> The rifle’s effective firing range underwater is about 25 meters at a depth of 30 meters and 18 meters at a depth of 20 meters, according to Russia Today.
Earlier in the article they associate an increase in depth with a lessened range, but with this one it's the reverse? Is that a typo or a nuance of the technology?
I'm only guessing, but it's probably a nuance of the technology. The earlier weapons used cavitation created by the dart's blunt tip to stabilize it and give it range below the surface. The last gun uses a gas bubble to create a supercavitation [1] effect, which based on my quick skimming of the linked Wiki article, seems to be more effective at greater depth.
Anyone with a more thorough (read: any) understanding of fluid dynamics is encouraged to correct or expand this.
Ok, let's try: boiling and (super)cavitation are two sides of the same thing. A liquid will turn to vapor (boil) when a physical property of the liquid called "vapor pressure" becomes greater than the actual pressure.
When you normally boil water, you increase the temperature of the water, which increases the water property "vapor pressure" until it becomes equal to atmospheric pressure at 100 C. Or if you are at Mt. Everest, where the pressure is lower, water boils at 71 C.
When cavitation occurs, the opposite is done: you lower pressure until you reach the vapor pressure of the liquid at the current temperature.
In supercavitation, you create a shockwave in front of the bullet/torpedo/propeller which raises and then lowers pressure such that the water boils/cavitates. Why is this good?
Because it lets you put most of the velocity difference between the bullet and the water inside a gas layer. This gas layer has much less resistance to velocity differences than water. Running in water is much harder than running in air, right? So far so good.
So what happens when a supercavitating bullet is fired at greater depth? The pressure is higher, so it's harder to create cavitation, because you have to lower the pressure more to get to the vapor pressure.
Why is that good? Doesn't it reduce the nice gas layer? Yes, it does. My guess is that it also reduces the length of the cavitation bubble behind the bullet, and that reduction in drag is bigger than the increase in drag from a thinner gas layer.
In the older guns, the trailing gas bubble is much shorter because of the projectile shape, and the reduced bubble length at greater depth causes part of the projectile to be outside the bubble, increasing resistance.
You could probably put some decent numerical estimates up by spending an afternoon with White's Viscous Fluid Flow and a steam table.
But the density of water does not vary much by depth (like 1% per 2 kilometers depth). Similar deal for temperature, density lowers by 1% per 20 degrees Celsius (and given that the temp of ocean water stays very much between like >0 and <<35, basically no difference in density.
In fact some of these properties of density of water are what make it such a weird substance, and enable the use of submarines, etc.
> The rifle’s effective firing range underwater is about 25 meters at a depth of 30 meters and 18 meters at a depth of 20 meters, according to Russia Today.
Earlier in the article they associate an increase in depth with a lessened range, but with this one it's the reverse? Is that a typo or a nuance of the technology?