Water verses air
How much higher is the power required to propel a bullet through water than air at the same velocity?
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your question has no valid answer with air we know how fast a given bullet shape will lose it's velocity so if a bullet starts out at 3000 feet per second and travels say 500 yards we know how fast it will then be going. But in water there is much more DRAG on the same bullet and it most likely will not travel even 100 feet starting at same speed maybe not even 20 feet. 0 -
Wow I haven't thought about a fluid mechanics problem in a long time.
Density of air 1.225 Kg/cubic meter Water 1000 Kg/cubic meter
Both air and water are dampening forces and bullets are initial velocity being dampened to zero.
Rockets are propelled. Wind resistance is a cubic function.
A underwater torpedo had about 600 horsepower making about 50 knots, a small slick plane with 600 hp. should do 300 knots. The J3 Cub I learned to fly in could do perhaps 70 knots on 40 hp.0 -
In other words firearms are not functionally useful underwater. Kinda like lungs. 0 -
On Mythbusters they tested firing bullets into water and found that a subsonic bullet will travel farther into the water were the high velocity rounds breaks up. 0 -
Short answer- you will not drive a bullet thru water at the same velocity as thru air. Due to the greater density and the greater drag, and the fact that water cannot BE compressed, the bullet will be destroyed. 0 -
quote:Originally posted by penguin
How much higher is the power required to propel a bullet through water than air at the same velocity?
Its not the right question.
Muzzle velocity is the same, velocity drops up much faster in water. IE "same" power will get you the same velocity when the bullet comes out of the gun.
Since drag is roughly proportionate to the square of velocity, firing your in-water bullet faster, only means it will slow down that much faster. Within a very short distance its going no faster than your ordinary bullet at the muzzle, meaning it will lose all effective velocity within only a few more yards.
More simply, in practice there is NOTHING you can do to make a conventional bullet get the same sort of trajectory underwater as the same bullet in air.
Its not entirely hopeless, though.
The Russians came up with a special modified AK that uses special bullets with supercavitating type noses, like certain torpedos use.
In short, at high velocity the special tips turn the water in contact with the tip into a gas, and then the bullet (or torpedo, etc) "floats" through this gas "envelope", significantly reducing water drag. For torpedoes this lets them get much higher velocity, and for bullets, significantly greater practical range and accuracy underwater.
Claimed effective range of these AKs is 25 meters (yup, that's it) under water. That's not much, but its still a LOT better than an ordinary rifle, which underwater is incredibly inaccurate and has a lethal range of at most a few yards.
Technology is commercially available elsewhere, but I don't know if "just anyone" can buy these.
See here for a pretty good description of the technical capabilities of this type of ammo: http://www.defensereview.com/revolutionary-dsg-supercavitating-rifle-ammo-for-shooting-underwater-into-the-water-or-out-of-the-water-multi-environment-ammunition-mea-makes-its-debut-photos-and-video/0 -
quote:Originally posted by He Dog
In other words firearms are not functionally useful underwater. Kinda like lungs.
Conventional guns still function underwater, just effective range is limited to a few feet with conventional ammo.
EG a few inches of water isn't "armor". If you're in shallow water, rifle shots from outside the water can still kill you. Point blank or contact shot underwater can still be lethal. And as mentioned above in my last post, there are specialized supercavitating rounds that can give considerably enhanced performance underwater. 25 yds effective range underwater is actually quite a bit; its effective above water handgun range, for example.
As to lungs underwater, well, they still let you use SCUBA equipment. . .so I wouldn't say they're "useless"! Filling lungs with air also assists in buoyancy, so even if you can't use the ones you've got to breathe underwater, they're still potentially useful for swimming.0 -
It would seem to make quite a difference whether or not the gun barrel fired under water is filled with water or with air. A gun submerged in water but with the barrel still filled with air should produce a muzzle velocity (right at the muzzle, not some distance in front) close to that obtained if fired in open air I would think. But if the barrel is full of water when fired, the bullet has to push all that water out ahead of it resulting in a much lower velocity for the bullet once it reaches the muzzle. Actually, I why wouldn't a water filled barrel act the same as any other barrel restriction potentially leading to major failure? 0 -
Speed of sound in air 343 m/s and in h20 1482 m/s with densities air 1.21 kg/m^3 and h20 1000 kg/m^3. If the bullet holds together and depending on the its shape "back of the envelop calculation" using work energy theorem gives less than 10 meters. Checked if there was anything out there on the internet to support this range and found Soviet frogman had a underwater pistol lethal to 17 meters greater than spear gun range of 3 - 4 meters - so beware of pistol packing frogmen hahahaha. 0 -
Alas, water can be compressed. It can't be compressed very much, but it can be compressed.
quote:Originally posted by 11b6r
Short answer- you will not drive a bullet thru water at the same velocity as thru air. Due to the greater density and the greater drag, and the fact that water cannot BE compressed, the bullet will be destroyed.0
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