Question about bullet kinetic energy
Johnny Tindalos
approximately equiv-tech to what we have now) were converted (very
efficiently) into laser light, how strong would it be?
And, supposing that one were hit square-on by such a rifle bullet, would
the force of its impact be sufficient to knock one off one's feet (assuming
reasonably good footing and that one were a post-human sufficiently tough
to avoid sustaining injury from the bullet itself)?
Would very much appreciate any help you can give!
Sincerely,
Johnny T.
Christopher M. Jones
> If the kinetic energy of an incoming bullet (assume a rifle/machine gun of
> approximately equiv-tech to what we have now) were converted (very
> efficiently) into laser light, how strong would it be?
Figure oh 2,000 foot-pounds energy for a rifle bullet
(~2700 Joules). So that works out to about a 3 kiloWatt
laser with a dwell time of a second, or more like maybe
a 300 kW laser with a 10 millisecond pulse duration.
That's quite a lot of power as lasers go. That's enough
to vaporize around 200 milligrams of Iron. Which may
not seem like much, only 25 cubic millimeters, but
with a small hole that could end up being pretty deep.
> And, supposing that one were hit square-on by such a rifle bullet, would
> the force of its impact be sufficient to knock one off one's feet (assuming
> reasonably good footing and that one were a post-human sufficiently tough
> to avoid sustaining injury from the bullet itself)?
Probably not. It might unbalance them or throw them off
their stride and confuse them enough so they stumble,
but actually knocking someone down with good footing is
enormously unlikely. The momentum involved is just too
small, except for maybe with really, really large caliber,
high powered bullets. If you think about it for a moment
you'll realize that the momentum transfered to the target
won't be greater than the recoil from the gun as the bullet
is fired (conservation of momentum and all that).
Johnny Tindalos
news:bdps7m$v72l0$1...@ID-137880.news.dfncis.de:
> Figure oh 2,000 foot-pounds energy for a rifle bullet
> (~2700 Joules). So that works out to about a 3 kiloWatt
> laser with a dwell time of a second, or more like maybe
> a 300 kW laser with a 10 millisecond pulse duration.
> That's quite a lot of power as lasers go. That's enough
> to vaporize around 200 milligrams of Iron. Which may
> not seem like much, only 25 cubic millimeters, but
> with a small hole that could end up being pretty deep.
>
Brilliant, that's just what I was hoping for! I guess the beam would
attenuate somewhat with distance in an atmosphere, probably more so if
the air had a high-ish water vapour content?
>> And, supposing that one were hit square-on by such a rifle bullet,
>> would the force of its impact be sufficient to knock one off one's
>> feet (assuming reasonably good footing and that one were a post-human
>> sufficiently tough to avoid sustaining injury from the bullet
>> itself)?
>
> Probably not. It might unbalance them or throw them off
> their stride and confuse them enough so they stumble,
> but actually knocking someone down with good footing is
> enormously unlikely. The momentum involved is just too
> small, except for maybe with really, really large caliber,
> high powered bullets.
Hmmm...does that mean that if our posthuman were caught in a hail of
machine-gun bullets aimed directly at her, the combined momentum transfer
(momental? moment transfer?) could be enough to send her hurling
floorward? I suppose it would depend on range and the rate of bullet
impact...
> If you think about it for a moment
> you'll realize that the momentum transfered to the target
> won't be greater than the recoil from the gun as the bullet
> is fired (conservation of momentum and all that).
>
>
Thank you very much!
Yours off to put his heroine through more impossible odds,
Johnny T.
Christopher M. Jones
> "Christopher M. Jones" <spic...@dualboot.net> wrote in
> news:bdps7m$v72l0$1...@ID-137880.news.dfncis.de:
>
> > Figure oh 2,000 foot-pounds energy for a rifle bullet
> > (~2700 Joules). So that works out to about a 3 kiloWatt
> > laser with a dwell time of a second, or more like maybe
> > a 300 kW laser with a 10 millisecond pulse duration.
> > That's quite a lot of power as lasers go. That's enough
> > to vaporize around 200 milligrams of Iron. Which may
> > not seem like much, only 25 cubic millimeters, but
> > with a small hole that could end up being pretty deep.
> >
>
> Brilliant, that's just what I was hoping for! I guess the beam would
> attenuate somewhat with distance in an atmosphere, probably more so if
> the air had a high-ish water vapour content?
That would depend upon the wavelength of the beam.
The obvious choice is to pick a wavelength which is
not attenuated much by the atmosphere (the near
infrared is a typically favored wavelength range).
All in all I would say that beam attenuation would,
under most circumstances, be much less than the
reduction in bullet velocity due to aerodynamic
drag over the same distance.
> >> And, supposing that one were hit square-on by such a rifle bullet,
> >> would the force of its impact be sufficient to knock one off one's
> >> feet (assuming reasonably good footing and that one were a post-human
> >> sufficiently tough to avoid sustaining injury from the bullet
> >> itself)?
> >
> > Probably not. It might unbalance them or throw them off
> > their stride and confuse them enough so they stumble,
> > but actually knocking someone down with good footing is
> > enormously unlikely. The momentum involved is just too
> > small, except for maybe with really, really large caliber,
> > high powered bullets.
>
> Hmmm...does that mean that if our posthuman were caught in a hail of
> machine-gun bullets aimed directly at her, the combined momentum transfer
> (momental? moment transfer?) could be enough to send her hurling
> floorward? I suppose it would depend on range and the rate of bullet
> impact...
Here the details are important. A hail of bullets from
a tommy gun or an AK-47, probably not (nless it was
many machine guns fired by many attackers). A hail of
bullets from a high-caliber machine gun mounted on a
sturdy platform (such as a car, a tank, or even just a
tripod), quite possibly. An interesting tidbit of
information you might want to consider, the GAU-8/A
Avenger cannon (a 30mm caliber, high cyclic rate, electric
gatling gun), which is the primary weapon of the A-10
Warthog / Thunderbolt II ground attack aircraft, generates
so much thrust (i.e. throws out so much momentum) when it
fires that it actually overwhelms the thrust generated by
the aircraft's two jet engines (thus, while the weapon is
firing the plane will actually slow down, even while the
jet engines are in operation). The A-10 weighs over 20
tonnes, and its two engines generate around 9 tonnes of
thrust together. That's more than enough to toss a car
around, let alone a human. A well-mounted .50 caliber
machine gun on full-automatic would also be able to toss
around a human body.
Luke
> "Christopher M. Jones" <spic...@dualboot.net> wrote in
> news:bdps7m$v72l0$1...@ID-137880.news.dfncis.de:
>
> > Figure oh 2,000 foot-pounds energy for a rifle bullet
> > (~2700 Joules). So that works out to about a 3 kiloWatt
> > laser with a dwell time of a second, or more like maybe
> > a 300 kW laser with a 10 millisecond pulse duration.
> > That's quite a lot of power as lasers go. That's enough
> > to vaporize around 200 milligrams of Iron. Which may
> > not seem like much, only 25 cubic millimeters, but
> > with a small hole that could end up being pretty deep.
This depends a lot on the characteristics of the beam. With a
continuous laser, you'd have a hard time getting your target to stay
still for long enough to bore a deep hole into it. I have estimated
that for continous beams, you'd need about 500 kilojoules to cause
incapacitation and/or death, with the best option to defocus the beam
to expose much of the frontal area of the target to the beam. This
will cause third degree burns to exposed skin, second degree burns
underneath light clothing (like cotton t-shirts, demin, or typical
summer miliatry uniforms) and will ignite cloth.
Short pulses will tend to be absorbed near the surface, causing an
explosion, and will be obscured by their own debris and smoke. A
single explosion could probably cause rapid incapacitation and/or
death with about 10 to 50 kilojoules delivered to the target. Smaller
energies would probably just maim or injure but not kill.
The best idea seems to be to deliver sub-nanosecond pulses. At these
time scales, most of the energy of the beam is transfered to a shock
wave in the material the beam is incident on, turning light energy
into mechanical energy. It is much easier to damage people or
materials with mechanical damage than thermal damage, by about two or
three orders of magnitude. To cause damage to deep internal organs,
deliver a train of these sub-nanosecond pulses spaced, say, a
microsecond or so apart. Each one will cause an explosion at the
surface where it is incident on the target. This will drive the
expansion of a cavity in the medium upon which the beam is incident.
Comparing this to the cavity caused by the passage of a bullet, this
cavity is likely to stay open for about a millisecond before
recollapsing. This allows subsequent pulses to be incident on the
back of the expanding cavity, each of which explodes and starts a new
cavity expanding. These cavities reinforce each other in the forward
direction, causing enough stress to rip flesh or other whatever
material we are shooting at, allowing the beam to propagate deeply.
Each pulse can thus have an energy of around a joule or so, and the
full train of pulses would carry about a kilojoule. This idea may or
may not actually work, if it does, you could cause damage very similar
to that of a bullet for about the same amount of energy.
To avoid the problem of dust and debris, and to maintain focus on the
back of the hole you are driving through the person, we can use a
trick from non-linear optics. Femptosecond pulses focused to a very
small area are known to be self-focusing, capable of keeping
approximately the same beam area for long distances and also
penetrating clouds and haze better than non-self focusing beams. So
reduce your pulse duration down to a femptosecond. Focus the beam
near the surface of your target. As the beam comes to a tight focus
just before it hits the target it becomes self focusing, maintianing
tight collumation. Now it will bore through the debris caused by
previous pulses and will remain in focus by the time it gets to the
back of the hole it is ripping through the target. You probably want
to avoid having the beam become self focusing too early, as the self
absorption regime is lossy, and your beam will be absorbed with
increasing distance.
To cause more damage than simply driving a hole through your target,
try focusing two beams of femptosecond pulses onto your target, close
enough together that the cavities from their explosions will overlap.
This may cause the flesh (or other material) caught between the
expanding cavities to rip, connecting the cavities. Rather than
driving a hole, it cuts a swath out of your target. This will cause a
greater surface area of the wound from which to bleed from; it gives
you more of a chance of cutting major blood vessels, nerves, tendons,
or the spinal cord; and more chance of damaging vital internal organs.
Use enough closely spaced beams and you will be cutting your target
in half (or nearly so). Again, this idea may or may not work.
> Brilliant, that's just what I was hoping for! I guess the beam would
> attenuate somewhat with distance in an atmosphere, probably more so if
> the air had a high-ish water vapour content?
This depends a lot on both the wavelength of light used and the
atmospheric conditions. There is a "window" of transparency in the
thermal IR between 14 microns and 8 microns at which air is pretty
much transparent, but water vapor can have a significant effect on
attenuation of the beam due to absorption. Between 3 microns and 2
microns is another window of transparency with similar behavior. In
the near IR at 1.5 microns and shorter wavelengths, through the
visible (0.7 to 0.4 microns) and near UV (0.4 to around 0.2 microns),
is the window with the greatest tranparency, and in this frequency
range, water vapor has nearly no effect at all. Water droplets
suspended in the atmosphere (fog, clouds) and other suspended
particulates (haze, smog, smoke) will attenuate the beam in this
wavelength range due to scattering (rather than absorption). However,
at longer wavelengths (such as at the 14 to 8 micron window and the 3
to 2 micron window), the beams will be much less attenuated due to
particulates and suspended water droplets than the wavelengths in the
window centered on visible light. There is also scattering from the
air itself, this is always larger for shorter wavelengths, but
generally not much of a concern unless you want to focus a laser on a
target many kilometers away.
Also, keep in mind that the shorter the wavelength, the longer the
distance at which you can focus the beam to a given spot size. Thus,
a green beam can focus at a longer range than a near IR beam, but the
near IR beam will be scattered less. Which one will have a longer
range depends on the atmospheric conditions, as there is more junk in
the air (haze, fog, etc) the near IR beam will be favored, while the
green beam will work better in clearer air or vacuum.
A couple more things to worry about when choosing beam wavelengths --
between 1 micron and about 0.2 microns, the lens, cornea, and vitreous
humor of the eye are transparent. This means that laser light in
these frequency ranges can be focused by the lens of the eye onto the
retina. Even a tiny glint of a reflection of the main beam can thus
cause permanent damage to your vision or even blindness. Outside of
this range, the eye is pretty much opaque, so a 1.5 micron near IR
beam would probably be safe to use without special goggles.
Luke
Johnny Tindalos
Thank you very much; that sort of hard data is certainly going to make
writing this novel a lot easier (at least, the bits where beam meets flesh,
which are pretty important, and I'd like to get them right...the
characters, being from societies with experience in EM weapons, would
definitely be aware of wavelength issues, etc. Repeat pulses to the
same/nearby location look like the way to go...)
Thanks again,
Johnny T.
Johnny Tindalos
> Here the details are important. A hail of bullets from
> a tommy gun or an AK-47, probably not (nless it was
> many machine guns fired by many attackers). A hail of
> bullets from a high-caliber machine gun mounted on a
> sturdy platform (such as a car, a tank, or even just a
> tripod), quite possibly.
Well, she's being attacked by soldiers armed with rifles, fully-automatic
machine-guns, and scimitars, so it looks like they'd be able to knock her
over (and thus suit my plot) when they manage to get a straight shot (she's
making a fair job of dissuading them from so doing, but it can't last...)
The info on the Warthog was also v. interesting (the villains lost all six
of their jets in the process of knocking out her flight module, but they've
got more, and it helps to know what could happen to vehicles / buildings /
individuals targetted by such craft...)
Thanks for your help!
Johnny T.
Luke
No prob.
You might also be interested in the "special effects" of a laser
weapon's beam. Needless to say, beams outside the range of visible EM
radiation will be close to invisible - except that very short, intense
pulses can cause ionization of the air. This is generally something
to be avoided, it saps power from the beam and decreases its range.
In some cases, the patch of ionized air absorbs the rest of the beam,
preventing the beam from propagating any further. This is another
reason why you want a wide beam where it exits from the laser, and
then focus the beam to a tight spot at your target - it cuts down on
the intensity of the beam over most of its path, so ionization need
only be a problem right near the target (the other reason, of course,
is that the wider the beam as it leaves the laser, the farther away it
can focus to a tight spot, and thus the longer its range). Anyway,
with ionization, you will get bright, glowing beams.
The amount of ionization you get depends on the wavelength of light
you use, long wavelengths are more prone to producing a cascade
ionization where free electrons or ions in the air are oscillated back
and forth by the electric fields of the light hard enough and fast
enough that they knock loose more electrons. This is a real problem
with intense, pulsed, focused CO2 lasers (which operate at about 10
microns), especially since this tends to cause complete absorption of
the beam when ionization starts. Short wavelengths are more prone to
multiphoton ionization - short pulsed UV beams often glow green when
two or three photons can be simultaneosuly absorbed by an atom to
eject an electron.
Normally, weapon designers would want to avoid ionization issues
althogether. However, if you want to use self focusing pulses, this
cannot be avoided. A self focusing pulse creates a line of plasma
where the beam propagates. Interestingly, the light emission from
this plasma is mostly directed along the beam direction and also
backwards, antiparallel to the beam. People in front of or behind the
beam can see the plasma streak more clearly than people off to the
side.
In air with lots of dust or pollen, you might also get a sparkly beam
along the beam's entire path, as the bits of dust and pollen burn up.
Of course, when the beam hits the target, it will produce a bright
flash and a bang from the explosion.
So, when you are shooting someone or something with a femptosecond
laser weapon operating in the near IR, you probably will not see
anything of the beam near the laser itself or the person shooting it.
At the target, you will see a bright flash with a short streak of
light coming out of it pointing towards the laser. If the beem has
enough oomph to punch all the way through the target, you will see a
much longer streak of light pointing directly away from the laser and
possibly extending up to several hundred meters (unless it encounters
something else first). The flash is from the explosion, the streak is
the plasma generated from the air when the beam becomes self focusing.
Then, you get hot sparks and sizzling bits of flesh and gore shooting
off of the target, a sharp Bang! from the target exploding, and maybe
a bit of oily smoke and a nasty stench of charred meat.
If you are using visible beams, atmospheric scattering will make the
beam show up clearly at these intensities. The shorter your
wavelength, the more the scattering, so blue beams will appear much
brighter than red ones of the same intensity. There is the issue of
the response of the human eye to the wavelength of light as well, so a
beam far into the violet may not be very visible, since it is so close
to the UV that our eyes do not respond to it much.
When a visible light beam hits its target, there will be an incredible
bright flash of the same color as the beam. This is likely to be
enough to dazzle anyone looking at the target at the time.
On top of this, you get the same sort of effects you got with a near
IR beam, except that they are likely to be obscured by the brilliance
of the visible light scattered from the beam or diffusely reflected
from the target. The bang, flying chunky bits, and smell are all
going to be the same, though.
Hope this helps,
Luke
Johnny Tindalos
What can I say? That's got to be the best and most useful
answer I've ever recieved on Usenet! (Which is saying
something, as I've met some very helpful and resourceful
people online.)
Thank you very much for taking the trouble to respond in so
much detail - it's exactly what I needed and has had me
rethinking all the combat scenes in the novel - I think they
will now end up being much more visceral, and certainly more
vivid! All manner of cool visuals, tight situations, and
deadly ramifications have been springing to mind, and the
combatants and their gear have become that much better
defined.
(Also, all my characters are now demanding beam weapons,
even those I'd previously managed to satisfy with flechette
pistols and fractal-edged blades. And I'm not sure all of
them know better than to point one at a mirror...)
Thanks again,
Yours sincerely,
Johnny T.
Johnny Tindalos
> You might also be interested in the "special effects" of a laser
> weapon's beam.
I most certainly am! Anything that makes it possible for me to show the
reader a more realistic, vivid scene is most welcome.
I do have a few questions, but of course I will understand if you don't
feel like answering (having taken the time to give me such brilliant
replies already!)
I expect some of the questions are going to reflect the fact that I
really don't know as much physics as I'd like to (I know, typical
biologist, eh?) but here goes...
> In some cases, the patch of ionized air absorbs the rest of the beam,
> preventing the beam from propagating any further.
Other things such as beam width and wavelength being equal, would such
absorbtion be down to chance / hyper-local atmospheric conditions? Could
make for some lucky escapes if so...
Am wondering if it might be possible, given the right technology, to
somehow control/alter the air nearby to increase the chances of its
stopping a beam...I'm afraid my knowledge of physics is woefully
inadequate for this sort of thing; could one manipulate clouds of ionised
gas, somehow?
Probably better to spend more time polishing ones mirrored armour...
> This is another
> reason why you want a wide beam where it exits from the laser, and
> then focus the beam to a tight spot at your target
How wide are we talking about? Could width-of-beam issues limit the
lethality of portable laser weapons, as opposed to larger, mounted
versions? Or would this only apply at extremely long range (surface-to-
air? space-to-ground?), rather than reasonably-close infantry combat?
> (the other reason, of course,
> is that the wider the beam as it leaves the laser, the farther away it
> can focus to a tight spot, and thus the longer its range). Anyway,
> with ionization, you will get bright, glowing beams.
Would it be necessary / useful to have weapons that automatically adjust
the distance at which the beam focuses, according to the distance to the
target?
And would the bright beams produced by ionisation be enough to dazzle? To
leave an afterimage on the retina? I expect that at least, seeing one
would ruin your night vision...
> Normally, weapon designers would want to avoid ionization issues
> althogether. However, if you want to use self focusing pulses, this
> cannot be avoided. A self focusing pulse creates a line of plasma
> where the beam propagates. Interestingly, the light emission from
> this plasma is mostly directed along the beam direction and also
> backwards, antiparallel to the beam. People in front of or behind the
> beam can see the plasma streak more clearly than people off to the
> side.
Would this antiparallel-directed light be likely to pose a threat to the
user / people nearby, or would it just be a cosmetic effect? I'm guessing
that the light would be scattered rather than focussed... (Not that
"cosmetic" is really the right word anyway, especially if the targets of
the beam could see it more easily than their nearby allies...)
What would a plasma beam look like? Am guessing very bright white
light...hmmm, does what you wrote above mean that a plasma rifle would
actually be a laser weapon, set to produce plasma via self-focusing
pulses, rather than something that shot a "bolt" of electron-stripped
matter (as one usually sees in SF) ?
> In air with lots of dust or pollen, you might also get a sparkly beam
> along the beam's entire path, as the bits of dust and pollen burn up.
Very useful to know, given the environment my characters are using these
weapons in! I expect clouds of small insects might give a similar effect.
I wonder what it would look like if you used one of these weapons in the
rain? (Or shot one at a puddle of water in zero-G....danger of mirroring
in that situation, I'm guessing...)
> Of course, when the beam hits the target, it will produce a bright
> flash and a bang from the explosion.
And (I hope) a gasp from my readers!
> So, when you are shooting someone or something with a femptosecond
> laser weapon operating in the near IR, you probably will not see
> anything of the beam near the laser itself or the person shooting it.
> At the target, you will see a bright flash with a short streak of
> light coming out of it pointing towards the laser.
> If the beem has
> enough oomph to punch all the way through the target, you will see a
> much longer streak of light pointing directly away from the laser and
> possibly extending up to several hundred meters (unless it encounters
> something else first). The flash is from the explosion, the streak is
> the plasma generated from the air when the beam becomes self focusing.
> Then, you get hot sparks and sizzling bits of flesh and gore shooting
> off of the target, a sharp Bang! from the target exploding, and maybe
> a bit of oily smoke and a nasty stench of charred meat.
>
Brilliant description!
Just to make sure I've got it right: if the beam was beefy enough to
pierce the target all the way through, the long streak of light would be
in front of the target, as a result of the beam becoming self-focusing,
rather than behind it?
> If you are using visible beams, atmospheric scattering will make the
> beam show up clearly at these intensities. The shorter your
> wavelength, the more the scattering, so blue beams will appear much
> brighter than red ones of the same intensity.
I think my protagonists might well be using visible light, in order to
help with aiming - that is, if the Niven rule of "fire, then aim" (and
thus use the laser like a hose, as in _Ringworld_ and other stories)
would apply. Hmmm...mightn't that be a little hazardous to the beam-
wielder, like using tracer? And I'm not sure that one could actually fire
a laser continuously...wouldn't it be much more energy-expensive? Place
more strain on the weapon?
>
> When a visible light beam hits its target, there will be an incredible
> bright flash of the same color as the beam. This is likely to be
> enough to dazzle anyone looking at the target at the time.
Goggles for all combatants are a must, then.
> On top of this, you get the same sort of effects you got with a near
> IR beam, except that they are likely to be obscured by the brilliance
> of the visible light scattered from the beam or diffusely reflected
> from the target. The bang, flying chunky bits, and smell are all
> going to be the same, though.
Yay! Corruscating rays of ravening energy - with accurate physics too!
;-)
Thanks muchly,
Johnny T.