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[flashtwo]

Hi all,

Last weekend at the Wicksteed 2010 event, my Steam Jet powered experimental boat had its maiden voyage (see Wicksteed 2010 page 10 for description - http://www.modelboatmayhem.co.uk/forum/index.php?topic=21652.msg238462#msg238462 ).

Having developed a flash boiler for the Vital Byte experimental boat,  I thought the light boiler could be used as the basis of a steam jet engine and last year tried out a couple of unsuccessful ideas and soon discovered that the steam condensed in the air before producing a force.

Last February, I had the idea of expanding the steam through a nozzle into the very hot combustion gases of the boiler much in the same way as an exhauster on a steam loco which uses the cylinder exhaust steam to draw the fire in the furnace.

A prototype engine (boiler? - I call it a Bengine) was built and gave an encouraging performance - the jet exhaust actually set alight a piece of wood! The "Bengine" was about 1200mm long though, quite impractical for a model boat. Then, as I was winding a flash boiler coil on a piece of 22mm copper pipe as a former, I had the bright idea of keeping the copper former as the exhaust pipe through which the steam jet would be aimed through.

This exhaust pipe not only supported the boiler coil, but also passed back through the furnace and eventually was used as part of the  butane/propane gas  pre-heater. Using this arrangement the length was reduced to 350mm and also the construction was simplified.

I built a “swinging” test rig to test the thrust produced by the Bengine and was confused by how little the swing moved horizontally compared with force exerted against a sheet of aluminium held in the jet exhaust stream. The swing indicated a thrust of less than 1 Newton (1kg held in the hand exerts a downward force of approximately 10 Newtons) – this equated to about 3 ounces of force in old money.

The force on the aluminium sheet was telling me a different story. At a very rough estimate, it was about 0.7 kg , which was about 7 Newtons. Well, according to Newton’s Third Law every action has an equal and opposite reaction – so what was happening here.

I also rigged up a simple Pitot Tube (the thing aircraft use for measuring their air-speed)  to measure the exhaust velocity of  the Bengine – astonishingly it was calculated to be in excess of 250mph (400km/hr for our Antipodean friends). I used this figure to calculated the mass flows, momentum and kinetic energy of the exhaust and, sure it enough, it confirmed the small amount of thrust being developed, but didn’t explain the force on the aluminium plate.

So, being a Mayhemer, I thought let’s build a boat and get some real world experience to confirm or contradict the theory. Having built the boat in record time off I set for Wicksteed 2010 ( the home test tank being too small for a proper test).

One little side feature was the addition of a jet vectoring device in place of the usual rudder. This device was based on the Coanda effect where a fluid flow follows a curved surface.

The maiden voyage confirmed the lack of thrust being developed, confirmed that the vector thrust steering was practical and, as chance would have it, the equivalent of the aluminium plate sailed by the exhaust in the form a metre long model of a destroyer. The model destroyer was heavily blown over to port by the Bengine exhaust blast – the steam jet boat was no friend to other users on the pond.

There was the dichotomy – the jet could knock over another boat, but could hardly move the boat it was suppose to be  propelling.

What was going on with Newton’s Third law????

Ian

[Colin Bishop]

Is it simply the fact that the exhaust needs something to react against? Maybe the energy is simply dissipated too quickly against normal atmospheric pressure as the steam expands on leaving the nozzle. Think about a hovercraft which only lifts a few inches off the surface, if you increaased the power it wouldn't rise much further as all the excess air would simply escape and provide no extra lift. I supspect something similar is going on here. What would happen if the jet was directed to emerge under water I wonder?

Colin

[Martin (Admin)]

... photos AND video to follow! 

[snowwolflair]

You need smaller nozzle with a higher velocity jet.  Think about a nozzle on a hosepipe.

[Martin (Admin)]

I'm thinking it's more volume of steam needed....

[snowwolflair]

boat mass times velocity = jet mass times velocity

too much mass and you run out of steam, so it has to be big velocity.

[Colin Bishop]

Where is Dodgy Geezer when we need him? 

Colin

[dreadnought72]

Interesting results.

I think you need to physically treat the device as a rocket. It doesn't "need" to push against a plate or the air or anything else: the thrust is generated purely from flinging reaction mass out the back.

What you might be finding is that the instant expansion of the steam when it reaches atmospheric pressure, is occurring in a hemispherical volume from your nozzle. This may well be accelerating ambient air generally backwards, thereby producing the effects you see on your aluminium plate and passing destroyers, but represents a net loss of useful thrust to the boat.

Look into rocket nozzles - they are there for a purpose, and the perfect one is shaped so that, in your case, it'll end with a length and diameter to allow your expanding steam to reach one atmosphere at the lip of the bell. (The steam'll still has substantial velocity at this point, of course, and this is now directed directly aft.)

Can you post some figures for pressure and steam rate, please? That'd give a good guide to developing the perfect plenum.

Andy

[Colin Bishop]

That makes a lot of sense Andy. This is an interesting subject.

Colin

[Martin (Admin)]

http://www.youtube.com/watch?v=gJvpVYJY_cs

[benjaml1]

Together with a correctly designed "jet" nozzle as previously suggested, I was also wondering if an augmenter tube would be beneficial ??

[Fleetcom]

A jet engine works by sucking air in and expelling it - a propellor in air is doing the same thing more or less. A rocket engine does none of that but expells a huge amount of energy. A steam jet is a de-facto rocket engine but it would not be possible to create enough energy due to the small size of the nozzle (a larger nozzle cannot get enough steam), hence the lack of thrust. The augmenter tube might work because it would draw air in by venturi effect and thus increase the volume on exhaust.

[HS93 (RIP)]

if you put a engine on the end of the steam pipe with a fan , it may go quicker. 


Peter

[flashtwo]

Thanks for all your interesting comments..

I wasn't the first to try a jet propelled boat, see this site....

http://www.dself.dsl.pipex.com/MUSEUM/TRANSPORT/lucyasht/lucyasht.htm

Apparently, from what I can gather from other sites, the jet powered version (remember a twin Derwent Meteor aircraft could exceed 600mph) was no faster than when it was powered by its Victorian reciprocating steam plant.

Another unusual application for a Derwent engine was to dry out a power station boiler after a major fire - I was the instrument engineer on the project and one concern was bending the boiler drum with uneven heating by the jet exhaust. Not many, I can tell you, stood too close when the engine was sitting there at 10,000rpm!

Colin, I think you are correct to say that the exhaust needs something to react against in the sense that it needs to accelerate more ambient air or the pond water. I've tried putting the jet under the water, but only get a large amount of bubbling.


Ian.

[sheerline]

Could your lack of thrust problems possibly be caused by the very use of steam itself. This is ejected from the nozzle with apparent great velocity but it is rapidly contracting as a gas/vapour the moment it leaves the nozzle. There is lack of mass being ejected and what is ejected is diminishing as it mixes with the cold atmosphere.  A rocket ejects an expanding gas, a jet passes hugh masses of hot gas at high velocity so it would appear the steam powered 'rocket' boat would be rather innefficient and a poor use of the fuels energy.
I would be inclined to attach your steam generator  to a well designed turbine where the energy and gas/vapour velocity will impinge directly onto blades which will absorb this energy much more efficiently.
Power stations use steam turbines to good effect, I've often considered playing with such a power plant but there is little to beat pistons and cylinders at our scale really.
It's great to experiment however and perhaps you could keep us posted from time to time on any firm findings you make.
 

[steamboatmodel]

If you ran the steam through a venture with a pipe with water in the center to inject water into the stream would it work. Sort of like a Boiler injector? I remember something about fire hoses that worked like that.
Regards,
Gerald.

[sheerline]

Thats a ruddy good idea Gerald, why didn't I think of that.... must be getting old! It would use the inertia of the steam to throw a greater mass of material but I reckon it will still need a larger nozzle and possibly a bigger burner so could still be quite wasteful on energy.

[flashtwo]

Thanks again for your comments,

The part of the problem that I couldn't understand was the Newton's Third Law bit. On my test pond I filled a large ice cream tub with gravel and placed it in the path of the jet exhaust and again the ice cream tub moved away much quicker than the boat could move. I could feel a good force on the tub but hardly a thing on the boat - there were no equal and opposite forces present.

The experiments that were unsuccessful last year were the ones which the steam was in direct contact with the cold water or just expanded in the relatively cold ambient air - the initially invisible steam just condensed in the water causing lots of visible vapour and bubbles or condensed in the air thus loosing its energy.

The video of the boat at Wicksteed was taken after its dunking (see the Wicksteed thread for details) and all of the boiler insulation was soaking wet. Normally with the Bengine up to temperature the 250mph exhaust jet is invisible for all its length indicating that the steam has not cooled and condensed. On startup I can see that the steam vapour doesn't expand and disperse sideways very much, but stays in a fairly confined jet.

As you can See ts the Third Law bit which is the problem.

Keep the ideas coming in, I think we will all benefit.

Ian

[benjaml1]

It's all about thrust ( If in doubt watch "Chicken Run"...   ) Nothing to do with impact velocity, expansion of gasses or vapours... It's about thrust & the ability of a steam boiler to deliver such. As an example, a small Maccsteam boiler delivers  300 cu ins/min (4916ml/min) of steam at 50psi with 65/35 mix gas. In pounds per minute ( thrust) that is quite small...  0.026 lbs/min to my calculations, someone peease check as steam calculations weren't my forte'...

http://science.howstuffworks.com/rocket2.htm

[benjaml1]

http://cgi.ebay.co.uk/JETEX-MODEL-PLANE-ENGINES-augmenter-tube-/190402270927

An augmenter tube may possibly increase the mass of ejecta, I would look at a nozzle design that maximises exit velocity of the steam. Guessing, I would make sure no ( or little) expansion occurs within the tube as that may nullify the effect.

Good luck & have fun...  

[flashtwo]

Hi Ben,

I've tried trying to augment the exhaust flow by using a blown-edge aerofoil much like the Buccaneer aircraft used for low speed flight.

The exhaust flow was directed up through a slit nozzle around the curved leading edge of the aerofoil and then, using the Coanda effect, bent through 90 degrees to direct the thrust astern. The flow was boxed in at the top and air was sucked through to augment the exhaust flow.

It certainling sucked in air from the front and chucked it astern and the thrust may have been better but no greatly so.

With regard to your suggestions about thrust and the website example, what happens if I throw a cricket ball astern at 20mph and I'm already moving forward at 20mph - do I get any thrust?

Here's a couple of pics of the augmentor aerofoil.

Ian.

[benjaml1]

With regard to your suggestions about thrust and the website example, what happens if I throw a cricket ball astern at 20mph and I'm already moving forward at 20mph - do I get any thrust? Ian.

It's all relative....    Never mind the velocity part of the equation ( for now).... you also have to have the mass per unit time to throw.. May I give a crude example. Stand on roller skates & throw a marble as fast as you can. Now throw a cannon ball & compare the distance you have moved. You have imparted the same amount of energy but you have moved further with the greater mass...

Looking at the pics it looks like you have been having fun with this...

[Martin (Admin)]

If that doesn't work, you could try "Dempster Turb-O-Prop"   

 http://jetex.org/motors/motors-accessories.html

[benjaml1]

If that doesn't work, you could try "Dempster Turb-O-Prop"   

 http://jetex.org/motors/motors-accessories.html

Cathrine wheel comes to mind... 

[flashtwo]

Right,

I think I have an understanding of how propulsion works, be it conventional prop, gas turbine or bengine.

The steam / combustion gas emitted from the boats exhaust at high velocity has two components;-

1) The momentum = mass x velocity
2) The Kinetic energy = (mass x velocity^2)/2

Notice that the kinetic energy will always be much larger because the velocity is squared.

If the boat is stationary, or moving very slow compared with the exhaust velocity, then practically all of the energy is in the form of kinetic.

Now, here's the rub, the because kinetic energy is totally "frame referenced"  the boat cannot "see" that form of energy, it can only feel the force caused by the momentum.
Only an observer can "see" the kinetic energy (strains of Einstein's Relativity creeping in here would you believe!). Hence, in the example on the pond above, the "observer" was the model destroyer which was hit by the kinetic energy which exerted a force on the model  causing it to keel heavily to port.

So, there is no Newton's Third Law associated with the kinetic energy from the jet boat's point of reference!

All we need to build now is a "frame reference" eliminator!

What do you think?


Ian.