The nation's space agency said yesterday it has dropped seven operational flights of the space shuttle because of anticipated delays in the delivery of new lightweight fuel tanks.

"We've been told by the vendor that the tank delivery rate will be less than anticipated," Dr. Stanley Weiss, associate administrator of the National Aeronautics and Space Administration, said at a news conference. "There is therefore not a high certainty of having the tanks available for all our scheduled flights."

NASA has scheduled 35 operational flights through 1985. Weiss said that only 28 lightweight fuel tanks are sure to be delivered in time.

Weiss said dropping the seven missions means that at least two commerical U.S. communications satellites and at least five for foreign countries will have to be delayed from 1985 to 1986.

"We've given everybody else who'd booked space on the shuttle at least 30 days to decide if they want to get off the shuttle and book space on an expendable launch vehicle," Weiss said. "If they stick with the shuttle, we'll try an accomodate them as soon as we can at the best price we can."

Weiss explained that the delays in delivery of the lightweight tanks are the result of conversion of the tank assembly lines at the Michoud, Miss., factory of Martin-Marietta Aerospace. He said there is also more machining and hand-finishing involved in manufacturing the lightweight tanks, which take as long as 18 months to complete.

The lightweight fuel tanks weigh 6,000 pounds less than the tank carried by the Columbia on its maiden flight last April. The heavier tank will be used on the next five shuttle flights, then the lightweight ones will be used, allowing more weight in the shuttle's 60-foot-long cargo bay.

The tank was made lighter by removing crossbeams, reducing wall thicknesses, elminating a fire retardant paint and using a lighter titanium alloy in some parts.

Each tank holds 1.5 million pounds of liquid hydrogen and oxygen, which power the shuttle's three main liquid rocket engines. The external fuel tank is the largest single piece of hardware carried by the shuttle, measuring 154 feet in length and 27.5 feet around.

The delay in manufacture and delivery will add about $80,000 to the cost of eachy lightweight tank, which were expected to cost an average of $6 million apiece.