Formula for thrust?

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bluechipx
Let's say we have a tank with 1000 psi pushing in all directions, if we remove one square inch from an end, now the tank has 1000 psi more at one end than the other. Does this mean that there is a 1000 lb push in the opposite direction of the removed square inch?

Gold Member
Let's say we have a tank with 1000 psi pushing in all directions, if we remove one square inch from an end, now the tank has 1000 psi more at one end than the other. Does this mean that there is a 1000 lb push in the opposite direction of the removed square inch?
If the force on that square inch jet of water is indeed 1000pounds force then Newton's Third law tells you that there is 1000pounds reaction force on the tank. The forces must be equal.
PS IS someone pumping water into the tank to keep the internal pressure constant?

Mentor
Let's say we have a tank with 1000 psi pushing in all directions, if we remove one square inch from an end, now the tank has 1000 psi more at one end than the other. Does this mean that there is a 1000 lb push in the opposite direction of the removed square inch?
Yes, and that's one way of explaining how a rocket generates thrust in a vacuum where "there's nothing to push against". The combustion chamber of a rocket is basically a high-pressure tank with an opening at the back.

sophiecentaur
bluechipx
If the force on that square inch jet of water is indeed 1000pounds force then Newton's Third law tells you that there is 1000pounds reaction force on the tank. The forces must be equal.
PS IS someone pumping water into the tank to keep the internal pressure constant?

I am making a test fixture that will monitor thrust which is still under construction. I will test both ways, just air and air pushing water. I'll try to post pics soon. The tank is a full size high pressure welding type cylinder rated for 4500 psi with a test pressure of 6750 psi. The ball valve is a two inch with a pressure rating of 5000 psi. With no measuring system, I started out with a mere 250 psi yesterday for the first test, then tried 600 next. I am focusing on snapping the valve open as quickly as possible for the best effect, which was very impressive for the low starting pressure.

Gold Member
, just air and air pushing water
There will be a big practical difference between to two situations. You will need to shift a lot of air, fast, to maintain the sort of pressure compared with doing it with water. But there is a lot of Potential Energy stored in compressed air, compared with the Energy stored in a compressed liquid.
Perhaps you could go into some more detail to help us make the leap between your actual problem and the original question. Was that the 'right' question?

bluechipx
Although the actual test will be what matters, I was just curious if there was an accurate way of knowing beforehand what to expect. I have a hydraulic fixture with a one square inch piston (1.128") that applies pressure to a 10,000 psi gauge. The gauge has a telltale pointer that records max pressure. The tank will be pressurized to 4500 psi and released instantly though a two inch dia ball valve. I will be testing both ways, air over water and air only. Anybody care to venture a guess as to the actual thrust both ways? The valve has an area of just over three square inches (3.14). Hopefully I may see 3.14X 4500 = 14,130 lbs, but I suspect in reality the figure will be much lower for reasons I am unaware of.