The Defense Department, which burns 78 per cent of all the energy consumed by the federal government and more than any single corporate user sees little hope of saving much fuel in the future.

The nation's planes, tanks and ships must keep burning fuel at the current rate to stay ready for war, a Pentagon energy executive said yesterday. And the Pentagon already has a drive under way to save fuel at military bases.

John R. Noland, deputy director of the Pentagon office that recommends energy policy to the Secretary of Defense, said the present-day military vehicles - which are fuel guzlers - are here through the year 2000 and cannot be changed in design.

Asked if it would make sense to go back to coal-burning warships, Noland replied: "Teddy Roosevelt's Great White Fleet went around the world under a towering pillar of black smoke. But going back to that would be wildly impractical."

Today's ships have neither the bunkers for coal nor the furnaces to burn it, and the Navy would not want to return to that kind of propulsion in any case.

Noland said it was warships that pioneered use of oil and then nuclear energy for propulsion because of the advantages each offered for military operations.

The Army, Navy, and Air Force cannot turn to alternative sources of fuel, such as solar energy, to run tanks, ships and planes. Therefore, Noland predicted, if oil does run out, the military will derive liquid hydrogen-type fuels from coal or even seawater.

The Pentagon's energy executive said, however that military forces would be the last to be cut off from American sources of fuel and probably can count on conventional supplies of it for 50 more years.

Defense worries were translated last year, Noland said, into a conservation program that reduced military operations at sea and in the air to save fuel. Also, Noland said, the Pentagon has earmarked $722 million to save fuel at military installations through such steps as better insulation of buildings, an element of the Carter energy program.

The Defense Department, including its military forces, burns the equivalent of 225 million to 250 million barrels of oil a year, Noland said. This is 1.7 per cent of the total energy used in the United States yearly.

Adm. H. G. Rickover, head of Navy nuclear propulsion, has argued that U.S. warships should be nuclear powered, partly to lessen the danger of running out of fossil fuels. But opponents of an all-nuclear Navy have argued that the supply of uranium needed for nuclear propulsion is limited too.