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Contra-Rotating Bullet Sections

The front part spins left, the back part spins right
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This allows a spin stabilized bullet to have a core containing a camera and or guidance mechanisms that aren't spinning thousands of RPM.

There are two riflings to achieve this. One turning one direction is a standard bore rifling that contacts the front part of the bullet in standard fashion imparting a left handed twist. The back of the bullet has extended "fins" that fit into an outer rifling consisting of deep notches in the barrel that rotate opposite the standard rifling giving the articulated back portion of the bullet a right handed twist, opposite the front part of the bullet.

The body of the back part of the bullet is slightly narrower than the front part so it doesn't hit the inner rifling, only the outer rifling into which its extended fins fit.

doctorremulac3, Jun 05 2016

Well I'll be darned. https://www.youtube...watch?v=bzbVwiIeM0M
[doctorremulac3, Jul 19 2016]

Wikipedia's explanation of the dihedral effect https://en.wikipedi...zes_the_spiral_mode
see my anno [notexactly, Jul 21 2016]

[link]






       The obturation is therefore going to be at the rear of the front portion, where the pivot is. The propellant gases will impinge on the pivot, requiring a high-speed rotating seal.   

       The front portion has to pull the rear portion. That's a lot of tension on the coupling.   

       It would make more sense to have the full width portion at the rear and the contrarotating portion in front, being pushed. A much simpler seal would be required.   

       How is the reliable mating of the splines with the anti-rifling achieved, other than by manual loading ?
8th of 7, Jun 05 2016
  

       //It would make more sense to have the full width portion at the rear and the contrarotating portion in front, being pushed. A much simpler seal would be required.//   

       The wider part is at the back if that's what you mean.   

       //How is the reliable mating of the splines with the anti-rifling achieved, other than by manual loading ?//   

       Not sure I catch what you're saying, could you dumb it down a bit? Are you talking about possible issues with the gas seal?   

       Again, I think I should probably throw together a drawing of this so it's clear what I'm proposing. Re- reading it, it is a bit confusing.
doctorremulac3, Jun 05 2016
  

       I'm liking the idea of this, but am not sure it's the best approach to achieve your objectives. You are effectively suggesting three parts to the bullet; one going clockwise, one anti-clockwise and one non-rotating. That might be overkill - I believe there are tank-fired missiles which use a rotating sabot to launch a non-rotating projectile from a rifled barrel.   

       A couple of decades or so ago, I read (IIRC in Scientific American) about a suggested guided air to air (large caliber) bullet. It was proposed as illumination targeted, i.e. the attacker would illuminate the target plane with a laser, and the bullet would home in on that. To do so it had a camera in the nose, and could flex very slightly in the middle to change direction. It would only need to flex in one axis because the bullet is rotating, but would have to do so rather quickly.
You might say that's not particularly relevant, but I found it interesting, and it does lead me to the following suggestion:
Don't worry about preventing the rotation, just use a very high-speed camera (which you'll need anyway, otherwise most things will be 'warp- speed' effect blurred), and synchronise the frame rate to the rotation rate.
Loris, Jun 05 2016
  

       There must come a point at which it's easier to rotate the target and use a non-rotating bullet.
MaxwellBuchanan, Jun 05 2016
  

       Why counter-rotating ?   

       concentric - the outer catches the lands and spins, imparting stability; the inner is the camera... doesn't spin. Bearing, bushing, whatever, between. Picture a .17 bullet (the camera) wedged into a ball bearing wedged into a hollowed out .40 .
FlyingToaster, Jun 06 2016
  

       Well, just having bearings isn't going to do much since just a little friction on an outer part spinning at 180,000 rpm is going to get that camera spinning really fast.   

       As far as using the spin for power, yea, I guess you could do that.
doctorremulac3, Jun 06 2016
  

       As a guy who deals with bearings quite often this seems like asking quite a lot from them.
RayfordSteele, Jun 06 2016
  

       As I said on your other recent stabilized-bullet idea:   

       I guarantee this will not work even a little bit, and the bullet will be just as unstable as it would be without the two cylinders spinning (barring some unexpected stabilization from the Magnus effect or something). Any two equal-in-magnitude-(the site tells me I need to put a space here)-moment-of-inertia counter-rotating rotors will cancel out each other's momenta of inertia, leaving you with no stabilization. (I know because I have looked into this before, for CMGs.)
notexactly, Jul 03 2016
  

       So you're saying a contra-rotating helicopter blade pair has no stability because each one cancels the other one out?   

       I know that's not the case, they're extremely stable and don't even need a tail rotor except one facing backwards for forward thrust only, so maybe I'm misunderstanding you.
doctorremulac3, Jul 05 2016
  

       Yes, that's exactly what they do, and indeed the main reason they exist. Copters with contrarotating blades don't need a tail boom and rotor for yaw stability. All directional and attitude control is applied via the swashplate linkages.   

       They're stable because of the active control of the airfoils. A bullet is purely passive and relies on gyroscopic effects.
8th of 7, Jul 05 2016
  

       Yea, nothing unstable about two opposing gyroscopes, quite the contrary.   

       Bearing issues, possibly. Stability issues, no.
doctorremulac3, Jul 05 2016
  

       No. Still wrong. [8th] is right.   

       I'm not claiming that two counter-rotating or contra-rotating rotors of any kind impart negative stability to the thing that holds them. I'm claiming that they don't impart positive stability like a single rotor does. (As long as their angular momenta are equal in magnitude; if they are different, then the effect is be the same as a single rotor whose angular momentum is equal to the difference.)   

       A regular helicopter needs some kind of sideways thruster on its tail only because the one main rotor imparts a reaction torque to the body, and this needs to be counteracted or else the whole helicopter will rotate all the time. This can be counteracted using a tail rotor (conventional or Fenestron), a NOTAR system (jet exhaust blowing out one side of the tail boom), or—only in RC helicopters so far—a fixed drag plate at the end of the tail boom. The important thing to understand is that THE TAIL THRUSTER IS NOT A STABILIZER in the sense of gyroscopic stabilization. However, the main rotor DOES provide gyroscopic stabilization. It just also produces a torque that needs to be counteracted.   

       A two-rotor helicopter (counter-rotating or contra- rotating) does not need the tail thruster because the two rotors counteract each other's torque directly. However, they also counteract each other's angular momenta, removing the gyroscopic stabilization effect on the helicopter as a whole. (However, each one still attempts to stabilize itself (but in opposite directions, obviously), and this results in torques and forces being transmitted through the helicopter's frame or rotor axle assembly, where they cancel each other out as they meet.)   

       However, such helicopters are still stable because helicopters have another source of stability: active stabilization by the pilot or autopilot. Helicopters deviate from upright quite slowly (unless doing so deliberately), making it easy for the pilot to maintain stability, and their rotors are powerful enough to correct even quite large deviations from upright under the control of the pilot. (On the other hand, a quadcopter or Harrier deviates very rapidly (and a Harrier's stabilization jets are much less thrusty relative to its moment of inertia), so those require much more rapid reactions to remain stable, which only an autopilot can handle.)   

       Another possible source of stability in helicopters is the upward flex of the rotor blades under load. This, I would think, acts like dihedral of an airplane's wings. (The mechanism of the dihedral producing roll stability of a plane (or roll/pitch stability of a helicopter rotor, if I'm right) is quite unintuitive and I can't remember how it works, but it does.)
notexactly, Jul 19 2016
  

       So I guess counter rotating gyroscopes DO cancel each other out. (see link)   

       Thanks NotEx, learned something.
doctorremulac3, Jul 19 2016
  

       That's actually quite a good demonstration. I'll have to remember to show it next time someone doesn't believe it.
notexactly, Jul 20 2016
  

       // Thanks NotEx, learned something. //   

       Oh good... you can still have the sadistically violent beating if you want, as an aide-memoire perhaps ?   

       // upward flex of the rotor blades under load.... is quite unintuitive and I can't remember how it works, but it does. //   

       It's very intricate. The rotor tips follow a complex sinusoidal path with respect to the hub, different on the advancing and retreating segments of the cycle because of the change in relative velocity. It's one of those little quirks of rotary-wing aerodynamics that attracts a very particular breed of pointy-headed twisty-brained Übernerd to the profession.... which is a good thing, because without helicopters to work on, people like that might design anything...   

       // [8th] is right //   

       Only because it's not possible for us to be wrong.
8th of 7, Jul 20 2016
  

       Pretty sure gyroscopes use black magic or some kind of voodoo to do their thing.   

       I'll leave this post up just because the science discussion is interesting.
doctorremulac3, Jul 20 2016
  

       Basically angular momentum is a vector quantity, and so vector addition applies.
RayfordSteele, Jul 20 2016
  

       //mechanism... dihedral.. unintuitive// I was going to say Wikipedia, but their article is rather unintuitive. Look at an aircraft, flying level, fom the front. Both wings have the same amount of lift so it flies level. But, if you roll it a bit to one side or the other then the wing that's closest to being horizontal has the most lift... so it rolls back.
FlyingToaster, Jul 20 2016
  

       That effect can escalate into Dutch roll.   

       Unfortunately, it's an extremely complex problem not really amenable to simple analogies; and extrapolating from a (relatively) stiff fixed wing to a flexible rotary wing really isn't possible - no criticism intended [FT].
8th of 7, Jul 21 2016
  

       // But, if you roll it a bit to one side or the other then the wing that's closest to being horizontal has the most lift... so it rolls back. //   

       Nope (if you don't mean anything more complicated by that). That's the #1 dihedral misconception, IIRC. Looking at the Wikipedia article now, though, it doesn't seem to clarify that well.   

       Lift doesn't know or care which way is 'up' relative to the outside world; it only pushes the plane up along the plane's own vertical axis, which may be tilted relative to the ground. If the plane is rolled to one side, the lift vector rolls with it, to still point straight up in the plane's coordinate system.   

       According to Wikipedia, with a dihedral, this somehow causes a sideslip, which causes the plane to yaw in the other direction, which increases the angle of attack on the down-rolled wing and decreases it on the up- rolled wing, causing differential lift, which restores roll angle toward zero.   

       The net effect is that the wing that is rolled down does experience more lift, but this is not due to it being closer to horizontal. (In fact, it should work just as well if that wing is below horizontal.) It is instead due to it being yawed such that it leads the other wing and points more in the direction of travel, resulting in higher angle of attack.   

       Interestingly, I think that would mean that if you were flying with wings and/or in a regime where increased angle of attack reduced lift, then dihedral would destabilize and anhedral would stabilize. Maybe.   

       And I'm still not clear on why the sideslip causes yaw. But I'm pretty sure it won't work for a helicopter rotor, which is always yawing anyway.
notexactly, Jul 21 2016
  

       While //lift doesn't know or care which way is 'up'//, gravity still does, and is a constant force pulling the airplane straight down. If the plane had wings at 90deg to each other, which way would it roll if one was straight out and the other straight up ? At what angle would it end up balanced at ? (For simplicity's sake let's say the CG is right at the wingroot).   

       It's reasonably obvious that sideslip happens... no clue how that would affect roll except maybe something like the wing in the lee of the wind (which is the up-pointing wing) gets less air because it's blocked by the fuselage, or because it sticks up more and is pushed back down by the crosswind.   

       It makes no sense to me whatsoever that the plane yaws in the opposite direction, ie: the left wingtip dips down and the plane yaws right ? Since there's a crosswind happening (from sideslip), the plane should yaw left, that being what the tailfin is for.   

       Helicopters ? What breed of chicken they sacrifice before each flight probably has more to do with stability than dihedral, but the same principle should apply.
FlyingToaster, Jul 21 2016
  

       Chicken ? You could ritually sacrifice a whole flock of sacred virgin ostriches on top of a Ziggurat built entirely from flight safety test documents and dedicated to the Gods of Aeronautics, and the bloody helicopter still wouldn't be safe, or even a little bit less lethally dangerous.   

       Sp. "helicopter", Pr. "Assisted Suicide".
8th of 7, Jul 21 2016
  
      
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