- This topic has 43 replies, 12 voices, and was last updated 14 years, 8 months ago by
Kyle Parsons.
Kyle Parsons.-
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Chris LaPanse
This seems like a really interesting idea, but I do see a couple of problems.
1) You’ll need to get the water EXTREMELY hot, since when it flashes to steam, it also cools substantially (energy is absorbed by the phase transition). Then, when a gas flows through a converging-diverging nozzle, it cools further, and if you aren’t careful, the steam could condense in the nozzle. This would absolutely wreck your thrust. Of course, the issue with getting the water hot enough is that the pressure in the tank will rise the hotter the water is. I can run some numbers on this tomorrow, to get a better idea for the initial temperature needed…
2) In a rocket, the chamber will maintain a pretty high pressure, so it will remain mostly full of water until a substantial amount of the propellant has exited the rocket. In order for a rocket nozzle to work, you need a gas (not a liquid) flowing through it, so you’ll effectively have to have the nozzle hooked to the top of the tank. If you have the nozzle on the bottom (or side) of the tank, it will act as a glorified water rocket spewing boiling water. It will work, but it will be drastically lower efficiency than if you had gas flowing through the nozzle.
3) Why does your nozzle design have an extremely long throat? You want to keep your nozzle length relatively short, and if you have to extend a section of the chamber, you want to extend the part prior to the nozzle. Basically, the narrow tube you’ve drawn (the 12 inch long one) needs to be substantially larger than the throat of the nozzle, so the flow through that region is relatively slow. The nozzle should then contract, have a fairly short throat, and then expand out into the divergent section. If you have a long region of minimum area, I suspect you will have subsonic flow in the divergent section, once again wrecking the thrust.
As I said, I can run some calculations if you want – I’m quite familiar with the relevant formulas, so it shouldn’t be too hard.
Do you have any details on your current design? Chamber temp, pressure, etc? Rough numbers would be a nice starting point, but if you don’t have any, I could always invent some plausible exit values and work from there. I’d also be interested to see the equations you’ve been over with your profs…
Chris
(Aero engineering graduate student who has taken far too many fluid dynamics courses)Bruce R. Schaefer
Chris, you are dead-on in everything you’ve said. A “steam engine” emits gas not water (regardless of temp). Otherwise, it is a bottle rocket (and, not the fireworks kind), i.e., not taking advantage of the con/div nozzle, although the “bottle rocket” does… because it is a rocket. Anything coming out of a rocket engine that is not gas is not a rocket, by definition. Period. Otherwise, the escaping gas won’t be supersonic at the end of a con/div nozzle. Another… period. Pursue it, Chris.
Kyle Parsons
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.
Bruce R. Schaefer
Chris, have at it! There are so many falsehoods in Shredders’ post…
Kyle Parsons
Hey be nice…..
Chris LaPanse
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.
Uploaded with ImageShack.us
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.
Kyle Parsons
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.
Kyle Parsons
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.
Chris LaPanse
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.
new2hpr
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
Chris LaPanse
Bruce R. Schaefer
Kyle Parsons
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