The Carter administration plans to spend as much as $400 million to transform Egypt's outmoded airfield at Ras Banas into a modern launching pad for the Rapid Deployment Force, government sources said yesterday.

Under the Pentagon's blueprint, Ras Banas, a point of land east of Aswan jutting into the Red Sea across from central Saudi Arabia, would be improved to accommodate American warplanes and a division of troops.

The Rapid Deployment Force is being organized to rush to remote trouble spots, like the Persian Gulf, in hopes of influencing a volatile situation before it gets out of hand.

The unit's planes and troops would not be stationed at Ras Banas permanently. They would fly there in periods of tension in the Mideast. To keep the American profile low in normal times, buildings for Rapid Deployment Force troops would be kept to a minimum at the Egyptian outpost.

But in contrast to the low-profile effort, internal Pentagon budget documents confirm that President Carter is counting heavily on Egyptian President Anwar Sadat to provide a Staging area for U.S. forces in the Mideast.

In one such document, Defense Secretary Harold Brown directs the Army in drafting its "basic" budget for fiscal 1982 -- the budget going to Congress in January -- to "provide funds for construction of a staging facility and one-division cantonment at Ras Banas. Detailed project description will be provided be separate correspondence."

A U.S. Army division numbers about 18,000 people, not counting the rear-area support forces for it.

Sources said the airfield will be first class, presumably with runways big enough to accommodate the long-range B52 bombers. Officials were reluctant to go beyond saying that B52s out of Ras Banas would be an attractive option for making a show of force all over the area, including the oil-rich Persian Gulf.

Although Sadat is warmly welcoming the costly improvement of his air base at Ras Banas sources said, the Carter administration does not want to advertise what kind of U.S. warplanes would use the base for fear of projecting an image of an American takeover of the Egyptian facility.

But it would make military sense for B52s, spy planes like the SR71 Blackbird, refueling aircraft and troop transports to use Ras Banas, not just short-legged fighters that can fly only about 300 miles with a full load of munitions before having to return to base for fuel. Refueling in flight extends their range.

As for barracks, mess halls and other facilities to support up to a division of Rapid Deployment Force troops, Pentagon planners stressed they were not going to build a Fort Benning in the Egyptian back country.

Aside from such basics as water and sanitation facilities, they said, the idea is for the American troops to bring with them most of what they would need -- including tents to live in during their stay.

The Pentagon estimates it will cost from $200 million to $400 million to upgrade Ras Banas, with the higher figure likely to be nearer to the ultimate price tag.

Sadat has publicly put out the welcome mat for U.S. military forces to use his facilities on a temporary, but not permanent, basis.

Not only does the upgrading of Ras Banas promise to strengthen Sadat and other friends of the West, in the Carter administration's view, but it also would help Egypt militarily and economically.

Sadat's air force would get full use of the Ras Banas airfield improved at U.S. expense, U.S. officials assert, and the American-financed construction would help develop southern Egypt.

Its far-south, remote location in Egypt also gives Israel less cause to worry about a modern air base at Ras Banas, in the administration's view.

A story in The Washington Post Aug. 7 disclosing that Ras Banas was part of the blueprint for Rapid Deployment Force bases for the Mideast drew a denial from the State Department. Administration officials said this was "a technical denial" because the deal has not yet been signed, sealed and delivered even though it is proceeding with Sadat's full blessing.

The fact that money is being set aside for Ras Banas in the Army and Air Force budgets evidences a high degree of confidence by the administration that the Egyptian outpost will play a major role in extending the U.S. reach in the vital Indian Ocean theater.

Oman, Somalia, Kenya, Diego Garcia and Israel also figure prominently in Pentagon contingency plans for bringing U.S. military power to bear in a crisis and to combat what planners see as a growing Soviet threat to the West's oil line.

Oman's air and naval facilities at the island of Al Masirah and the Thamarit airfield inland in Dhofar provide vital staging areas in the crucial Persian Gulf, in the Pentagon's view.

As in the case of Ras Banas, the United States, under an agreement made with the sultan intends to spend millions to improve those existing facilities in Oman. U.S. ships and planes operating from Al Masirah and Thamarit would be able to guard the Straits of Hormoz, a choke point for tankers carrying Iranian and Saudi oil out of the Persian Gulf. The bases also put North and South Yemen in easier reach.

Somalia's formal agreement last week to allow U.S. forces to use its naval ports at Berbera on the Gulf of Aden, and at Mogadishu on the Indian Ocean, is being hailed by military planners as a mighty assist in protecting another vital passageway, the Straits of Bab al Mandab connecting the Red Sea with the Indian Ocean.

Military planners consider the Straits of Hormoz, Bab al Mandab and the Suez Canal as the three most strategic naval passageways in the volatile theater.

The Soviets, too, seem to appreciate the value of those straits. Pentagon planners see the Soviet navy's use of South Yemen's port of Aden and its island of Socotra as menacing to the Straits of Bab al Mandab. Ethiopia's port of Assaba, its airfields and its island of Dahlak in the Red Sea also are viewed as potential launching pads for Soviet power in eastern Africa.

Further south, Kenya has agreed to allow the United States to use its port of Mombasa.

The British island of Diego Garcia is being turned into the warehouse for the Rapid Deployment Force as part of a U.S. investment expected to run up to $1 billion. Runways on the island are being enlarged to make it easier to handle B52s as part of the master plan for extending the reach of the U.S. military.

At least one U.S. military leader would like to put B52s closer to the hot spots than the remote island of Diego Garcia, suggesting Israel.

"Consideration should be given to making the Negev bases we will build B52 and KC 135 capable to provide the United States with the broadest possible Middle East-Indian Ocean crisis options," Gen. R. H. Ellis, commander of the Strategic Air Command, wrote Defense Secretary Harold Brown in a secret letter dated April 9, 1979. The KC 135 is an aerial refueling plane.

Ellis' B52 suggestion for Israel has not been acted upon, according to Pentagon sources.