The thread was way back in '14, The OP was run off by too many puns after the initial post. What is scarier, flying a high power washing machine agitator or enduring the outrageous slings and arrows from TRF peanut gallery?⁷
Lakeroadster's Helical Fin Rocket.
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Scarier heck; which is hadrer?
Not too hard to fly an agitator with a powerful tractor motor. Impossible to avoid puns on TRF.
Yes, the gallery here can be really pun-ishing, can't they?
Oh, don't. Just... Just don't.
boatgeek
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[walking past the pun minefield... I will not go in. I will not go in... ]
My theory is that there is relatively little chance of buckling the body tube with canted fins. My thinking goes like this. You need a lot of torque to buckle the tube, so a relatively high angle of attack at relatively high speeds. The rocket has a low moment of inertia about its centerline, so it spins up quickly at low speed when there's relatively low torque on the tube. By the time it gets to high speed, it's spinning fast enough that the angle of attack is low or nil.
An exception might be very fast motors (Super Thunder, Warp 9 or VMax) where you might get to high speed so quickly that the rocket hasn't spun up yet. In those cases, I'd guess that the linear acceleration would be more likely to crumple body tubes than the torque.
This is a "thumb-in-the-air" analysis, testing or further thoughts welcome.
]My theory is that there is relatively little chance of buckling the body tube with canted fins. My thinking goes like this. You need a lot of torque to buckle the tube, so a relatively high angle of attack at relatively high speeds. The rocket has a low moment of inertia about its centerline, so it spins up quickly at low speed when there's relatively low torque on the tube. By the time it gets to high speed, it's spinning fast enough that the angle of attack is low or nil.
An exception might be very fast motors (Super Thunder, Warp 9 or VMax) where you might get to high speed so quickly that the rocket hasn't spun up yet. In those cases, I'd guess that the linear acceleration would be more likely to crumple body tubes than the torque.
This is a "thumb-in-the-air" analysis, testing or further thoughts welcome.
Rschub
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Look up Shaft whip if you want to understand why the tube may crumple once you get to a certain critical rotation speed.
lakeroadster
When in doubt... build hell-for-stout!
[walking past the pun minefield... I will not go in. I will not go in...
My theory is that there is relatively little chance of buckling the body tube with canted fins. My thinking goes like this. You need a lot of torque to buckle the tube, so a relatively high angle of attack at relatively high speeds. The rocket has a low moment of inertia about its centerline, so it spins up quickly at low speed when there's relatively low torque on the tube. By the time it gets to high speed, it's spinning fast enough that the angle of attack is low or nil.
An exception might be very fast motors (Super Thunder, Warp 9 or VMax) where you might get to high speed so quickly that the rocket hasn't spun up yet. In those cases, I'd guess that the linear acceleration would be more likely to crumple body tubes than the torque.
This is a "thumb-in-the-air" analysis, testing or further thoughts welcome.
I cant find any reason to disagree with this analysis.
boatgeek
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Eh, I'm not really convinced shaft whip is a big deal. Using a worst case calc from here for an 18" piece of BT-70 tube:
E ~ 2 GPa = 2 E 9 N/m^2
I ~ 3.1 E -8 m^4
m = 0.037 kg
L = 0.46 m
Nc = 107 rev/s
I don't believe that any hobby rocket with canted fins is going to crack 600 rpm, let alone 6000. Also, the calc above assumes a horizontal cantilevered shaft. With a mostly-vertical rocket, you're not going to have the weight of the rocket trying to pull it off centerline. While it's possible that out-of-center weights would cause some more displacement, I would guess that effect would be smaller than the weight of the rocket itself.
Rschub
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Are you including the far from CG motor mass in your calculations? What is the rocket cantilevered off of? Once it's off the rail, it's kinda not.
Isn't air resistance the only damping force here? All these things would tend to lower the critical speed.
So the case would be more like:
Non-Central rotating mass - Short Bearings but with no fixed bearing support.
I am just saying that it may be a factor that is usually not considered.
Isn't air resistance the only damping force here? All these things would tend to lower the critical speed.
So the case would be more like:
Non-Central rotating mass - Short Bearings but with no fixed bearing support.
I am just saying that it may be a factor that is usually not considered.
Very cool!
I did something similar, so maybe some helpful info there?
Spiral Spin Can
Was a lot of fun and I plan to revisit it again some day (perhaps with ball bearinged can), so I'll be watching how yours turns out. 
I did something similar, so maybe some helpful info there?
Spiral Spin Can
Was a lot of fun and I plan to revisit it again some day (perhaps with ball bearinged can), so I'll be watching how yours turns out.
Also from what I recall, the spiral fins will make the rocket really spin fast and for larger builds this can even twist up the recovery stuff in addition to dorking the altimeter readings. YMMV
lakeroadster
When in doubt... build hell-for-stout!
Points of reference:
My Thunk! model rocket used spin stabilization and the roll speed calculated to be 45 rev/s (BT-55 w/ a D12)
My Mighty Saturn V also uses spin stabilization and it calculates out at 24 rev/s (BT-300 w/ a cluster of (3) C11's and (2) D12's)
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lakeroadster
When in doubt... build hell-for-stout!
Just for ***** and grins I ran the simulation on THUNK based on a F39 motor. The roll speed calculates to 122 rev/s.
Would it actually do that... 
Would it actually do that...
boatgeek
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I see where you're coming from. I tried to get at this earlier, but I don't think I did a good job of explaining. In the cantilever beam model I cited, the weight of the shaft causes it to sag off of CL, increasing the chances of the shaft whirling out of control. In a spinning rocket, there's no gravity to contend with--everything is more or less centered on the rocket centerline. The only thing that would cause wobbling is off-center weights.
Color me skeptical until I see a high-speed video of roll patterns.
That said, I'll ask the OR development crew how they calculate roll rate. If it doesn't incorporate some flavor of angle of attack adjustment for the rocket's spin, it's going to overstate pretty dramatically.
lakeroadster
When in doubt... build hell-for-stout!
It's less draggier than the sim.
The sim assumes 5 independent sets of 3 fins, all of which have their entire leading edge out in the wind.
In reality a portion of 4 of those fin sets have part of their leading edges blocked, thus less frontal area, thus less drag.
Will all those fins be shaped to the perfect airfoil and finished to a glass like surface? The Wright Brothers would be proud. Maximum spin efficiency!
lakeroadster
When in doubt... build hell-for-stout!
Sandblasted glass... yep.
boatgeek
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Ooooh, just enough shark-skin texture to really optimize drag characteristics. Mumble mumble Reynolds Number mumble flow attachment mumble vortex shedding. Fancy!
lakeroadster
When in doubt... build hell-for-stout!
Ooooh, just enough shark-skin texture to really optimize drag characteristics. Mumble mumble Reynolds Number mumble flow attachment mumble vortex shedding. Fancy!
We can get away with such blatant disrespect for surface finish here at 8,200 feet above sea level...
boatgeek
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You and your fancy lack of oxygen lording it over the rest of us.
Not with the typical old dude, hardened Cold Warrior, Space Cowboy, Top Man you will find flying at 8,800 feet in South Park! Years of high grade balsa airfoil sanding to perfection, then several coats of Dope with more sanding, has warped thier minds in strange ways. Light, strong and zero grain showing! A deep color glass like finish that perfectly slices through the thin, dry, mountain air. This is the way!
Each fin pre painted in a psychedelic scheme suitable to the Timothy Leary crowd as it spins through space and time. Maybe a good dip paint job to revive the colors and groove of good ole days in the late 60's!
