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scheav t1_jahunkm wrote

Reply to comment by MassProductionRagnar in Does the spinning of a bullet have any effect on its drag? by Eastern-Ability8411

“Filling the empty void with gases” IS pushing the shell. It might not have a net positive acceleration, but it’s like pushing a car that’s out of gas - you’re still pushing even though it’s not accelerating.

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ramk13 t1_jahw8on wrote

I think their point was that the purpose wasn't propulsion, but a reduction in drag. If you look at the momentum imparted by the gas released it will be much smaller than the decrease in momentum caused by drag if the gas weren't there.

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MassProductionRagnar t1_jahwc4l wrote

>IS pushing the shell

By a negliglible amount. There are actual range-boosted shells with rocket motors which gain significant thrust from the shells actually accelerated in flight.

Base-bleed is not that and for simplicities sake these shells aren't said to be propelled by it, they just have less drag.

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haplo_and_dogs t1_jai6e2x wrote

Vacuum doesn't exert a force. The gases released do.

You can look at this from a force diagram. Having a vacuum behind the shell just means there isn't gas pushing on the shell. Adding a gas does push on the shell.

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Phoenixdive t1_jaje6is wrote

Out of curiosity, in your force diagram, what happens to the matter around a vacuum? What forces are created by the pressure differential? If you found a way to decrease the pressure differential, what would happen to those forces? Which one would exert a greater net gain in speed, the equalization of the pressure differential or the ejection of mass to fill said void in this particular scenario?

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haplo_and_dogs t1_jajjp4n wrote

>what happens to the matter around a vacuum?

Matter in a vacuum has no forces exerted on it.

>What forces are created by the pressure differential?

Pressure is just Force / Area.

Pressure ( in this example ) is the result of the summation of forces of gases bouncing off of a surface. There is very high pressure at the front of the shell, and very low pressure at the rear of a shell when fired.

>If you found a way to decrease the pressure differential, what would happen to those forces?

You can reduce drag if you change the shape of the shell. If you have more pressure at the rear of the shell the net force on the shell is reduced. You can do this with a boat-tail shell.

>Which one would exert a greater net gain in speed, the equalization of the pressure differential or the ejection of mass to fill said void in this particular scenario?

The gas sent out of the rear of the shell does not act like a rocket. In a bleed gas shell the gas is going nearly the same speed as the shell, and is trapped there for multiple interactions. The trapping of this allows the gas to exert a pressure onto the shell. If your force diagram model has this gas as part of the shell you have a new drag coeffienct, if you model the shell alone then you have this gas exerting a force ( or a pressure over an area ) on to the shell.

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scheav t1_jaiaxfq wrote

I didn’t say they are propelled by it, I said they are pushed by it. It exerts a force, which reduces its acceleration due to drag. This is called pushing.

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