Fuel
The problem announced itself the moment Arista actually calculated the zeppelin's projected flight duration against her standard steam engine's wood consumption rate, a sobering number that made her sit back from her own notebook with genuine dismay.
"...That can't be right." She recalculated it twice, hoping for a different answer, and got the same grim result both times. "If I'm burning wood at the rate every other steam engine I've built has needed, a flight long enough to actually reach unconfirmed territory and return would need more than half the gondola's total volume just for fuel storage. That's not a reconnaissance vehicle anymore. That's a flying woodpile with a passenger compartment bolted on as an afterthought."
She sat with the problem for a while, running through her existing toolkit, lava heat had solved this exact issue for the Hell-based systems, but open ocean obviously offered no equivalent free heat source. This needed an entirely different approach.
Charcoal, she wrote first, working through the logic methodically. Considerably higher energy density than raw wood, the same mass took up less volume while burning hotter and longer, precisely because the slow, controlled burning process that produced charcoal in the first place had already driven off the water and volatile compounds that made ordinary wood burn less efficiently. Less bulk, more heat per load, a straightforward improvement over her baseline assumption.
Pressed charcoal briquettes, next: an even better refinement, she realized, once she actually thought through the storage problem properly. Loose charcoal packed unevenly, leaving wasted air gaps between irregular chunks. Compressed into uniform briquettes, the same mass could be stacked considerably more densely, squeezing out exactly the kind of inefficient empty space that had inflated her original wood calculations in the first place.
This tale has been pilfered from NovelPhoenix. If found on Amazon, kindly file a report.
She sat back, satisfied with the improvement, but the numbers still weren't quite where she needed them for genuine confidence in a long-range ocean crossing.
...A hybrid system, she added finally, the idea arriving sideways the way her best ones always did. A smaller steam engine than she'd normally build, not the powerhouse driving Buzzy or the metro's full network, just enough continuous output to maintain cruising speed and altitude control. And alongside it, sails, properly rigged along the envelope's structure, catching genuine wind the same way any oceangoing vessel would, doing the bulk of the actual propulsive work for free whenever conditions allowed.
"...Steam for control and reliability," she murmured, sketching the combined system. "Sails for the actual heavy lifting, whenever the wind cooperates. The engine doesn't need to do everything alone anymore. It just needs to handle what sails can't, steady, predictable altitude and directional correction, regardless of what the wind's doing at any given moment."
She ran the fuel math again with the revised system, briquettes instead of raw wood, engine output scaled down to a supporting role rather than sole propulsion, and finally landed on a number that made actual sense, fuel storage shrinking from an unworkable majority of the gondola's volume down to something genuinely proportionate, leaving real room for crew, supplies, and Colm's carefully requested optics equipment.
"...Okay," she said, grinning at the finally-workable design. "That's viable. Charcoal briquettes for density, hybrid steam-and-sail so the engine's not carrying the whole burden alone." She glanced up at the envelope's rough sail-rigging sketch, already imagining the actual shape of it, something between an airship and a proper sailing vessel, entirely unlike anything she'd built before.
"...We're basically building a ship that sails through the sky instead of across water," she murmured, delighted by the framing. "Nettle's going to hate that I said it that way, given the whole race thing."
She said it anyway, out loud, to the empty workshop, already reaching for fresh materials to start the actual sail rigging.
"...Race it is, then."