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Countdown to Launch
This chart depicts the major launch countdown events for launching a Titan IV with a Centaur upper stage (click for bigger image). The countdown is divided into phases and tasks but controlled by a single launch countdown procedure. The countdown begins with launch status preparations and includes manual activities and an automatic launch sequence. Countdown operations begin at L–26 hours; however, the final countdown begins at T–780. The countdown clock refers to two times, L-time and T-time. L-time shows the actual time remaining to launch, while T-time takes into account preplanned holds. Weather balloons are released periodically to measure winds and weather conditions in the upper atmosphere. Terminal Countdown starts at T–5 minutes. During this phase, control is transferred to the launch control computer.
| T–25:00: | Countdown initiation | |
| T–24:55: | Launch vehicle power on | |
| T–22:30: | SRMU and core vehicle safing-pin removal | |
| T–20:30: | Guidance nulls and gains | |
| T–15:30: | Core vehicle pressure adjust | |
| T–14:00: | Begin final closeout | |
| T–13:05: | Comm checks and poll | |
| T–13:00: | Final countdown initiation | |
| T–12:25: | Range holdfire checks | |
| T–10:20: | C-band open-loop checks | |
| T–9:50: | Tower roll preparations begin | |
| T–9:50: | Launch vehicle readiness begin | |
| T–7:50: | Launch vehicle readiness complete | |
| T–6:50: | Guidance Control Unit (GCU) to alignment begin | |
| T–6:35: | Tower roll preparations complete | |
| T–6:35: | Tower rollback begin | |
| T–5:40: | GCU to alignment complete | |
| T–5:25: | Tower rollback complete | |
| T–5:25: | Tower securing and complex evacuation begin | |
| T–3:00: | Pad securing and complex evacuation complete | |
| T–3:00: | Planned 50 minute hold | |
| T–3:00: | Centaur propellant loading begin | |
| T–1:13: | Centaur liquid oxygen at 100 percent | |
| T–0:45: | Centaur liquid hydrogen at 100 percent | |
| T–0:45: | Range flight termination checks complete | |
| T–0:40: | Final command receiver decoder checks | |
| T–0:10: | Core pressurized to flight pressure | |
| T–0:10: | Load countdown abort sequence | |
| T–0:09: | Verify pulse beacon system on | |
| T–0:09: | Programmable Aerospace Ground Equipment refresh and hardline TM monitor | |
| T–0:05: | Planned 10 minute hold | |
| T–0:05: | Load countdown sequence | |
| T–0:05: | Winds update complete | |
| T–0:05: | Inertial Navigation Unit health checks | |
| T–0:05: | Readiness poll | |
| Terminal Countdown | ||
| T–0:05: | Resume countdown | |
| T–0:05: | Initiate terminal countdown auto sequence | |
| T–0:04:58: | Start emergency cameras | |
| T–0:04: | Verify ignition enabled and armed | |
| T–0:03: | Start range recorders | |
| T–0:02: | Announce T–2 minutes and counting | |
| T–0:02: | Start commit sequence | |
| T–0:01:59: | Establish abort monitors | |
| T–0:01:55: | Centaur to internal power | |
| T–0:01:40: | Vehicle Power Supply/Instrumentation Power Supply power transfer | |
| T–0:01:40: | Launch enable | |
| T–0:01:38: | SRMU power transfer | |
| T–0:01:36: | Centaur Data Station power transfer | |
| T–0:01:30: | Arm core and SRMU destruct | |
| T–0:01:14: | Arm Thrust Vector Control (TVC) ignition High Energy Firing Units (HEFUs) | |
| T–0:01:07: | Start Event Sequence Programmer (ESP) timer | |
| T–0:01:02: | Flight Control System to flight mode | |
| T–0:00:40: | Remove TVC heater power | |
| T–0:00:35: | GCU to flight mode | |
| T–0:00:29:30: | Open prevalves | |
| T–0:00:13: | Ascent vent door open | |
| T–0:00:10: | Start SRMU gas generator | |
| T–0:00:07: | Turn on pad water | |
| T–0:00:06: | SRMU nozzles to flight position | |
| T–0:00:04: | High-speed cameras on | |
| T–0:00:02: | Open Overpressure Suppression System valves | |
| T–0:00:01: | Lockout holds | |
| T–0:00:00:70: | Disconnect Centaur fill and drain lines | |
| T–0:00:00:20: | SRMU ignition | |
| T–0:00: | Vehicle liftoff | |
