Future defense budgets could run 30 percent over Reagan administration estimates if recent history on weapons buying is allowed to repeat itself, a Pentagon cost analyst said yesterday.

Francis (Chuck) Spinney, a cost analyst and aeronautical engineer in the Defense Department's Program Analysis and Evaluation Office, made that estimate after showing a joint meeting of the Senate Armed Services and Budget committees scores of slides documenting how weapons costs had soared far above the Pentagon's estimates in the past.

"The problems are structural," Spinney stressed during his long presentation in the cavernous Senate Caucus Room. One problem, he said, is overestimating in future budgets how much money will be saved by large-scale production on the assumption that the work force will become more efficient.

Although his presentation, which the Pentagon had tried to block, focused on past procurement practices, the analyst said that "the pattern appears to be the same" in the latest defense budgets of President Reagan.

After the briefing, Budget Committee member Charles E. Grassley (R-Iowa) said he would press the Pentagon to take "the next logical step" and reprice its future budgets to make them more realistic. This would help "reestablish the credibility of the defense budget, which is deteriorating," Grassley said.

Budget Committee Chairman Pete V. Domenici (R-N.M.) asked what would happen to Reagan's future budgets if past cost overruns were repeated. Spinney replied that the overrun "would be a big number. It could be as much as 30 percent."

Armed Services Committee Chairman John G. Tower (R-Tex.) said the administration was taking steps to make sure history did not repeat itself. "I don't think we should visit the sins of the past on this administration," Tower said.

David S.C. Chu, director of the Program Analysis and Evaluation Office and Spinney's boss, took the same line in rebutting the cost analyst's report. Chu said that Spinney's briefing fails to take adequate account of the administration's remedial actions in estimating weapons costs and keeping them under control.

Among the administration's reforms that Chu cited were assigning defense programs more realistic inflation rates, wider use of independent cost estimates, and placing orders over several years to achieve greater savings from mass production.

"While we recognize that this is a long-term process, there are already some encouraging signs that these changes are beginning to take effect," Chu said.

Sen. Barry Goldwater (R-Ariz.) blamed Congress for some of the extra defense costs, citing the House's addition last year of the Fairchild A10 anti-tank plane that the Air Force did not want. He said money for the plane was added to the fiscal 1983 defense budget by the chairman of the House defense appropriations subcommittee, Rep. Joseph P. Addabbo (D-N.Y.), and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) because it would bring business to their home states.

Tower concluded the hearing by saying, "I am somewhat at a loss as to why the Defense Department was reluctant to have Mr. Spinney testify before Congress." Tower said Spinney underlined the danger of cutting Reagan's defense budget this year by saying that he believed more money was needed for defense and that programs would have to be canceled if it were not forthcoming.