Cold start: ion thruster warm-up in deep space
The procedure most crews use was written for station-adjacent operations. Out past the beacons it will cost you thrust when you need it most.
Every operations manual has a thruster warm-up procedure. Almost all of them assume you are within a few hours of a station and can afford to be conservative. Deep space is a different problem, and the standard procedure quietly makes it worse.
Why the standard procedure exists
Conventional warm-up ramps propellant flow gradually over roughly four minutes to avoid thermal shock at the nozzle throat. Sound reasoning: a cold throat hit with full flow can crack, and a cracked throat is a nozzle you no longer have.
The assumption underneath is that you have four minutes. Near a station, you always do.
Where it breaks
Out past the beacons the situation that calls for thrust is usually the situation that does not give you four minutes: a drifting mass on a converging track, a docking approach going wrong, a tumble that started for reasons you have not diagnosed yet. Crews trained on the gradual ramp will either wait out the ramp they do not have time for, or override it and crack a throat.
The deep-space procedure
The IT-12 supports an eleven-second cold start because the throat is pre-conditioned rather than gradually eased into service. The procedure:
- Keep two nozzles warm. Idle two of the twelve at minimum flow continuously. Cost: a negligible amount of propellant per day. Benefit: two nozzles always available instantly.
- Pre-condition on watch change. Run the full array to 20% for ninety seconds at each watch change. The throats stay above the thermal shock threshold for roughly six hours afterwards.
- Full cold start only when pre-conditioning has lapsed. Then, and only then, use the eleven-second ramp — and log it, because the throats accumulate cycles.
- Never override the ramp on a genuinely cold array. There is no emergency that is improved by losing a nozzle.
What this costs
Idling two nozzles continuously uses about 0.3% of your propellant budget on a ninety-day transit. Every operator I have talked to who has needed instant thrust in the deep has concluded that this is the cheapest 0.3% they spend.
A note on propellant
Refined tibanna conditions differently from xenon — it holds throat temperature longer but is less forgiving of a hard start. If you are running tibanna, extend the pre-conditioning burn to two minutes and shorten the interval to four hours. The array's own telemetry will tell you when it is outside the window; it is worth putting that readout somewhere the watch officer can see it without asking.