An independent review panel said yesterday that NASA still does not have adequate backup designs for the shuttle booster rocket if there are serious problems with the new design about to be tested.

This omission takes on added urgency because a key joint in the rocket booster "is likely to fail" the test, according to the panel that was established to monitor the National Aeronautics and Space Administration's redesign following the Jan. 28, 1986, Challenger disaster that killed seven astronauts.

The report comes as the space agency nears completion of the redesign of the booster rockets and moves into the testing phase. The complicated process has already caused "substantial delays" in the effort to return the shuttle to flight, the panel noted, but the booster test program continues to face "considerable pressure."

"More attention is now being paid to contingencies, but we remain concerned that alternatives for certain critical aspects of the design are not being pursued with sufficient vigor to minimize delays in the program should it become necessary to turn to alternatives," the panel said.

The board had previously criticized NASA for failure to pursue backup designs. But NASA believes, according to the panel, that the only way to meet the schedule is to focus its limited manpower and money "intensely" on the main redesign and that "the risks to the program are acceptable."

The group was formed by the National Research Council, an arm of the National Academy of Sciences, as an outgrowth of the Rogers presidential commission which investigated the Challenger accident. The commission blamed the accident on the failure of a poorly designed solid rocket booster joint that leaked hot gases.

The most critical features of the new design -- the field joint that was blamed for the Challenger accident, and the joint that joins the rocket case to the rear nozzle -- are scheduled for their first test in a full-scale firing of a solid rocket motor on Aug. 23.

The solid fuel rockets that launch the shuttle cannot be test-fired before launch because the firing consumes and destroys much of them. Instead, special techniques must be used, and tight controls are needed to assure that materials and processes are "as nearly as practical identical" for all hardware, the panel noted.

One of these test techniques is to introduce intentional flaws into the rocket motors used for testing.

The panel strongly suggested that NASA may be introducing flaws that would never occur in actual flight and said this "may yield meaningless or misleading results."

In the instance of the case-to-nozzle joint, "the area of our greatest concern," the board said, the agency plans to make a hole through two barriers in the joint that could allow heat damage in a primary O-ring -- something the agency's current requirements prohibit. The panel said the agency may be needlessly introducing a flaw that the hardware cannot deal with and thereby produce a test failure.

"If that happens, that's serious business," said panel director Myron Uman.

The agency would then face either a delay of months, while it starts almost from scratch on an alternative design, he said, or the politically controversial course of altering the test requirement so that the hardware could "pass," raising questions about safety. The board intends to "hold them to that requirement," he said. "Consequently, we recommend that the redesign team address this problem with urgency," the report said.

John Thomas of Marshall Space Flight Center in Alabama, head of the redesign team, said, "I think we're doing the work necessary to arrive at a reasonably credible flaw scenario." As to the likelihood that the new joint would fail the test, he added, "That issue is still open to judgment."

The panel, in its fourth report on the redesign, also said that the Challenger accident "demonstrated the importance of conducting an on-going program" to review the performance of the booster and provide for improvements as needed, a costly measure.

The lack of such a program was blamed in part for NASA's failure to fix the faulty booster joint before the accident.

NASA had earlier told the panel it intended to do this but the plan was eliminated partly because of budget considerations.