SPACE CENTER, Houston (AP) _ Officials at the Johnson Space Center and at Rockwell International are making tentative plans to build a new space shuttle to replace the destroyed Challenger, and most believe it will be the pride of the fleet - the safest, ''cleanest'' spacecraft ever built.

Congress has not approved the construction of a new orbiter, but experts have estimated the cost at about $2.8 billion and the time it would take to build at about three years. And they have suggested a name - Challenger II.

''That's the one I like,'' said Richard Colonna, manager of the space shuttle projects office at the Johnson Space Center.

Colonna said no firm plans have been made as yet, but preliminary, informal studies have shown that assembly lines to start building a new shuttle could be organized quickly.

''The first thing we would do is to try to assess what we are going to need and what is available,'' he said. ''Those parts that are short are the ones we would move on first.''

NASA was in the process of building up spare parts for the four-orbiter fleet of shuttles when Challenger exploded just after its launch Jan. 28. Many of those spare parts could be used in the construction of new orbiter, but the parts, in turn, would have to be replaced to keep the fleet flying.

Colonna said building a new shuttle would require a delicate balancing act between the parts demands of the assembly line and the needs of shuttles that are flying. Flights are suspended now, but are expected to resume before a new shuttle could be completed.

''The needs of the factory versus those of the field would have to be balanced over the next three years. The flight program always takes precedence,'' he said.

Colonna said that within two months after getting the authority to build a new orbiter, Rockwell International of Downey, Calif., the spacecraft's prime contractor, would start hiring new workers. Rockwell would slowly add 1,700 to 2,000 new employes, peaking about two years after construction began.

Subcontractors also may have to add new workers, but Colonna said he had no estimate of those numbers.

''I've got 131 suppliers, or subcontractors, and to my knowledge all of those are capable of coming right back up with a short amount of lead time,'' he said.

Within three and half years after getting the go-ahead, Colonna said the new shuttle would be ready to launch.

Though it would look like its sister ships, there would be important diffences.

The new shuttle, Colonna said, would have all of the latest innovations, with a heavy emphasis on safety.

Johnson Space Center engineers are reviewing a list of almost 750 ''criticality 1'' items, those that must work for the shuttle to operate safely, and changes will be made where it is indicated. For the existing shuttles, the changes will be added on. For the new shuttle, the changes will be part of the assembly.

Colonna said the new shuttle would have improved general purpose computers, the five mechanical brains that control the craft. There also would be improved electrical power cells, a new steering system, a new braking system, and even a new space toilet. In the cockpit, there would be enhanced avionics to make flying the craft easier.

Included in the cost of the new orbiter would be the construction of key parts that were lost in the Challenger explosion.

NASA had only three flight-qualified robot arms, the cargo bay crane that is operated remotely from inside the cockpit. One of those arms was lost on Challenger and will have to be replaced. The originals cost $14.2 million in 1982 dollars.

Also lost on Challenger were two space suits, the pressurized garments used in space walks. One of the lost suits was an extra large, the only one that size in the NASA inventory. This will have to be replaced since some of NASA's most experienced spacewalkers require that size. The new suits cost about $2.15 million each.

Cost of the new orbiter costs would include about six months of pre-flight testing after it was delivered to the Kennedy Space Center.

NASA in the past seldom has let any spacecraft parts be wasted. Enterprise, the shuttle craft used only for drop tests during the developement of the spacecraft, was gutted of parts after the tests were completed. The Enterprise sent to a museum was just a hollow shell of the craft that once dropped from the back of a jetliner and glided to Earth during development tests.

But Colonna said there is little chance of salvaging anything useful from Challenger, ''and even if there was, nobody would ever agree.''

The surviving fleet of shuttle craft, Columbia, Discovery and Atlantis, all appear identical, but there are subtle differences. Columbia, the first to orbit, is the heaviest, and Atlantis, the last to be built, is the lightest and most versatile.

Discovery and Atlantis can be launched into a polar orbit from Vandenberg Air Force Base, Calif., but the heavier Columbia can only be sent into a west- east orbit from the Kennedy Space Center in Florida. Atlantis now is the only shuttle capable of launching the liquid-fuel Centaur rockets that are scheduled to be used to send spacecraft on voyages of solar system exploration. Columbia is outfitted primarily to fly the spacelab, an orbiting laboratory carried in the cargo bay.

A new orbiter would incorporate the versatility of Atlantis, and the best of new technology, Colonna said.

''I would anticipate that we would build a duplicate of Atlantis with only minor differences,'' he said. ''And the differences would be anything to make it better. It will probably be the cleanest vehicle. I think it will be the best.''

Critics, however, have said that no matter how a new shuttle is modified, it still will be based on technology of the 1970s, when the shuttle concept was developed. Some have suggested that instead of building another shuttle, the money could be spent more usefully for development of a new generation of spacecraft.

Furthermore, some experts have told Congress that the shuttle will never be operated economically.

Engineers at Johnson have estimated that a totally new spacecraft would take 15 years and many more billions to develop.

They also say that a fourth orbiter is needed to meet the demand for launch services, and to build the nation's space station, a permanent orbiting fixture set for operation in the mid-1990s.