Change in Mars Observer’s Flight Plan May Have Caused Its Demise : Space: Pressurizing the $500-million craft’s propellant tanks was delayed for 11 months. Valves were not designed for altered conditions.

Seven months before the launch of NASA’s Mars Observer, managers made a change in the flight plan that might have caused the $500-million spacecraft to vanish on Aug. 21, according to sources familiar with the program.

Instead of pressurizing the craft’s propellant tanks five days after the launch, as originally planned, NASA managers decided to delay the procedure for 11 months--until the Observer reached the red planet. They did so, ironically, to avert a potentially serious leak, officials said.

But the valves had not been designed to operate under the altered conditions, the sources said, and the result was probably an even worse leak that caused a catastrophic rupture in a fuel line and spun the craft out of control.

The fact that the change in plan was made did not come up during a press briefing Wednesday at NASA headquarters, where an independent investigating panel reported its conclusions about what happened to the spacecraft and why.

Sources familiar with the program expressed surprise that mention of this management decision was omitted from the briefing and also from the report’s executive summary and overview that were released to the press.

The change is described, however, in at least two places deep inside the report’s eight-inch-thick, four-volume documentation, which was not released to the press but was available for review at NASA headquarters.

Timothy Coffey, chairman of the investigating board, was traveling and not available for comment, his office said. Others on the panel said they were not sure why the management decision to delay pressurizing the tanks was not mentioned.

The disappearance of the Observer is one of several embarrassments for the National Aeronautics and Space Administration in recent years.

In 1989, it launched the Hubble Space Telescope with what was later found to be a faulty mirror; corrective optics were installed in a spectacular spacewalk mission last month. And about the same time that the Observer disappeared, the antenna on the Galileo probe to Jupiter malfunctioned. NASA also has been plagued by cost overruns on its space station project.

The decision to change the pressurization plan for the Mars probe was made in February, 1992, because someone reminded the team that a similar propulsion system used on the Viking missions to Mars in the late 1970s had run into leakage problems when the fuel tanks were pressurized early in the flights, Glenn E. Cunningham, Mars Observer project manager at the Jet Propulsion Laboratory in Pasadena, said last week.

As to why this concern developed so late in the program--after it was too late to make what would have been a simple mechanical fix and still meet the launch date--Cunningham said: “That’s the $64 question.”

The decision to delay pressurization was not questioned by anyone at the contractor firm, JPL or NASA headquarters.

“With benefit of 20-20 hindsight,” Cunningham said, “it appears that probably all of us did not do as much analysis on this new condition as probably should have been done.”

Controllers at JPL lost contact with the Observer late on Aug. 21, 1993, while trying to perform the procedure they had decided to delay: pressurizing the propellant tanks. The tanks have to be pressurized to fire the braking rockets, which would put the Observer into orbit around Mars.

The investigators said last Wednesday that the most likely cause of the spacecraft’s catastrophic loss was a “massive failure in the pressurization side of the propulsion system.”

Pressure in the tanks was required for any major maneuvers by the spacecraft, but the Observer team had determined that none would be needed between Earth and Mars--assuming the launch went well. (They had left a small amount of residual pressure stored in the tanks before launch for minor course corrections en route.)

Pressurizing the tanks involves opening previously closed valves and releasing high-pressure helium gas, whose function is to squeeze the hydrazine fuel and nitrogen tetroxide oxidizer out of separate tanks to mix and ignite in a controlled way.

If the investigators’ conclusions are correct, the failure’s most likely cause was the escape of small quantities of the oxidizer through a system of check valves during the spacecraft’s 11-month, 450-million-mile cruise through space.

When JPL controllers tried to pressurize the tanks, minute amounts of condensed oxidizer may have been carried prematurely into contact with the hydrazine fuel.

The combination would have ignited inside the plumbing and ruptured it, spewing jets of propellant into space and putting the spacecraft into a rapid, uncontrolled spin that rendered it useless.

“There was no problem with the check valves,” said a planetary engineer who asked not to be identified. “The valves were never intended to be operated that way.”

When the Observer was designed and built, the assumption was that the valves would have to operate five days, not 11 months, and not in such cold temperatures.

The valves, made by two different subcontractors, were regarded as assembly-line items that had been proven in other spacecraft. But they had operated only in the relative warmth of near-Earth orbit, investigators said, never in the deep cold of an interplanetary voyage.

In 1992, the Observer team debated the change in procedures extensively before it was approved, said Peter G. Wilhelm, a member of the investigating panel and director of the Naval Center for Space Technology. He focused on the propulsion system.

“Was the decision prudent? That’s what the debate was all about,” he said, adding that changes so late in the game are acceptable, “assuming you make the right change.”