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### Questions about pressure and flow rate

Posted: **Tue Nov 22, 2022 4:36 am**

by **yyt**

Assuming that there are two combustion chambers A and B, the volume of A is twice that of B, and the initial pressure of B is twice that of A, then theoretically, A and B should use the same amount of fuel, so their calorific values should be the same. Then the question comes, which one, A and B, can provide a higher muzzle velocity on the premise of the same barrel and caliber?

### Re: Questions about pressure and flow rate

Posted: **Tue Nov 22, 2022 4:41 am**

by **yyt**

To put it another way: If your cannon does not inject additional fuel and air, but only compresses the mixture at normal atmospheric pressure, will its performance change?

### Re: Questions about pressure and flow rate

Posted: **Tue Nov 22, 2022 8:35 pm**

by **hectmarr**

yyt wrote: ↑Tue Nov 22, 2022 4:36 am

Assuming that there are two combustion chambers A and B, the volume of A is twice that of B, and the initial pressure of B is twice that of A, then theoretically, A and B should use the same amount of fuel, so their calorific values should be the same. Then the question comes, which one, A and B, can provide a higher muzzle velocity on the premise of the same barrel and caliber?

In chamber B, the volume is half, for this reason, the pressure will be higher, and the speed of the projectile will be higher. I imagined a 100 cm3 motorcycle engine that has no compression, compared to another engine with the same volumetric ratio, 100 ml, but with a usual compression ratio, say 7 to 1. In the first case the motorcycle will not even move, but in the second case, it will roll perfectly. Is the same.

### Re: Questions about pressure and flow rate

Posted: **Tue Nov 22, 2022 8:37 pm**

by **jackssmirkingrevenge**

yyt wrote: ↑Tue Nov 22, 2022 4:36 am

theoretically, A and B should use the same amount of fuel, so their calorific values should be the same.

They use the same amount of fuel, but B generates higher pressure and will therefore outperform A by a considerable margin, because projectile acceleration will be greater.

Edit: here is a comparison modelled in HGDT to have an idea of the numbers:

### Re: Questions about pressure and flow rate

Posted: **Wed Nov 23, 2022 5:26 am**

by **yyt**

hectmarr wrote: ↑Tue Nov 22, 2022 8:35 pm

yyt wrote: ↑Tue Nov 22, 2022 4:36 am

Assuming that there are two combustion chambers A and B, the volume of A is twice that of B, and the initial pressure of B is twice that of A, then theoretically, A and B should use the same amount of fuel, so their calorific values should be the same. Then the question comes, which one, A and B, can provide a higher muzzle velocity on the premise of the same barrel and caliber?

In chamber B, the volume is half, for this reason, the pressure will be higher, and the speed of the projectile will be higher. I imagined a 100 cm3 motorcycle engine that has no compression, compared to another engine with the same volumetric ratio, 100 ml, but with a usual compression ratio, say 7 to 1. In the first case the motorcycle will not even move, but in the second case, it will roll perfectly. Is the same.

I see.good example

### Re: Questions about pressure and flow rate

Posted: **Wed Nov 23, 2022 5:30 am**

by **yyt**

jackssmirkingrevenge wrote: ↑Tue Nov 22, 2022 8:37 pm

yyt wrote: ↑Tue Nov 22, 2022 4:36 am

theoretically, A and B should use the same amount of fuel, so their calorific values should be the same.

They use the same amount of fuel, but B generates higher pressure and will therefore outperform A by a considerable margin, because projectile acceleration will be greater.

Edit: here is a comparison modelled in HGDT to have an idea of the numbers:

Great, maybe it shows that the compression ratio is proportional to the efficiency.