Oxygen
Claire found her mid-sketch, cross-referencing the new fuel chamber's dimensions against the rest of the hull layout, and set a fresh cup of tea down beside her without asking.
"Question," Claire said, settling onto the nearest crate. "What's your plan for oxygen?"
Arista paused, stylus hovering. "...Oxygen."
"Breathable air. For five people, sealed inside a hull, for however many hours we're planning to stay submerged." Claire said it plainly, the same brisk, practical tone she'd used catching every previous oversight. "You solved heat without needing atmospheric oxygen for combustion, which was the right call. But the crew still needs to breathe, and a sealed hull doesn't refresh its own air supply. What's the plan?"
Arista opened her mouth, then closed it, running back through weeks of design work and realizing, with a familiar sinking feeling, that she genuinely didn't have one.
"...I have ideas," she admitted slowly. "None of them concrete. I was sort of assuming we'd carry compressed air, or maybe cycle the atmosphere through some kind of filtration, but I haven't actually worked out the mechanism for either. I got so deep into heat and propulsion and defense that I think I just... assumed breathing would sort itself out."
"It won't," Claire said, not unkindly. "Five people breathing in a sealed space consume oxygen and produce carbon dioxide at a fairly predictable rate. Without a plan, you're looking at a hard, calculable time limit before the air itself becomes the actual danger, regardless of how good the hull, the fuel, or the defenses are."
Arista sat with that, the specific weight of yet another overlooked fundamental settling in. "...Okay. I clearly need to think about this properly. Compressed air tanks, maybe, something we carry and vent slowly—"
"I already have an idea," Claire said, and there was something in her tone: calm, assured, already halfway to a solution, that made Arista stop talking entirely and actually listen.
"...You do?"
"Electrolysis." Claire said it simply, like it should have been obvious. "You're already generating electrical charge for the defense net, off the crystal chamber. Water splits into hydrogen and oxygen when you run a current through it, basic reaction, well understood, doesn't require anything exotic. Run a small, continuous current through a contained volume of seawater, separate from the main defense system, and you get a steady trickle of breathable oxygen as a byproduct."
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Arista stared at her, the pieces clicking together with the specific, satisfying speed of a solution that had been sitting right there the entire time, hidden behind a system she'd already built for an entirely different reason.
"...I'm already generating the electricity. I just need to redirect a fraction of it toward a separate reaction chamber instead of only the defense net."
"Exactly. Small, dedicated electrolysis cell, tapped off the same crystal-generated charge, running continuously whenever the boiler's active. Hydrogen vents externally, it's not remotely useful to you, and you don't want it accumulating inside a sealed hull regardless, for fairly obvious reasons. Oxygen gets collected, filtered, and cycled into the crew compartment."
"And the carbon dioxide buildup?" Arista asked, already sketching furiously, the whole system reorganizing itself around Claire's addition.
"Separate problem, smaller one. There are absorptive compounds that scrub carbon dioxide from air reasonably efficiently, I'd want Fen's input on the exact chemistry, but the mechanical side is simple. Circulate cabin air through a filtration chamber packed with the right material, let it absorb the excess CO2, vent the scrubbed air back into the cabin." Claire shrugged, like she'd been carrying this solution in her back pocket the entire time, waiting for the right moment to hand it over. "You already know how to build filtration systems. You built one for the metro's ventilation. This is the same principle, just scaled down and sealed."
Arista sat back, staring at the completed sketch, crystal chamber powering both the defense net and, now, a dedicated electrolysis cell, hydrogen vented safely away, oxygen cycled into the cabin, a CO2 scrubber handling the other half of the equation.
"...You just solved the single biggest gap in this entire design in about ninety seconds."
"I've been thinking about it since the zeppelin," Claire admitted, a small, satisfied smile crossing her face. "Altitude and depth have more in common than people assume, both environments where the air you'd normally take for granted isn't just sitting there waiting for you. I started sketching this before you even finished the electric net design, actually. Just wanted to see if you'd catch the gap yourself first."
"I did not catch the gap myself."
"No," Claire agreed, entirely unbothered. "You didn't. That's why I asked."
Arista laughed, equal parts relieved and genuinely impressed, already integrating the new system into the master schematic. "...Okay. Oxygen's solved. Electrolysis cell, hydrogen vent, CO2 scrubber, all running off the same crystal charge as the defense net." She glanced up at Claire, grinning. "You know, for someone who joined this crew because I forgot boilers can fail mid-flight, you've become suspiciously essential to basically every system on this thing."
"That's rather the point of bringing a specialist," Claire said, already standing, coffee forgotten, heading toward the workbench to start drafting the actual electrolysis cell specifications. "You handle the impossible ideas. I handle the part where people actually survive them."