Fantasy engineering: steam-driven turboprop?

[HareBrain]

As far as I understand it, a turboprop engine uses the thrust from a jet to drive a turbine that turns an aircraft propeller. Is there any reason the turbine couldn't be driven by steam, like a ship's screws? (The system wouldn't need to generate lift, only forward propulsion, like in an airship.) If the firebox had magical fuel, making it hotter and lighter than a coal-burning one, would it be credible (or credible enough for a fantasy novel) that steam-turboprops could make an aircraft go more than a hundred miles an hour, or would it be impossible to make a boiler system strong enough to stand the strain? (The only magic available is the fuel; the rest of the engineering has to be mundane.)

Many thanks for learned opinions.

 

[Venusian Broon]

Have you come across this?

Steam aircraft - Wikipedia, the free encyclopedia

The messerschmitt 264a seems closest to what you are describing.

I guess it all about trying to get the power to weight ratio up in your steam engine. Concievably a steam turbine for a plane is practical, just that in our reality air craft design for us went along the lines of petrol engines, then onward to jet engines, so no one really bothered to develop it.

Edit - whoops, I missed that you were saying airship. It's the relentless humid heat that is melting my mind. Anyway - if they could put it in a fixed-wing plane then I don't see any problem at all putting it in an airship, in fact I think it should be easier.

 

[ctg]

I don't see why not. The fantasy landscape especially in the steampunk era is far bigger than what people realise. And if you keep the airship altitude under commercial airlines, you should be golden.

People will buy it and believe in it. However, the thing is that it has been done before. but it doesn't mean you couldn't do it either because you can associate so many things to the steam-driven technologies. For example think about using the steam provide power to the cryo-technologies. You could drop something like that in your airship as it sails over the continents and visits harbours.

 

[HareBrain]

Thanks guys, that's set my mind at rest.

Have you come across this?

Steam aircraft - Wikipedia, the free encyclopedia

No -- to be honest, it never crossed my mind to check to see if anyone had actually built or designed a steam-powered aircraft; I thought the idea was ridiculous. I have a character in my book declaring that a steam-engine alone could never generate enough power to overcome its own weight -- shows what he knows!

(It's not an airship as such, but the lift is provided by an air elemental, with a fire elemental to boil the water and drive the propellers).

 

[Tywin]

I was going to say that the the biggest problem with getting an airship to go over 100 miles per hour would be the big balloon on top. I guess with an air elemental you could just make him really aerodynamic?

Also, it warms my American heart to see 100 miles per hour instead of 160 Kilometers per hour

 

[Montero]

Just remember you need to be able to get water refills. Cloud scoop?

 

[HareBrain]

Also, it warms my American heart to see 100 miles per hour instead of 160 Kilometers per hour

Almost no one uses kilometres in the UK either. But even mph was a concession to modernity -- I wanted to use furlongs per epoch.

Just remember you need to be able to get water refills. Cloud scoop?

How difficult would it be to get the steam to condense to water again? I realise it's easier on a train or ship to take on extra water, but could you do it with cooling pipes running along the outside of the aircraft? (I am not, by any stretch of the imagination, either an engineer or a physicist.) I guess the presence of a water elemental might hasten the process.

 

[Montero]

The steam re-condenses in the cylinders and runs back to the boiler - the condensing of the steam creates a vacuum in the cylinder which is part of the motive force on the piston. However there are losses in the system at valves. Picture a steam locomotive and all the wisps of steam rising from it.
(Edited - On second thoughts - its been a while since my steam nut days and it may be the condensate drips out of a drain rather than going back to the boiler. I've forgotten. Losses at pistons and moving parts is true though.)

In the days of steam trains there were regular watering points along the railway lines. For shunting engines and branch line it was a big reservoir with an overhead arm and a pull chain. Park locomotive in the right spot, open the filling point, swing arm over, pull chain.
For main line expresses, they had scoops under the locomotive that would pick up water from a long trench between the rails without stopping the train.
One of the "joys" of running steam locomotives like the Orient Express trains on modern mainline is the total lack of watering points. They have to stop at places where there is a siding beside a car park and have water bowser lorries waiting to fill - which is very slow compared to the purpose designed cisterns that there used to be.

Further thought - don't know what the losses are like on a turbine system. By the way, with turbines, remember they don't start from cold like a petrol engine. Because of the different metals from which they are constructed, with different coefficients of thermal expansion, you have to trickle steam through well below motive power rates to warm the whole thing up to operating temperature before opening the throttle on the steam. If you don't, you just strip the turbine blades off the axle.
Electricity power stations are kept on standby for times of high demand - so they are kept hot with steam trickling through, then they can be asked to deliver on a timescale of a few minutes, rather than many hours.

Petrol and diesel engines have advantages.

 

[HareBrain]

By the way, with turbines, remember they don't start from cold like a petrol engine.

Ha -- I like the way you used the word "remember", as though you might be talking to someone not in lifelong ignorance of that fact!

Because of the different metals, with different coefficients of thermal expansion, you have to trickle steam through well below motive power rates to warm the whole thing up to operating temperature before opening the throttle on the steam. If you don't, you just strip the turbine blades off the axle.

That's useful to know, thanks.

So how might the steam aircraft VB linked to above have coped with water loss, given the desirability of keeping weight to a minimum? (Edit: you've already said you don't know about turbines, so maybe losses are lower.)

(BTW, fascinating about the express-train scoops. I would never have guessed.)

 

[Montero]

No idea about aircraft - didn't even know that someone had tried. There were steam cars and steam lorries at one point as well as traction engines. I think all piston not turbine. Don't know if looking at them would help.
BTW, I'm not an engineer myself, just hung around a lot of engineers.

Depending on who comes along on here next, I'd suggest looking for a steam enthusiasts website and asking them about how a steam powered aircraft would operate.

Further thought
Materials tech. The materials needed for turbines are a lot harder to create than for steam piston engines. (Mind you, you have to have pretty good metals tech to create cylinders and boilers which don't explode.)
When you have anything spinning, then you have a lot of forces involved, which can cause part of the thing spinning (e.g. the end of a turbine blade) to snap off. (Google centrifugal and centripetal forces.) So the materials have to be very advanced for it to work. The first steam turbine was invented by Parsons (google Turbinia - its a great story) and turbines are still being refined today - Rolls Royce's jet engines with extremely advanced ceramics and cooling ducts and design shape.
So your world's material tech will limit the speed of the engine. It also gives you several versions of nasty accidents for your plot.

One chemistry text book I had, had a rhyme in the front which went something like:

First the test tube, then the pail,
then the semi-working scale,
ever bigger, ever faster,
faster, faster, then disaster.

 

[chrispenycate]

The steam condenser pipes could be used to heat the gasbag, thus giving extra lift. But 100MPH airspeed!? Air friction goes up, as I remember it, as the surface area times the square of the velocity. Even with something like dolphin skin friction reduction, and optimum aerodynamic shape (which changes with velocity, but isn't far from that ellipsoid shape dirigibles are recognised by) the gasbag is enormous, so frictional forces will be too, just from pushing the air out of its path.

However the nacelle can't hang beneath the gasbag; it has to be incorporated into the shape, partly to reduce turbulent flow, partly for the transfer of many tons of motive force. In our timeline dirigibles had big propellors turning quite slowly. This also reduces cavitation and unbalanced forces attempting to distort the skin. For a turbine I suspect you're not going to try and gear down, but run full power all the time, which involves small, fast-turning props as on present day aircraft.. An airship has much less tendency to "coast" (less mass, more friction). It is also much harder to brake than a locomotive or anything on wheels, so would probably be worth having modern variable pitch props that can be feathered for reverse thrust (parachute braking might well be effective at 100 mph – it's got to be better than dropping an anchor over the side to ensnare a church steeple.

Turbines generally use lower pressures than cylinder steam engines, which is good because the boiler can be made lighter (not by any means light, just slightly less excessively heavy). Perhaps even made of a lighter alloy than steel (somehow my brain rejects magnesium, though). But it does demand a higher throughput, which means more weight of water to start with.

Lift goes up as the cube of the radius, but so does weight of just about everything

 

[HareBrain]

The first steam turbine was invented by Parsons (google Turbinia - its a great story)

No need -- I'd already pinched the fleet review episode for the first book.

But 100MPH airspeed!?

Oh dear ... when I said "it's not an airship as such" I should have made it more clear how much it wasn't. It looks more like a primitive Heinkel 111. Sorry. But I am now more educated about airship propulsion, so thank you.

I think that's about all the technical stuff I need, thanks all. For better or worse, none of my POV characters are going to be interested in how this thing works; I just needed to think it was possible.

 

[Montero]

OK - definitely not an airship.

I was about to come back to point out H2 vs He, and that more advanced technology is needed for extracting the latter.

 

[Mark R]

You can "cheat" this by simply having a metal that has all the properties of steel but weighs a great deal less. The biggest problem with a steam powered aircraft is the power to weight ratio, and this fixes that in an instant.

 

[Gumboot]

As far as I understand it, a turboprop engine uses the thrust from a jet to drive a turbine that turns an aircraft propeller.

Not really. A turboprop is an engine that uses a gas turbine to drive a propeller shaft.

A jet engine such as a turbojet or a turbofan uses the gas turbine to drive a compressor which accelerates airflow through the engine, producing a high pressure exhaust which provides thrust.

Is there any reason the turbine couldn't be driven by steam, like a ship's screws?

In theory, yes. There's no reason a steam turbine couldn't replace a gas turbine. The only issue would be weight. As opposed to a gas turbine, where the fuel is what drives the turbine, in a steam turbine the turbine is driven by heated water, which means you need to carry water for the turbine, as well as a fuel for heating the water.

 

[sknox]

All chemistry textbooks should have poetry in them!