The Margin

The tug Beduin carried no computer that Okonkwo trusted with his life, which was why he was alive.

That was the old hand's joke and also the literal truth. The flight computer had taken a cosmic-ray hit eleven days out and now insisted, cheerfully, that the ship massed four hundred tonnes and was presently on fire. Okonkwo had switched it to telemetry-only and gone back to the way his grandfather had crossed water: by hand, on paper, with a slide rule worn so smooth its scales were ghosts.

He was a propulsion tech, not a navigator, but out here past the inner belt those were the same job done lonelier. The Beduin was hauling a fueling depot to a rendezvous with the freighter Cataract, and the entire economy of the run came down to one merciless equation. To change your velocity by some amount delta-v, you burn fuel; the more you want to change, the exponentially more you spend, because every kilo of fuel you carry is mass you also have to push. Tsiolkovsky had written it down three hundred years ago and the universe had not once granted an appeal.

The transfer window to the rendezvous opened in nineteen hours and stayed open for ninety minutes. Miss it and the next geometry was four months out — longer than his air, longer than his nerve.

He ran the burn budget the way he ran it every morning. And this morning, for the first time, the numbers did not close.

He had thedelta-v he needed for the transfer. He had, separately, the delta-v he needed to brake at the far end so he didn't ram the Cataract at two kilometres a second. What he did not have, when he added them honestly, was fuel for both. Somewhere a tank had been mislabelled at loading, or had vented slow for eleven days, and he was short. Not by much. By a margin.

A margin was the difference between a clever man and a dead one.

Okonkwo set down the slide rule, and the cursor — the little glass slider that read the scales — chose that moment to crack along its hairline and fog.