In his first seven months on the job, NASA Administrator Michael D. Griffin has announced the replacement of eight of the agency's 14 top career civil servants, a mass beheading seldom seen in a city where the political folks are the ones who routinely get the ax.

Griffin is getting away with it because, unlike most new agency chiefs, he is not a stranger who arrived in town with a couple of trustworthy sidekicks, a head full of big ideas and no idea how to implement them.

The ideas are big enough: Griffin seeks nothing less than the transformation of NASA from an agency focused for 25 years on the space shuttle and the international space station, to a visionary enterprise rebuilt to implement President Bush's call to return humans to the moon by 2020 and eventually send them to Mars.

But Griffin, 56, brings more to the job than vision. A blunt-spoken scientist-engineer with 35 years in the aerospace business, he has also worked at NASA, the Defense Department, and as a consultant and expert frequently summoned by Congress to testify on the shortcomings of the agency he now leads.

"I say this with all humility," Griffin said in an interview. "I don't believe there is someone else who knows the broad span of NASA better than I. . . . When I was offered the job, I showed up at the front door knowing what needed to be done. I did not want to waste any time, nor did I feel I needed to."

And he didn't. Two months after his April 13 confirmation, all four of NASA's mission directors, in charge of human space travel, science, aeronautics and exploration, announced their departures. Also leaving are four of 10 directors of the agency's major installations, among them Houston's Johnson Space Center.

"It's a rare moment when you get a housecleaning like this," said New York University's Paul Light, an expert on the federal bureaucracy. "It could presage a strong turnaround for an agency that's been adrift for years if not decades."

Griffin hopes so: "I don't share the view that a good manager is a good manager is a good manager," he said. Instead, he wants an old-style NASA leadership steeped in technical and scientific know-how.

"I'm looking for domain knowledge, and it's a non-negotiable requirement. It's core for me. To do this you do need to be a rocket scientist," Griffin said. "The managers we had in place were not in my judgment what we needed for the change of direction we wanted to take."

Besides bringing technical expertise, Griffin has revamped NASA's chain of command by swapping out his deputy administrator and chief of staff -- both political positions -- and putting a civil servant into the newly created job of associate administrator, who oversees NASA's day-to-day operations.

His model, he said, is the triumvirate that ruled NASA during the run-up of the Apollo program in the early 1960s. Then-administrator James E. Webb was NASA's public face, Deputy Administrator Hugh Dryden provided the technical expertise, and Associate Administrator Robert C. Seamans Jr. handled operations.

"It was the best NASA ever," Griffin said. The difference this time, he added, is that he will be the technical expert, and Deputy Administrator-designate Shana Dale, who has extensive Capitol Hill experience and is currently deputy director of the White House Office of Science and Technology Policy, will be the policy expert. The associate administrator is Rex Geveden, NASA's former chief engineer.

In a series of interviews to discuss the changes, sources in Congress, private industry and the government generally gave Griffin high marks for his new choices but suggested that pitfalls may lie ahead. They agreed to talk about NASA on the condition their names would not be used, because they were not authorized to speak for their organizations or did not want to become part of a dispute over agency policy.

One source suggested that Dale, a protege of former House Science Committee Chairman Robert S. Walker (R-Pa.), was a "mole" installed by the Bush administration to keep an eye on Griffin. Walker, now a lobbyist, has close White House ties.

Not true, Griffin said. The administration "consulted me, and I did pick Shana," he said. With "an extensive background in policy dealing with technical matters in Congress and the White House," he added, she will be "a deputy who will complement what I bring."

Two industry sources also said Dale may prove to be a tactful buffer for Griffin: "Mike says what's on his mind, and he realizes that it can cause problems," one source said. "He's bringing in Shana Dale to improve communications and interpret what he says."

This source and several others said Dale might have been able to help with a recent article in USA Today in which Griffin described the shuttle program as "a mistake." Griffin in the past has frequently expressed such sentiments, but the story caused an uproar within NASA, forcing him to explain his remarks in an agency-wide e-mail.

Congressional sources also suggested Dale could have helped with a recent spat between NASA and the Senate Committee on Homeland Security and Governmental Affairs over a recent Government Accountability Office report on misuse of agency airplanes for executive travel before Griffin arrived.

"The committee called over to NASA, and they tried to be argumentative and to stonewall," the source said. "It could have been handled much more smoothly." This source blamed the misunderstanding on NASA's new chief of staff, Paul Morrell, hired by Griffin from the White House National Security Council. Morrell, according to this source, "is in over his head."

Griffin defended Morrell's hiring and his qualifications: "He came in for an interview, and we hit it off beautifully," Griffin said. "And there wasn't a solitary soul in the White House that cared one way or the other whether I hired him."

Griffin acknowledged that his high-level clean sweep has "displeased" some of those affected, and "some of those who supported them," but insisted that he made his changes "without prejudice."

Those who left "are not bad people or bad managers," he said. "Their skills were just not the ones I needed to go forward, and that's my judgment to make. The most important thing I do is pick the team."

He may not be finished: "Some more evaluation is required to see whether we are done," Griffin said. "I need a team which is compatible -- and if that's not the case, I will make a change, and it's not something I will hesitate to do."