Greg Egan - The Clockwork Rocket

What would Tullia have done? Joked, reasoned, mocked, argued, probed all the competing theories for weaknesses, shone some light into other people’s blind spots, then followed her own imperfect instincts like everyone else. Yalda had never stopped missing her, but she was confused enough already without resorting to begging ghosts for advice.

The light on the floor brightened; Yalda turned back to the window. A fresh, dazzling streak of violet had appeared; it spread out slowly, parallel to the older trails.

She needed to make some kind of decision, if only to let herself sleep. So… she would take an interest in Eusebio’s plans and offer him whatever guidance she could, trying to help him spot the pitfalls in his strange endeavor. She could do that much without agreeing to be a passenger on his mad flying mountain—and without abandoning hope that the cosmic pyrotechnics that obsessed them both might yet turn out to be as harmless as a swarm of mites.

Eusebio led Yalda on foot for the last few saunters across the dusty brown plain. “Sorry to make you walk so far,” he said. “But it’s not a good idea to bring the trucks too close.”

Yalda took a moment to understand what he meant. The liberator for truck fuel wasn’t identical to that used with sunstone, but it could still cross-react. A pinch of gray powder spilt from a tank and carried the wrong way on the wind could heat things up very quickly.

“So how did you get the sunstone out here in the first place?” she asked.

“The way it’s always transported: well wrapped, in small pieces.”

They reached the test rocket: a cone of hardstone about Yalda’s height. Apertures near the top revealed an intricate assembly of cogs and springs secured within. “Most of this is for attitude control,” Eusebio explained. “If the sunstone burns unevenly, that puts a torque on the rocket and the whole thing will start to swerve. The mechanism needs to detect that and respond quickly, adjusting the flow of liberator between the combustion points.”

“Detect it how?”

Eusebio took a crank from a box of tools that had been left beside the device, and began winding the main spring. “Gyroscopes. There are three wheels set spinning rapidly on gimbals; if their axes shift relative to the housing, that means the rocket is veering off a straight course.”

When he’d finished winding the spring he squatted down and gestured with the crank at the bottom of the cone, which was held half a stride above the ground on six stubby legs. “There are four dozen tapered holes drilled into the sunstone, lined with calmstone most of the way. The conical plug of sunstone that was cut out to make each hole is also lined and put back in, but with grooves carved into it that leave a gap between the pieces. The liberator flows down through the gaps and ignites the unlined part at the bottom. As the sunstone burns, the lining’s corroded, progressively exposing the fuel.”

Yalda joined him and peered up at the orderly array of not-quite-plugged holes. Sunstone lamps used the tiniest sprinkling of liberator, diluted with some kind of inert grit, to keep them blazing for a night’s performance in the Variety Hall—and they still killed a few careless operators every year. Deep inside this rocket, waiting to trickle down into the fuel, was a whole tank full of the stuff in its pure form.

“I remember when you were afraid to visit the chemistry department,” Yalda said.

“I was never afraid!” Eusebio protested. “You told me not to waste my time with chemists, because they couldn’t get their energy tables straight.”

“Yes, of course, that’s exactly what I would have said.” Yalda straightened and stepped back from the rocket.

Eusebio tossed the crank into the toolbox, then looked back across the plain toward Amando, one of the three assistants who’d been working at the site when Yalda arrived. Eusebio waved broadly with two hands, and received the same gesture in reply. Then he reached into the cone and released a lever.

“What does that do?” Yalda asked.

“One chime to ignition.”

“Thanks for the warning.”

“One chime!” Eusebio scoffed, picking up the toolbox. “We’ll be behind the barrier long before then.”

Yalda was already ten strides ahead. “Did your father really let you design his trains?” she called back to him.

“Yes, but I just tweaked someone else’s plans,” Eusebio admitted. “Trains are complicated. I wouldn’t want to have to invent them from scratch.”

When Yalda reached the truck closest to the rocket she saw the other two assistants, Silvio and Frido, lying chest-down on the tray. A sturdy timber barrier some four strides high had been attached to the side of the vehicle; the two men were propped up on their elbows behind narrow slits in which theodolites had been fitted. The theory was that the rocket would ascend vertically, run out of fuel, rise a little farther from momentum alone, then plummet back to the ground to make a crater at more or less the point from which it had risen. Knowing the height it reached would allow the team to quantify the amount of energy produced that actually ended up doing something useful. It was one thing to measure the effects of a quarter of a scrag of sunstone incinerated in a workshop experiment, where the gases produced in a sealed chamber drove a piston against a load. Expecting the same yield when gas was spilling off the sides of the rocket and fragments of heat-cracked fuel were free to sprinkle down onto the desert might be a bit optimistic.

Eusebio caught up and joined Yalda and Amando behind the shield. Amando said, “One lapse to go”; he’d synchronized his own small clock with the one Eusebio had set ticking inside the rocket. Yalda stared anxiously at the clock’s face. If the sunstone burned hotter and faster than expected, it wasn’t inconceivable that the flame could rise up and eviscerate the tank full of liberator, bringing everything together in one mighty flash when the rocket was too high for the barrier to protect them.

“What am I doing here?” she muttered, trying to decide how low to squat for safety without completely obscuring her view.

“Watching history being made,” Eusebio replied, not entirely seriously. Yalda glanced at Amando, but his face gave nothing away.

“Three, two, one,” Eusebio counted.

The barrier concealed the flash of ignition, but by the time Yalda felt the ground tremble a dazzling line of white light had risen into view. A moment later a deafening hiss arrived through the air.

Yalda shielded her eyes and sought the rocket at the top of the afterimage, but something wasn’t right; the line seared on her vision had been joined by an arc, a circle, a widening helix. Then the point of radiance that had been inscribing these curves dropped below the barrier, and the ground shuddered. She stiffened her tympanum protectively, deafening herself to the sound of the impact, then tensed for a larger explosion.

Nothing followed. Either the fuel had all been consumed, or the liberator had ended up scattered.

Yalda turned to Eusebio. He looked shaken, but he recovered his composure rapidly.

“It reached a good height,” he said. “Maybe ten strolls.”

It took them more than a bell riding around the desert before they found the remains of the rocket. If it had stayed in one piece it might have made a spectacular crater, but the fragments of hardstone casing and mirrorstone cogs strewn across the ground had barely gouged the surface, and some were already half-buried in the dust. If Yalda had stumbled upon them unawares, she would have called for an archaeologist.

“Attitude control,” Eusebio said. “It just needs some refinement.”

He left the others sorting through the debris and gave Yalda a ride back to the city.