Downstream of the engine, robots were also needed to maintain the shape of the rocket nozzle. This was a long duct with a wide mouth on the aft face of the shard, tapering to a narrow throat near the reactor. The throat had been constructed on Earth and launched up with the reactor. It was made of a corrosion-resistant alloy called Inconel. Any other material would rapidly wear out from the hot steam blasting through it. Conditions in the long spreading bell of the nozzle, however, were more benign, and so it worked fine for that to be sculpted from ice. Nonetheless, it changed its shape as it was used. Deeper in, where the exhaust was hot, it grew wider as its walls were melted by the torrent of steam. Closer to the exit, where the exhaust had cooled to below freezing, it accumulated on the walls and narrowed the passage. So robots had to scuttle around reshaping the nozzle. This was a fine task for the Nats that Larz had experimented with in Seattle.