The Pentagon has responded to congressional demands for cuts in the president's so-called "Star Wars" research program, in part by reducing five demonstration projects this year, but putting the money back in the president's proposed budget for the next.

In outlining how Congress' $377 million reduction in the president's fiscal 1985 Strategic Defense Initiative (SDI), or Star Wars, research program would be carried out, Defense Secretary Caspar W. Weinberger wrote that cuts totaling $141 million were applied to five Army antiballistic missile (ABM) demonstration programs. However, the fiscal 1986 budget, sent to Congress last week, shows substantial increases in each of those programs, indicating plans to reinstate them to their former funding levels.

"Because these budget cuts will cause delays in many of the elements of the SDI program, they also will delay the time in which the nation will be able to make an informed decision on strategic defense," Weinberger said in a letter to the House Appropriations Committee.

Some key Reagan officials along with defense experts on Capitol Hill believe the Army's proposed ABM system, which has been in development for 19 years, offers a more realistic approach to ballistic missile defense for the 1990s than does President Reagan's more ambitious space-based plan. Even if the Star Wars plan proves workable, it could not be deployed until the next century.

One key congressional military expert said yesterday that members were "surprised" by the Weinberger cuts in the Army demonstration programs.

Another Weinberger cut was made in Talon Gold, an older Defense Department space-based satellite program that last year was integrated into the Star Wars research. Talon Gold was designed to find, track and target Soviet missile warheads in space using lasers.

Weinberger said $70 million of the planned $105 million for this year was cut from Talon Gold, "so that we could provide a major restructuring of our efforts into a new more effective program."

In a related development, the Federation of American Scientists yesterday criticized the cost of the president's SDI. The group, which opposes the program, released a report that said the "initial development phase of the SDI will run through the early 1990s, at a cost of about $70 billion."

In a news conference, Associate Director John Pike said the Star Wars program which "last year was described as a five-year, $26-billion research program has already become a six-year, $33-billion program and will continue to grow until a deployment decision is made in the 1993 time frame." The original description of it as a five-year program was an "optical illusion," he said.

Pike also pointed out that one of the bigger increases in next year's Star Wars budget was for "systems analysis" which grows from $56 million this year to $163 million. These analyses are needed, Pike said, to determine "the missions and required capabilities of components of the SDI system," which he said is still in the formative stage.

Dr. Gordon Adams of the National Campaign to Save the (1972) ABM Treaty appeared with Pike and said the Reagan Star Wars program is "now laying track for a spending train to roll along for the next two decades."

The president's fiscal 1986 budget asks for $3.7 billion for the Star Wars program, more than twice the $1.4 billion Congress approved this year and a "totally unprecedented" increase, Pike said.

In fiscal 1987, according to the federation report, the administration plans to seek $4.9 billion.

Pike said it was "far from clear the administration can spend the research money it is seeking."

A few of this year's cuts were made in what Weinberger described as "inherited programs" designed to develop laser and other directed energy systems considered basic to the Star Wars concept.

Next year's budget, however, shows a sharp increase in expenditures on such programs. Ground-based laser systems grow from $171 million this year to $385 million next year.

The more exotic research on space-based particle beam weapons is scheduled to rise from $36 million this year to $150 million next.