President Carter will ask Congress on Tuesday to cut the Pentagon budget he inherited from President Ford by about $2.7 billion - an action that will open this year's debate on how much is enough for defense.

The cuts, if approved by Congress, would reduce military spending in fiscal 1978, which begins Oct. 1, by only about $500 million because most of the military programs Carter wants to cutwould have been financed over several years.

Defense officials confirmed yesterday that this first whack at Ford's Pentagon budget, which called for $123.1 billion in total authority to obligate money and $110.1 billion in spending, is only the beginning.

Later this year, they said, Defense Secretary Harold Brown will propose to Congress a much bigger package of cuts, calling for closing some military bases and schools, cutting the Pentagon payroll, reducing military moving expenses by keeping families at one base longer, and overhauling the retirement system.

Brown has decided it is necessary to risk the minefield of political protests because manpower costs have gotten out of hand, sources said.

Already manpower and related costs eat up about 56 per cent of the Pentagon's total budget, with retirement pay for military personnel running $9 billion for fiscal 1978 alone.

Brown's strategy will be to put all the possible savings together in one multi-billion package and argue before Congress that the nation would be stronger militarily by investing that money inweaponry.

Defense officials said yesterday they have not determined yet how big the manpower overhaul package will be or when they will present it to Congress. But Brown definitely will make the fight in his first year as secretary, they said.

One idea is to present the package to Congress this year but try to implement the revisions in the fiscal 1979 Pentagon budget. This would mean voting on the revisions in 1978 as the lawmakers review the first budget Carter will write from scratch.

Even if the manpower changes are implemented, Defense officials said, Brown holds no hope for lower Pentagon budgets in the future. Any savings would simply go into paying faor new weapons, most of them already on order, these sources said.

During the election campaign carter promised to reduce the Pentagon budget by $5 billion to $7 billion. Defense officials are now saying he intends to save that much through better management, which will not necessarily show up as a net reduction in the total Pentagon budget.

All the military services, and several contractors, will get nicked if Congress approves the cuts Carter has already decided on.

The Army would lose the non-nuclear version of its Lance battlefield missile on grounds it is ineffective against moving targets and not powerful enough for fortified positions.

The Navy would lose its nuclear powered strike cruiser and some A-7 fighter-bombers, and end up with an austere version of its experimental surface effects ship - a craft that skims over the water on a bubble of air produced by giant fans.

The Air Force would be able to buy fewer B-1 bombers before Carter's go or no-go decision on producing a fleet of them is made; its purchases of F-15 fighters would be reduced, and development money for the MX nuclear blockbuster, a giant intercontinental ballistic missile, would be reduced.

The Marine Corps would be ordered to make fixes in its heavy-lift helicopter before proceeding further toward production.