The nation's oldest (and many believe smartest) military man said yesterday the government should junk its 55 different retirement programs and put all federal civilian and military personnel under a single pension system.

Adm. Hyman Rickover, 77, said his plan would end the practice of "double dipping" in which retired military personnel in government are allowed to keep their entire pension while drawing full civilian salaries. And he would stretch out most military careers - which now end in 20 years - by an additional 10 years.

Rickover was the leadoff witness at House hearings on dual compensation or, as critics call it "double dipping." But he went far beyond that single problem, and, in what he said was a "labor of love," outlined a program that would totally revamp military retirement concepts and the federal system now used to pay more than 2 million retirees.

Rep. Morris K. Udall (D-Ariz.) said, at the conclusion of Rickover's 22-page tatement, that the 77-year-old admiral's ideas could "slop over" to save the financially "endangered" Social Security system, the primary pension backup for virtually all Americans.

Chairman Robert N. C. Nix (D-Pa.) said Rickover's presentation was "the most complete and factual statement it has been my privilege to listen to in 20 years" in Congress.

Rickover said the up-or-out in 20 years retirement system of the military is antiquated and encourages the mediocre to hang on while forcing some of the best talent in the Army, Navy and Air Force to quit at the most productive time of their lives. He said the unfunded military retirement system now has future debts of about $200 billion, and that it will get worse.

Rickover himself would have been forced into mandatory retirement in 1962. But the "father of the nuclear Navy" has so impressed the late five President's that he has had nine two-year extensions, the latest coming from Jimmy Carter, who once served directly under four-star admiral.

His program would:

Prohibit waivers of the dual compensation law that permits some retired military officers to keep their full pension, and take jobs with the government that pay them - in combination - more than the Vice President or the Chief Justice.

Apply dual compensation laws across the board, ending exemptions that penalize "regular" retired officers but permit retired reserve officers and enlisted personnel to draw two full pension and salary checks from the government.

Treat retired military personnel who come into government the same as federal retirees who are rehired. Their civil service salaries are reduced by the amount of their pension. Rickover said military retirees should continue to earn retirement credits while working as federal civilians, but not be paid any more than the regular civil service salary for that job.

Require all agencies to annually budget and provide funds enough to cover retirement costs.

Consider making the military pension plan a contributory system like the civil service program, into which employees pay 7 per cent of their salary.

Put all 50-plus government retirement programs into a single system including the military. Retirement credits would be interchangable but nobody would get double credit, as at present, for military service when civilian retirement is computed.

The new pension system should defer all retired pay until "at least" age 55. "Our citizens should not be encouraged to believe they can expect a salary, and retired pay during their working years," he said.

All federal pensions should provide rights to deferred income with payments starting at age 62 for individuals who have served a minimum time of 5 years.

Military personnel discharged before completing a full career (30 years under the plan) would get lump sum payments, Rickover said, "rather than . . . offering relatively young people lifetime retirement income . . ."

Rickover said exceptions to the dual compensation cutoff might have to be made for people already on the payroll. But, he said, "as a matter of public policy, no agency should have to depend on retired military personnel to staff its organization."

The admiral indicated that many generals and admirals now working for the government here had used their influence to reserve top-paying civilian jobs for themselves while still in uniform. He said that the Navy, and other services, frequently take care of their own by hiring retirees from "think tanks" to do work for them. "Thus, we have a curious phenomenon - an officer becomes sufficiently expert to perform Navy work only after he has left the Navy."

The hearings will continue. But Rickover - like him or not - is a tough act to follow.