Gun barrels have rifling to spin the projectile and thereby keep it from tumbling in flight. Experience shows that this works. The usual explanation, that gyroscopic force maintains the projectile's direction in space, is clearly lacking. Consider a howitzer shell. Its detonator fuse is in its nose. It is fired upward at a steep angle, yet impacts on the detonator. An American (prolate) football may offer a clue. The closure stitching moves its center of gravity, and hence its axis of minimum moment of inertia, off the axis of geometric symmetry. I observe that the spin axis of a well thrown spiral pass precesses, and I assume that the displaced CG causes that. I also observe that the axis of precession points in the direction of travel throughout the arc. I can imagine that this is the result the interaction of aerodynamic (thank you, Rune) and gyroscopic forces. I can imagine, but not explain. It's pretty clear that the geometric and dynamic axes of rifle and handgun bullets coincide, but these are rarely expected to be effective with trajectories that depart much from flat. Could it be that artillery shells are deliberately unbalanced? Jerry
OT. Spin Stabilization
Started by ●December 13, 2010
Reply by ●December 13, 20102010-12-13
On Mon, 13 Dec 2010 09:31:40 -0800 (PST), Jerry Avins <jya@ieee.org> wrote:>Gun barrels have rifling to spin the projectile and thereby keep it >from tumbling in flight. Experience shows that this works. The usual >explanation, that gyroscopic force maintains the projectile's >direction in space, is clearly lacking. Consider a howitzer shell. Its >detonator fuse is in its nose. It is fired upward at a steep angle, >yet impacts on the detonator. > >An American (prolate) football may offer a clue. The closure stitching >moves its center of gravity, and hence its axis of minimum moment of >inertia, off the axis of geometric symmetry. I observe that the spin >axis of a well thrown spiral pass precesses, and I assume that the >displaced CG causes that. I also observe that the axis of precession >points in the direction of travel throughout the arc. I can imagine >that this is the result the interaction of aerodynamic (thank you, >Rune) and gyroscopic forces. I can imagine, but not explain. > >It's pretty clear that the geometric and dynamic axes of rifle and >handgun bullets coincide, but these are rarely expected to be >effective with trajectories that depart much from flat. Could it be >that artillery shells are deliberately unbalanced? > >JerryI've kind of wondered about that myself, that shells still wind up impacting on their nose, even when the trajectory has a pretty steep curve. I do suspect that the aerodynamics helps, and some bullets don't maintain stability and begin to tumble (I think the early M16s had a problem with that). And now there's this stuff: http://www.foxnews.com/scitech/2010/12/10/navy-railgun-shoots-bullets-electromagnet/ I'm curious whether they spin-stabilize those guys or not. It's hard to tell from some of the vids: http://www.youtube.com/watch?v=F6ioLh7boMc Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com
Reply by ●December 13, 20102010-12-13
Jerry Avins wrote:> Gun barrels have rifling to spin the projectile and thereby keep it > from tumbling in flight. Experience shows that this works. The usual > explanation, that gyroscopic force maintains the projectile's > direction in space, is clearly lacking. Consider a howitzer shell. Its > detonator fuse is in its nose. It is fired upward at a steep angle, > yet impacts on the detonator. > An American (prolate) football may offer a clue. The closure stitching > moves its center of gravity, and hence its axis of minimum moment of > inertia, off the axis of geometric symmetry. I observe that the spin > axis of a well thrown spiral pass precesses, and I assume that the > displaced CG causes that. I also observe that the axis of precession > points in the direction of travel throughout the arc. I can imagine > that this is the result the interaction of aerodynamic (thank you, > Rune) and gyroscopic forces. I can imagine, but not explain. > > It's pretty clear that the geometric and dynamic axes of rifle and > handgun bullets coincide, but these are rarely expected to be > effective with trajectories that depart much from flat. Could it be > that artillery shells are deliberately unbalanced?The nose of a projectile gets oriented in the direction of travel entirely due to the aerodynamic force. Spinning increases the seeming moment of inertia of the projectile, keeping it dynamically stable from tumbling. It is a well known that there is an optimal spin rate for a given shape and weight of the bullet; lighter bullets need faster spin rates. Unbalanced load is a disaster from the accuracy standpoint; perfect balancing is one of the parameters which distinguishes the expensive target grade ammo from the regular stuff. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●December 13, 20102010-12-13
Eric Jacobsen wrote:> On Mon, 13 Dec 2010 09:31:40 -0800 (PST), Jerry Avins <jya@ieee.org> > wrote: > > >>Gun barrels have rifling to spin the projectile and thereby keep it > >>from tumbling in flight. Experience shows that this works. The usual > >>explanation, that gyroscopic force maintains the projectile's >>direction in space, is clearly lacking. Consider a howitzer shell. Its >>detonator fuse is in its nose. It is fired upward at a steep angle, >>yet impacts on the detonator. >> >>An American (prolate) football may offer a clue. The closure stitching >>moves its center of gravity, and hence its axis of minimum moment of >>inertia, off the axis of geometric symmetry. I observe that the spin >>axis of a well thrown spiral pass precesses, and I assume that the >>displaced CG causes that. I also observe that the axis of precession >>points in the direction of travel throughout the arc. I can imagine >>that this is the result the interaction of aerodynamic (thank you, >>Rune) and gyroscopic forces. I can imagine, but not explain. >> >>It's pretty clear that the geometric and dynamic axes of rifle and >>handgun bullets coincide, but these are rarely expected to be >>effective with trajectories that depart much from flat. Could it be >>that artillery shells are deliberately unbalanced? >> >>Jerry > > > I've kind of wondered about that myself, that shells still wind up > impacting on their nose, even when the trajectory has a pretty steep > curve. I do suspect that the aerodynamics helps, and some bullets > don't maintain stability and begin to tumble (I think the early M16s > had a problem with that).Military guns have to shoot regular bullets as well as light loads like tracers or incendiaries. So, the rifling is made pretty steep to accomodate for the light loads. Because of that, the regular bullets are made artificially destabilized (hollow pocket at the nose, lead knocker at the tail). This keeps the ballistics more or less similar regardless of the load.> And now there's this stuff: > http://www.foxnews.com/scitech/2010/12/10/navy-railgun-shoots-bullets-electromagnet/No real accomplishment. A typical tank cannon shoots sabots at ~1.8km/s. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●December 13, 20102010-12-13
On Dec 13, 1:12�pm, eric.jacob...@ieee.org (Eric Jacobsen) wrote: ...> And now there's this stuff: > > http://www.foxnews.com/scitech/2010/12/10/navy-railgun-shoots-bullets... > > I'm curious whether they spin-stabilize those guys or not. � It's hard > to tell from some of the vids:I don't expect to learn much detail from an author who writes, "... the intended target could be gone by the time it reaches its destination."> http://www.youtube.com/watch?v=F6ioLh7boMcWhy the smoke and fire? Jerry
Reply by ●December 13, 20102010-12-13
On Mon, 13 Dec 2010 12:14:21 -0600, Vladimir Vassilevsky <nospam@nowhere.com> wrote:>Unbalanced load is a disaster from the accuracy standpoint; >perfect balancing is one of the parameters which distinguishes the >expensive target grade ammo from the regular stuff.http://www.theboxotruth.com/docs/bot49.htm
Reply by ●December 13, 20102010-12-13
On Dec 13, 1:33�pm, Vladimir Vassilevsky <nos...@nowhere.com> wrote:> Eric Jacobsen wrote:...> >http://www.foxnews.com/scitech/2010/12/10/navy-railgun-shoots-bullets... > > No real accomplishment. A typical tank cannon shoots sabots at ~1.8km/s.The big gun on the Abrams M1A1 main battle tank is a German-designed (and produced) 120 mm smooth bore. It replaces the A1's rifled 105 mm one. Jerry
Reply by ●December 13, 20102010-12-13
On Mon, 13 Dec 2010 11:16:35 -0800 (PST), Jerry Avins <jya@ieee.org> wrote:>On Dec 13, 1:12=A0pm, eric.jacob...@ieee.org (Eric Jacobsen) wrote: > > ... > >> And now there's this stuff: >> >> http://www.foxnews.com/scitech/2010/12/10/navy-railgun-shoots-bullets... >> >> I'm curious whether they spin-stabilize those guys or not. =A0 It's hard >> to tell from some of the vids: > >I don't expect to learn much detail from an author who writes, "... >the intended target could be gone by the time it reaches its >destination."I think that was just a simplification of why reducing time to target has value.>> http://www.youtube.com/watch?v=3DF6ioLh7boMc > >Why the smoke and fire?From other descriptions my understanding is that the energy involved essentially ignited part of the payload. It's not propulsive, but a by-product of whatever it is that they're doing. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com
Reply by ●December 13, 20102010-12-13
Jerry Avins wrote:> > Gun barrels have rifling to spin the projectile and thereby keep it > from tumbling in flight. Experience shows that this works. The usual > explanation, that gyroscopic force maintains the projectile's > direction in space, is clearly lacking. Consider a howitzer shell. Its > detonator fuse is in its nose. It is fired upward at a steep angle, > yet impacts on the detonator. > > An American (prolate) football may offer a clue. The closure stitching > moves its center of gravity, and hence its axis of minimum moment of > inertia, off the axis of geometric symmetry. I observe that the spin > axis of a well thrown spiral pass precesses, and I assume that the > displaced CG causes that. I also observe that the axis of precession > points in the direction of travel throughout the arc. I can imagine > that this is the result the interaction of aerodynamic (thank you, > Rune) and gyroscopic forces. I can imagine, but not explain. > > It's pretty clear that the geometric and dynamic axes of rifle and > handgun bullets coincide, but these are rarely expected to be > effective with trajectories that depart much from flat. Could it be > that artillery shells are deliberately unbalanced? > > JerryChanging the shape of the bullet does a lot to stabilize and increase the range of a round. Some artillery rounds have the body necked in to play with center of pressure and center gravity. I suspect that aspect ratio (length to diameter) would alter flight characteristics as well. A long time ago I worked on a control systems for a dropped bomb. These were fin stabilized and could be reasonably modeled aerodynamically. Does anyone know how rounds fired from the smooth bore Abrams tank are stabilized? Walter..
Reply by ●December 13, 20102010-12-13
Jerry Avins wrote:> On Dec 13, 1:33 pm, Vladimir Vassilevsky <nos...@nowhere.com> wrote: > >>Eric Jacobsen wrote: > > > ... > > >>>http://www.foxnews.com/scitech/2010/12/10/navy-railgun-shoots-bullets... >> >>No real accomplishment. A typical tank cannon shoots sabots at ~1.8km/s. > > > The big gun on the Abrams M1A1 main battle tank is a German-designed > (and produced) 120 mm smooth bore. It replaces the A1's rifled 105 mm > one.British still use rifled barrels on their tanks. No wonder the longest distance kill (>5km) was done by the British tank. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
