Personal Forcefield-Vests

[Starglider]

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.

That isn't too implausible, for a normal infantry situation. If you already have the magic-tech to apply a force to an incoming bullet, sufficient to make it stop in mid air, you can probably apply an opposing force to the ground the person is standing on, such that the net force on the generator is minimal. Some cleverness may be required to avoid creating an inconvenient torque, but really avoiding the knockback problem is a minor issue compared to the difficultly of making any kind of 'force field' in the first place.

[Sea Skimmer]

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.

Actually the impulse for the shooter will be much higher because a very substantial portion of the recoil from a firearm comes from all the propellant gas being blasted out the muzzle. That’s why muzzle breaks work to reduce recoil without any loss of projectile energy; they divert the gas flow into a more favorable direction. Your point however remains mostly true.

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.

The main reason why they want EM guns is because it would reduce recoil and remove the need to transport and protect gun propellant, which is a much greater hazard then the explosives inside of shells or bombs. Its propellant that makes British battle cruisers splint in half, and T-72 tanks engage the turret ejection system and burn like bonfires.

Super high velocities do not really work for armor penetration, because the projectile will likely shatter on impact. We hit the limit for steel in that respect back in WW2 and now we’ve basically hit it with tungsten and DU ammo too. The latest US 120mm DU sabot ammunition has actually been getting slower in ordered to use a heavier projectile with more mechanical strength to better resist explosive reactive and internal deformation based armor systems. Velocities over 2,000m/s are thus unlikely to be useful for this purpose, though a super fast projectile might have other applications like air defense.

[ThomasP]

That isn't too implausible, for a normal infantry situation. If you already have the magic-tech to apply a force to an incoming bullet, sufficient to make it stop in mid air, you can probably apply an opposing force to the ground the person is standing on, such that the net force on the generator is minimal. Some cleverness may be required to avoid creating an inconvenient torque, but really avoiding the knockback problem is a minor issue compared to the difficultly of making any kind of 'force field' in the first place.

I was just about to make a post asking about this possibility.

If you've already got the force-field, what's to prevent having some kind of counter-impact system to deliver momentum into the surroundings? Say you take the car from another example and drive it into your force-field wearer - could the force of that impact be distributed through air, rock, etc, by a counter-field, instead of into the target?

Is this reasonable, or is there some CoM issue I'm missing?

[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.

Actually the impulse for the shooter will be much higher because a very substantial portion of the recoil from a firearm comes from all the propellant gas being blasted out the muzzle. That’s why muzzle breaks work to reduce recoil without any loss of projectile energy; they divert the gas flow into a more favorable direction. Your point however remains mostly true.

Thats not something I'm really able to estimate too reliably. If we take the mass of the propellant into account and assume that the mean gas velocity component along the barrel is approximately the same as muzzle velocity then you can get an estimate of how much recoil that will contribute, could double the force for a normal load of propellant massing about the same as the projectile.

The initial point is certainly valid for the railgun case though!

If you've already got the force-field, what's to prevent having some kind of counter-impact system to deliver momentum into the surroundings? Say you take the car from another example and drive it into your force-field wearer - could the force of that impact be distributed through air, rock, etc, by a counter-field, instead of into the target?

Is this reasonable, or is there some CoM issue I'm missing?

There isn't a CoM problem as the fact that the impact force was being matched would mean that the momentum was being transferred to other objects. You would run into a problem with something being crushed however. You would really hope that you could project through your body, because having another generator on your back pushing the other way on you back would make stuff much worse for you as it would then be as if you were being rammed by a car on each side. If the single generator can do the job then it is going to be the one getting crushed at a certain stage (depending on how it works).

[Starglider]

You would really hope that you could project through your body, because having another generator on your back pushing the other way on you back would make stuff much worse for you as it would then be as if you were being rammed by a car on each side.

That just means you need a rigid torso section, no big deal. In fact all you'd need is a solid metal hoop going around your chest, with a generator mounted on each side. Providing 360 degree protection would complicate the situation but in should still be less of a mobility hinderance than conventional body armor - unless the generators and/or power packs are very heavy.

[Sheridan]

It depends on how the force is transferred; if it is being simply spread over the wearer's body by the field, then, yeah, Joe Soldier is gonna be chunky salsa if the impact carries enough force.

If it is all being absorbed somehow by the field generator, then the generator is either going to have to transfer that source to another object or objects (probably crushing it/them in the process, if the impact is great enough), or it is going to have to transform the energy from kinetic energy to some other form--such as heat (not as seemingly farfetched as having a force field in the first place, in my opinion). In which case, Joe had better be wearing his asbestos underwear.

[Steel]

It depends on how the force is transferred; if it is being simply spread over the wearer's body by the field, then, yeah, Joe Soldier is gonna be chunky salsa if the impact carries enough force.

If it is all being absorbed somehow by the field generator, then the generator is either going to have to transfer that source to another object or objects (probably crushing it/them in the process, if the impact is great enough), or it is going to have to transform the energy from kinetic energy to some other form--such as heat (not as seemingly farfetched as having a force field in the first place, in my opinion). In which case, Joe had better be wearing his asbestos underwear.

How much physics do you actually know? Did you read anny part of the thread? People have been going on about the main problem being the momentum for about the last 2 pages, and actually quantifying these effects.

[Serafina]

It depends on how the force is transferred; if it is being simply spread over the wearer's body by the field, then, yeah, Joe Soldier is gonna be chunky salsa if the impact carries enough force.

If it is all being absorbed somehow by the field generator, then the generator is either going to have to transfer that source to another object or objects (probably crushing it/them in the process, if the impact is great enough), or it is going to have to transform the energy from kinetic energy to some other form--such as heat (not as seemingly farfetched as having a force field in the first place, in my opinion). In which case, Joe had better be wearing his asbestos underwear.

The proposed concept is that the forcefield stops the impactor dead.
But the Momentum hast to be transferred somewhere - it can not simply disappear.

It is generally assumed that the Momentum would be transferred to the forcefield-generator.
A small generator would gain significant speed and may hurt its wearer - the smaller, the worse it gets.
That's why i proposed a large generator - it would gain less speed, and the force it would apply to the wearers body would be spread over a wider area.

For the sake of discussion we are assuming perfect stopping power from the forcefield and an equal spread over the generator.
That is somewhat arbitrary, but usefull to get an upper limit.


Another question:
What would be the differences in projectile size, and how would the forcefield react to explosive shockwaves?
How would shrapnel affect the whole thing?

[Steel]

This setup would provide no real protection against a shockwave. If the pressure alone was going to fuck you the shield isn't going to spread it any more so you're buggered. Shrapnel it would do better against as I believe a hand grenade throws out many small bits that penetrate, so it like the bullet case. Artillery shells there are bits big enough to ruin your day though.

[Serafina]

This setup would provide no real protection against a shockwave. If the pressure alone was going to fuck you the shield isn't going to spread it any more so you're buggered. Shrapnel it would do better against as I believe a hand grenade throws out many small bits that penetrate, so it like the bullet case. Artillery shells there are bits big enough to ruin your day though.

Does a normal frag-grenade produce a strong enough shockwave to kill you (since the shrapnel is stopped)?
What would the effects be?

That artillery or tanks would kill you is pretty obvious.

[Steel]

This setup would provide no real protection against a shockwave. If the pressure alone was going to fuck you the shield isn't going to spread it any more so you're buggered. Shrapnel it would do better against as I believe a hand grenade throws out many small bits that penetrate, so it like the bullet case. Artillery shells there are bits big enough to ruin your day though.

Does a normal frag-grenade produce a strong enough shockwave to kill you (since the shrapnel is stopped)?
What would the effects be?

That artillery or tanks would kill you is pretty obvious.

The hand grenade is going to do nothing at all apart from the (significant) effect of the bits of metal flying about.

I beleive the drill if you cant get away from a grenade is to dive face down on the floor so that you are pointing away from the grenade and you take the hit to the soles of your boots, (and in that situation you'll obviously present a much smaller cross section and so get hit by less stuff) but apparently that makes you pretty survivable, so the blast produced by a grenade etc is pretty small.

I think you need to get up to those massive bojmber dropped thermobaric thingies that pop peoples lungs outside before a pressure wave becomes significant.

[Sea Skimmer]

Actually the impulse for the shooter will be much higher because a very substantial portion of the recoil from a firearm comes from all the propellant gas being blasted out the muzzle. That’s why muzzle breaks work to reduce recoil without any loss of projectile energy; they divert the gas flow into a more favorable direction. Your point however remains mostly true.

Well if the need to do such math comes up again, this calculator will tell you how much recoil you are getting from a given weight of powder. The same site also has one that will do muzzle energy and momentum
http://www.handloads.com/calc/recoil.asp

I like this one for muzzle energy more though, as it has a bunch of calibers preprogrammed to display and gives sectional density if you input the bore diameter.
http://billstclair.com/energy.html

The initial point is certainly valid for the railgun case though!

Yes and no. I say this because while it wouldn’t matter on a typical sci fi railgun, even a semi realistic rail gun is almost certainly going to use either a discarding sabot or separate armature that pushes the projectile (this controls bore erosion which is currently still a insoluble problem, and it makes ammo design absurdly easier as the ammo doesn’t need to conduct the current), so you’d still generally be in a situation in which the gun has to fling out more mass out of the barrel then will ever hit the target. The extra mass is likely to be proportionally less then that of the powder charge for a bullet, but it’s still present.

With a coilgun it’d be much easier to design ammo that has no ‘extras’ like this as interior of the barrel does not conduct electricity, but a sabot is still not unlikely.

[Sea Skimmer]

The hand grenade is going to do nothing at all apart from the (significant) effect of the bits of metal flying about.

Depends on the grenade and the situation, in a confined area that blast can be non trivial. Meanwhile concussion grenades exist which are specifically blast weapons. The idea is to give the user a grenade he can throw without needing cover from the fragments (they used to separate frag grenades from concussion grenades under the terms defensive and offensive, no one really does anymore). This is useful if you are attacking the enemy across open ground, it’s also very useful for fighting inside buildings with only thin walls fragmentation would fly through.

Concussion grenades contain much more explosives, usually twice or more compared to a fragmentation grenade, and use a casing made of plastic or cardboard instead of metal.

I beleive the drill if you cant get away from a grenade is to dive face down on the floor so that you are pointing away from the grenade and you take the hit to the soles of your boots, (and in that situation you'll obviously present a much smaller cross section and so get hit by less stuff) but apparently that makes you pretty survivable, so the blast produced by a grenade etc is pretty small.

Yeah that’d be the drill. Also when you dig a fighting position like a foxhole, one of the things you do is dig a grenade sump, which is basically a smaller deep hole positioned in such a way that you can easily kick an enemy grenade down it, and then jump clear of the opening. The sump contains the fragments and muffles the blast, this works all the better if you can also fill the sump with water.

One thing to keep in mind also is that fragmentation from a exploding shell or grenade is not truly omni directional nor are fragments uniform in size. Designers try to make it so by internally scoring the grenade casing, but the end reality is the casing will tend to shatter in certain directions first, and that can mean a person close by at a favorable angle survives while someone much further away may be killed. Indeed frag grenades are normally said to have about a 15 meter wounding radius, 5 meter kill radius, but large fragments like an end piece can go more then 200 meters and retain lethal force. Some really advanced warheads (mainly for anti aircraft missiles) actually exploit this natural desire for directionality from explosions by using multiple detonators to allow the warhead to actually aim its explosion towards the target. Basically it’s a kind of shaped charge.

Artillery shells are particularly prone to directional fragmentation because of the initial velocity of the shell, and because of the shape of the shell isn’t uniform. This chart shows the kind of patterns you would get from a typical solid casing (no internal scoring) shell with ground and air bursts. Notice how fragments go way out to the sides, but not very much forward because the nose of the shell is very strong and directs the blast away from itself.

I think you need to get up to those massive bojmber dropped thermobaric thingies that pop peoples lungs outside before a pressure wave becomes significant.

It’s really a matter of details, endless ones unfortunately (it’d really be helping wounded guys from Iraq if we understood this better). Usually a normal frag grenade isn’t a serious threat from blast, but it can be. Blast injuries are pretty erratic things. In WW2 many frontline reports of enemy chemical weapons use were traced to people being killed by blast overpressure when it didn’t really make sense. For example in one case the entire crew of a Sherman tank was killed by the blast of a nearby 150mm shell air burst entering an open hatch, something that did not normally happen. It was suspected that the commanders open hatch may have served to deflect the blast into the tank at exactly the perfect angle for max effect

In another instance a shell passed through the front armor of a Sherman, right between the hull gunner and the driver. Neither man was hit by fragments, but the driver was killed by the overpressure of the shell passing by at supersonic speed, while the gunner an equal distance was uninjured. No explanation on that one.

In general though any high explosion detonation is capable of creating lethal overpressure and vibration (the two are linked, but sometimes only vibration it really present), if you are close enough. It’s just with most weapons if you are close enough, then the fragmentation or flying debris has already killed you a split second earlier.

They now have thrown thermobaric grenades BTW, as well as ones for small rocket launchers. However keep this in mind. The peak overpressures for thermobaric/FAE weapons are much lower then those of conventional explosives. This makes a thermobaric weapon bad for demolishing for example a big chunk of reinforced concrete. However a thermobaric weapon creates a much wider area of low-medium strength overpressure which will still kill people and destroy typical civilian buildings. This is because the fuel disperses to mix with the air before it detonates, creating a wider but less dense initial point for the blast. This also makes thermobaric weapons unreliable, though small ones are pretty good. Big ones, not so much with dud rates as high as 70%. And after all that typing I need another beer and some sleep. Explosions are fun, too bad we use them to tear people to pieces so much instead of the god given job of annihilating tree stumps and building railroads

[Winston Blake]

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.

There are two points I can see here which would effectively increase the projectile length p. First is that body armour is not infinitely stiff and 'dents' to a certain depth when hit - Google tells me that the NIJ blunt-trauma standard is 44mm depth. Even if the forcefield is skintight, it could be designed to 'give' like modern armour does. Further, the deformation of the bullet will increase the deceleration time; the time taken for a bullet to traverse its own length is clearly a lower limit for the duration of the impact, and thus gives an upper limit for the force applied. So the target damage may not be as bad as your previous calcs suggest.

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.

1. 1m is as long as an entire M16 from butt to muzzle. A full-size rifle barrel will be about 20 in / 508mm long.
2. With a mushrooming bullet (x2 - I admit this is a guess), and a 44mm 'dent depth' (+44m), the effective bullet length could be more like 14.4cm.

These modifications give a target:firer force ratio of merely 3.5x, vs 20x.

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.

With my modifications, let's consider a Barrett M82 .50BMG rifle - barrel length: 508mm, bullet mass: ~50g, bullet length: ~55mm, v0: 835 m/s. Assuming constant acceleration, the lower limit for the recoil force is 34 kN (for 1220 us), similar to your 32 kN. Target force: 226 kN (for 184 us), different to your 640kN.

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.

With my numbers, it's: Shooter: 3.4 MPa vs Target: 1.5 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...

I don't think we can say it's equivalent just based on pressure. I'm sure the duration matters, but to be honest I don't know what parameter to use to compare 'survivableness'. All I can say is that I think the target's torso will feel much less pressure for a much briefer duration than the shooter's shoulder. Then of course, in the end, our hypothetical target just survived a .50 cal shot to the gut at point blank range, so I think we can say that such a forcefield offers fairly decent protection in exchange for a moderate level of discomfort.