A previous Titan 4B rocket is readied for launch as the mobile service tower is retracted. Photo: NASA/KSC

A U.S. Air Force Titan 4B rocket is finally ready for liftoff Monday after a tumultuous stay on the launch pad that included an oil leak scare and two recalls for parts installed inside the booster.

The Lockheed Martin-built launcher -- the most powerful currently in the U.S. fleet -- is slated to depart Cape Canaveral Air Force Station at 9:30 a.m. EDT (1330 GMT), the opening of a four-hour window of opportunity.

Atop the $400 million rocket is the 20th Defense Support Program satellite that will join the Pentagon's early detection system designed to spot missile launches around the globe.

The $250 million DSP satellite is identical to the one lost a year ago. In that mishap, one of a trio of failures to plague the Cape's last three Titan missions, the spacecraft was placed into a worthless orbit because its upper stage kick motor malfunctioned.

The Boeing-built Inertial Upper Stage, needed to boost the satellite from the temporarily orbit achieved by the Titan 4 to geostationary altitude 22,300 miles above Earth, failed to separate its two stages properly. Human error in the application of tape on connectors between the stages was blamed.

Bumpy road to liftoff
Originally scheduled for launch late last year, the Titan rocket has faced more than its fair share of delays that have extended its wait at Space Launch Complex 40.

An artist's concept of a Defense Support Program satellite in orbit. Photo: USAF

The headlining trouble started on December 22 as the rocket's protective payload fairing, or nose cone, was being installed around the DSP satellite in advance of a January liftoff.

An overhead crane used to hoist the fairing's multiple sections into place dripped oil, creating worry that the super-sensitive spacecraft could have been contaminated.

After extensive tests, engineers determined the satellite was fine and preparations resumed for a late February launch.

Then came word that hydraulic devices, called actuators, which move the rocket's engine nozzles for steering during flight might be faulty. The actuators are crucial to keep the rocket on the proper course during its nine-minute trip to orbit.

"During the acceptance testing of the Moog-made actuators, it was discovered that they had inadvertently been over-stressed in violation of their qualification," Air Force officials said in a written statement to Spaceflight Now.

"This resulted from a miscommunication between the prime contractor, Lockheed Martin, and their subcontractor, [Moog]. As a consequence, all the actuators which had been tested to that point were no longer deemed flightworthy."

With the seven actuators installed on the Titan rocket's first and second stages and twin solid rocket boosters, officials ordered the units removed and replaced at the launch pad with ones tested correctly.

Although the actuators "in all likelihood" would have worked without incident during the launch, they were replaced to ensure a successful flight, officials said.

The Titan 4B rocket is comprised of the two-stage core vehicle, two strap-on solid rocket boosters and payload fairing. Photo: USAF

"A mainstay in the space business is that we do not fly anything that exceeds it's qualification test levels," the statement went on to read.

The most recent problem involved six so-called High Energy Firing Units, electronic parts that start vital actions of the two solid rocket boosters.

The firing units on the rocket had to be replaced after a design flaw was uncovered with similar parts during manufacturing for future launches, the Air Force said in a separate statement to Spaceflight Now.

"During the production of these units for vehicles which will fly two years from now, there was a failure of an electronic filter designed to eliminate electromagnetic emissions from the unit when it fires. It was found that the filter mounting design in the unit was marginal and that it was possible for the filters to be damaged during installation and later fail under vibration.

"After reviewing the units already built, it was decided to fix the mounting design and replace all the filters."

Each solid rocket booster has three High Energy Firing Units -- two located in the nose cone and one in the aft skirt of the motors.

The devices initiate detonators inside the motors: one ignites the solid rocket booster at liftoff; the second starts the gas-driven turbine used to provide high pressure hydraulic fluid for steering the motor nozzle; the third fires the staging thrusters which push the boosters away from the core vehicle after they have completed their burn.

Focused ahead
Through the setbacks, the rocket has remained fully assembled at the launch pad with its cargo still attached. The Air Force said the rocket was in "excellent health."

Now with the glitches resolved and an important mission rehearsal completed last Friday, the Air Force is preparing to start the countdown to liftoff.

The launch will mark the 29th for a Titan 4 rocket since 1989.

Once this rocket is off the ground, the Air Force will next use the Cape's Complex 40 to launch another Titan 4B in about six months. It will carry a Milstar military communications satellite into space just like the one stranded in the wrong orbit last April due to another upper stage rocket failure.