Chris,

1. Yes the possibility is very high in the steam condensing or cooling off before its exits the system. So testing will have to be done to make sure we do get the water hot enough so it keeps above boiling until it exits the nozzle.

2. I don’t understand exactly what you mean by "top of the tank" I’m going to add another picture of our new design. I do understand though that, lets say for example, we don’t heat the water at all we just push it through the system as water. No nozzle is needed all we would need is a hole at the end for it to "squirt" out. Now that its going to be steam were messing with gases which is similar to solid propellant. With steam we can harness the power of it expansion using an actual nozzle.

3. Your right the throat is much to long, new design is attached.

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The new design is a little simpler. We will put the water in the tank before anything, then we will close the valve that’s in front of the nozzle, open the other valve then pressurize it. Once it’s pressurized we will close all valves and heat the tank. Once it is ready we will open the valve in front of the nozzle.

The problem remains on what valve to use that can release it quickly and handle the stresses were putting on it.

Wait, so it's pressurized before heating the tank? Why? Simply containing it will allow for plenty of pressure buildup (which is the idea behind a pressure cooker). As for what I mean by the top of the tank? Exactly what it sounds like. For this to work, the nozzle entrance has to be supplied by gas, not liquid, so it has to be in the region of the tank above the waterline.

Okay, this picture is geometrically incorrect. The system will be pointing downward. Just like an actual motor would be in a rocket, that’s why I was confused. You’re also taking my words to literal; I have no idea of the step by step process that we are going to have to do to cycle the motor. I am just giving you a generalization of how it’s going to work. Doesn’t matter if we heat it first or pressurize it first at this point in the design process.

Even if the system was laying on its side it shouldnt matter if the nozzle is above or below the container. There should be no air in the system and as soon as the water hits the atmospheric pressure its going to turn into steam and follow the least resistance which would be out the nozzle. There should be no water in the system past the tank at all as soon as you open the valve, so all you have is gas.

Even if the system was laying on its side it shouldnt matter if the nozzle is above or below the container. There should be no air in the system and as soon as the water hits the atmospheric pressure its going to turn into steam and follow the least resistance which would be out the nozzle. There should be no water in the system past the tank at all as soon as you open the valve, so all you have is gas.

As soon as the water hits atmospheric, it will turn to steam. However, I suspect that if you have the nozzle inlet below the water surface level, this transition to steam will happen in the expansion section of the nozzle, and I would be surprised if that gave you a supersonic expansion. I could be wrong, as I am just guessing here, but I would think you'd be better off pulling gas directly.

Chris,
I think what Shredder is getting at, is assuming that all water in the tank has been heated to the gas phase, no liquid in the tank.

Shredder,
I believe what Chris is getting at is, with the tank upright and the nozzle down, you will have some in the liquid phase, and it will be right at the nozzle end. If you plumb the tank outlet,valve,nozzle out the top of the tank (u-bend?), then you'll always be getting the gas phase (steam).

Just trying to translate.
-Ken

I wonder if there is a way to do this with 2 chambers: One containing the liquid, from which the water exits as a superheated liquid, and then a plenum where the water vaporizes due to a pressure drop before exiting the rocket nozzle as a gas. If liquid goes out the nozzle, you would lose a lot of the Isp, I would think. Maybe a top-exit for pulling the vapor from above the waterline is the right approach.

So if there might be a problem of the water transfering to steam before exiting the nozzle, how can we fix that? should we make the distance between the tank longer so it has more time to change into steam?

Chris,
I think what Shredder is getting at, is assuming that all water in the tank has been heated to the gas phase, no liquid in the tank.

If that's the case, the pressure in the tank will be absolutely absurd, as will the temperature (assuming it started full of liquid). Some rough calculations indicate that you'd be in the supercritical fluid part of the phase diagram, with pressure of >30kpsi and temperature of > 800F.

So if there might be a problem of the water transfering to steam before exiting the nozzle, how can we fix that? should we make the distance between the tank longer so it has more time to change into steam?

It's not a matter of time, it's a matter of pressure. For the liquid to change to a gas, there has to be a pressure drop, and with a nozzle designed for gases, I'm not sure you'd see enough of a pressure drop for the phase transition until you reached the diverging section of the nozzle, which is too late. Adrian's idea seems potentially usable though, If you effectively went through a small expansion chamber to vaporize the water, and then went through the nozzle, that could work. Pulling from above the waterline guarantees that it will work, but it also has some other issues with the geometry of the motor.

Would the pressure between the tank opening and the nozzle not be enough to allow the water to flash into steam? That original picture, its twelve inch section, I was thinking in that distance the water would turn into steam. And no the tank wont be full of gas at all. The tank will only hold water and the transition will happen outside the tank. So pressures should be close to a solid rocket motor, 500-1000 PSI and temperatures between 212-400 degrees. Concerning the nozzle being above the water line, I can’t think of any design where this would be possible to build, unless we tested the motor upside down pointing up. Which is realistic for testing but not realistic for an actual launch attempt.

Some equations we went over was of course bernoullies, and we also looked at relationships between pressure and volume of water/steam.

The benoulli relation will be relatively useless in this case, since the flow in a rocket motor is definitely compressible (and bernoulli basically assumes incompressible flow). The phase diagram of water is useful though.

Oh, and Ken's suggestion does bring to mind an interesting possibility. If you intentionally filled the tank only partway, and then heated it such that the final state (just prior to release) was something like 600F, 1000-1500 PSI water vapor. That would make the design of the motor substantially simpler, at the expense of propellant density.

Otherwise, with the primarily liquid chamber conditions, the issue I see right now is simply that the propellant entering the converging section of the nozzle absolutely has to be a gas for it to work properly. If it's a liquid entering the nozzle, then the throat flow will probably not be choked, which ruins your efficiency. I don't know any good way to calculate this either - perhaps a small scale test?

I just ran some numbers with a chamber pressure of 1000PSI, a chamber temperature of 550F, and an exhaust pressure of 25 PSI (slightly underexpanded), here's what I get (assuming that the stuff entering the nozzle is 100% pure water vapor, with no liquid):

A/A* = 4.45 (nozzle expansion ratio)
Exit temp = -106 fahrenheit (yes, negative)
Exit mach: 3.05
Exit velocity: 3470 ft/s
Specific impulse: 108 seconds

That exit temp is a problem too, since this analysis assumed gas for the entire time. In reality, at 25psi and -106F, the vapor would quickly condense into liquid water.

After running a couple more numbers, I've managed to convince myself that you pretty much can't do this the way you want to. The problem is that in a nozzle, the flow cools too much, so if it was on the vapor dome in the chamber, it will be solidly within the liquid part of the phase diagram in the nozzle itself. Your best bet would probably be to start with superheated steam rather than a saturated liquid-vapor mix of steam and water.

Darn. Double post and I can't find the delete button...

So basically that idea would mean you fill the tank partially of water and heat it till its all vapor? Then release it throught the valve and nozzle? Wouldnt that limit the amount of vapor you could produce? If you have a full tank of water and transform that all to steam you could almost double the amount of steam. If you only partially fill the tank you would only get a fraction of steam.

What equations are you using for those numbers? I cant even start to understand the dynamics books I looked at.