A U.S. military weather satellite, grounded since January 2001 by technical problems, has been removed from its Titan 2 rocket at a Vandenberg Air Force Base launch pad and returned to the hangar to be repaired for the second time in a year.

The Titan 2 rocket won't launch its DMSP weather satellite cargo until late April at the earliest because of a thruster problem. Photo: William G. Hartenstein

The Defense Meteorological Satellite Program (DMSP) F16 spacecraft was seconds from blastoff 12 1/2 months ago, but an epic series of troubles have delayed the launch. Most serious was a broken cable that knocked out the satellite's guidance computer brain, forcing the Lockheed Martin-built craft to be removed from the Titan 2 and shipped back to its processing building to be fixed.

With the spacecraft repaired and returned to the launch pad at the end of 2001, all appeared ready for liftoff on February 1. But technicians uncovered a glitch with the satellite's propulsion system while performing a vacuum leak check before the spacecraft was to be fueled in January. An investigation showed a faulty valve inside a thruster failed to operate when commanded to do so.

"The troubleshooting we were able to do revealed there is malfunction with that thruster valve and as a minimum we will have to remove and replace that thruster," Col. Randy Odle, DMSP program director at Los Angeles Air Force Base, said in an interview last week.

Although the spacecraft was not loaded with hydrazine propellant during this most recent stay on the pad, it was fueled a year ago for the initial round of launch attempts. In order to remove the craft from atop the Titan 2 last year, it was drained of fuel. However, small amounts of the hazardous hydrazine remained inside the satellite.

Odle said working with the thruster on the pad, given the hydrazine factor, would be too cumbersome. So it was decided to demate the satellite and again head back to the hangar, which occurred last Tuesday, February 5.

The suspect thruster valve is being removed for testing to determine why it isn't functioning correctly. The tests are expected to take a couple weeks to complete.

"It's not going to be until this analysis is complete that we'll have a better idea about what the actual cause of that thruster failure was. Hopefully we can pinpoint that and it could lead us into having to replace more parts of the spacecraft, the propulsion system, but we'll keep our fingers crossed that its isolated to that one thruster valve."

The satellite carries four of these thrusters, each delivering 100 pounds of thrust. They are crucial during the launch, providing the boost to separate from the Titan 2 rocket and then to keep the satellite on course when its kick motor is firing to achieve orbit.

"These are very critical to us in our ascent phase and clearly if we had a thruster malfunction that would be a serious problem for us. Obviously when we convinced ourselves that Thruster Four was not working properly we had to do something to fix that before we launched."

A DMSP weather satellite. Photo: Lockheed Martin

Under the best-case scenario, just the one thruster will have to be fixed. That would allow the DMSP satellite to be ready for launch in late April or early May.

If all four thruster valves on the spacecraft need to be replaced, Odle said the work could occur in parallel, leading to launch in the same time frame.

But if the investigation calls into question the satellite's entire propulsion system, which Odle said is "a possibility, certainly one of the options you have to consider right now", the extensive work to remove and replace that section of the spacecraft would delay the launch six to eight more months.

"There are certainly concerns if you introduce air into a system that has hydrazine in it. As we go through our destructive analysis we are hoping that analysis points to whether there was a hydrazine issue that really affects this valve or not. Clearly if it points to a hydrazine issue, then that might lead us to having to replace the entire propulsion system that has been contaminated, or has the hydrazine in it."

Once it is clear when the DMSP satellite will be ready for launch, which Odle said would be known by the end of February, Titan officials will then decide whether to keep this mission next on the schedule or delay it until after a planned June 25 flight of a civilian NOAA weather satellite.

Titan 2 rockets are tailored for specific spacecraft and the DMSP launcher would have to be destacked and removed from the pad to make way for the NOAA booster.

Col. John Insprucker, Titan's deputy program manager at Los Angeles Air Force Base, said the Titan 2 rocket for the NOAA-M satellite launch would have to be assembled on the pad by mid-to-late March to support a June 25 date.

Titan 2 officials are watching the outcome of the valve testing to ensure similar thrusters on the rocket aren't deemed suspect. Although the satellite and rocket thrusters are different, they are made by the same company.

"We're clearly interested in what they find, but to date there is nothing that indicates the Titan has a problem," said Insprucker. He added that the Titan 2's valves have passed their on-pad testing.

"It's been pretty tough," Odle said of the rocky road to launch. "I think everybody's pretty frustrated that it just seems like everytime we turn around and get close something else bites us here, but I think the team is staying very motivated.

"We are still all postured to do our very best to launch a very healthy satellite that last for many numbers of years on orbit."