How does the fuel and oxidiser in a liquid fueled engine keep from beeing pushed back through the injector with all the extream high preassure in the combustion chamber?
Because the injector openings are so small, they don't see enough pressure to keep a hefty pump from pushing the liquid out. The pressure inside the combustion chamber presses against all surfaces inside the chamber. Since there is an opening on the back end the pressure is unequal, and the pressure from the burning fuel only presses against surfaces on the front and sides of the chamber (this is what pushes the rocket forwards).
To see the difference, a 1000 psi pressure on the surfaces of a combustion chamber will only exert 12.2 psi on a 1/8" diameter opening (Unless my calcs are off). Not that this is the pressure or the openings in rockets, just using these numbers to show how little pressure there is on a small opening compared to an entire square inch of chamber wall.
Mushtang,Because the injector openings are so small, they don't see enough pressure to keep a hefty pump from pushing the liquid out. The pressure inside the combustion chamber presses against all surfaces inside the chamber. Since there is an opening on the back end the pressure is unequal, and the pressure from the burning fuel only presses against surfaces on the front and sides of the chamber (this is what pushes the rocket forwards).
To see the difference, a 1000 psi pressure on the surfaces of a combustion chamber will only exert 12.2 psi on a 1/8" diameter opening (Unless my calcs are off). Not that this is the pressure or the openings in rockets, just using these numbers to show how little pressure there is on a small opening compared to an entire square inch of chamber wall.
You are confusing pressure and force. The force on the 1/8" opening is lower but the pressure is still 1000 psi. There smaller the area, the smaller the force for a given pressure. This is in part how hydraulics work. A small force over a small area can be converted to a large force on a greater area at the expense of the distance the fluid/piston travels. The burning fuel also presses evenly (relatively) on the sides of the combustion chamber as well (Pascal's principle). There is a pressure drop along the length of the combustion chamber as the mass flows out the nozzle. The pressure would be equal throughout if there was no mass flow.
Doug
That's so funny, because last night I was at a gas pump and for some reason my post came to me and I was thinking to myself, "Wait, did I say that the reduced number is also PSI? I hope not, that's not what I meant!! I'll have to correct that if I did. And I bet someone has already caught my error."
Sure enough, you did. Heh. So yes, I agree with you completely. The reduction is not in PSI. A 1000 psi pressure on the surface will produce a force of 1000 lbs force on a one square inch area. On a 1/8" hole there will only be a 12.2 lb force against it.
The numbers were right, the units were jacked up.
Sorry about that.
I read on another website that rockets can't work in space. Since the website used "math" and "science", it must be true.
How does the fuel and oxidiser in a liquid fueled engine keep from beeing pushed back through the injector with all the extream high preassure in the combustion chamber?
Because the injector openings are so small, they don't see enough pressure to keep a hefty pump from pushing the liquid out. The pressure inside the combustion chamber presses against all surfaces inside the chamber. Since there is an opening on the back end the pressure is unequal, and the pressure from the burning fuel only presses against surfaces on the front and sides of the chamber (this is what pushes the rocket forwards).
To see the difference, a 1000 psi pressure on the surfaces of a combustion chamber will only exert 12.2 psi on a 1/8" diameter opening (Unless my calcs are off). Not that this is the pressure or the openings in rockets, just using these numbers to show how little pressure there is on a small opening compared to an entire square inch of chamber wall.
Don't go there.
One more difficulty you get with pumping fuel/oxidizer into the combustion chamber is that the combustion tends to happen in pulses, it's very difficult to get it to burn at a constant rate. The fuel/ox burns and the gases expand through the nozzle, and afterwards there is a pressure drop in the chamber until the inflowing mixture ignites. This causes the infamous "pogo" effect, that makes a rocket engine behave more like a pulse-jet, with significant resulting vibration. You get the same issue, albeit somewhat less pronounced, with hyrid engines (motors?...) too. That's wny the U/C hybrids like the Ratt Works have a floating injector, to help keep the tank pressure relatively constant. Solid fuel motors don't have as much of an issue, because the fuel/ox is just sitting there waiting to burn... it doesn't need to be introduced into the chamber. Pogo oscillation has been a problem for propulsion engineers since Day One, and nearly killed the Saturn V early on.
Pressure is pressure, regardless of surface area... it doesn't care if it's pressing on a 1/8 inch diameter injector orifice or a 10,000 square inches of combustion chamber wall or nozzle wall. Now the FORCE is different
Thanks for correcting me, after someone else had already pointed it out, and then I came back and explained that I understood the mistake (typo) I'd made.
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