Vapors stream away from the Delta 4 rocket during Thursday's liquid oyxgen tanking as the sun rises.

Boeing technicians pumped super-cold liquid oxygen into both stages of the inaugural Delta 4 rocket for the first time early Thursday, completing the initial step in a series of fueling tests at Cape Canaveral's rebuilt launch pad 37B.

The 330-foot tall mobile service tower was retracted from around the 205-foot tall rocket Wednesday evening in preparation for the fueling exercise. Since the vehicle would be exposed for the better part of a day, officials opted to perform the activities at night and into the morning hours when ground winds are less of a threat.

The company said the liquid oxygen loading was successfully completed overnight. The cryogenic oxidizer was then drained from the rocket and the tower was rolled back into position to enclose the vehicle.

The test -- the first of four fueling demonstrations scheduled prior to two full-up countdown dress rehearsals -- was pushed back about a week to give more time to get ready, officials said.

Chilled to Minus-298 degrees F, the liquid oxygen was pumped from the ground systems into the first stage via a tail service mast umbilical. The flow line is connected to the rocket's base from one of six box-like masts atop the pad's launch table.

The new Rocketdyne-built RS-68 main engine on the first stage will guzzle liquid oxygen and liquid hydrogen during launch.

The Delta 4 rocket on the launch pad. The first stage liquid oxygen tank is the middle orange band. The upper stage liquid oxygen tank is enclosed within the middle white section of the vehicle. The tail service masts can be see at the base of the vehicle. Photo: Justin Ray/Spaceflight Now

The second stage receives its fuel from one of the pad's three swing arms that extend from the Fixed Umbilical Tower to the rocket. The middle arm is dedicated to servicing the second stage.

The upper stage has a single RL10B-2 engine that also burns liquid oxygen and liquid hydrogen.

With the liquid oxygen loading test completed, engineers are now looking ahead to the first liquid hydrogen tanking test, which will see the cryogenic fuel loaded into both stages. The hydrogen demonstration is expected to occur within the next few days, Boeing said.

Later tests will feature both liquid oxygen and liquid hydrogen loaded into the rocket on the same day as will be the case during a launch countdown.

Liftoff remains scheduled for October 9, but Boeing is still preserving the option of moving the launch up to September 25.

The Air Force-controlled Eastern Range -- the vast network that provides tracking, communications and safety services to all Cape Canaveral and Kennedy Space Center launches -- will be closed to Delta 4 and Atlas 5 missions from September 9 through October 4 while upgrades are made to systems used to support the next-generation rockets.

If the Range blackout period is moved, then the Delta 4 debut launch could be advanced to the end of September.