Clarification on the kinetic energy

[gigabytelord]

Hey guys I've had several arguments with a friend concerning the effects of kinetic energy, and what it would do to a man sized suit of armor.
I don't really know how to explain it properly so I'll describe it instead.

The argument goes like this, we were talking about the armor used in a particular universe when the subject of getting hit by really big kinetic weapons came up, this setting is rifts earth rpg, in rifts earth the standard body armor type is MDC, or 'mega damage' where as the average human is 25 to 35 SDC, or 'standard damage', I'd go into this more, but it's really pointless to the argument.

Basically I'm of the belief that no matter how much MDC your personal body armor is, if you're wearing man sized armor and get hit by a tank round, it may not puncture the armor but the kinetic energy transfer alone will at least knock you on your ass, if not straight up kill you, where as getting hit a particularly powerful kinetic based light weapon will knock you around but may not kill you.

However the bigger and heavier the armor you're wearing is(mobility may be an issue, buts its not important to the argument), the more kinetic energy it will be able to absorb without killing the occupant, you may be knocked around a bit, but you'll be fine.

I know this whole thing is kind of dumb but I find it a little hard to believe that Mr. Wolf, who is wearing MDC body armor can run out in front of a tank, take a direct hit and then get right back up and keep running! it makes no sense... I always though that you may be able to successfully stop a projectile but stopping the kinetic energy it transfers into your armor, and therefore you, is not so easy, and can be lethal by its self.

In other words, human sized body simple can't do that without some serious hand wavium, at least that's what I was always told, if I'm wrong let me know, and a nice link or two to read while I'm at work today would be nice as well.

[Imperial528]

Well, the first problem I can think of is that you have to find a way to either distribute the energy across a surface (gonna be hard to do with a tank round) or a distance (which means bigger suit, or some sort of magic shielding). Failing those, you can try an active defense system (like explosive reactive armor, although that can cause the same problems as the shell in the first place) or some way of bleeding off the energy into a part of the armor that then moves away, due to conservation of momentum. And with that you'd need something that either relies on being hit in a certain way, since if you put this stuff all around the entire suit you run into bulk issues.

Essentially, the only way you're going to solve this realistically is with padding, but that only goes so far. You could make the suit able to lock itself into the ground and lock up all of the joints, so that there is no actual movement, or at least, not more than the padding can handle. Good luck doing that in an instant's notice, though. Oh, and this will increase the strength requirement of the materials, too, since now they have to take the full force of the tank round, with none of the luxury of bleeding off the round's energy into motion.

[Purple]

Do that and your armor will just slide along the ground or be thrown into the air.

[Xess]

Think of it like modern day body armor. The armor stops the bullet from penetrating but you're still left with a nasty bruise because the bullet's momentum hasn't magically gone away. The super armor act the same, stop the tank round from penetrating but because it has much, much more momentum instead of a bruise you have a shattered ribcage and burst organs.

[Simon_Jester]

Is the question whether a real life tank could hurt through the armor, or whether tanks in the setting could do so?

In either case, it's a threat. Things can get hurt by a weapon that doesn't pierce armor. Spalling and shock come to mind. Heck, there are types of tank gun ammo that are designed to NOT penetrate armor, and just blast a bunch of shrapnel off the inside to destroy everything in the interior of the target...

All bets are off, though, if the 'suit of armor' is so big and heavy that it provides the same level of protection as a tank. Say, a very squat, slab-sided 'mecha,' or a one-man tank. This would weigh many tons, though.

[Terralthra]

You're more or less correct. Kinetic energy of sufficient magnitude will knock the armor-wearer on their ass or flying, regardless of the MDC properties of the armor itself. This is trivial to demonstrate with math:

Assuming an inelastic collision (unlikely, but let's go with it for argument's sake).

Let's say an average-sized man, around 85 kilograms. Body armor adds another 5 kg, for 90 kg total. An [url=http://the_kitsune.tripod.com/Rifts-Earth-Vehicles/GAW_M1A2_Abrams.htm]arbitrary RIFTS tank[/url] fires a 120mm tank shell, weighing 18 kg with a nominal speed of 1,679 m/sec.

Ke = 1/2 mv^2.
Momentum = mv.

Momentum is conserved: m1v1 + m2v2 = (m1+m2)v(combined)

m(shell)*v(shell) + m(man)*v(man) = (m(shell) + m(man))*v(combined)

18 * 1679 + 85 * 0 = momentum afterward (let's assume he's standing still)

So, the momentum of the colliding entities combined is 30222 kg*m / sec

Divide this back out by the mass of the combined entities to arrive at the speed:
30222 / 108 (90 + 18) = 279 m/sec

Simple unit conversion leaves your dude in body armor hugging a tank shell at ~960 km/hour (625 miles per hour). Whatever eventually stops him is going to leave a mark somewhere, yeah. Even with shock-absorbing layers in the interior padding or what have you, humans don't go from 0 to 1000 km/hr and back to 0 without serious harm.

[Imperial528]

Do that and your armor will just slide along the ground or be thrown into the air.

Note that I said "Lock into the ground" which really isn't the sort of thing you could do in a minute (Or even a half hour), let alone the milliseconds you have between seeing a tank fire at you and getting hit.

Well, if your armor can do that in a minute, you've got bigger problems than a tank. Or you're just an idiot who is attached to a gigantic pile driver.

[Baffalo]

Simple unit conversion leaves your dude in body armor hugging a tank shell at ~960 km/hour (625 miles per hour).

Given the speed's we're talking, it will take less than a tenth of a second to overcome all compaction, stretching, deformation, etc. that will occur when the shell strikes the armor. Even if the person inside is alive, you're talking about an acceleration from 0 to 266.66 m/s in .1 seconds (at best). So, a = dv/dt = (266.66 m/s - 0 m/s) / .1 s = 2666.6 m/s2 = 272g.

If Wikipedia is accurate, a shock greater than 100g on the human body is about the highest survivable shock acceleration from explosions, so you're dealing nearly 3 times that force. Assuming the suit survives, you'd be better off attaching a spigot and just pouring the guy out than trying to pull him out.

[gigabytelord]

You're more or less correct. Kinetic energy of sufficient magnitude will knock the armor-wearer on their ass or flying, regardless of the MDC properties of the armor itself. This is trivial to demonstrate with math:

Assuming an inelastic collision (unlikely, but let's go with it for argument's sake).

Let's say an average-sized man, around 85 kilograms. Body armor adds another 5 kg, for 90 kg total. An [url=http://the_kitsune.tripod.com/Rifts-Earth-Vehicles/GAW_M1A2_Abrams.htm]arbitrary RIFTS tank[/url] fires a 120mm tank shell, weighing 18 kg with a nominal speed of 1,679 m/sec.

Ke = 1/2 mv^2.
Momentum = mv.

Momentum is conserved: m1v1 + m2v2 = (m1+m2)v(combined)

m(shell)*v(shell) + m(man)*v(man) = (m(shell) + m(man))*v(combined)

18 * 1679 + 85 * 0 = momentum afterward (let's assume he's standing still)

So, the momentum of the colliding entities combined is 30222 kg*m / sec

Divide this back out by the mass of the combined entities to arrive at the speed:
30222 / 108 (90 + 18) = 279 m/sec

Simple unit conversion leaves your dude in body armor hugging a tank shell at ~960 km/hour (625 miles per hour). Whatever eventually stops him is going to leave a mark somewhere, yeah. Even with shock-absorbing layers in the interior padding or what have you, humans don't go from 0 to 1000 km/hr and back to 0 without serious harm.

I love you, no seriously.

Also just to note, the "Glitter Boy" (I know, I know XD) has a rail gun that fires a round at Mach 7 or 7,808 fps if I remember correctly, something I don't think anyone in man sized body armor could get back up from.

[Batman]

Actually humans go from zero to near lightspeed and back in a matter of minutes pretty regularly (in a matter of seconds depending on the universe) but that usually involves starships. A Perryverse pre-Hyperimpedanz SERUN could probably do the trick (I'd provide a link but it inevitably being in german likely would make that pointless), as could one of the upper end Iron Man suits, both thanks to technobabble, but ordinary limited to the laws of physics body armour?
Unless the stuff's really resilient and really expensive you might as well skip the effort of siphoning the remains of the guy wearing it at the time out of it and bury them together.

[gigabytelord]

Actually humans go from zero to near lightspeed and back in a matter of minutes pretty regularly (in a matter of seconds depending on the universe) but that usually involves starships. A Perryverse pre-Hyperimpedanz SERUN could probably do the trick (I'd provide a link but it inevitably being in german likely would make that pointless), as could one of the upper end Iron Man suits, both thanks to technobabble, but ordinary limited to the laws of physics body armour?
Unless the stuff's really resilient and really expensive you might as well skip the effort of siphoning the remains of the guy wearing it at the time out of it and bury them together.

Well for all intents and proposes most rifts body armor is fairly normal, if that word can be used here, very durable, but relativity normal (e.g. no technobable).

[Simon_Jester]

Then yeah, the only thing that helps is to bulk up the armor so much that momentum transfer isn't a problem. The tank that fired the gun presumably doesn't go flipping over backwards from recoil. So a suit of armor that weighs a large fraction of the tank's mass, and cannot be penetrated by the main gun round, offers some chance of survival.

But most settings don't bother with twenty-ton suits of power armor, and even if you had one there'd be huge drawbacks. It wouldn't be 'armor,' it'd be a squat, vaguely bipedal tank.

[Terralthra]

Then yeah, the only thing that helps is to bulk up the armor so much that momentum transfer isn't a problem. The tank that fired the gun presumably doesn't go flipping over backwards from recoil. So a suit of armor that weighs a large fraction of the tank's mass, and cannot be penetrated by the main gun round, offers some chance of survival.

But most settings don't bother with twenty-ton suits of power armor, and even if you had one there'd be huge drawbacks. It wouldn't be 'armor,' it'd be a squat, vaguely bipedal tank.

Worth noting that the Glitter Boy, the aforementioned power armor variant with a Mach 7 projectile railgun, weighs 2,400 lbs (1,090 kg) per the RIFTS sourcebook, has "laser-tipped" drills in its feet that instantly drill down into the ground below the suit and anchor it in place, else it would be "tossed backwards 30 feet" every time it fired. The immediate corollary to this is that every time it fires, its feet lock in place and the rest of the Glitter Boy smashes into the ground behind it with triple-digit N*m of torque, with either knee or ankle joints (or a combination of the two) bending due to immense jerk.

Oh, also, the railgun is over-the-shoulder-mounted, meaning that the force of the recoil doesn't even act through its center of mass. Hello, broken spine!

[Lord Revan]

the thing about "bulking up" armor is that unless there's anti-grav tech or all the fighting happens in very sturdy materials, you'll end up with problem on ground pressuare (there's a reason why most heavy machinery is tracked).

[gigabytelord]

Oh, also, the railgun is over-the-shoulder-mounted, meaning that the force of the recoil doesn't even act through its center of mass. Hello, broken spine!

Well to be fair, if I remember correctly it says somewhere in there that the suit is hardened or stabilized so it doesn't do stuff like just twisting around at the waist like a bottle cap when it fires, however what "hardened" or "stabilized" means in this sense I'm not sure.

[Baffalo]

If the armor is being thrown backwards due to the gun, then in order to ensure it doesn't get twisted like a screw-cap is to configure the armor to withstand the torque (because now there's a significant moment of inertia centered around the armor's own center of mass) and to withstand being propelled backwards. That's extra work going into the design just to make it somewhat bipedal, and would make much more sense if placed on a tracked platform like most tanks already are. I'm not really seeing the appeal in having a bipedal firing platform other than it can withstand tipping over.

[Terralthra]

the thing about "bulking up" armor is that unless there's anti-grav tech or all the fighting happens in very sturdy materials, you'll end up with problem on ground pressuare (there's a reason why most heavy machinery is tracked).

I'm wary of the "ground pressure" argument, because it's often somewhat overblown. While it is one important measure of mobility for vehicles, don't take it as the only one. A human female wearing narrow high heels exerts a ground pressure of ~13 mPa, after all, and I see them walking across grass or moderately soggy mud all the time.

[Lord Revan]

the thing about "bulking up" armor is that unless there's anti-grav tech or all the fighting happens in very sturdy materials, you'll end up with problem on ground pressuare (there's a reason why most heavy machinery is tracked).

I'm wary of the "ground pressure" argument, because it's often somewhat overblown. While it is one important measure of mobility for vehicles, don't take it as the only one. A human female wearing narrow high heels exerts a ground pressure of ~13 mPa, after all, and I see them walking across grass or moderately soggy mud all the time.

sure it's not the only one, but we shouldn't dismiss it as totally irrelevant either.

[Simon_Jester]

the thing about "bulking up" armor is that unless there's anti-grav tech or all the fighting happens in very sturdy materials, you'll end up with problem on ground pressuare (there's a reason why most heavy machinery is tracked).

Or very big, clompy, flexible feet. Which isn't really a practical solution; I'm just trying to come up with something that would sort of work. Armor that could 'tank' such large-caliber gun rounds without being bowled over and ruining the wearer's day (and torso) would probably evolve into a very dense, slab-sided one-man tankette. It would still have high ground pressure, but would at least be vaguely workable.