With cracks on its launch pad freshly patched up, the giant Delta 4-Heavy rocket has returned to the oceanfront complex aiming for a middle-of-the-night blastoff in late August to haul a missile observation satellite into orbit for the U.S. Air Force.

The Delta 4-Heavy rocket was placed on its pad last week after cracks in the launch table were repaired. Credit: United Launch Alliance

The booster had rolled out to the pad once already, traveling from the assembly hangar to Cape Canaveral's Complex 37 in early January for a planned April launch on its first operational flight.

The Delta 4-Heavy is created by taking three hydrogen-fueled Common Booster Core stages and strapping them together, then adding a powerful upper stage to form a launcher capable of delivering hefty amounts of cargo into space. It is the biggest unmanned rocket available in the U.S. inventory today.

But plans for an early spring launch were dashed when the pad's massive table that supports the rocket cracked during a countdown dress rehearsal on February 28. Supercold liquid oxygen leaked from plumbing inside the table, enabling the frigid temperatures of the escaping cryogenic oxidizer to create small fractures in the metallic structure.

"The direct cause (of the leak) was found to be a high cycle fatigue failure in the flexible bellows sections of the vacuum-jacketed liquid oxygen fill and drain lines in the launch table. This suggests these cracks may have resulted from flow-induced vibrations in the lines," according to information provided by the Launch and Range Systems Wing at the Air Force's Space and Missile Systems Center.

While the cracks didn't put the rocket in jeopardy, the Launch and Range Systems Wing said, repairs had to be made before the mission could lift off.

"The rocket was never at risk of damage. However, we could not have attempted a launch from SLC-37 without possibly incurring additional damage to the pad."

At the end of March, crews removed the United Launch Alliance rocket from the pad and put it in temporary storage within the nearby hangar while repairs were made to the table and fuel lines.

Two locations on the table had sustained the structural cracks - in the starboard and center bays below those two rocket stages, where the liquid oxygen equipment is situated - and each fracture had several "spider" cracks that were much smaller in length. Engineers implemented repair plans to restore the pad and remove the troublesome fuel lines.

"The launch table has been repaired to its 'as designed' requirements. The repairs consisted of cutting crack arrest slots, performing full penetration seal welds, welding vertical and horizontal doubler plates and replacing the vacuum-jacketed fill and drain lines," the Wing said.

In addition, new safeguards to prevent such a problem from repeating in the future are being formed.

"Mitigation efforts are underway to include local leak containment and drain lines being added. Recommendations to procedure changes are being assessed," the Wing said.

With the repairs completed and the launch complex once again ready to welcome a rocket, the Delta 4-Heavy rode horizontally to the pad aboard a diesel-powered transporter last Monday, June 18. The pad's erector system powered by hydraulic pistons lifted the rocket vertically atop the pad table on Tuesday.

And now the second pre-launch campaign for this rocket flight has commenced.

"The vehicle will conduct its normal pre-launch preparations from this point on," the Wing said.

Retests of the propulsion and electrical system checkouts occur in early July, followed by another countdown dress rehearsal at the end of July. The mock countdown sees the rocket stages fueled as part of a launch day simulation.

The rocket's payload, the 23rd and final in the Defense Support Program (DSP) satellite series, will arrive at the launch pad in early August for attachment.

If all goes well, liftoff is targeted for August 28 during a window extending from approximately 2:37 to 4:30 a.m. EDT (0637-0830 GMT).

The rocket will fly directly into geostationary orbit 22,000 miles above Earth -- a trip lasting about six hours and featuring three firings by the upper stage -- to deploy the DSP spacecraft for its mission to detect enemy missile launches and nuclear weapon detonations.

The Northrop Grumman-built DSP satellites use infrared telescopes to spot the heat from missile and booster plumes against the Earth's background, giving the U.S. and allies early warning of impending attacks.

Assuming the upcoming DSP 23 launch happens in late August, another Delta 4-Heavy rocket is waiting at the Cape to take its place atop the Complex 37 pad this autumn. That booster will loft a classified spy satellite payload for the U.S. National Reconnaissance Office. The tentative launch date is January 25, 2008.