Aero-steam
...
Myeah. Closed-circuit boilers exist; they're found in ships and basements. Making a reliable, lightweight model in 1920 is no small feat, and likely why the whole project failed to get off the ground.
It's not the boiler as much as the condenser that's the problem; engine cooling was tricky enough for IC engines where the cooling is auxiliary, when it's the working fluid that has to be cooled down to liquid temperatures to be reused the heat flux is much greater.
Steam had some apparent advantages to consider, especially in the days before superchargers were developed to the standards available in WWII era.
For one thing, the engine itself can be quite light--if you don't consider it needs a boiler and a condensing system. But the boiler need not be very heavy, and at a great price (needing to carry an expendable stock of boiler water) one can dispense with the condenser, as I believe t
he only steam airplane I've ever heard of, the
Besler, did.
A steam engine's performance would not be degraded at higher altitudes (in fact the cooler air there would help with condensing the steam, and a steam engine that simply discharged spent steam would benefit from lower ambient pressures) so they seemed like a possible approach to solving the problems that OTL were solved with superchargers.
A steam engine can be run at variable speeds without loss of efficiency, so instead of variable-pitch props one can simply run the prop at a lower RPM at lower airspeeds. This is good for cruising at optimal lift-drag speeds at various altitudes--not so good for getting both good take-off thrust and good cruise efficiency, which is what the variable-pitch prop designed to allow constant engine speed delivered. Of course a steam engine
and a variable-pitch prop could be a very nice combination! But far better for airships, which want efficiency at a great range of airspeeds, than airplanes which typically fly in a narrow range of airspeeds.
An airship also has a good trick available to it, recently (in the past couple of decades that is) patented by
Tom Goodey--the steam balloon or ballonet as condenser. If you are using a gasbag full of steam, it gives respectable aerostatic lift--but unless jacketed inside some other gas maintained at higher than boiling temperatures, it will be cooling and the steam condensing--so if you are using a steam engine, you make lemonade from this lemon--that's your condenser, just match the radiating area to the expected output of the steam engine.
But we aren't talking airships here; for an airplane condensation is just a hassle.
And worse than that, steam engines are just plain inefficient compared to IC engines. The engine itself can be far lighter for a given power output, but you have to factor in the weight of a separate boiler (not necessarily too severe if it's a flash boiler, but the design and operating control requirements are a bit stringent) and the condensing system which probably also adds to the air drag, and withal even a very high-tech steam engine is limited by thermodynamics versus the maximum sustained temperatures available materials can handle while under stress to lower efficiencies than IC, meaning more fuel per air-mile covered, which negates the advantage steam has that the range of possible fuels is much extended (anything that will burn versus the exacting demands of either a petrol-type or diesel-type fuel--actually diesel fuels cover a broad range too though not as broad as steam boiler candidates).
That's with condensing systems. If one dispenses with the condenser then the engine, even with a boiler, can be nice and light--the price you pay is that in addition to needing more fuel due to the lower thermal efficiency, you need to also carry water which will be expended just like fuel; the combined weight of both more fuel and also boiler water is much greater per mile than for an IC engined plane, so it's a nonstarter for any long-range application.
Given that it could however perform well at high altitudes, I can see a scenario, assuming the funding was there in the relatively peaceful late 1920s and early '30s, for developing a steam-engined point-defense interceptor, designed to climb to high altitudes and swoop down on attacking bombers at very high speeds, back in the days before variable-pitch props and superchargers became standard items in high-performance plane design. The poor ratio of fuel/water weight doesn't matter so much if the mission is limited like that. Of course in those days there was no radar and the question would be, would the defenders have any suitable early warning of the impending attack so they have enough time to scramble and climb, but not have to maintain a sustained air patrol, using up fuel/water at a high rate even in efficient cruise mode...?
Given both that the attitude in the 20s and early 30s was that war could, should, and would be avoided, and the specific doctrines of air power that did prevail when governments could be persuaded to fund development at all, it is not too surprising no one ever seems to have taken the concept of the point-defense interceptor too seriously and certainly never funded a steam-powered version--by the time the need for such fighters had been demonstrated (and made more practical by the development of radar) supercharging and variable-pitch props had both been developed to the point that no one would even consider the steam engine.
To close, I've had my own extreme and wacky idea for some time--could a combination of steam engines and very large steam-inflated delta-winged fabric hybrid airship/airplanes have been used to enable extremely high-altitude bombers?
At low altitudes, the combination of steam lift in the huge "wing" volume and the very large wing area would reduce takeoff speeds to practically nothing; climbing, the steam-inflated wings would need to either vent steam or climb slowly enough to allow condensation to keep the pressure within limits, but as the buoyant lift declines and air densities fall, the need for airspeed to maintain dynamic lift rises--this plus the cooling air of higher altitudes helps with condensation and steam engines can be more and more powerful while the supply of water for them remains constant. I wonder if very extreme altitudes, so high that neither interceptor fighters nor ground-based artillery could take them out (despite the evident fragility of a huge fabric wing dependent on pressure to maintain its shape) and they could thus fly over targets to bomb them with impunity?
Of course if one side can make these things capable of climbing to that height, the other can presumably scrape together some kind of interceptor to match it. And targeting accuracy for bombs dropped from say 20 or 30 kilometers altitude would be terrible, unless there were some kind of terminal guidance--say the bombers also carried imaging radar or powerful optics for the bombardiers, and guided the bombs (with steerable tailfins and simple radio receivers designed to only listen to signals coming from above?) to their targets.
Anyway if they are using steam engines to allow operations at altitudes beyond what superchargers could enable in the WWII era, they are paying a penalty in fuel weight--they'd have to be quite aerodynamically efficient to offset that.
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Sorry OP, I tend to agree with the consensus that on the whole biplanes have seen their day. I have yet to understand why the Belphegor (that's that Polish biplane crop duster) was designed in the first place--it seems plausible that its agricultural role is a cover for its gas-warfare real purpose, but still the question comes up--why use a jet engine when props are more efficient at those lower airspeeds?
According to one of my books, when it was first displayed at Farnsborough, Western wags suggested that as a cropduster it didn't need to carry any actual poisons--just fly it over the fields and the insects would die of revulsion at its mere appearance!
Anyway I offer you the steam airship and the superstratospheric steam-inflated giant hang-glider bomber in compensation for my coming up empty on cool biplanes!
--Though come to think of it, a steam-engined 1930 high-altitude, high-powered interceptor might conceivably be a biplane (of very clean design, anyway) at that...
