Workers pulling night-owl duty will begin countdown activities in the predawn hours Saturday, prepping the debut Delta 4-Heavy rocket, its Cape launch site and ground systems for fueling and liftoff.

File photo of the Delta 4-Heavy rocket exposed on the launch pad for an earlier countdown simulation. Credit: Justin Ray/Spaceflight Now

The 33-story mobile service tower enclosing the rocket at pad 37B is scheduled for retraction before sunrise as the 9-million pound structure is wheeled 100 yards away from the Delta booster's launch mount.

Clearing the pad of all personnel happens at 8 a.m. EST as the countdown clocks enter a 60-minute planned hold. During this pause, the Boeing and Air Force management teams will conduct a series of polls to ensure all systems are ready to start the Terminal Countdown and begin loading super-cold cryogenic propellants into the rocket. Terminal Count starts at 9:01 a.m. EST from the T-minus 5-hour, 15-minute mark.

Fueling operations commence with the "chill down" thermal conditioning of the liquid oxygen systems on the three Common Booster Cores at 9:31 a.m. Once that prep work is completed, the liquid oxygen tanks in each CBC will be filled with the Minus-298 degree Fahrenheit oxidizer.

Liquid hydrogen fueling of the CBCs starts with cold gas chilldown at about 9:40 a.m. Loading of the rocket's fuel tanks with the Minus-423 degrees Fahrenheit propellant should start just before 10:30 a.m. with the "slow fill" mode before transitioning later to "fast fill."

Chilling of the upper stage's liquid oxygen and liquid hydrogen systems will start shortly after 11 a.m. The liquid oxygen tank is scheduled to be loaded first, starting around 11:20 a.m. Hydrogen fill will start around 11:30 a.m.

Complex 37 has two giant sphere-shaped fuel tanks to store the cryogenic liquid oxygen and liquid hydrogen. The LOX tank holds 250,000 gallons and LH2 sphere about 850,000 gallons.

The cryogenics are fed from the storage tanks through pipelines to the pad. For the three Common Booster Cores, the propellants are routed up to the launch table upon which the rocket sits. Tail service masts, the large box-like structures at the base of the vehicle, feed the oxygen and hydrogen to the boosters via separate umbilicals. The upper stage receives its cryos from the middle swing arm that extends from the Fixed Umbilical Tower to the front-side of the rocket.

Once the rocket is fueled up, all eight cryogenic tanks in the CBCs and upper stage will be topped off through the final minutes of the count to replace the cryogenics that naturally boil away.

A series of engine nozzle motion checks occur at about 1:15 p.m. These "slew" tests ensure the engines will provide good steering for the rocket's ascent. Range Safety conducts tests of the command receiver decoders a half-hour later. This system would be used to destroy the vehicle if it veered off course during launch.

At 2:11 p.m., the countdown enters a planned 15-minute hold at the T-minus 5 minute mark. This offers managers a chance to perform final readiness polls of the entire launch team to verify there are no issues or concerns before entering the last phase of the countdown.

Assuming all systems are go, clocks will resume ticking at 2:26 p.m. During those final five minutes, the rocket will switch to internal power, ordnance will be armed, all eight cryogenic tanks will be secured and the Range will announce a clear-to-launch.

At T-minus 13.5 seconds, the sparkler-like radial outward firing ignitors -- or ROFIs -- are started beneath the main engine nozzles. The Terminal Countdown Sequencer will grab control at T-minus 8.5 seconds to manage events in the crucial last seconds and oversee the rocket's status. The ignition sequence for the three RS-68 powerplants follows at T-minus 5.5 seconds as the main hydrogen fuel valve in each engine is opened. As fuel floods through the engines, spectacular flame erupts at the base of the rocket as free hydrogen reaches the ROFIs.

"As that hydrogen makes it through the engine and hits those sparks, you are going to see the flame rise along the side of the rocket," said Dan Collins, the Boeing vice president of expendable launch systems.

The oxygen valves in the engines are opened at T-minus 2 seconds as the RS-68s begin roaring to life. The engines must rev up to full throttle -- 102 percent thrust level -- and undergo a rapid computer-controlled health check to ensure all parameters are met.

"If (the control sequencer) determines all three RS-68s, as well as the rest of the vehicle is in good health, at T-0 it will send the signal to release the hold-down bolts and the rocket will take off. There is a very short period of time where the rocket is under full thrust, being monitored for health, before we let it go."

If any problem is detected before T-minus 40 milliseconds, the engines will shut down and the liftoff aborted, Collins said.

A successful engine startup leads to T-0 as the 12 hold-down bolts that have been restraining the rocket to Earth finally detonate. The 23-story, 1.6-million pound vehicle blasts off at 2:31 p.m. EST (1931 GMT) to begin the maiden voyage of Boeing's Delta 4-Heavy rocket on its qualification flight.

"It will clear the tower in about the same amount of time as the (Delta 4) Medium vehicle did -- somewhere a little quicker than 20 seconds, but not a lot. In the Delta 4 family, the Medium vehicle and the Heavy -- so the ones without solid rocket motors -- tend to rise at about the same rate. They have essentially the same thrust-to-weight ratios," Collins said.

The slow-rising rocket's trajectory, coupled with conservatism for this first flight, resulted in an unusually large hazard area covering Cape Canaveral Air Force Station. Virtually the entire base will be evacuated Saturday morning except for 437 mission-critical personnel, said Col. Mark Owen, the 45th Space Wing commander at Cape Canaveral.

"It is prudent to make sure the public safety is well taken care of. And I think what the Range has done is right in line with good engineering practice," Collins said.