steam rocket

[stoffe64]

hello everyone!

i have these compressed airbottles from fire extinguishers which i want to use as the base for a small steam rocket engine,they are rated for 200 bar air pressure so they are very strong,i wonder if anyone in here have any drawings of the exhaust nozzle for best performance, any help appreciated!
best regards/stephan

[Johansson]

Hi!

I have a vague memory of promising you a drawing of a steam nozzle, sorry for not remembering to send it to you. Much schnapps during the christmas weekend does wonders to the short time memory...



Here is a picture of steam nozzle scaling, I am not saying that it is perfect but it worked fine for a nozzle with a 12mm throat. I have calculated it from the dimentions of a smaller nozzle so it should be fairly accurate for smaller nozzles as well.







And here is a video of the first test, nozzle expansion seems spot on from the look of the steam plume.

[racket]

Hi Stephan

If you want a good book get hold of a copy of ................Model Steam Turbines .................how to design and build them , by H H Harrison first published in 1927 reprinted 1994 by TEE Publishing ............ISBN 1 85761 075 X www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=ISBN+++1+85761+075+X&x=12&y=18 lotsa good "numbers" for DIY work

Cheers
John

[racket]

Hi Anders

Would you have some more data on the steam rocket , that nozzle looks more like a wide angle "rocket" nozzle than the usual De Laval steam nozzle with its ~10 deg included angle ( 5 plus 5 deg ) .

What sort of pressure did you have initially ,

Was there water in the steam

Was it all water coming out that flashed off as steam once the pressure dropped in the nozzle ??

Lotsa questions

You needed a freepower turbine on that exhaust , probably several hundred horsepower ;-))

Cheers
John

[Johansson]

Hi John,

There is unfortunately little data about the nozzle, I shamelessly copied and scaled it from a booklet about Robert Truax steam rocket engines.

Tank pressure is 50 bar, the nozzle is meant to be injected with 100% water which transforms to steam down the nozzle. That would be the reason for the non-DeLaval looking nozzle.

In the video there is a few seconds of pure water injection, but since no G forces presses the water towards the nozzle in the static run steam starts to blend with the water. Subjected to G forces the tank empties in aprox. 6-7 seconds.

Yeah, a freepower on that thing would be interesting!

[stoffe64]

thank you guys,big thanks to both Anders and John!
i hope that my "small" air bottles will produce atleast some thrust,its just a fun project to see how it works really, thanks guys!
best regards/stephan

[Johansson]

Just make sure that you don´t add any external heat to a spot on the bottle where there isn´t water on the inside, otherwise the metal will soften and possible burst.

Truax stated in his book that he used oxygen bottles and a maximum pressure of 35 bar, so I suggest that you consider that pressure the maximum "safe" pressure.

Good luck and keep the thread updated on the progress!

[ernie wrenn]

John

Make sure you DO NOT USE a aluminum bottle at any rating with a carbon based heat source. They will change molecular structure and explode. During their production they are work hardened and any carbon mixing with the aluminum will cause it anneal and weaken rapidly.

Use a STEEL cyl. only!!

Ernie

[Richard OConnell]

stay away from those carbon fiber bottles too. It looks like you should be creating some good thrust there. Is that one rope to keep it secured? And if so would it be possible to put a strain-gauge inline with it to give an idea of lbs thrust?

[Johansson]

We had an analogue scale tied to the rope and saw 900N of thrust for the initial 5 seconds or so, after that steam started to mix with the water in the nozzle and thrust dropped considerably.

A steel bottle is of course the only option for a steam rocket.

[Richard OConnell]

you might actually be using too much water. The reason i say this is because the boiling point of water is lowered as pressure decreases. So in your case, as the pressure increases, the water inside is becoming harder to boil. Although the water flashes to steam when the nozzle opens and the pressure drops, you are still losing a lot of water in liquid form, which i guess is to be expected as the gasses are making a high velocity escape through the bottle and out the nozzle. I really dont know much about steam rockets, but is it possible to preheat a water holding tank below your bottle, then when the nozzle on the bottle is opened, the gas escapes upward into the bottle and pehaps carries less water and more steam? Or do I just need to pick up a book and do some more research on it?

[Johansson]

The whole idea with steam rockets is to eject the overheated water through a nozzle where it boils, expands and produces thrust, when the valve opens the remaining water inside the tank boils and keep the pressure inside the tank fairly constant until the tank is empty.

If steam was ejected (rocket engine upside down or with a siphon tube) you would find that the bottle pressure drops instantly since the water can´t boil off fast enough to produce the amount of steam needed to produce decent thrust.

[racket]

Hi Anders

I did some numbers using the steam tables and it looks like you'll only "boil" ~25% of the water exiting the nozzle throat as the latent heat of evaporation is so high compared with the difference in boiling points between 15 bar pressure and ambiant .

I'd say that you'd have got virtually the same thrust without the diffusing nozzle section, ...............LOL, I wouldn't be surprised if Traux merely copied a normal rocket nozzle and didn't do any research/development

Cheers
John

[Johansson]

Can´t argue with numbers, but I am fairly certain that the nozzle does some work at least.

A couple of guys in the southern part of Sweden tried to copy our kick after it became a bit famous in a couple of tv stations and newspapers, they didn´t care to fit a CD nozzle so they had only a straight hole as nozzle and they couldn´t even get near the speed we got out of our kick even though they ran at double pressure (100bar, crazy bastards).

[racket]

Hi Anders

Your nice bellmouthed inlet up to the nozzle throat would have contributed to maximum mass flow , a square edged hole will only flow ~60% of its area with very poor efficiency .............LOL, it pays to do things "right" like you guys did :-)

Cheers
John