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  1. #1
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    Recycling Container Oddroc

    The project we’ll be describing here is the build of a recycling container odd rock using the same recycling container as the one in SMR’s “Recycling container Jayhawk” thread. The two projects are related, as I will describe below.

    Since this is our first build thread on TRF, I’ll start by telling you a little bit about ourselves and about how we got involved with this project.

    JLRockets is the mother-son team of Judy and Jackson. We are members of both NAR and Tripoli. Our home clubs are WOOSH (NAR #558) and Tripoli of Wisconsin. Judy is Level 3 and Jackson is Jr Level 1. As Chief Public Relations Officer for the team, Judy will be handling all of the posting. Jackson is Chief Engineer on this project.

    This will be our first high power odd rock, and our first high power scratch build. We have 4 certs between us and 3 of those 4 certs were done with, essentially, the exact same rocket kit, sized appropriately for the cert (all from the Wildman line – a junior, a 3 inch and an ultimate dark star). So, although we have proven ourselves capable of building Wildman rockets, it remains to be seen whether such skills transfer over to the world of large scratch builds. This will be the test.

    On his post, SMR described how this odd rock project got started. Fellow WOOSH member Marc S. originally had the idea and found a recycling container supplier with a minimum order of 4. Marc, Bill and Sather (SMR, as you know him) were the first three takers, and then they “persisted until finding enough rocketeers willing to participate in this endeavor.” This is where we came in. When we joined the WOOSH email list last August, one of the first emails we received was the invitation to take up the 4th position. We jumped in. (By "we", I definitely mean Jackson.) Later, we discovered that Sather, Marc and Bill had been waiting for their 4th participant for over 3 years! And they had been sending out the invitation email periodically during those 3 years – in vain, until we newbies came along. In economics, we call this the “winner’s curse” – the tendency for the winning bid on any project to come from the team that is the least knowledgeable about what the project actually entails.

    What this project actually entails is strong scratch building skills. That’s why we’re posting here. We appreciate any and all advice that the TRF community might have. We have already gotten a great deal of very helpful advice from our fellow bottle-teers Sather, Marc and Bill. Keep it coming!

    Here is the bottle in its native environment. Note that the object itself is actually a recycling container impersonating a bottle. So, this project has an extra layer of “oddness”. It is a container made to look like a bottle made to work like a rocket. Call it a super odd rock. Or, just a very odd rock. Or, double insanity.

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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  2. #2
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    Judy,

    It sounds to me like you are confident in your ability to construct Wildman kits.

    I say, go with your strengths, work with wildman to assemble a "stuffer tube" that will be the center body of the oddroc. That way, you don't need to worry about getting a motor mount to fit into the body of the recycling container, just fit one inside a Wildman BT.

    I'm sure that you can get some large sheets of G10 cut to your fin spec from Wildman, or Giant Leap Rocketry offers the same service. Then cut slots in the recycling container and mount the fins to the Wildman BT.

    Plus, by using a Wildman kit, you have a real excuse to buy the biggest motor that will fit into your chosen motor mount...

    Good luck, and I look forward to watching the build!

    G.D.


  3. #3
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    Welcome to TRF, Judy (and Jackson). You are going to love scratch-built rocketry. The biggest differences, in my opinion, between a Wildman / Performance G-10 kit and a scratch-built are time and personal satisfaction. With kits, pretty much everything is there, pre-cut, and ready to assemble. If you don't include epoxy curing time, one could assemble a G-10 kit in an hour. While there is some skill involved in component alignment and epoxy application, the kits are generally well engineered and forgiving. Scratch-built rocketry, on the other hand, is more expensive and time-consuming, and not the least bit forgiving. You can easily spend a week building a single part, change the design spec, and have to throw it away. But, you get a real sense of pride flying something unique, of your own design, personally crafted from raw materials. Add to that the unique difficulties inherent in the recycling bottle. It's PLASTIC. No epoxy on Earth will stick to it, it shatters when cold, and it is not particularly aerodynamic. But, to develop complex problem solving skills, one has to start with a complex problem.

    I love this quote from Teddy Roosevelt... “It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat.”

    We look forward to watching your success.

    cheers, Sather
    Last edited by SMR; 7th January 2011 at 09:37 PM.
    Proud member of WOOSH, FVR, IR, QCRS, (RC)2
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  4. #4
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    Quote Originally Posted by SMR View Post
    Scratch-built rocketry, on the other hand, is more expensive and time-consuming, and not the least bit forgiving.
    Bring it on! We are up to the challenge.

    We have already noticed that scratch building is less forgiving, and that it is fraught with feedback loops. For example, we're planning on using clear fins. Jackson wants the bottle to look like a bottle. The problem with lexan is that it is heavy if you use the strength necessary to survive the rocket landing on it. But using the heavier lexan means more nose weight, which means the rocket will be heavier, which means the lexan must be stronger and therefore heavier. Arghhh!

    We have already tossed dozens of designs into the trash bin (make that the recycling bin). Our first attempt used half inch thick lexan and required 45 pounds of nose weight. It weighed in at over 100 pounds unloaded and could only be launched on high initial thrust M motors or bigger. And that didn’t even include the weight of the steel bands we would need to prevent zippers from the 45 pound nosecone (feedback loop). Into the recycle bin with that idea!

    So, here are some initial thoughts on the design (nothing is final at this point, except for the use of the recycling container)

    • An internal tube running the length of the rocket, as Gdiscenza suggests (Thanks!). Sather (SMR) is also using this design, and we will implement it in much the same way. The bottle will be a “dress” that the inner rocket wears to the launch. Fiberglass would be too expensive, though, so we’re going with a 7.5 inch Loc tube. Unfortunately, the inner rocket design cuts the rocket into two, which means that the motor section is too short to fit the biggest possible motor for the motor mount. Too bad, because the initial designs show that the rocket will be heavy, and need big motors.

    • 98mm motor tube. The current design suggests a need for at least 2000 N of average thrust to make the 5:1 ratio. This means 75mm or 98mm, unless we use v-max propellant. This is not the right type of rocket for v-max.

    • Lexan fins. The rocket will look like a bottle (or a recycling container impersonating a bottle). And, I should note, they will be planar fins. We have given up on grid fins. RockSim does not support them, so we can’t simulate them. It was worth a try, though (or maybe it was grasping at straws).

    • Removable fins. Mainly to handle the feedback loop described above. Removable fins will also handle our transportation and storage problems. The need to get the rocket into the minivan to transport it to launches limits the wingspan to 54 inches, unless fins are removable (although cost will likely keep us within this range anyway). And, there is no fin configuration, other than removable, that would allow us to build the rocket in our workshop (which has a 29 inch wide door) and save us from exile to the very cold garage.

    • Our most likely approach for the removable fins will involve some kind bracket system, probably metal, attached to the inner 7.5 inch tube, and again to the inside of the bottle. The fins will be bolted into the brackets. We would keep the bottom centering ring removable also, and use this access point to bolt in the fins. More thought needs to go into this.

    • A second removable fin design idea is to copy Team Vatsaas’s design: http://www.vatsaas.org/rtv/construct...able/fins.aspx Detailed design can be found here: http://www.vatsaas.org/rtv/arsenal/r...cket-fins.aspx The essential elements of the Vatsaas design are slots in the centering rings into which the fins slide and tabs in the top and bottom centering rings which hold the fins in once a locking device is attached. We’ve never met the Vatsaas brothers, but we greatly appreciate the wealth of information that they provide on their website.

    • Lighter lexan, thanks to the removable fin design. We’re considering quarter inch thick, but may have to go up to 3/8 to discourage fin flutter. With removable fins we only need to make the fins strong enough for ascent, not for a tough landing. We will almost certainly break a fin using quarter inch, but we won’t have to worry about it. The lighter lexan allows us to use less nose weight, which lessons the need for stronger lexan (feedback loop working the other way – yea!)

    • Anti-zipper design TBD. Although our more recent design attempts have allowed us to get nose weight down to about 16 pounds, we’ll still be facing high zipper potential with the nose weight needed for this rocket.

    Initial Rocksim drawings coming in the next post. (I’m still making them look presentable.)
    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  5. #5
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    Okay, my husband just sent me this suggestion for our next project, when we finish this one. He's a great supporter, but this might fall into the category of "enabling".
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  6. #6
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    initial fin designs

    Getting closer on the fin design – still far from final, but closer. We’re trying to get down some basics on shape, placing and overall size. Here are two representative configurations. Neither are at all final. We still have to consider the potential for fin flutter, have to spend some more time weighing the trade-offs from the different fin configurations, and tweak the actual shape once we choose the basic shape.

    Thanks are owed to Marc, who put the bottle itself into a RockSim file so the rest of us can design our rockets.

    We’re aiming for a half caliber on the stability. This is based on advice from SMR, who has far more experience with short, fat rockets than we do (and far more experience with any type of rocket than we do). Note that RockSim is calculating the margin based on the diameter of the first body tube, which is 9 inches. The rocket is 20 inches in diameter, so RockSim’s margin calculation is incorrect in these diagrams. We’ve got about 13 inches of difference between Cg and Cp on both these designs. (a bit more than the half caliber guideline, but we haven’t yet added in the weight of fin attachment, etc). We’re using an M3700 in the sims, just to get a feel for loaded weight. This is a 6 grain 75mm White Thunder motor that I currently own, so it might just get used in reality.

    Design 1 uses black brandt style fins, swept back to move the Cp back. With 16 pounds of nose weight, we have just over a half caliber stability margin. Total weight so far is about 70 pounds. These fins will be acutely prone to breakage. Removable fins is a must with this design, especially with thin lexan. But, as SMR pointed out, this fin design can be prone to flutter, especially when removable, because its wider at the mid chord than at the root chord. We’ve played with adding an extra fin tab forward of the start of the fin. It adds weight, but may increase attachment strength.

    Design 2 uses what we’ll call regular fins. This design was done before we came upon the idea of removable fins, and offers a little more protection to the fins during landing. Still, with quarter inch thick lexan, these fins will break on landing, and so will have to be removable. Same approximate stability margin as design 1, but with 25 pounds of nose weight. Total weight is about 80 pounds.

    Our Chief Engineer is spending the day testing the “shatterproof” claims on the lexan. So far, the lexan is surviving, but the workshop itself has taken heavy damage.
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  7. #7
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    Quote Originally Posted by JLRockets View Post
    Okay, my husband just sent me this suggestion for our next project.
    oooooh, I like that one. Is there a minimum order?
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    Proud member of WOOSH, FVR, IR, QCRS, (RC)2
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    Highest altitude achieved: 21,981' AGL

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  8. #8
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    Judy, First off WELCOME to the club...

    Great project and I am really looking forward to seeing what you do with this.

    In your two rocsim pics I would be more for going with the one on the right. the fins seem to be a little stronger. that would be the biggest worry I have about the design. FIN FLAP.

    Clear fins are cool BUT think about the strength of the clear plastic. it is your weakpoint. Lexan tends to crack if it is "flapped" really hard.

    Thats just something to think about

    I have a BIG R2D2 cooler I am going to fly sometime.. My thoughts on clear fins.. maybe Clear fins AND aluminum Channels along at least one or two edges for strength... I have not tried it but I am thinking it is going to work really well and they are something that you could remove if you wanted to

    Good luck
    Stickershock23.com

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  9. #9
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    Fin flap and lexan strength

    Quote Originally Posted by stickershock23 View Post
    that would be the biggest worry I have about the design. FIN FLAP.

    Clear fins are cool BUT think about the strength of the clear plastic. it is your weakpoint. Lexan tends to crack if it is "flapped" really hard.
    Thank you, Mark, for bringing up fin flap. We haven’t used lexan, so we are not personally familiar with fin flap, but we have heard about it. With what thickness of lexan have you experienced fin flap? It does not look like the half-inch would flap anywhere, but its very heavy. We thought about the aluminum channel concept as we browsed the hardware store (the lexan was shelved near the shower door department). We’ll look into that some more.

    After further investigation, we are increasingly moving towards a design that will have a removable bottom center ring combined with a system in which we bolt the through-the-wall-fins to double brackets both on the inner tube (at the root end of the fin) and inside the bottle just before the fin leaves “the wall”. Thoughts on how this will stand up to fin flap?

    Meanwhile, we have made some headway into testing the strengths of the various thicknesses of lexan. With the help of two 14 year-olds and a sledgehammer, we have proven, beyond a shadow of a doubt, that lexan is, as advertised, shatterproof. And we have given ourselves confidence that it can handle all manner of impact. After being hit with 50 pound rocks and the aforementioned sledgehammer, the quarter-inch lexan was occasionally marred, but never broken or shattered. The thicker pieces looked untouched. Q.E.D.

    Speaking of sledgehammer-wielding 14 years olds, Team JLRockets is pleased to welcome some new members to this project! Jackson’s friend Augie is currently L0, but his Jr L1 rocket is built and ready to cert at the next semi-warm day. And, he knows how to work all of the machinery at his father, Frank’s, carpentry shop. Jackson and Augie will be in charge of cutting centering rings and fins (with adult supervision, of course).

    The boys are thinking up a mechanism to test the fin flap scenario. We’ll let you know if they come up with good data.
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  10. #10
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    Judy,

    I'm not a materials engineer, but as I understand it, fin flap increases in proportion to the projection of the fin into the airstream. Since CP is dependent on total fin surface area (not completely, but for the purposes of this argument it works) couldn't you use more, smaller fins?

    You have already decided on a removable aft ring of material, so the additional slots in the "shroud" should have minimal impact on the structural integrity. And using aluminum angle brackets at both point of attachment, and internal point of penetration, you should end up with *more* structural integrity, given the additional reinforcement from the shroud to the internal airframe tube.

    One test your jr. engineers might want to try is to take a nasty looking section of the Lexan, and using some all-thread, plywood and 2 cinder blocks, make a sandwich of sorts. ( Ply -- C.B. -- Lexan -- C.B. -- ply (with about 10 to 15 inches of protruding Lexan to simulate a fin)) Bolt it all together, and drop the whole shebang from the top of a ladder onto the edge of the Lexan to see what kind of edgewise impact the fin material can withstand. Once the edge strength properties are more well understood, you can design your fins with much more confidence as to what will happen when the rocket comes down at 15-20 fps, fins first.

    Watching with anticipation!

    G.D.

  11. #11
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    Quote Originally Posted by gdiscenza View Post
    One test your jr. engineers might want to try is to take a nasty looking section of the Lexan, and using some all-thread, plywood and 2 cinder blocks, make a sandwich of sorts. ( Ply -- C.B. -- Lexan -- C.B. -- ply (with about 10 to 15 inches of protruding Lexan to simulate a fin)) Bolt it all together, and drop the whole shebang from the top of a ladder onto the edge of the Lexan to see what kind of edgewise impact the fin material can withstand. Once the edge strength properties are more well understood, you can design your fins with much more confidence as to what will happen when the rocket comes down at 15-20 fps, fins first.
    ooooh! They would love this! Too bad they have school tomorrow. They'll work on it on the weekend.

    The sims in the prior post use a 4 fin design (sorry, I didn't make that clear). We simmed 6 fins, and even 8. It seemed a bit much, but that was before the removable fin idea. We'll look at those designs again. Thanks!
    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  12. #12
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    Great info there...

    I have not done HUGE lexan fins but I did break a set on a 5" crayon during flight. that is the only reason I mentioned it.

    the reason mine broke (i believe) I gave it a place for the crack to start.
    the fin had an Inside corner where the fin hit the body tube thats where it broke

    So my suggestion NO inside corners only outside corners. (does that make sense?) for the same reason airliners have ROUNDED windows. square corners are a place for a crack to start.

    I like the suggestion more small fins.. seems to be a win situation.
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  13. #13
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    make the inside corner have a small radius, also make sure not to leave a kerf cut in the corner ...that will increase the strength of the corner dramatically
    Andy

  14. #14
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    You should find a way to leave the hole in the side and stand it next to your setup on the flight line. That way you can collect bottles and cans so that when you have enough deposit refunds the flight is free.
    Al Gloer - TRA/NAR L3 (President Emeritus - CATO)
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  15. #15
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    Quote Originally Posted by H_Rocket View Post
    You should find a way to leave the hole in the side and stand it next to your setup on the flight line. That way you can collect bottles and cans so that when you have enough deposit refunds the flight is free.
    A great idea - in theory. Though I can't vouch for Illinois, there is no deposit/refund for bottles and cans in Wisconsin. 35 cents/lb. for aluminium, that's about it. Gonna take a while to get loads at that rate!
    Unstable by design
    www.wooshrocketry.org NAR Sec. 558
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  16. #16
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    Talking

    Quote Originally Posted by chadrog View Post
    A great idea - in theory. Though I can't vouch for Illinois, there is no deposit/refund for bottles and cans in Wisconsin. 35 cents/lb. for aluminium, that's about it. Gonna take a while to get loads at that rate!
    Well, that gets us right back to Sather’s point that scratch building is more expensive. Darn, I had really counted on those bottle return funds. And, our other funding idea is falling short, too. Sather reported on his post that Pepsi turned him down for a sponsorship opportunity. This puts the kibosh on the idea that we had been kicking around since Sather initially slated his bottle to be Coke. We were going to contact Pepsi and see how much they would pay for film footage of a Pepsi bottle soaring into the skies, while the Coke dud sat on the pad after we switched out Sather’s igniter. Of course, the real flaw in that plan was that there would be no way to switch out Sather’s igniter – he’s not one to let you pull the wool over his eyes.
    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  17. #17
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    Quote Originally Posted by stickershock23 View Post
    the reason mine broke (i believe) I gave it a place for the crack to start. the fin had an Inside corner where the fin hit the body tube thats where it broke

    So my suggestion NO inside corners only outside corners. (does that make sense?) for the same reason airliners have ROUNDED windows. square corners are a place for a crack to start.

    I like the suggestion more small fins.. seems to be a win situation.
    That makes a lot of sense. And so do the other suggestions about rounded corners and more, but smaller, fins. We had sort of noticed the rounded corner effect. As part of the shatterproof testing, Jackson threw the sample squares at the ground. When he hit the corner, the corner dented. It didn’t shatter, just dented. But hitting either the side or a rounded corner did not produce dents. So, we were thinking about rounding all corners on the fins, but we hadn’t yet gotten to the understanding that corners or attachments can be places where a crack starts.

    There’s a post in the scratch build section that discusses lexan versus acrylic in models.
    http://www.rocketryforum.com/showthread.php?t=17858
    Someone there mentions that if you score the lexan, it will break along the score, but otherwise, it will bend. We noticed that it will scratch, which could act like a score. We also noticed that big sizes of thinner lexan are actually quite warbly and will clearly flutter at a certain size. We’ll have to take care about how we attach the fins, to ensure that there is not unintended scratching and scoring action going on. The lexan post also mentions that it will crack and shatter at below freezing temperatures. Good thing we’re unveiling this at WOOSH’s Eat Cheese or Fly launch in August.

    We’re going to do some more lexan throwing over the weekend. Should be fun. And, we’ll work on refinements in fin design.
    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  18. #18
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    Quote Originally Posted by JLRockets View Post
    We were going to contact Pepsi and see how much they would pay for film footage of a Pepsi bottle soaring into the skies, while the Coke dud sat on the pad after we switched out Sather’s igniter.
    "Et tu, Judy?"
    Proud member of WOOSH, FVR, IR, QCRS, (RC)2
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  19. #19
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    Quote Originally Posted by SMR View Post
    "Et tu, Judy?"
    Ooooooooooh. Seven months to go, and it's on!
    Unstable by design
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  20. #20
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    fin redesign for flap control

    Quote Originally Posted by chadrog View Post
    Ooooooooooh. Seven months to go, and it's on!
    We only have 7 more months?! Enough clowning around. Time to get back to work!

    Back to the question of fin size and fin flap. It turns out that, with this design, the suggestion to use more, but smaller, fins is not as functional as we would have liked. From a stability perspective, the optimal fin number is 4 (well, 5 actually, but 5 fins is weird). Going from 4 fins to 6 fins, ceteris paribus, actually decreases the margin of stability because the weight of the additional fins moves the Cg back more than the additional fins moves the Cp back. This means that a 6 fin configuration would need bigger, not smaller, fins to keep the same margin of stability. This is the case only because the fins are so big in the first place. For very small fins, 6 fins is more stable than 4 of the same size fins, but this holds only when the fins are far too small for this rocket. This is with quarter-inch thick lexan. The effect is even more pronounced with heavier lexan.

    So, we’re staying with 4 fins. But, we can still tweak shape to minimize fin flap. We have recognized that the relationship between fin size and stability is rather flat. We can increase or decrease the fins quite a bit and only change the stability margin a small amount because any change in fin size moves the Cg almost as much in one direction as it moves the Cp in the other.

    The fins in this new design attempt actually have less surface area than the fins in the original design, but almost the same margin of stability. Nose weight was held constant at 25 pounds. I’ve shown it without engines because the engine hides the Cp. This design looks like it would give better flap protection. The fins project less into the airspace (total span diameter is almost 6 inches less) and have a better ratio of root chord to mid-chord length. Once we see the sheets of lexan and judge the propensity for movement on bigger pieces, we can even cut 2 more inches off the span of each fin, with a cost of 3 extra pounds of nose weight to return to us to the same stability margin.

    Of course, the fins may still flap. Because the fins will be removable, we aren’t concerned about the potential for fin flap to cause a weakness in the fin, which could lead to breakage on the impact of landing. We would only be concerned if the flap created a shred condition. It seems to me that if we were careful to round corners and careful about the way that the fin interacts with the body tube, we’d be okay.
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  21. #21
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    Centering rings

    After a long absence, JLRockets is pleased to introduce . . . . the centering rings! We have three freshly cut rings made of ½ inch Baltic birch. The top two are 18 inches in diameter, and the bottom started as 18 inches in diameter before the sides were cut to accommodate indents in the bottom of the bottle. All three have a 7.5 inch hole for the stuffer tube.

    Jackson and Augie cut the rings with the help of Augie’s father Frank. I forgot to bring the camera to Frank’s shop, so you’ll have to make do with photos of the finished product. After a great deal of measuring and re-measuring, the boys rough cut the rings with a jig saw. Then they trimmed to size with a shaper. When we liked the fit, they used the jig saw to rough cut the 7.5 inch center hole in one of the rings. Then, they got the inner hole perfect using an oscillating spindle sander. That became the template for the other two. They used the template and a router to cut the center holes on the remaining two rings. They also cut the hole in the bottom of the bottle with a router. The best part is, they both still have all 10 fingers! Job well done boys.

    Although we have the basics for the core, we can’t start the attachment until we finalize the fin design. More on that later.

    Photos are: the core of the rocket beside the bottle; the finished centering rings; the motor hole in the bottom of the bottle; the bottom centring ring fitted into the indents; dry fit view from the top.
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  22. #22
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    Quote Originally Posted by JLRockets View Post
    The best part is, they both still have all 10 fingers! Job well done boys.
    Nice looking rings. And, yes, keeping all your fingers IS the most important part. Great job!
    Proud member of WOOSH, FVR, IR, QCRS, (RC)2
    2013 APCP: 48,397.3 Ns (18% P)
    Highest altitude achieved: 21,981' AGL

    "Gravity is a cruel and unpredictable mistress"

  23. #23
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    Fin design

    After much deliberation, hemming and hawing and back and forth, JLRockets has – finally - chosen a fin design.

    The credit for this design must be given, in large measure, to the newest member of our project advisory team, Judy’s father Florian, a retired aeronautical engineer who has designed parts of the F-series fighter jets. After reviewing the challenging aerodynamic situation, Florian solved the problem with a single utterance: “winglets” (like most engineers, he’s a man of few words).

    Some of you may have noticed that the winglet idea is already in use by fellow bottleteer Sather (SMR). Great minds think alike. The winglet solution clearly goes back to their shared history – Florian has designed fighter jets, and Sather has flown fighter jets. So, the winglet connection is not just a random coincidence. Apparently, winglets were the tool of choice to fix post-production aerodynamic problems in the days before simulated flying. (Okay, we’re just copying Sather. After all, this whole bottle thing was all his idea.)

    Lexan is too heavy and has too many flap issues (even with the winglets), so we have officially abandoned lexan. No more pretending that our oversized rocket is not really a rocket, just a flying soda bottle.

    After considering every single conceivable fin material, we have settled on making our own fin material by laminating thin pieces of G10 onto a piece of foam. These fins will be much lighter than lexan fins, and even lighter than plywood fins. With the lighter fins, we may even be able to keep it light enough for a skid.

    Because of the winglets, the span on the main fins will be small enough to get the rocket into the minivan. We figure if we make the winglets removable (they will bolt onto the main fins), we can get away with permanently attaching the main fins. This clears up a number of engineering challenges.

    So, the final fin design is: permanently attached winglet-ed fins made of foam core G10 laminate.

    Up next: results of the scale model test
    Attached Files Attached Files
    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  24. #24
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    Fin design attachements

    My apologies. I attached the RockSim drawings as pdf's, not jpg's, so you have to click on them to see them. Here are the jpgs, which will display even if you don't click.

    Final fin design: permanently attached, winglet-ed fins made of foam core G10 laminate.
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  25. #25
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    Successful scale model launch

    To test our fin concept, Jackson built a one-eighth scale model. We chose one-eighth because we had 2.6 inch tube sitting around, and that scales to one-eighth of the recycling bottle’s diameter. We built the model so that the stability parameters would exactly match those in the current design. That meant a margin of .6 loaded. The CG was 46% from the top of the rocket in both the scale model, and the predicted real thing.

    We had to add about 10 ounces of nose weight to get the CG in a comparable spot on the model. Check out the fancy nose weight technique – Jackson put large washers into the soda top and held them in with clay. An interesting look with a clear nose cone.

    Our first test did not go well. We had major skywriting, and off angle flying soon after leaving the rod. I was convinced that it was a stability issue, but Jackson held to his conviction that we didn’t use a powerful enough motor. Combining what we had in our motor stock with the available field size that we could assure was empty on the first nice day after a run of bad weather, we went with an Aerotech E20 White Lightening. The motor package said max weight was 16 ounces – the exact weight of our test rocket. But Jackson’s later calculations showed that to be only a 4.45 thrust to weight ratio. That’s why we pay him the big bucks to be the Chief Engineer (okay, we don’t pay him anything, but the job has some good perks.)

    So, we put it up on an Aerotech F32 Blue Thunder. Straight as an arrow! A perfect flight – except that the rocket is not retrievable. We can see it, so its technically not lost, but we can’t get at it. No need, though. Its served its purpose. Our design is stable. QED

    Just the same, we may extend the fins out a bit, just to give a better margin of error in case we’re too heavy in the build.

    Winglets rule!
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  26. #26
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    Quote Originally Posted by JLRockets View Post
    (Okay, we’re just copying Sather. After all, this whole bottle thing was all his idea.)
    Well, let's give credit where credit is due... Marc was the original bottle advocate. Technically, we're all just copying him. And to add the proper perspective, he's using it for his L3 cert, whereas the rest of us are just having fun!

    Sather
    Last edited by SMR; 29th April 2011 at 12:46 AM.
    Proud member of WOOSH, FVR, IR, QCRS, (RC)2
    2013 APCP: 48,397.3 Ns (18% P)
    Highest altitude achieved: 21,981' AGL

    "Gravity is a cruel and unpredictable mistress"

  27. #27
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    Quote Originally Posted by SMR View Post
    Well, let's give credit where credit is due... Marc was the original bottle advocate. Technically, we're all just copying him. And to add the proper perspective, he's using it for his L3 cert, whereas the rest of us are just having fun!
    Yes, its true. This was all Marc’s idea. But since our entry into the fray came as a response to an email from you, Sather, its your name that gets mumbled under our breath when the going gets tough.

    But your right, we are having fun. And Marc’s design is an engineering marvel. Marc, if you’re out there, do us the favor of posting your design!
    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  28. #28
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    Quote Originally Posted by JLRockets View Post
    To test our fin concept, Jackson built a one-eighth scale model.
    I like your idea (and implementation) of testing a scale model. And keeping the Cp and Cg in corresponding locations. Very well done.

    Quote Originally Posted by JLRockets View Post
    Apparently, winglets were the tool of choice to fix post-production aerodynamic problems in the days before simulated flying.
    As clearly demonstrated by the Beech 1900. How many aerodynamic band-aid add-ons can we count on this airplane?
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    Last edited by SMR; 10th May 2011 at 07:51 PM.
    Proud member of WOOSH, FVR, IR, QCRS, (RC)2
    2013 APCP: 48,397.3 Ns (18% P)
    Highest altitude achieved: 21,981' AGL

    "Gravity is a cruel and unpredictable mistress"

  29. #29
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    Fin material

    Quote Originally Posted by SMR View Post
    How many aerodynamic band-aid add-ons can we count on this airplane?
    The scary thing is that our winglets are not aerodynamic band-aids, but rather are the center piece of our design. It may time be to get a second opinion! Although its actually too late for that. We have moved irrecoverably forward with the winglet design.

    Although we’re behind in the postings, we have made some headway on the fins. Back in May (sorry, the posting has been really slow), we headed out to the Wildman’s workshop to get a tutorial on how to use our new vacuum bagging supplies. The customer service that Tim Lehr of Wildman Rocketry provides is unsurpassed. Thank you, Tim. Its people like you who make rocketry fun. We left Tim’s house with our first block of fin material. Two sheets of thin G10 (1/16) with about ½ inch foam sandwiched in between. The fins are strong and, in contrast to the lexan, do not wobble at all, despite being 15 inches by 21 inches before shaping. At 3 pounds for the uncut fin material, we should come in about where we had hoped with respect to weight.

    We made the other three fin blocks at home. Clearly, we had a bit of a learning curve. The next two fins looked pathetic compared to the one we made at Wildman’s. Turns out, we had melted the foam with the heat lamp that we used to speed up the cure. After the first bad fin, the cause of the odd indentions in the foam was still a mystery. But, there’s nothing like doing something wrong twice to help you figure out what you did wrong. We nixed the heat lamp for fin block number 4, and that did the trick. We re-made the two bad ones. Now, we have four usable pieces of fin material, each 15 inches by 21 inches.

    Next up, cutting the fin material into actual fins.
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    Judy, L3
    Flying solo now that Jackson has his own L3
    WOOSH (NAR #558), Tripoli Wisconsin
    Indiana Rocketry, QCRS

  30. #30
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    Quote Originally Posted by JLRockets View Post
    The scary thing is that our winglets are not aerodynamic band-aids, but rather are the center piece of our design. It may time be to get a second opinion!
    I hope you don't think I was being critical of your design... I think it looks and will fly great. I am actually fond of winglets, since Jayhawks, Fireball XL-5, and TIE fighters all use them. Your fins are looking good.

    (I do agree with your father, and I WAS making fun of the Beech 1900, though.)

    Proud member of WOOSH, FVR, IR, QCRS, (RC)2
    2013 APCP: 48,397.3 Ns (18% P)
    Highest altitude achieved: 21,981' AGL

    "Gravity is a cruel and unpredictable mistress"

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