The upturned nozzle sat atop a short, fat cylinder, and two smaller bells protruded from the cylinder’s sides. The cylinder was the pressure shell which contained the radioactive core, and the smaller nozzles, gimbaled, were attitude control rockets. She saw the ring of cone-shaped actuators at the base of the engine; the actuators operated the control drum, which moderated the reactor. A huge spherical hydrogen tank sat close to the engine, and pipes snaked from it to swaddle the pressure shell and nozzle. Plumes of vapor vented from the tank, and sheets of ice encrusted its sleek metal walls.

Adam Bleeker helped her trace out the engine’s operation.

“Liquid hydrogen works as both propellant and coolant — it’s called regenerative cooling. A pump pushes the hydrogen through that cooling jacket surrounding the pressure shell and the bell. Then the hydrogen is forced through the radioactive core, where it flashes to vapor, and drives its way out through the bell…”

There was still no trap for the vented hydrogen, York noted absently.

Bleeker showed her how an efflux pipe from the reactor carried a proportion of the hot hydrogen gas to a turbine, to power the engine’s pumps in flight. The turbopump exhaust was used as attitude control gas, vented off through the small supplementary nozzles.

“What’s the problem tonight?”

“Cavitation. Gas bubbles in the liquid hydrogen flow. We raised the core temperature to its working regime, and we’d started the hydrogen flow. We reached nominal thrust, for about half a second. But then the core temperature started to climb. We were cavitating, somewhere below the pump: hydrogen bubbles, stopping the circulation of coolant. And that was why the core temperature was climbing. We had to shut down.” He sounded tired. “You can imagine the safety restrictions we’re under here. If the pressure shell had been breached, we’d have had radioactive products reaching the atmosphere, and there would have been hell to pay. So as soon as we saw the problem, we had to obey the rule book, which said to shut off the hydrogen, and flood the whole damn core with water to ensure the temperature comes down. Now we’re going to have to siphon off all that radioactive water, and take the core apart by remotes, and make sure the propellant flow cylinders haven’t been damaged in the heat… It will be days before we’re set for the next test.”

“Christ. What a mess.”

York studied his profile; picked out by the powerful floods, Bleeker’s skin looked thin, almost translucent. She found it hard to read Bleeker’s own reaction to all this. Was he impatient at the restrictive rules, the need for safety? Did he have any qualms about handling lethal substances in such an unstable, unproven rig? She couldn’t tell. Just as when she’d first met him, Bleeker struck her as utterly calm, cool. Or, perhaps, completely without a soul.