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Model Forum / General / Rockets / August 2007(RMR) High thrust construction technique
I've started purchasing parts for one of next year's projects, a 7.5" x 13' rocket boosting on 75mm hybrids. The motor I intend to use is an L2525, and since I've never boosted anything quite that beefy I have a question or two. The peak thrust of the motor is well over 1000lbs Generally a very standard 3FNC, dual deploy rocket. The only real oddity is the 3" x 60" MMT. I've seen a lot of folks use all-thread in the motor mount assembly, and it seems an excellent idea I intend to copy. However, it seems to me that while all-thread adds significantly to the tensile strength of the MMT (great for recovery) it does little to add to the compressive strength. Since the whole point is to transfer the forces from the motor to the airframe it seems there would be significant compression on the MMT assembly as it drags the rest of the airframe up to speed. My thought is to use small diameter phenolic tubes (say 1/2"-5/8" diameter) to fit over the 1/4-20 all-thread, gluing them directly to each bulkhead / centering ring in turn. I've mocked this up without epoxy, and the assembly is able to take over 200# sustained load, directly applied (I stood on it, bounced up and down a few times). Is this overkill? Also, I'm seriously interested in some sort of composite fins for this rocket. I've seen articles on foam cored / fiberglassed fins. I've heard that some folks use a 'built-up' technique, involving rib/spar structures along with foam & fiberglass. Can anyone point me at further information? I didn't have much luck with Google. Thanks! Kevin OClassen > I've seen a lot of folks use all-thread in the motor mount assembly, and it > seems an excellent idea I intend to copy. However, it seems to me that while [quoted text clipped - 3 lines] > there would be significant compression on the MMT assembly as it drags the > rest of the airframe up to speed. You are correct that the all-thread would only help on recover (and my opinion is that I would rather the airframe fail gracefully than stay intact on impact). The thrust will be distributed where the fins attach to the MMT (for TTW mounting) or at the centering rings otherwise. If you are really worried about it use a rear ring that is installed so it over the end of the body tube (not internally mounted) and flush with the MMT so the motor actually presses against the ring (essentially a thrust plate). FWIW my L3 used TTW construction with internally mounted rings. I have no concerns about it taking the thrust. As far as fins go, look at the infocentral article on fin flutter and/or do a google search for fin flutter. > I've started purchasing parts for one of next year's projects, a 7.5" x 13' > rocket boosting on 75mm hybrids. The motor I intend to use is an L2525, and [quoted text clipped - 26 lines] > > Kevin OClassen All thread is over used. The strength of a 3" diameter phenolic motor mount tube is much greater than the forces you will apply. See the axial tube tests at: http://www.rocketmaterials.org/ Which show failure at about 3,000 pounds. So the motor mount tube all by itself is strong enough with plenty of margin. Add in the remaining structure and you have even more margin. I made built up fins for my L3 project. Details at: http://home.earthlink.net/~david.schultz/level3/t22/construction.html There is no all thread in this rocket.  SignatureDavid W. Schultz http://home.earthlink.net/~david.schultz ------ "What you don't know should be a goad to make you try and find out, not an excuse to bliss out and sit drooling into your filet mignon." -- PZ Myers > All thread is over used. > > The strength of a 3" diameter phenolic motor mount tube is much greater > than the forces you will apply. See the axial tube tests at: > > http://www.rocketmaterials.org/ <snip> Dave, My concern isn't the thrust loading; it's the shock loading at deployment. My fear is that a hard (late) deployment might pull the forward ring - where the shock cord attaches - out of the fin can. I suppose a bit of coupler just forward of the ring ought to take care of that, but my take was that the all-thread was mainly to spread deployment loads among the multiple rings in the fin can. In that vein, having seen how brittle phenolic can be - eg, Tim Sapps rocket wrecking at McGregor a couple years ago when the fancy, aftermarket MMA broke resulting in vectored thrust - trusting the MMT to transfer alone the deployment loads to the other rings seems inadequate. Of course that igores the airframe doing the same thing which may make it a moot point. Anyway, my take was that the all-thread was about increasing tensile strength and distributing the jerk load. But I suspect it's overkill for that, too. Doug >> All thread is over used. >> [quoted text clipped - 26 lines] > > Doug If you look at the construction of my L3 rocket you will see how I handled this. Speaking of which, the Rocksim file for that rocket has been amazingly popular for the last month with over 400 hits. No other page has been hit anywhere near as many times. I have no clue why it is suddenly so popular. In any case, Kevin's concern was about compression loading. SignatureDavid W. Schultz http://home.earthlink.net/~david.schultz ------ "What you don't know should be a goad to make you try and find out, not an excuse to bliss out and sit drooling into your filet mignon." -- PZ Myers > >> All thread is over used. > >> [quoted text clipped - 29 lines] > If you look at the construction of my L3 rocket you will see how I > handled this. (The following questions are to satisfy my curiousity, nothing more. Don't take them as an "attack") How do you inspect the knot before flight? Can you see it? How hard is it to replace that strap? Are there multiple knots (one behind each centering ring)? A piece of allthread attached to the recovery harness attachment point and to the other centering rings does serve to distribute recovery stress well. SignatureTweak > In article <13cpk4jaei92...@corp.supernews.com>, ab...@127.0.0.1 says... > [quoted text clipped - 47 lines] > > - Show quoted text - Same way you insect the condoms on your chute packer buddies TWEAKie..........get a clue you moron. I swear, I wonder sometimes how some of you even manage to dress yourselves without your mommas help.........I bet Freddie boi even has his momma help him put his "raincoat" on before he goes out for the night..........I hear tell Freddie is pretty well a hopeless idiot in love with his "self" > help.........I bet Freddie boi even has his momma help him put his > "raincoat" on before he goes out for the night..........I hear tell > Freddie is pretty well a hopeless idiot in love with his "self" My mom passed away a year ago next week. You really are a moron Roid. Take this to the bank; " if I ever find out who you are for sure, you're making daisy fertilizer,". Nothing personal, just red neck justice.. > tynksvag...@yahoo.com wrote: > [quoted text clipped - 5 lines] > Take this to the bank; " if I ever find out who you are for sure, you're > making daisy fertilizer,". Nothing personal, just red neck justice.. Ahhhhhhhhhhh, so since mom is history, I guess your love life went to hell then huh? So who is helping you put your raincoat on now Freddie Boi or are you ridinig bareback ...........oh wait, w.nking your own dick does not require a raincoat does it Freddie Boi. that is since mommy is not there to holler at you for getting it all over her sheets! > > tynksvag...@yahoo.com wrote: > > [quoted text clipped - 11 lines] > dick does not require a raincoat does it Freddie Boi. that is since > mommy is not there to holler at you for getting it all over her sheets! Any time you want to come out of hiding roid. Do you feel lucky; well do you punk?? Hey Freddie Boi, take Tweaks dick out of your mouth, your hard to understand mumbling like that! Or is it that Bobs chute packing dick is tickling your tonsils and its making you mumble and gag Now go bury that stinkin mess you call your momma! SignatureVia: http://www.jumlers.com/rec.models.rockets/ > > In article <13cpk4jaei92...@corp.supernews.com>, ab...@127.0.0.1 says... > > [quoted text clipped - 54 lines] > "raincoat" on before he goes out for the night..........I hear tell > Freddie is pretty well a hopeless idiot in love with his "self" Get a clue, none of my buddies keep their condoms in foam packed cardboard tubes. How about that beer? Talledega gran prix raceway in Anniston, Saturday night. Me and a whole bunch of my chute (well, motorcycle exhaust, I guess) packing buddies will be there. We can even meet you somewhere, and I'll buy. SignatureTweak >> If you look at the construction of my L3 rocket you will see how I >> handled this. [quoted text clipped - 5 lines] > it to replace that strap? Are there multiple knots (one behind each > centering ring)? You can't inspect it, you can't see it, (except for the visible portions of course) and it is impossible to replace. Which is of course identical to all thread. But it is in a nice cozy location protected from the hazards of life. Not to mention being embedded in urethane foam. There are only the two knots. One in each end. I know that the Kevlar(TM) has a finite life. But I also know that I will not fly this rocket very much (75mm motors are not cheap, but I am :-) and I expect that it will be retired long before the Kevlar(TM) shows any wear. I have used the same technique on other rockets that have more flights and none of them show any signs of wear. > A piece of allthread attached to the recovery harness attachment point > and to the other centering rings does serve to distribute recovery > stress well. SignatureDavid W. Schultz http://home.earthlink.net/~david.schultz ------ "What you don't know should be a goad to make you try and find out, not an excuse to bliss out and sit drooling into your filet mignon." -- PZ Myers > >> If you look at the construction of my L3 rocket you will see how I > >> handled this. [quoted text clipped - 9 lines] > of course) and it is impossible to replace. Which is of course identical > to all thread. Aside from allthread not being a wear item, it won't come loose, and it distributes recovery shock throughout the assembly better, yes, it's pretty much identical. ;-) There are drawbacks to allthread (weight, complexity in construction, weight, and, uhhh, weight), but there are benefits as well. At what size rocket do the benefits out"weigh" the drawbacks? That's the question. My answer is "about that big". > But it is in a nice cozy location protected from the > hazards of life. Not to mention being embedded in urethane foam. [quoted text clipped - 6 lines] > shows any wear. I have used the same technique on other rockets that > have more flights and none of them show any signs of wear. SignatureTweak |
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