steam rocket engines

[adam]

does any body know where to get good plans for a steam rocket because they seem to have a pretty easy concept of how they work and produce incredible amounts of thrust for there size but i dont really know much about them, any help would be apprieciated.

[Irvine.J]

Rossco is doing a considerable amount of work right now on that very thing and has had some really successful tests, i'd be talking to him/ waiting for his response. I'll let him know.

[leo]

look at this topic please.
http://www.pulse-jets.com/phpbb2/viewto ... highlight=

[adam]

hey thanxs leo!, well i found a little more info about these steam rockets but man i want to build one because it sounds so simple and you get a lot of thrust, well i guess just for a short burst at least, well if any body knows how they work then tell me if my understanding of these engines are right, >> the water is in a tube that is closed of at both ends and the tank is made out of something that can hold high pressure as well as high temp as well and then the water is filled all the way up in the tube or maybe half way i dont know, but then the water is heated up to its flash point and since it has nowhere to go the water doesnt turn into steam yet but instead the water is just under high pressure but once the valve is opened at one end and goes through the convergent/ divergent nozzle, and i dont know why it flows through a convergent nozzle first but any way the supper heated water turns into steam as it is released into the atmosphere or the divergent nozzle and then expands rapidly in the divergent nozzle which in turn produces thrust from pushing outwards on the side walls of the nozzle?

[PyroJoe]

Here is a site with some pictures. Their construction is a bit rough and heavy, but at least it lifted off.

http://www.muthaship.freeserve.co.uk/BJ ... page2.html

http://www.canosoarus.com/10X1Skycycle/Skycycle02.htm



Here is info on the Laval nozzle:

A good review of Laval's work reveals how important his simple breakthroughs have carried our knowledge forward.

http://en.wikipedia.org/wiki/Laval_nozzle

The higher the pressure in the CC, the better results you will get from a
convergent/divergent nozzle. The nozzle pushes the choked flow up to supersonic, it is a VERY useful tool for creating thrust.

[larry cottrill]

adam -

In a convergent nozzle, the idea is to achieve as much flow speed as possible through a pressure drop from inside (chamber) pressure to outside (sea level air, for us) pressure. What deLaval found is that, once the chamber pressure is so high that sonic speed is reached, increasing the pressure doesn't work to increase this flow speed - that is, through the sonic shock in the narrow neck, only sonic speed is achieved! However, at the same time, there is no increase in pressure DROP - so, the gas coming out of the nozzle has "excess" pressure that causes outward expansion. In a perfectly designed deLaval nozzle, the rear cone contains the gas flow as it completes the pressure drop to outside pressure, which results in further acceleration of the gas to supersonic speed. And, just as you've said, this creates a reaction force on the surface of the cone, increasing net thrust.

The "sonic speed" mentioned above is, of course, NOT the speed of sound in normal air - it is usually something MUCH higher than that, because we are usually dealing with very hot gas.

L Cottrill

[PyroJoe]

Larry explains it in a much better light.

[adam]

well thanxs guys, kinda hard to understand the convergent part a little still but only because im a slow learner :(. but the reason for me not under standing it is because right before you have the gas flowing threw a convergent duct, isnt the gases that are about to flow through the convergent duct all ready choked by a tiny hole that lets the gasses out from the chamber, so my question is, right were the whole is where steam comes directly out before reaching the nozzle isnt the gasses allready movig faster since its going threw a restricted whole and instead of having the nozzle convergent to divergent can it just be divergent right after the little orrifice or will it not work at all or still work but produce poor thrust?

[adam]

hey larry, im thinkin about researching this more and making one, so far every thing i read says that its really safe but that is only compared to other rockets that have chemicals and can cause fires, but with the a steam rocket, is there possibilies of an explosion? say i get a tube and have one end sealed off and the have a valve and nozzle at the other end with a pull valve type mechanism so i can tie a string to it and get really far from it and pull on the string to pull the valve open, so lets say with this setup i fill the tube with water and let it sit with a blow torch on the side off the metal for maybe 15 minutes or so, do you think it can blow up from to much pressure and be more like a dangerous time bomb? and also should the metal be pretty thick?

[heada]

High pressure explosions are very dangerous and if not designed carefully, a pressure rocket could be the source of a high pressure explosion. Look back in history at the number of water heater explosions that would almost level a house until laws and codes were written to force a pressure relief valve be installed.

In the situation you're talking about, if the pressure increases above the bursting point of the pipe, you now have a very hot liquid spraying everywhere along with pipe shrapnel flying at high speeds. You would need to find the burst pressure of your pipe before starting and then monitor the pressure within the pipe while performing your experiments. A safe way to do this is with hydrostatic pressure testing (not cheap) and a pressure transducer (also not cheap)

-Aaron

[PyroJoe]

I've been thinking about building one also. I would consider putting in a portion of PVC or other plastic that would intentionally burst before things reached to high pressure, as a fail safe. Scalding water is very dangerous. Even more so under pressure.

[larry cottrill]

well thanxs guys, kinda hard to understand the convergent part a little still but only because im a slow learner :(. but the reason for me not under standing it is because right before you have the gas flowing threw a convergent duct, isnt the gases that are about to flow through the convergent duct all ready choked by a tiny hole that lets the gasses out from the chamber, so my question is, right were the whole is where steam comes directly out before reaching the nozzle isnt the gasses allready movig faster since its going threw a restricted whole and instead of having the nozzle convergent to divergent can it just be divergent right after the little orrifice or will it not work at all or still work but produce poor thrust?

Well, ideally, you would have the convergent nozzle as the bottom "wall" of the chamber. If you have some kind of restriction ahead of the exit nozzle, you won't have a true deLaval nozzle even if it's "perfectly" shaped. The reason is that for the deLaval nozzle to work as such, the pressure ahead of the convergence must be at least the value called the "critical pressure", that is, the pressure that's required to create sonic speed in the throat. Calculation of the critical pressure is not simple (even though the math is not difficult), because it depends on the exact characteristics of the gas at the local gas condition - in other words, you have to know what gas, what temp, what density, etc. to get it right.

I don't see any reason why the throat couldn't be extended as a long pipe (say, to reach up through liquid to get at the vapor head at the top of a tall chamber) as long as you provide a decent convergent entrance (inside the top of the chamber) at the top end with the outlet cone at the bottom. It would introduce some inefficiency, but it should work. But again, you'd need to know that you can achieve critical pressure to make use of the cone. The conical end is counter-productive if the pressure is inadequate. Even with critical pressure and sonic speed at the throat, it is a mistake to make the cone too long (allowing separation of the flow from the cone). Also, for good efficiency, the cone must be narrow (10 or 12 degrees total included angle, or some such).

For air, the chamber pressure has to be almost twice the outside pressure - a value that we can't sustain with pulsejets or low-speed ramjets, though amateur chemical rockets exceed it easily. The pressure ratio would probably be very different for high temperature steam. You would need "steam tables" or something to determine it - but you would still have to have a good idea of what steam temperature you think you can actually get.

There are reasons some people can make their living designing this stuff ;-)

L Cottrill

[leo]

I don't see any reason why the throat couldn't be extended as a long pipe (say, to reach up through liquid to get at the vapor head at the top of a tall chamber) as long as you provide a decent convergent entrance (inside the top of the chamber).

Larry I always understand it was the opposite, the pipe has to be on the bottom off the tank.
The idea is that the liquid water flashes to a mixture of vapour and water in the nozzle.
There is a fixed amount of energy stored in the water, to get the most out of it you will have to use al the water as trust mass.
When we use the vapour only, the pressure in the tank will fall quickly, its the same problem we al have with the propane tank, liquid verses vapour feed.

[larry cottrill]

Larry I always understand it was the opposite, the pipe has to be on the bottom off the tank.
The idea is that the liquid water flashes to a mixture of vapour and water in the nozzle.
There is a fixed amount of energy stored in the water, to get the most out of it you will have to use al the water as trust mass.
When we use the vapour only, the pressure in the tank will fall quickly, its the same problem we al have with the propane tank, liquid verses vapour feed.

leo -

I wasn't meaning that was an ideal way to do it, just that it's a possible geometry. You're right, the water mass needs to be expended (or as much as we can use of it), just as in an air-driven water rocket. We then have the problem (I think) that the conversion from liquid to vapor in the throat will probably be incomplete, in which case sonic speed won't be approached there, so the deLaval nozzle will be ineffective. But, perhaps I'm just ... wrong. Some experimentation required.

(By 'incomplete' I mean that I think it will come out as a bunch of vapor with a large charge of liquid droplets. That can't possibly follow the "ideal gas laws" of pure vapor, like you would have with true superheated steam.)

L Cottrill

[Mark]

I wonder if any water/steam rockets reach the critical point before blastoff or if/how it would affect things for a short time?

"In water, the critical point occurs at around 647 K (374 °C or 705 °F) and 22.064 MPa (3200 PSIA or 218atm)."
http://en.wikipedia.org/wiki/Critical_p ... ynamics%29