What did you do rocket wise today?

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neil_w

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That's quite interesting. And you have to love *anything* called "smerd".

I do wonder, though, if fin flutter isn't pretty much a solved problem in the vast majority of cases, with existing composite techniques. The sort of sandwich you're describing would certainly be as much work as doing the composites.
 

jqavins

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What I imagine (from a standpoint of ignorance regarding composites) is that such a solution might allow for thinner and lighter fins. If I understand the matter correctly, the existing technique basically amounts to making the fin thicker so it's stiffer and stronger. That means reliance on preventing oscillation or surviving small oscillation. If a fin could become better at dissipating flutter energy and thus keeping oscillations very small, that might solve the problem with less bulk and weight.

Neil, I'm pretty sure the rules of geek fun allow solutions in search of a problem.
Yeah, that too. :) Hey, the LASER started life as a solution in search of a problem, and look where that got us.
 

boatgeek

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What I imagine (from a standpoint of ignorance regarding composites) is that such a solution might allow for thinner and lighter fins. If I understand the matter correctly, the existing technique basically amounts to making the fin thicker so it's stiffer and stronger. That means reliance on preventing oscillation or surviving small oscillation. If a fin could become better at dissipating flutter energy and thus keeping oscillations very small, that might solve the problem with less bulk and weight.

Yeah, that too. :) Hey, the LASER started life as a solution in search of a problem, and look where that got us.
Disclaimer: I am not a vibrations person, so take this with a block of salt...

I see two potential problems with this:
1. If the elastomer core is significantly "stretchier" than the skins, then the skins will be bearing most of the load. They'll have to be strong enough on their own to support flight loads. You'd probably end up with a very thin core and thicker skins, which may not reduce weight and definitely wouldn't reduce total thickness.

2. A damping layer would definitely take out resonant behavior, so when you tap the fins, they wouldn't ring like a bell. However, most of our common fin materials are pretty good at that already. I'm willing to listen to anyone's fin-ringing concert, though. 😀 The elastomer core might change the resonant frequency, but I'm not sure by how much in the grander scheme of things and it might change in unpredictable ways. The fins are still going to be excited into vibration by the flight forces (mumble mumble von Karmann vortex street). The question to me is whether fin flutter failures are a resonance problem or a simple structural problem from exciting forces that are higher than the materials can withstand.

As always, I'll sit behind a folding table with a "Prove me wrong" banner. :D
 

kuririn

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Mostly finished the Zeta.
Just needs fillets. Tail ring is dry fitted on, will be permanently glued once filler and primer is on the fins.
And some putty to blend in the transition/tail cone lip to the payload section.
0126210543[1].jpg0126210547[1].jpg0126210553_Burst01[1].jpg

Too wet and windy to fly or do yardwork so might as well build, build, build!
Laters.
 

Adam3836

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Did some more work on the MC PAC3. 4inch
Motor mount fully epoxied in and all the fins epoxied and fin fillets done. Aft end motor ring has a nice fillet as well
Shock cord attached
Next up.... coupler attachment and upper BT/nose cone.. chute... and aero pack retainer
She’s a beast lol
 

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jqavins

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Disclaimer: I am not a vibrations person, so take this with a block of salt...
I double dog disclaim you. I'm also not a vibration expert. Also, as they say, a little knowledge is a dangerous thing, and a little knowledge is exactly what I have. So all this is what I'm presenting for consideration, not anything I'm planning or suggesting to be done.
1. If the elastomer core is significantly "stretchier" than the skins, then the skins will be bearing most of the load...
There are other practices with, if not stretchy then weak cores, e.g. FG over balsa. The balsa (or other light but weak material) provides a form to lay the FG over. Also, some thickness is necessary for stiffness (as of course you know) and the core provides that.

Hard rubber, in my purely qualitative experience, is not nearly as stretchy as it is bendy, and I know it's very lossy when bent. (Bend it back and forth a few times and it gets hot.) SMRD is, at least as far as I can test by hand, not stretchy at all and barely bendy, and maybe other materials like that are available. Now, either of these is a lot denser than balsa or birch. The only way this saves weight or bulk is if the vibration damping allows for thinner, i.e. less strong and less stiff skin layers to be acceptable. Slightly thinner skin layers would save bulk and quite possibly cost weight if the core dimension remains the same. If the damping allows for a thinner core by allowing less stiffness (because vibration is damped through this other method) then it might be possible, maybe, to save weight. I think.

2. A damping layer would definitely take out resonant behavior, so when you tap the fins, they wouldn't ring like a bell. However, most of our common fin materials are pretty good at that already. I'm willing to listen to anyone's fin-ringing concert, though. 😀 The elastomer core might change the resonant frequency, but I'm not sure by how much in the grander scheme of things and it might change in unpredictable ways. The fins are still going to be excited into vibration by the flight forces (mumble mumble von Karmann vortex street). The question to me is whether fin flutter failures are a resonance problem or a simple structural problem from exciting forces that are higher than the materials can withstand.
Fin shredding is surely the latter, but most oscillations of this sort are completely resonance driven. Think of the stop signs you see waggling behind the poor TV reporters who get sent outside storms just to say "It's really windy out here, Steve!" A flat plate wants to turn broadside to the wind. It only oscillates because the post has torsional springiness and a resonance is struck between that and the inertia of the sign and post system. It has long been my assumption that fin flutter is essentially the same. I could be wrong in this, but I doubt it.

When a lossy element (a.k.a an energy dissipater) is added to such a system, it not only alters the resonant frequency but also reduces the Q of the system. In other words it (pardon me) flattens the curve, so the amplitude of the oscillation at resonance is reduced. Circling back, when the amplitude is reduced it may be possible to get by with a less stiff and less strong fin.
As always, I'll sit behind a folding table with a "Prove me wrong" banner. :D
I will leave proving you or me (or even both of us) wrong to greater minds than mine.
 

Old School Doug

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I've had a bug about having one model dedicated to utilizing the Apogee long burn E6 motor. Three attempts resulted in two seriously unstable rockets that found the ground long before the motor burned out and one that did a tail wagging corkscrew after about 500' and did achieve apogee and deploy only to be lost to the fields of SE Wisconsin. Sims on all three looked good in OR but actual results were less than nominal. So I've decided on a different approach. I'm taking the Shelax design from the Apogee free plan file (already built one according to the plans for backyard flying with the Grandson) and I'm up-sizing it to a BT60 version with a dedicated 3" long, 24mm motor mount. The original version is a great flyer and the sim in OR for the up-size looks spot on. Fingers crossed that this time I'll have a winner. Feel free to eyeball the OR file as I'm always open to suggestions/ideas.

Long Burn Shelax.jpg
 

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boatgeek

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Fin shredding is surely the latter, but most oscillations of this sort are completely resonance driven. Think of the stop signs you see waggling behind the poor TV reporters who get sent outside storms just to say "It's really windy out here, Steve!" A flat plate wants to turn broadside to the wind. It only oscillates because the post has torsional springiness and a resonance is struck between that and the inertia of the sign and post system. It has long been my assumption that fin flutter is essentially the same. I could be wrong in this, but I doubt it.
Airflow around stop signs (or flagpoles) is a slightly different animal. There's a really cool phenomenon that I referenced above where you get alternating vortices on each side of the object that end up driving the object to oscillate perpendicular to the wind. If you look at a flagpole vibrating in wind, you'll see most of the motion is perpendicular to the wind direction. Of course, fins aren't cylinders, and the linked article shows how the vortex street gets dampened by a fin behind a cylinder. Of course, it doesn't seem to entirely go away , so that may be a factor driving the fin vibration frequencies.

Proposal: Canted fins are less likely to flutter since they are operating at a consistent-sign angle of attack (positive while under thrust, switching to negative during coast) rather than switching back and forth to even out small directional displacements. We'd have to fly a lot to prove it one way or the other.
 

jqavins

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I've had a bug about having one model dedicated to utilizing the Apogee long burn E6 motor. Three attempts resulted in two seriously unstable rockets that found the ground long before the motor burned out and one that did a tail wagging corkscrew after about 500' and did achieve apogee and deploy only to be lost to the fields of SE Wisconsin. Sims on all three looked good in OR but actual results were less than nominal...
Did you check the rod exit speed in the sims? 'Cause it sounds like at least the first two might have been two slow, and the low thrust makes that a concern.
 
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Mugs914

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Found out my two Stratologgers were shipped by Perfectflite! :music1:

I've been trying to get a Stratologger for a couple of years now and finally got the timing right. Can't wait to get 'em!
 

BruceS

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I was given this Estes Citation Bomarc bg. Managed to break the nose. Found one at thingiverse and printed it out. After much sanding and priming and final gold, am now ready to test glide it. My question is this: the instructions want the balance point at the base of the wing antennas. W/o the core current cg is at the front of the wing. Do I need to add tail weight?
 

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Old School Doug

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Did you check the rod exit speed in the sims? 'Cause it sounds like at least the first two might have been two slow, and the low thrust makes that a concern.
I don't have the files for the prior attempts. The current build is sitting at 54 f/s off the rod. But in digging a bit deeper I'm seeing a thrust/weight ratio of 4.10:1 which has me really concerned. I understand that NAR holds 3:1 as a safety floor but everything else I've read recommends a minimum of 5:1 on a dead calm day (as if I've ever been lucky enough to launch in dead calm). I may have to rethink my approach.
 

boatgeek

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I don't have the files for the prior attempts. The current build is sitting at 54 f/s off the rod. But in digging a bit deeper I'm seeing a thrust/weight ratio of 4.10:1 which has me really concerned. I understand that NAR holds 3:1 as a safety floor but everything else I've read recommends a minimum of 5:1 on a dead calm day (as if I've ever been lucky enough to launch in dead calm). I may have to rethink my approach.
Not sure which motor you are talking about, but you can fudge the 5:1 rule if your thrust to weight off the pad is 5:1. Some motors tail off toward the end of the burn.
 

Mugs914

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I was given this Estes Citation Bomarc bg. Managed to break the nose. Found one at thingiverse and printed it out. After much sanding and priming and final gold, am now ready to test glide it. My question is this: the instructions want the balance point at the base of the wing antennas. W/o the core current cg is at the front of the wing. Do I need to add tail weight?
Good looking Bomarc!

Proper balance is absolutely critical to achieve a proper glide, so you will either need to add tail weight or reduce the weight of the nose. I would look at reducing nose weight to achieve proper balance as lighter overall weight will result in better glide performance.

As I recall, the original Bomarc nose cone was a pretty light plastic part. I would guess the printed nose is quite a bit heavier. I believe Estes sells a plastic BT-55 nosecone that is nigh unto a clone of the old nose. It might be a better choice than the printed version, balance-wise.

Also, make sure the elastic that lifts the elevator is in good shape before every flight.

Get your balance squared away then go find some tallish grass and spend some time tossing it like a glider to get the trim set. Once it flies as an airplane you'll be in good shape for recovery.

Most of all have fun with it and get video!

Mike
 

Nytrunner

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I don't have the files for the prior attempts. The current build is sitting at 54 f/s off the rod. But in digging a bit deeper I'm seeing a thrust/weight ratio of 4.10:1 which has me really concerned. I understand that NAR holds 3:1 as a safety floor but everything else I've read recommends a minimum of 5:1 on a dead calm day (as if I've ever been lucky enough to launch in dead calm). I may have to rethink my approach.

Speed off the rod is the inportant part, and 54 ft/s is good.

Look at the thrustcurve of the motor and pay attention to the 1st 1/4 second or so. That's the part of the burn while the rocket is on the rod/rail.
 

jqavins

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W/o the core current cg is at the front of the wing. Do I need to add tail weight?
Adding to Mikes reply, What does "without the core" mean. One of the very few things I know about gliders is that you have to trim them in flight configuration without exception of compromise.

Speed off the rod is the inportant part, and 54 ft/s is good.

Look at the thrustcurve of the motor and pay attention to the 1st 1/4 second or so. That's the part of the burn while the rocket is on the rod/rail.
I'd agree that 54 ft/s is OK, though I here that 60 is a good rule of thumb. It was just a thought. I looked at the thrust curve after I posted. There is a pretty fair spike, but not a great one. A D12 would get you off the rod faster.

IF rail exit speed is the issue, a longer rod would help, particularly with the E6, who's initial spike rises more slowly than BP motors like the D12. And more speed off the rod never hurts. I'd repeat agreement with Nytrunner that 54 ft/s is probably OK, and then I'd advise a longer rod anyway. And if you feel like it, maybe a D12 test flight.
1611753364340.png
 
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BruceS

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Adding to Mikes reply, What does "without the core" mean. One of the very few things I know about gliders is that you have to trim them in flight configuration without exception of compromise.

I'd agree that 54 ft/s is OK, though I here that 60 is a good rule of thumb. It was just a thought. I looked at the thrust curve after I posted. There is a pretty fair spike, but not a great one. A D12 would get you off the rod faster.

IF rail exit speed is the issue, a longer rod would help, particularly with the E6, who's initial spike rises more slowly than BP motors like the D12. And more speed off the rod never hurts. I'd repeat agreement with Nytrunner that 54 ft/s is probably OK, and then I'd advise a longer rod anyway. And if you feel like it, maybe a D12 test flight.
View attachment 448176
The Bomarc ejects the power core which returns via streamer or parachute.
 

jqavins

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Ah. Good.

IF rail exit speed is the issue, a longer rod would help, particularly with the E6, who's initial spike rises more slowly than BP motors like the D12. And more speed off the rod never hurts. I'd repeat agreement with Nytrunner that 54 ft/s is probably OK, and then I'd advise a longer rod anyway. And if you feel like it, maybe a D12 test flight.
Oops! I misread the thrust curve overlay. The E6 spikes faster than the D12, not slower. My bad. So now I'm not so sure which gets off the rod faster, yet I'd bet (a little) on D12. Still, it doesn't spike as high. So the rest of the suggestions stand.
 
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BruceS

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Good looking Bomarc!

Proper balance is absolutely critical to achieve a proper glide, so you will either need to add tail weight or reduce the weight of the nose. I would look at reducing nose weight to achieve proper balance as lighter overall weight will result in better glide performance.

As I recall, the original Bomarc nose cone was a pretty light plastic part. I would guess the printed nose is quite a bit heavier. I believe Estes sells a plastic BT-55 nosecone that is nigh unto a clone of the old nose. It might be a better choice than the printed version, balance-wise.

Also, make sure the elastic that lifts the elevator is in good shape before every flight.

Get your balance squared away then go find some tallish grass and spend some time tossing it like a glider to get the trim set. Once it flies as an airplane you'll be in good shape for recovery.

Most of all have fun with it and get video!

Mike
Since we had some fluffy snow, I decided on giving it a glide test.
 

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jqavins

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Oops! I misread the thrust curve overlay. The E6 spikes faster than the D12, not slower. My bad. So now I'm not so sure which gets off the rod faster, yet I'd bet (a little) on D12. Still, it doesn't spike as high. So the rest of the suggestions stand.
OK, I'll stop beating the horse after this one last thing.

I went and did some ThrustCurve.org sims, since it's all I can do here at work. With a 1 m rod I got 47.7 ft/s off the rail on the E6 and 46.3 on the D12. On a 1.5 m rod I got 57.2 ft/s on the E6 and 61.5 on the D12.

More broadly, ThrustCurve states that there are 9 engines in the catalog that are 24 mm diameter and not longer than 77 mm long (the E6 length) that give safe exit speed from a 1 m rod. With a 1.5 m rod there are 20. All in all, this just seems to be a rocket that wants a long rod.
 

Long_Gone

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Found a nice 3" tube 25" long. Looks like another Der Red Max clone.......
IMG_20210127_094958.jpg
 

Old School Doug

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OK, I'll stop beating the horse after this one last thing.

I went and did some ThrustCurve.org sims, since it's all I can do here at work. With a 1 m rod I got 47.7 ft/s off the rail on the E6 and 46.3 on the D12. On a 1.5 m rod I got 57.2 ft/s on the E6 and 61.5 on the D12.

More broadly, ThrustCurve states that there are 9 engines in the catalog that are 24 mm diameter and not longer than 77 mm long (the E6 length) that give safe exit speed from a 1 m rod. With a 1.5 m rod there are 20. All in all, this just seems to be a rocket that wants a long rod.
Thanks for all the input. I use a 48" rod personally as well as having one available at the club launch. OR and Thrustcurve both show velocity off the rod above 51 ft/s so I think I should be OK with low wind conditions. I'll probably throw a spacer in the mmt and load up a D12 for the first flight just to test her out. The main reason for this build is for club launches. Nothing like a small diameter rocket that just keeps going and going and going to raise a few eyebrows.
 

Tobor

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I hear you, but part of what these boards are designed to do is minimize that interference in racing drones. Our stuff is pretty constant drain, especially compared to four drone motors. The exception would be on deployment charge firing, and that should be low current too with ematches unless there is a short. I'm doing this more in an effort to see how much I can cram into a small space than to reduce interference. I'm looking at it the other way, i.e. is the increased interference and loss of battery redundancy worth the space and wiring complexity savings in crammed electronics bays. When designing things to fit in 38mm rockets, the batteries are often the most frustrating part. I want to see if I can fit a camera, 2 altimeters and a tracker in one 38mm bay that is about 8" long...I think I can if this works, it will certainly be easier.

If nothing else it will be a cheap ($13) learning experience.


View attachment 446536 View attachment 446537
Question: Make, model and price?
 

Cape Byron

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Some more searching on the business name. The internet is such an interesting place...

I found our kits on sale at a number of sites, none of which I have heard of before and all selling our kits for half the price we sell them. Some kind of voodoo wholesale agreement? The Ouija board is still in its box...

We only sell on Ebay. Not through any other online stores such as:

fsbullety.com
alegitfx.com

Not sure of how to proceed, but if you're ever looking for something of ours message me or look on Ebay.

:mad:
 
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