President Carter told National Aeronautics and Space Administrator Robert A. Frosch yesterday that his administration will give the space shuttle its full financial support even if it means an extra $300 million in the fiscal 1981 budget to keep it going.

"I had he impression that he [President Carter] is prepared for that possibility," Frosch told a hastily called press conference after meeting privately with the president at the White House. "He expressed his full support for the program."

Frosch also disclosed that under NASA's latest schedule, the maiden space shuttle test flight, will take place in August or September of 1980. This means the first shuttle flight will be at least eight months behind NASA's original schedule.

Frosch said Carter committed the administration to the shuttle program "for a variety of reasons," in part because of its importance to national defense and in part because of its importance to the nation's commericial use of space.

While Frosch did not spell it out at the press conference, he has privately told Congress that the driving force behind keeping the shuttle schedule is the SALT pact soon to come to the Senate floor. The Pentagon plans to use the shuttle to carry its newest reconnaissance satellites into orbit to verify whether the Soviet Union is adhering to the treaty.

On the commercial side of space, it is known that at least four U.S. satellites may have to find more expensive ways of reaching orbit if the shuttle schedule is postponed again. This might mean going into orbit on top of the European Ariane rocket, which has already begun to compete with the shuttle for pay loads.

As he did when he testified before the Senate last summer, Frosch blamed the newest shuttle delay on engine testing and the production and installation of the unique foamed glass tiles that will cover the space shuttle and protect it from heat of reentry.

"I told president the problems and difficulties we are having are not major central technical problems," Frosch said at the press conference. "They are technical odds and ends, things that break and things that need redesign, which causes delay and time problems."

Early this month, a 520-second test firing of the shuttle's three main hydrogen engines was aborted when a seal failed in one engine and a hydrogen line broke in another. The seal failure shut down the engine and the hydrogen break caused the combustion chamber to overheat because the fuel in the chamber was too rich in oxygen.