated moisture. Although hesitantly, it managed to clear the pad.Numbers given by calculator yielded stable trajectory, OTOH, I’m not sure if it was not over-stabilized and didn’t weathercock (the low trajectory seemed more like loss of thrust).Unfortunately it seems, that reduction in spin wasn't enough, while judging from on-board sound, it might be that the camera was too slow. With stick it went like rapid whoshwhoshwosh, now it sounds like single whosh during lift-off and then just swhoooo trough the air. Gotta get some better camera in air.https://www.youtube.com/watch?v=M8i6Ly3MoRcI guess next time when I make engines I should seal them shut right after casting. Would hot glue "cap" over nozzle and pass-fire plug suffice or I need to stick some desiccant in spindle too? P.S. Since the Kdenlive doesn't fully support my on-board video here is unedited flight from it:https://www.youtube.com/watch?v=r2B2IJZLnIQ Edited June 11, 2015 by deer
stix Posted June 15, 2015 Posted June 15, 2015 (edited) Looks like you are having fun and I hope you get this sorted. From my experience the overall "shape" of the launched model above looks good. I don't think that the instability and spin of the model is due to the actual shape and drag co-efficient, but rather more likely the construction and attention to detail. What did you make the fins out of? If you view the 1st video in Post #7 @ approx. 0:18, you will see one of the fins (looking straight on). It looks a bit twisted and distorted. If the fins are too thin, not straight and able to flex, you will get unstable flight Don't bother with cardboard or thin plastic. Also the tape around the nose cone could cause an issue too. I don't go as far to use a jig for aligning fins, but I mark out the tube with equidistant parallel lines to help positioning. I use approx. 3mm thick balsa wood, cut with the grain running horizontal to the body - this aids the strength. I also shape the leading edges with glass-paper into a reasonable airfoil shape. I use 5min Epoxy which gives enough time to align them as best by eye. Having said all that, I've never put a camera on one of my rockets to check stability - I've always wanted to. So I'm interested to see you get this working. You'll probably never be able to stop the spin, but a much slower spin and you'll get some good footage. The "chuffing" of the motor indicates it not getting up to pressure quickly enough, likely due to the moist fuel. I immediately seal my motors in a ziplock bag. I put a small bag of calcium chloride crystals wrapped in paper towel and elastic bands. I then suck the air out with a straw in the corner and zip it shut. Then put that in another bag. A good free and open-source model rocket designer program is: http://openrocket.sourceforge.net/The program predicts stability, altitude and allows for "what if" scenarios. It is a fair bit of learning though. Looking forward to you next experiment. Cheers. Edited June 15, 2015 by stix 1
deer Posted June 15, 2015 Author Posted June 15, 2015 Oh wow, thanks for the app. recommendation and tips. Yeah, the fins were cardboard. Next one will be multi use design and thus with more decent components. Since currently I just build them around motor I try to use discarded stuff. Still, my main gripe is that I've failed to open chute every single time. I think I need wider tube for that, because its pretty tight in 20mm. Now I'm just wondering, should I include ejection charge in motor or main body. First would mean fitting motor snugly and securely in main body, so it can generate ejection pressure, second would mean simply making a pass fire channel to ejection charge in body.
stix Posted June 15, 2015 Posted June 15, 2015 The "vehicle" is the first part to get right - worry about the camera later. Do some "dummy" tests first. With a bit of patience, you will get there. Cheers.
ddewees Posted June 17, 2015 Posted June 17, 2015 No, that would have the opposite effect. Mil missiles usually have them for steering. But my rockets explode at the end so I'm no expert. Military Missiles explode too... 1
stix Posted June 18, 2015 Posted June 18, 2015 But my rockets explode at the end so I'm no expert. Yeah, I don't understand the issue either. What's the problem?
deer Posted June 18, 2015 Author Posted June 18, 2015 Well true, but I bet sometimes it might be quite useful to get by without the stick for fireworks too. e.g. takes less space, can reuse single stick for all launches, less rocket weight (safer, more useful payload) etc.Hell, you could even stack a string of multiple rockets on single launch stick:
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