Personal Forcefield-Vests

[Serafina]

Newtons third law clearly dictates that a forcefield that stops a kinetic impact will be propelled into the same direction.
Which means that a small forcefield generator will be propelled into the wearer, causing damage.

But it should be possible to reduce this damage by enlarging the generator - say, as a vest or suit similar to modern body armor. This way, the impact would be distributed over the whole body.

How much damage would a modern bullet cause if its kinetic energy was dispersed over the whole torso of a human?
What damage could be expected by such an impact?
And what about other weapons, such as shotguns or futuristic hypervelocity guns?

[Captain Seafort]

How much damage would a modern bullet cause if its kinetic energy was dispersed over the whole torso of a human?

None, I expect, other than perhaps some minor bruising, any more than firing a weapon would hurt them (less, in fact, given that the impact would be spread over the whole body, rather than concentrated on the shoulder).

[NecronLord]

How much damage would a modern bullet cause if its kinetic energy was dispersed over the whole torso of a human?
What damage could be expected by such an impact?
And what about other weapons, such as shotguns or futuristic hypervelocity guns?

Negligable. At worst some bruising. Remember, that the guy firing the gun handles the recoil.

Of course, it wouldn't be long before some forcefield missile or something was developed.

[Serafina]

Hm, good point about the recoil. So basically, we can assume that every handheld gun that is not based on rocket-driven ammunition (which would generate additional KE) could not damage a target protected by such a forcefield-vest, since the shooter has to handle the recoild with a smaller part of his body?

Is there any way to determine which amount of KE and respectively weapon would be necessary to inflict damage against such a target, assuming that 100% of the KE is absorbed by the forcefield and spread equally over the whole torso?

[GrandMasterTerwynn]

Hm, good point about the recoil. So basically, we can assume that every handheld gun that is not based on rocket-driven ammunition (which would generate additional KE) could not damage a target protected by such a forcefield-vest, since the shooter has to handle the recoild with a smaller part of his body?

Absolutely correct.

Is there any way to determine which amount of KE and respectively weapon would be necessary to inflict damage against such a target, assuming that 100% of the KE is absorbed by the forcefield and spread equally over the whole torso?

A car driving into the wearer at high speed would do it. Pushing the wearer off a fifty foot high cliff would do it too. In short; any impact where even spreading the force of the impact out would break ribs and rupture internal organs would do it. What sort of weapons would carry this much energy? Explosives. Large anti-materiel rifles.

[Serafina]

How about machineguns or rapid-fire weapons in general?
With a sufficient firerate, that should put a near-constant pressure on the wearers chest, presumably hindering him - but propably not inflicting damage unless heavy projectiles are used.

How much KE is carried by a handgrenade, and how much would impact the wearer (assuming that the forcefield is body-hugging)? Remember that we are not discussing penetration of the forcefield but rather damage by KE-transfer.

Would it be possible to build a handheld weapon that could harm the wearer under these circumstances?
Either by using explosive projectiles or rocket-propelled ammunition?

[Stark]

Don't be absurd. Splinters from grenades and area hits from machineguns would kill Mr No-Limits Torso Armour just fine.

[Sarevok]

Assuming the shield only covers certain areas instead of entire body.

[Shroom Man 777]

A fifty caliber HMG would not be kinetically conducive to Vest Man.

[Sheridan]

While it's not a huge consideration, note also that any sort of projectile weapon with a barrel to direct the force spreads out the kinetic energy of the recoil over time. Not a whole lot of time, but some. That's why nitrocellulose gives better muzzle velocities than blackpowder at the same amount of recoil--nitro burns (very quickly) instead of just exploding.

A corrollary to that is the question of whether the KE shield is simply spreading the impact over the wearer's surface area or if it is also slowing down the impact over a period of time. It will be more efficient if it is the latter.

[Oni Koneko Damien]

I doubt fully automatic weapons would be effective either considering that, once again, the shooter still absorbs the recoil. Calibers large enough to require tripods or other emplacements though...

[Darmalus]

A sticky web or entangler would be effective for disabling. If you must kill them, designing the web to contract after exposure to air could suffocate or crush the person wearing the vest. High-tech bear hug of death.

Edit: I am assuming there are other protections that make "Just use frag grenades." not as effective, somehow.

[Connor MacLeod]

You can't just think in terms of KE. There's force and momentum and pressure (REember mike's shield page? The IXJac entry in the hate mail page, etc) Hell, alot of this depends on how the "force field" interacts with the bullet. Maybe it deflects the bullet rather than slowing it down, for example (a glancing hit rather than a direct hit.)

[Serafina]

Don't be absurd. Splinters from grenades and area hits from machineguns would kill Mr No-Limits Torso Armour just fine.

Did i ever say that it has no limits?
Well yes, i actually did, kinda. Of course, thats merely a mental excercise to establish a upper limit for the forcefield - "what kind of impact would harm a person despite the spread of the impact over the whole body (torso to make the question easier)".

Of course, that can easily be explained that you would need enormous force to penetrate the forcefield, at which point you would smear the wearer into paste anyway. Say, you need a batteship gun to penetrate it - but you could certainly harm the person with way less, since the momentum gets transferred.
So yes, i do assume no limits for the absorption capabilities - but it's not a falacy but a valid tool.

You can't just think in terms of KE. There's force and momentum and pressure (REember mike's shield page? The IXJac entry in the hate mail page, etc) Hell, alot of this depends on how the "force field" interacts with the bullet. Maybe it deflects the bullet rather than slowing it down, for example (a glancing hit rather than a direct hit.)

Good points.
I must admit that i am not capable enough to do all the calculations on my own, but the idea seems like a good solution for the "KE-shields would punch a hole in you due to conservation of momentum"-problem.

Would it be reasonable to assume that the forcefields spreads whatever force impacts it all over the torso (or whole body, if you want to) to get an upper limit for it's protection?

Regards
Fina

[Sarevok]

A fifty caliber HMG would not be kinetically conducive to Vest Man.

If you have magic momentum trasfer shields a 50 caliber round won't hurt you anymore than a person trying to fire a 50 caliber bullet without a tripod. You will both be feeling same amount of force from the bullet. Painful yes but not so bad that one or two shots will incapacitate you.

[CaptainChewbacca]

I have a question on 'future guns' as I'm presently having some sort of brain fart. Is there any reason that electromagnetic 'rail' guns WOULDN'T have recoil? Does the fact that its an energy field change anything in what the person firing the gun feels?

[Gilthan]

Neglecting exotics (lasers, nerve gas, flamethrowers) or large weaponry (recoilless cannons, rocket launchers), frequent projectile-stopping forcefield vests might make the standard infantry weapon include a grenade launcher. To cause a blast wave knocking someone out despite their forcefield, fire concussion grenades, which have more explosive (less metal shrapnel) than frag grenades.

Here's one illustration, said to have 2 meter effective casualty radius in open areas from its overpressure wave, more in closed areas, from just 8 ounces of TNT (let alone futuristic higher-yield explosives):

http://www.fas.org/man/dod-101/sys/land/mk3a2.htm

I have a question on 'future guns' as I'm presently having some sort of brain fart. Is there any reason that electromagnetic 'rail' guns WOULDN'T have recoil? Does the fact that its an energy field change anything in what the person firing the gun feels?

Railguns have recoil. Conservation of momentum means that, anytime a projectile gets expelled by force exerted against an object, the object experiences an equal and opposite momentum change, whether the projectile is being expelled electromagnetically, by gunpowder, by rubber bands, by any method.*

For instance, if a loosely held 10 kg projectile weapon of any kind were to fire a 10 gram bullet at 1000 m/s, that weapon would move backward at 1 m/s until deaccelerated and deliver 10 kg * m/s of impulse to the shoulderpad or whatever stopped it.

* Of course, the preceding implicit assumption of only a two-body problem isn't always valid. Recoilless rifles and rocket launchers add backwards-going exhaust gas out the rear to eliminate or reduce momentum transfer to the launcher. Actually, conventional firearms have a little more recoil than corresponds to the bullet's momentum alone, as they expel both the projectile and hot gas directed forward.

Anyway, high railgun muzzle velocity means that railguns can deliver a given amount of kinetic energy by a relatively lightweight projectile with relatively low momentum (as kinetic energy scales exponentially up with velocity squared, in contrast to the slower linear scaling of momentum).

So, for instance, a railgun with lethality equivalent to an AK-47 could have less recoil than a AK-47, able to fire relatively tiny bullets if its bullets had multiple times as high velocity. It would still have a little recoil, though.

Of course, railguns are not the best weapon against this imaginary forcefield, although, in real life, reduced recoil is an advantage.

[Sarevok]

Tiny bullets would have to travel at extremely high velocties. Probably not a good idea inside an atmosphere since bullets would disintegrate, be deflected or slow down too quickly.

[Gilthan]

Tiny bullets would have to travel at extremely high velocties. Probably not a good idea inside an atmosphere since bullets would disintegrate, be deflected or slow down too quickly.

You're partially right. Railgun bullets in atmosphere certainly can't just be arbitrarily high velocity or arbitrarily tiny to an unlimited extent. There's a tradeoff and an optimal velocity (depending on how highly range, versus penetration, ammo capacity, etc. are prioritized in a given weapon).

Yet they could plausibly be multiple times higher velocity than the average contemporary firearm and a mere fraction the bullet mass.

Optionally, switching from primarily 8 g/cm^3 steel or 11 g/cm^3 lead to rather an alloy based mostly on 19 g/cm^3 tungsten would approximately halve the volume to mass ratio.

Elongate the projectile a bit more (even though there are some limits on such before diminishing returns, like form drag versus skin friction drag), and even air drag wouldn't stop a significantly smaller caliber from still having competitive range.

Such is really much like the evolution which occurred between 0.75" relatively low-velocity musket balls of the 18th century, versus the higher velocity, smaller, and elongated M-16 bullets of today, which are approximately 0.2" (5.56mm).

Current muzzle velocities of firearms are more due to the limits of the weapon and propellant method than the limits of projectile passage through atmosphere. Even comparing different conventional firearms, an AK-47 gets a figure typically like 2300 feet per second muzzle velocity, versus some rifles reaching 4000 feet per second.

There's room for a futuristic weapon to fire bullets at significantly higher velocity, even though not so fast that they'd burn up in the atmosphere, while firing significantly smaller bullets, giving more ammo capacity. For instance, if 2 km/s was workable, a couple kilograms of projectiles for a railgun or gauss gun could correspond to a soldier carrying 2000 rounds if each round was desired to be 2 kJ energy. The reduced recoil due to the lower ratio of momentum to kinetic energy would help burst or full-auto fire be controllable.

[Sarevok]

However it must be noted railguns are not the only way to create more powerful guns. Instead of firing smaller bullets at very high velocities larger ones can be used. For instance rifles used in WW 1 and 2 had considerably longer range and power than assault rifles used by most soldiers today. They were abandoned since people can not reliably hit anything at the ranges the rifles were capable of reaching. It should be possible to return to larger and more powerful conventional weapons should armored / shielded infantry make an appearance. The biggest limit on a gun meant for infantry is weight and recoil. If a soldier can not carry a certain gun he would not be able to carry the railgun equivalent in hitting power as well.

[Steel]

What the target feels is NOT the same as what the shooter feels.

Consider a rifle with a 1m long barrel. The bullet is accelerated all the way along the barrel (not necessarily uniformly, but certainly > 0 all the way) the bullet itself is only a couple of cm long, say 5 cm. When it hits something that doesn't move out of the way it has to stop over approximately its length, as in it crumples into a flat pancake as the back end travels through the front end.

With muzzle velocity V, a barrel length L and a projectile length p and assuming constant acceleration a, we have that in the barrel

V2=2*a*L

and hitting the target it decelerates at rate d such that

V2=2*d*p,

so trivially we can see that the deceleration will be greater by a factor of L/p and we don't care about muzzle velocity for determining the relationship.

This means that in the case of a rifle with a 1m barrel and a 5cm bullet (barrel longer than most assault rifles and the bullet about 2x as long) the person being hit receives the same IMPULSE, but the FORCE is TWENTY times greater. So you would be thrown backwards the same amount but the chance of something breaking would be much higher. Bear in mind that non constant acceleration can only increase the peak force applied. Constant acceleration gives the lowest max force a body feels for achieving a given speed in a given time.

Assuming a bullet weight of 100g length 5cm, barrel length 1m and muzzle velocity of 800m/s (this is something like a .50). The shooter has a recoil force of 3.2x104kg m/s2 for 1/400 = 0.0025s, wheras the target gets a force of 6.4x105kg m/s2 for 0.000125s.

If we have that the recoil of the shooter is taken over a 100cm2 shoulder stock and the target has a vest covering the torso, then the surface area of that will be approx 50cm x 30cm and so 1500 cm2, so we get the guy in the vest has a pressure applied to his body about 1/3 greater than the shooter, but applied to the whole torso. Shooter: 3.2 MPa vs Target: 4.3 MPa.

If this is survivable for the shooter, it will be survivable for the target, however they will be really fucked by it. I think that the equivalent of firing a rifle at every point on your stomach would be pretty debilitating for a while...

[Gilthan]

However it must be noted railguns are not the only way to create more powerful guns. Instead of firing smaller bullets at very high velocities larger ones can be used. For instance rifles used in WW 1 and 2 had considerably longer range and power than assault rifles used by most soldiers today. They were abandoned since people can not reliably hit anything at the ranges the rifles were capable of reaching. It should be possible to return to larger and more powerful conventional weapons should armored / shielded infantry make an appearance.

Although we're not at the stage of R&D for electromagnetic small arms yet, part of the reason the U.S. Army is doing research currently on light gas guns and railguns firing antitank projectiles as fast as even 3 or 4 km/s is that such is more optimal for penetrating armor, better than the lower velocities obtained by conventional cannons. I forget the exact figures, but it is something like a 30mm or 40mm projectile of such velocity could defeat armor which stops 120mm conventional shells, even though the former would have only a small percentage of the mass.

Such as maybe a 5mm railgun rifle having the penetration of a giant heavy 20mm old anti-tank rifle would be an advantage against futuristic body armor, a lot more portable and having much less recoil, also with a greater ammo capacity and practical firing rate.

The biggest limit on a gun meant for infantry is weight and recoil. If a soldier can not carry a certain gun he would not be able to carry the railgun equivalent in hitting power as well.

Pretend someone said:

"The biggest limit on a gun meant for infantry is weight and recoil. If a soldier can not carry a musket powerful enough to penetrate armor, he would not be able to carry the later-century rifle equivalent in penetration."

By going to higher muzzle velocities than old muskets, we are able to make modern rifles with a penetration that would have taken a small cannon back in the 18th century, developing more advanced weapons which have a higher ratio of kinetic energy delivered to recoil, relatively less recoil.

The difference between a musket and a modern rifle is rather analogous to the difference between a modern rifle and a hypothetical future railgun or gauss gun, of still higher velocity, of still less recoil relative to the KE or penetration delivered to the target.

Of course, there are other ways to counter futuristic body armor too, like a grenade launcher firing miniature HEAT warheads. The relative bulkiness of such heavily limits ammo capacity, though.

[Sarevok]

This means that in the case of a rifle with a 1m barrel and a 5cm bullet (barrel longer than most assault rifles and the bullet about 2x as long) the person being hit receives the same IMPULSE, but the FORCE is TWENTY times greater. So you would be thrown backwards the same amount but the chance of something breaking would be much higher. Bear in mind that non constant acceleration can only increase the peak force applied. Constant acceleration gives the lowest max force a body feels for achieving a given speed in a given time.

Why so ? I thought the forcefield acts like an elastic collision between two objects. The bullet and the body can be treated like particles. Here 100 N of force applied over 2 seconds is same as 10 N applied over 5 seconds. That is the beauty of the force field protection scheme since you are not being hit at an specific point which violently accelerates and rips away.

[Steel]

This means that in the case of a rifle with a 1m barrel and a 5cm bullet (barrel longer than most assault rifles and the bullet about 2x as long) the person being hit receives the same IMPULSE, but the FORCE is TWENTY times greater. So you would be thrown backwards the same amount but the chance of something breaking would be much higher. Bear in mind that non constant acceleration can only increase the peak force applied. Constant acceleration gives the lowest max force a body feels for achieving a given speed in a given time.

Why so ? I thought the forcefield acts like an elastic collision between two objects. The bullet and the body can be treated like particles. Here 100 N of force applied over 2 seconds is same as 10 N applied over 5 seconds. That is the beauty of the force field protection scheme since you are not being hit at an specific point which violently accelerates and rips away.

Ok, so your forcefield starts to decelerate the bullet 5 seconds out. That means you've started to decelerate the 800m/s bullet when it is a good TWO THOUSAND METRES away! Bullets are moving really fast, there really is a limit to how far out you can start to act on them and the form of your forcefield up to that point. Consider a brick wall: it will be interacting with the bullet the entire time, but it isn't until the bullet is practically inside the wall that electrostatic forces (or whatever) actually act on the bullet significantly and slow it down.

Also the question was asked about a "body hugging" force field. In the case of a "typical" scifi forcefield which has a steep gradient effectively looking like a wall a body hugging one is better; consider if the field is not body hugging then I can go into a similar demonstration and show that the problem then becomes a torque issue as a glancing hit will then impart some obscene force on your finite sized generator/projector/whatever apparatus leaving you in an even worse state than before...

[dragon]

So justs means we need the force fields to act like the personal shields from Stargate which violates the third law big time . Granted we know they are able to get around conversation of momentum and other pesky things like that.

Which if you think about it most scifi verses that have personal shielding tech also has the tech to get around this to some extent or another.