Tests planned for the space shuttle's redesigned boosters, although unprecedented in scope and on a tight schedule, should be expanded to answer more questions before the first post-Challenger flight, a panel of independent experts said yesterday.

The National Research Council panel praised the test program, saying it goes beyond that for any other large solid-rocket motor and has produced "heightened confidence" in the new design.

H. Guyford Stever, chairman of the panel, said he believes the recommended additional testing could be done before the scheduled June launch date if less critical tests are postponed.

NASA's John Thomas of Marshall Space Flight Center, head of the booster redesign team, agreed, saying, "I think we can accommodate" the recommendations.

The apparent success of the strengthened booster joint design prompted some of the panel's recommendations for more testing, an irony that Thomas described in a telephone interview as "a nice dilemma."

A failure in an O-ring seal in one of the booster joints caused the destruction of the shuttle Challenger and its crew of seven in January 1986. The new design adds insulation, a third O-ring and a metal grip to supplement the two original rubbery O-ring seals.

The new "upstream" barrier of bonded insulation performs so well that it, rather than the first and second O-rings, has become the seal, according to Stever, but it cannot meet the requirement that such a seal be redundant.

"This means you should find out as much about it as possible," he said.

Accordingly, the top priority in yesterday's report is the recommendation that the National Aeronautics and Space Administration do more to evaluate sealing characteristics of the new insulation with variations in temperature and other conditions.

The new barrier also means that if the O-rings were not tested in special ways -- by inducing flaws that allow hot gases to reach them -- the O-rings would never be called on to perform. The panel urged that NASA increase subscale testing of the O-rings under pressure and install additional sensors to measure internal pressure during test firings.

The expert panel was created by the National Research Council, an arm of the National Academy of Sciences, after a presidential commission investigating the Challenger accident recommended independent oversight of the booster redesign.