In the Wikipedia article on continuous-rod warheads, in the ‘Operation’ section, two statements are made that don’t make a lot of sense to me (and, unless I missed it, the cited references don’t explain things either).
“Since the net momentum of the rod relative to the missile is roughly zero its effectiveness will rapidly diminish as the broken ring expands.”
Isn’t the reason that the rod’s effectiveness decreases as the ring expands simply because the explosive force (of the warhead) decreases as the distance from it increases. What does the relative momentum to the missile have to do with it?
“The (lethally destructive) effect is only pronounced as long as the ring is unbroken, so multiple layers of rods are employed in practical weapons to increase the effective radius.”
It would seem to me that the only way to prevent the ring from breaking is to increase its circumference (which increases as the length or number of the component rods increases). Again, I ask what do “multiple layers of rods” have to do with maintaining ring integrity?
I think the reason for the greater destructive force from an unbroken ring must be something to do with forces being able to be transferred along it to the point of contact with the target (although I’m not really clear on the specifics of that).
The multiple layers bit - I think they’re talking about folding a single ring in two dimensions (i.e. zig-zag up and down the axis, and in and out of the radius) - folding it into multiple layers means you can pack a ring with a longer circumference into the space.
At the start, different pieces of the rod are all flying out at very high speed due to the explosion. But opposite pieces are flying in opposite directions, so the net momentum is zero. Once it reaches maximum expansion, the different parts will be pulling on each other, and slow them all down.
Imagine, if you will, a ring that never broke open at all. Once it reached maximum expansion, it would just plain stop. The actual device still has some destructiveness left at that point, as it breaks apart, but it’s much diminished, since a lot of the energy goes into breaking the ring.
Also, at the point where it is a continuous ring, it is presenting to the target in the manner of a cheese wire - capable of creating a long cut across delicate structures. When it breaks apart, the wire fragments aren’t much more destructive than flying nails.
It’s pretty fascinating really. I had no idea that’s how anti-aircraft warheads work. Pretty ingenious. I always imagined it as a spherical burst of shrapnel.
Its certainly hard to jump from momentum of the rod to effectiveness of the rod.
for example, where it says that the ring is only effective when its unbroken… ok but if the momentum is always zero, it hasn’t much to lose ???
I believe its getting at the issue that when the ring is fully expanded and breaks , the outward velocity is reduced … the kinetic energy goes into breaking the ring, and it becomes just debris in the air, rather than the high velocity projectile it was intended to be. More rods adds more radius to the ring, which is kind of obvious as the size of the ring is set by the sum of the length of the rods…
It was awkward to refer to momentum , since that is a vector and the sum of the ring’s momentum (ignoring that given to it by the missiles flight ) is always zero.
Many of them still are. For example, the Buk missile that probably brought down MH17 was a fragmentation-type rather than a continuous-rod. There are pluses and minuses for both types.
I was in the business, know how they work, and I found that wiki article to be very confusing. But Chronos & Mangetout in posts 3 & 4 have it explained properly.
To my eye, the whole bit about net momentum is a red herring. It’s accurate physics, but immaterial to the engineered weapon effect.
The point is to create a high velocity cheese wire. Between the explosive-driven ring expansion rate and the missile versus aircraft closure rate, the kinetic energy per unit of contact area at impact is big. Real big. Which is real hard on comparatively flimsy aircraft structure.
Thank you to all who’ve responded. Things are now making more sense.
Just to be clear though, with respect to my second question (i.e. the use of multiple layers of rods to increase the effective radius), has Mangetout nailed it in post #2 - that it’s not a need for multiple layers per se, so much as it is a clever way of packaging and/or folding the rods in order to create a ring of greater diameter?
