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
Not too hard to fly an agitator with a powerful tractor motor. Impossible to avoid puns on TRF.Scarier heck; which is hadrer?
Yes, the gallery here can be really pun-ishing, can't they?Not too hard to fly an agitator with a powerful tractor motor. Impossible to avoid puns on TRF.
Oh, don't. Just... Just don't.Yes, the gallery here can be really pun-ishing, can't they?
[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.
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:Look up Shaft whip if you want to understand why the tube may crumple once you get to a certain critical rotation speed.
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:
View attachment 566934
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.
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.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.
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.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)
It's less draggier than the sim.I guess it goes without saying that I bet it's a whole lot draggier than the sim.
Sandblasted glass... yep.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!
Ooooh, just enough shark-skin texture to really optimize drag characteristics. Mumble mumble Reynolds Number mumble flow attachment mumble vortex shedding. Fancy!Sandblasted glass... yep.
Ooooh, just enough shark-skin texture to really optimize drag characteristics. Mumble mumble Reynolds Number mumble flow attachment mumble vortex shedding. Fancy!
You and your fancy lack of oxygen lording it over the rest of us.We can get away with such blatant disrespect for surface finish here at 8,200 feet above sea level...
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!We can get away with such blatant disrespect for surface finish here at 8,200 feet above sea level...
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