Saudi Arabia has a legitimate need for the F15 fighter because that nation has opted for a defensive rather than an offensive war strategy, a report by the American Enterprise.Institute says.

The research group, sometimes referred to as the conservatives' Brookings Institution, rejected the argument that the F15s President Carter wants to sell Saudi Arabia could pose an offensive threat to Israel.

To fight over Carter's Mideast arms package, which includes 60 F15s for Arabia, is expected to heat up next week when Congress starts its formal review.

Dale R. Tahtinen, in a 45-page report entitled "National Security Challenge to Saudi Arabia" that was released Thursday noted that the F15s for Saudi Arabia" would not have bomb racks and related gear needed to the U.S. Air Force aerial dogfighter into light bombers.

Israel, which has received about 15 of the 25 F15s it ordered before Carter put together a new package of arms for Israel, Saudi Arabia and Egypt, already has a few of the bomb racks needded for the offensive role.

The Pentagon confirmed this on March 23, stating: "With the permission of the United States government, Israel entered into an agreement with McDonnel-Douglas [manufacturer of the F15] to borrow three racks for testing and evaluation." Each rack can carry three bombs weighing 750 pounds.

(Presumably, Israel and Saudi Arabia could manufacture their own F15 racks eventually with Israel using one borrowed, one for a model).

"For the next decade, the Saudis cannot hope to destroy without significant losses any attacking Israeli air strike force, but they can attempt to make that effort costly to the Israelis," the AEI report said.

"Applying the same type of pragmatism, Saudi Arabia would probably not deploy its F15s in an Arab-Israeli conflict in the mid-1980s, when they are operational" because it would lose an unacceptable number of its limited pilots.

The first 10 Saudi F15s would be delivered to Saudi pilots in the United States in 1981. The planes would not get to Saudi Arabia until 1982, according to the Pentagon.

Although Tabuk would the "the most logical base" from which to launch F15 strikes against Israel, continued the report, putting the planes there would make them "highly vulnerable" to a pre-emptive attack by the Israeli air force.

Other Saudi airfields in the north in easy striking distance of Israel, such as Turayf or Gurayat, "have asphalt airstrips but lack the military support facilities to allow for the sustained operation of F15s, and there are no known plans to build such facilities."

The report concluded that "all in all. Israel seems most likely to tangle with Saudi F15s if it attacks Saudi Arabia in the Dhahran area, where the oil fields are located: in the Mecca-Medina-Jeddah area, which is protected from Taif, or in the southern border area, whose defense is centered at Khamis Mushayt. Wisely or not, Saudi Arabia does not envision the type f defense that develops an offensive capability."

Saudi intentions for the F15s have caused concern in Congress. Sen. Richard Stone (D-Fla.), chairman of the Near Eastern and South Asian affairs subcommittee of the Senate Foreign Relations Committee, has expressed fears that Tabuk would be used as a launching pad for strikes against Israel.

On Feb. 23, Stone wrote the Pentagon asking for detailed pictures of Tabuk. Said Stone in that letter to Defense Secretary Harold Brown:

"I will appreciate receiving from the Department of Defense photographs of the Saudi base under construction at Tabuk, showing all facilities and weapons and communications and radar emplacements on the site and showing in outline anp further planned expansion or deployment of equipment or material at the site.

"It will be of help to have these photographs as soon as they can be readied. Preferably, we would like to have photos of poster size and, in addition, a set of a large photos for use in briefings by our staff.

"If your staff has any questions, please do not hesitate to get in touch with Steve Bryen of the committee staff."

The FBI is investigating charges that Bryen offered to supply Pentagon information to the Israelis. Michael P. Saba, formerly executive director of the National Association of Arab-Americans, made that charge last month in an affidavit filed with the Justice Department.

Saba has sworn that he overheard Bryen offering Pentagon information to a group of Israelis in the Madison Hotel coffee shop on March 9. Saba said he was sitting at a nearby table at the time. Bryen, who took personal leave from the committee after the charges were filed, has denied making any such offer.

Pentagon officials declined to supply photos of Tabuk to Stone, declaring "it would not be appropriate." However, the Pentagon offered to arrange a classified briefing for Stone or one of his staff by the Defense Intelligence Agency on Tabuk. No such briering was given to either Stone or Bryen, according to DIA and the senator.

Saudi Prince Bandar Sultan said in a recent meeting with Washington Post editors that Saudi Arabia has no intention of basing F15s in Tabuk, partly because they would present such an inviting target to Israel. Bandar is the son of the Saudi defense minister and was a highly influential member of the Saudi team which chose the F15.

The 60 F15s Saudi Arabia wants to buy from the United States would be based in the middle of Saudi Arabia, the prince said.

The air bases being readied for the F15s would enable the new long-route interceptors to combat what the prince said were prime military threats confronting Saudi Arabia on three fronts.

He said the F15 bases would be Taif, near Mecca on the Red Sea, which would enable F15s to cover Saudi Arabia's western border; Khamis Mushayt, which would put the fighters in easy range of the Horn of Africa and hostile South Yemen; and Dhahran, amid the Persian Gulf oil fields, from which F15s could fly a permeter defense along the borders of Iran, Kuwait and Iraq.

Rather than Israel, the prince said, the widening Soviet military presence in the Horn of Africa is one big source of worry and the communist government of South Yemen another. South Yemen, he said is being built up military by the Soviet Union to the point that it looks like a launching pad for attacks against Saudi Arabia.

The Soviets, he said, already have delivered Mig 21 fighters to South Yemen. The prince, who is one of the Saudi air force's pilots, said his country's aging squadrons of British Lightning fighters are no match for the more modern Mig 21s already in South Yemen.

Some of the Saudi Lightnings, Prince Bandar told The Post, have developed cracks in their fuselage from metal fatigue - a condition which develops when a plane wears out.