Miningman
I'm beginning to think that steam technology as applied to locomotives has millions, perhaps billions of variables, and like a lottery, out of all of it, all the variables, there is one correct grand winning ticket.
The problem there, just as with Marxist/Hegelian pseudoscientistic theory, is that there is no One True Design that optimizes everything at once. Porta grappled with this far more intensively than I ever have, with far more enthusiasm; in a sense his 'Argentina' still represents a kind of high-water mark in a surprising number of interrelated respects ... but is evidently not a guide for even 'Generation 2' steam.
Note that solutions for even something as simple as Andreas Schwander's steam monorail or 'Plandampf' conversion of Swiss commuter service are different from Porta's practical 'biomasa' projects. And both those are wildly different from Tom Blasingame's designs, or the 5AT optimization, or either the original or 'upgraded' Turbomotive 2.
The split becomes even more radical when you have to start with fuel technology 'politically' and practically suitable for purpose. (With the alternative always being properly-treated B100 or ethanol derivative from biomass, with any required additives, with DEF in an IC engine manufactured with many alternative uses and the usual higher engineered flexibility...)
I of course have beat the drum for forced waterwall circulation replacing pachinko staybolting in the radiant section of a 'Stephenson' boiler for some time. A combination of shipbuilding fab techniques and laser hybrid welding can produce a leakproof version of a Jacobs-Shupert firebox that contains suitable geometry for this, and even a 'multiple-thread screw' waterfall as in LaMont's ship boilers (each "thread" going to its own centrifugal inertial steam separator, etc.) should get you there -- the following convection stage being entirely tuned like a HRSG with common header into the separator vessels, and long-path economization down to below combustion-water condensation in the feed train -- etc.
The general consensus of whatever actual technologists remains active at SACA is that this approach scales to automobile size and is preferable to other 'modularly-scalable' boiler designs including all the flash or semiflash generators that modulate flame and water in counterflow.
If you want to go high-tech there is still promise in the enginion AG ultrasupercritical approach, which appears to have foundered more on politics and weird expediency (a German electrical-generating company bought a large stake in the company doing the development, then forced it into optimizing the design for distributed emergency grid-power generation!). This approach uses liquid fuel to generate very high-pressure steam and then superheat it separately at time of use, injected as a supercritical liquid for long expansion). While this requires relatively exotic metallurgy (with alloy elements from certain 'other lands') the absolute amounts are relatively small, as is the actual mass flow required to make power. Makeup is with highly distilled water actively treated for self-dissociation, which gets rid of the various problems with feedwater treatment cost and consequences otherwise requiring something like McMshon-Porta treatment.
And there is the Oxford Cycle, which uses a fairly cheap catalyst mixture to react methanol or ethanol fuel in water with roughly 30% hydrogen peroxide (distilled from natural sources) to produce steam directly at the maximum superheat for conventional locomotive-suitable expanders -- somewhere in the 800 to 840F range, largely hinging on required tribology. The controls are fascinatingly different from most historical steam plants, and the carbon emissions about as low as you can get with a practical non-carrier fuel formulation. Unfortunately terrorists can make a nifty hard-to-detect state explosive out of H2O2 and 'household chemicals' ... part of why we can't have nice things. (For the sake of completeness, Dave Klepper's beloved MIT has a version of this for space power that makes steam at something like 2200 degrees F; this is nifty for orbital applications but not so much for locomotive practice...)
And then there us the fascinating world of bottoming cycles, of which the poster child is the ALPS locomotive combined with the asynchronous compound. You can actually still get to much of the detail design via the University of Texas website resources (although you have to dig a bit now; start with the CEM published papers). Whenever this comes up in conversation it reminds me with the twinge of a bad tooth that it is a shame the JetTrain design was altered...
The rules of the game would be the laws of Physics.
Except that they are not. And, like it or not, seldom have really been. The laws of economics, and behind them the rules and principles of finance, have almost always dominated and dictated the "best solutions" for steam or other power. Note that while there are exceptions, they all have 'failed to thrive' over time; it would be nice if this were a conspiracy theory like NCL or the idea that GM set up a cabal of cognate interests to buy up all the makers of specialty or patent steam-locomotive auxiliaries either to shut them down (like Soros was accused of plotting to do with domestic firearm manufacturers) or simply and innocently pricing them in line with increasingly limited production... which, with the greater strikes against conventional recip steam, would have the same effect.
But, like the rules in say a hockey game, the outcomes are unpredictable despite the rules. Not surprisingly there is a winning ticket out there but it makes me wonder if we have yet to encounter it.
In my opinion, if there is, it would require the somewhat magical convergence (usually not properly acknowledged as such) of Sloan's management of EMC and Winton in the decade after their strategic acquisition. In other words, innovative engineering, savvy promotion, and both 'championing' (in the six-sigma sense) and broad and deep-pocket support from an organization stable enough to support it long-term to design and production and aftermarket maturity. No reason at all why that combination would 'only' work for diesels, and I think had there been financing and some kind of trade-in credit offered when railroads were relatively flush with cash after WWII not only might things have been different for a while, some of the 'new' approaches to steam might have had the chance to flourish or 'build share'...
Instead, Baldwin didn't even learn the lessons of the USRA, and Alco 'bet their company' on an alliance with GE and are gone largely as a result of it, and Lima missed the boat on where postwar Super-Power would come from...
Get that combination of savvy and morale, though, and I have some bingo moments for ya. But be advised that comparatively few of them resemble attractive legacy steam. There are very few powerplant foamers out there... perhaps for understandable reasons... 