Defense Secretary Caspar W. Weinberger yesterday revealed that President Reagan's new defense budget will top $255 billion, and he took a hard line when members of the Senate Armed Services Committee suggested that, given the state of the economy, this is too much.

Weinberger, according to those who heard him testify in the closed session, was adamant that projections of huge government deficits should not be allowed to derail Reagan's program to rearm America.

Shortly after taking office, Reagan called for increasing the amount of money approved for defense by 7 percent per year after allowing for inflation. His fiscal 1983 request of more than $255 billion in budget authority compares with about $222 billion for this fiscal year, a 15 percent increase.

Actual spending is expected to rise even more, from $182 billion to $215.8 billion in fiscal 1983, or 18 percent.

This guns-over-butter emphasis has sent politicians searching for ways to look like scrooges on defense as well as the domestic programs Reagan is pressing Congress to cut back.

Republicans on the Senate Armed Services Committee will take a stab at this Thursday in a hearing where the Army is expected to be lambasted for letting its new AH64 helicopter gunship pile up huge cost overruns. The General Accounting Office has estimated that the chopper will cost $13 million each, a price tag that is now considered conservative.

The open hearing by the Armed Services tactical warfare subcommittee will be chaired by one of the Pentagon's staunchest supporters in Congress, Sen. Barry Goldwater (R-Ariz.).

Goldwater and his allies are prepared to stress that they favor a strong national defense but will not tolerate such waste at the Pentagon in this era of high unemployment, high deficits and reductions in domestic programs.

Similarly, all the Republicans on the House Appropriations subcommittee on defense have gone on record as favoring cuts of up to $10 billion in the Pentagon budget this year to help shave federal deficits, which are expected to approach $100 billion this year and perhaps more next.

However, unless Congress is willing to leave American forces less ready to fight in the future than they are now, it will take huge cuts in the Pentagon accounts the lawmakers deal with to reduce the federal deficit significantly. The reason for this is the length of time it takes to build a modern weapon such as an aircraft carrier.

For example, if Congress approves $3 billion in budget authority for an aircraft carrier in one year, that amount will be spent out over eight years or so, with outlays in the first few years relatively low.

Rep. Les Aspin (D-Wis.), one of the many lawmakers trying to find a way to cut the defense budget without appearing soft on the issue or cutting into military muscle, recently sent a memo to the House Budget Committee outlining this problem of translating congressional budget cuts into immediate reductions in federal spending.

He said, for example, that to cut spending for war planes this year by $10 billion would require a $100 billion reduction in the procurement account Congress approves in the form of budget authority.

Given this situation, both Congress and the Pentagon in the past have often resorted to achieving immediate reductions in spending by keeping ships tied up in the docks for longer periods to save fuel, reducing flying hours for pilots, delaying the overhaul of equipment and going short in the purchase of spare parts.

All this reduces the readiness of military forces to fight, a condition that former president Carter was accused of creating during Reagan's election campaign of 1980.

Aspin warned the House Budget Committee that "given this choice" between eliminating weapons and waiting for the savings to show up years afterward in spending accounts, "It's a pretty good guess that the cuts are going to come exactly where they shouldn't be, in the operations and maintenance account that funds readiness. This is the way it has always been. and it's the major cause of weakness in our defenses."