NASA's last Atlas-Centaur rocket was severely damaged yesterday morning in a launch-pad accident at Cape Canaveral, indefinitely postponing the launch of its payload, an $83 million military communications satellite.

The launch was originally scheduled for June 11 but was delayed after another of the $78 million rockets, carrying an identical payload, was destroyed March 26 after it was hit by lightning.

That failure broke a string of seven successful launches by the space agency since last September and set back its effort to recover from the January 1986 Challenger disaster, which killed seven astronauts, and from the failure last year of Delta and Titan unmanned rockets.

Yesterday's accident occurred at pad 36B at about 11 a.m. when scaffolding hit the side of the unfueled Centaur second stage, puncturing its pressurized $4 million hydrogen tank, according to Richard Young, a spokesman at NASA's Kennedy Space Center.

Four General Dynamics technicians "vacated the area quite rapidly," but were unharmed except for scratches, said Young. It was not known what caused the scaffolding to hit the tank, he said.

The technicians were preparing to lift the Centaur stage off the Atlas stage to "troubleshoot a liquid oxygen leak" found earlier in one of the two Centaur rocket engines, according to NASA's official statement.

Efforts to assess the damage were delayed by a thunderstorm, but at first look, NASA spokesman Hugh Harris said, the damage to the tank "appears to be not repairable."

He said the rest of the Centaur, "including guidance system, engines and liquid oxygen tank, appears undamaged."

If no more 10-foot-diameter tanks exist and a new one has to be made, he said, "we're probably talking months here." The launch earlier had been rescheduled for this fall.

Jack Isabel, a spokesman for General Dynamics, the builder of the Centaur, said "there are no other Centaur stages or tanks, to my knowledge." He said he did not know how long it would take to build a new tank.

The military satellite is to join a network of five fleet satellite communications, or FltSatCom, spacecraft that enable the Pentagon to communicate with land, sea and air forces. Because the first four satellites have operated beyond their five-year design lives, according to Defense Department officials, the problems of the Atlas-Centaur are not as alarming as they might be.

The damaged rocket is the 68th in a line that began 25 years ago and, until March, had a success rate of about 95 percent. NASA had decided in the 1970s to phase it out along with other unmanned launch vehicles and rely primarily on the manned space shuttle.