Launch checklist · Terminal document
Halcyon‑4 / ORB‑12 count sequence
Fifteen sequential verification steps from pad clear through release. No step may be stamped until the step above it carries both initials. Any station may call a hold; the count does not resume until the hold condition is cleared on the record.
Pad Configuration
T‑06:00:00 → T‑02:40:00Pad crew still on deck. Everything in this block is reversible; once the block closes, the flame trench is sealed and re‑entry to the deck requires a director‑level waiver.
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001T‑06:00:00Pad Ops
Deck cleared to red line, headcount reconciled
All non‑essential personnel off the 40 m ring. Badge reader count must match the pad manifest exactly — no tolerance, no verbal reconciliation.
Hold condition H‑A1
Trigger: badge count and manifest differ by one or more, or any reader reports a stale scan older than 20 minutes.
Action: hold at 001, sweep the ring on foot in pairs, re‑badge at the north gate. Minimum hold 12 minutes.
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002T‑05:20:00Structures
Service tower retracted to park, pins witnessed
Tower at 12° park angle. Both retention pins visually witnessed by a second observer at the base — camera feed alone is not acceptable evidence for this step.
Hold condition H‑A2
Trigger: park angle outside 11.4°–12.6°, or either pin unwitnessed at the base.
Action: hold, re‑drive the tower under manual control, re‑witness. Structures lead signs the exception line.
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003T‑04:05:00Avionics
Flight computer triplex vote — all three strings agree
Boot all three flight strings, run the 90‑second cross‑vote, confirm zero disagreements in the guidance and sequencing channels.
Hold condition H‑A3
Trigger: any string disagrees on a guidance or sequencing channel, or a string fails to boot within 45 s.
Action: hold, power‑cycle the disagreeing string once. A second disagreement is a scrub, not a hold.
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004T‑02:40:00Safety
Flame trench sealed, deluge armed at 60 %
Trench doors latched, deluge header pressurised and held at 60 % of nominal. Closing this step ends deck access for the remainder of the count.
Hold condition H‑A4
Trigger: header pressure drift greater than 4 % over two minutes, or a door latch reporting intermediate.
Action: hold before sealing. Deck access remains open while the hold stands.
Propellant Load
T‑02:20:00 → T‑00:34:00Cryogenic operations. From here the vehicle is a pressure vessel and the pad is a hazard area. Holds in this block cost propellant; every minute of hold is 40 kg of boil‑off.
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005T‑02:20:00Fluids
LOX chilldown to −183 °C, transfer line stable
Chilldown across the full transfer line with less than 2 K of gradient between the farm skid and the vehicle interface.
Hold condition H‑B1
Trigger: gradient above 2 K sustained 90 s, or any bellows frost signature outside the mapped pattern.
Action: hold, recirculate for six minutes, re‑image the bellows before resuming.
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006T‑01:48:00Fluids
Stage 1 LOX fast‑fill to 94 %, topping armed
Fast‑fill at 780 L/min until the 94 % probe wets, then hand over to the topping loop. Ullage pressure tracked continuously against the fill curve.
Hold condition H‑B2
Trigger: fill rate off curve by more than 6 %, or the 94 % probe wets early.
Action: hold, drop to slow fill, verify probe against the mass gauge before resuming.
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007T‑01:02:00Fluids
Stage 2 fuel load complete, mass gauge reconciled
Upper stage load closed out. Gauged mass and flow‑integrated mass must agree within 0.3 % before the step is stamped.
Hold condition H‑B3
Trigger: reconciliation outside 0.3 %, or the mass gauge fails its self‑check.
Action: hold, settle for four minutes, re‑read. Persistent disagreement escalates to the director.
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008T‑00:34:00Range
Range green — sea, air and downrange corridors clear
Range safety confirms all three corridors clear with a 14‑minute forward projection. Weather balloon data no older than 30 minutes.
Hold condition H‑B4
Trigger: any intrusion inside the corridor, upper‑level shear above 22 m/s, or triboelectric field over 1.2 kV/m.
Action: automatic hold, no discretion. Resume only on a fresh range call, logged verbatim.
Terminal Count
T‑00:15:00 → T‑00:01:00Automated sequencer has authority; the room is now a monitoring function. Steps here are stamped as observations, not actions — the crew is confirming what the vehicle has already done.
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009T‑00:15:00Flight Ops
Go / no‑go poll — all twelve stations report go
Director polls each station by callsign on the loop. A silent station is a no‑go; there is no assumed go on this document.
Hold condition H‑C1
Trigger: any no‑go, any station silent through two calls.
Action: hold at 009, re‑poll after the reporting station clears its issue on the record.
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010T‑00:07:00Avionics
Vehicle to internal power, ground umbilical on standby
Bus transfer with no dropout longer than 8 ms. Umbilical stays mated and live until the release sequence.
Hold condition H‑C2
Trigger: dropout over 8 ms, or any bus voltage below 26.4 V after transfer.
Action: revert to ground power, hold, investigate. Two failed transfers is a scrub.
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011T‑00:03:30Propulsion
Tank pressurisation to flight, vents closed and latched
Both stages pressurised to flight values, vent valves closed and latched with position confirmed on two independent sensors per valve.
Hold condition H‑C3
Trigger: disagreement between paired position sensors, or pressure decay above 0.8 kPa/min.
Action: hold, vent to safe, re‑pressurise once. Cryo margin allows exactly one retry.
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012T‑00:01:00Flight Ops
Sequencer handoff — count authority to vehicle
Ground releases the count. From this stamp the vehicle owns the clock; the room retains only the abort function.
Hold condition H‑C4
Trigger: any open item on the exception line, or a station requesting time.
Action: this is the final discretionary hold. After 012 only the abort path remains.
Ignition & Release
T‑00:00:18 → T+00:00:00Sub‑minute events. These lines are stamped after the fact from the sequencer log; they exist so the paper record closes at T‑zero rather than at the last human action.
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013T‑00:00:18Propulsion
Igniter continuity confirmed on all nine chambers
Sequencer reads continuity across every igniter circuit and arms the start sequence. Logged, not commanded, by the room.
Hold condition H‑D1
Trigger: continuity fail on any chamber. Sequencer aborts autonomously — the room cannot hold here.
Action: safe the vehicle, recycle to T‑00:15:00 if the window allows.
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014T‑00:00:06Propulsion
Start command, thrust building to 96 % nominal
Chamber pressures climb through the health gate. Hold‑down clamps carry full thrust while the sequencer evaluates all nine chambers.
Hold condition H‑D2
Trigger: any chamber below 92 % at the gate.
Action: automatic shutdown on the pad, clamps retained. Not a hold — a terminated start.
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015T‑00:00:00Flight Ops
Clamp release — vehicle has cleared the tower
Hold‑down release, first motion, tower clear. The document closes on this line and transfers to the flight log.
Hold condition H‑D3
Trigger: none. There is no hold condition on this line.
Action: retained in the document only so the sequence is complete on the page.
Document closeout
This checklist is not valid until every line carries two initials and every hold called during the count has been released on the record. Partial documents are filed as scrubs.