Screens to keep sand out of the engines were removed from the eight helicopters sent over the Iranian desert to rescue American hostages in Tehran, the Pentagon confirmed yesterday.

Two of those helicopters broke down, and a third lost its way in a sandstorm during the first phase of the rescue attempt, prompting the commanders and President Carter to cancel the whole mission.

Second guessing about the sand screens was part of a larger, unofficial post-audit of the failed mission. The Pentagon was so stung by some of the criticism that Gen. David C. Jones, chairman of the Joint Chiefs of Staff who planned the mission, and Defense Secretary Harold Brown agreed to meet briefly with reporters at the Pentagon yesterday to respond.

Pentagon spokesman Thomas B. Ross said that "the removal of the sand screens had no impact on this mission." He said they were removed to provide a 3 percent increase in the helicopters' power, including their lifting power.

Jones said when asked about the sand screens that "the problems of the helicopters were not ones of propulsion," meaning it was not the engines that broke down, even though unprotected by screens.

Pentagon officials have said that the hydraulic systems on two of the Navy RH53 helicopters failed, not their General Electric engines. But several General Electric and government officials expressed astonishment yesterday at the decisions to leave off the screens since the helicopters would be flying low over sand, not over water as is the usual mission for the mine-sweeping RH53.

"The sand screen for the RH53 helicopter was designed to prevent long-term wear and tear on the engine," said Ross, adding that the rescue task force in rehearsals had "determined that by removing the screen overall engine performance could be improved by 3 percent without negative impact on the mission."

Government helicopter experts said the hydraulic system could be penetrated by sand on such a long flight as the 500 nautical miles from the Nimitz aircraft carrier to the refueling base in the Iranian desert. But whether the engine sand screens could have prevented this is unclear, since the helicopter hydraulic system is sealed off from the engine.

Here are points Jones and Brown covered at the Pentagon meeting:

Mission support. "We were not denied anything by anybody," said Jones. Brown said the mission originally called for sending in seven helicopters, in case one of the needed six broke down, but an eighth was added just to be safe.

One published reports claimed President Carter had scaled down the original plan.

Joint Chiefs' approval. Jones said "all of the joint chiefs" studied the plan that was executed and concluded that "we had a good chance of success. We thought we could be successful." He said the chiefs knew there would be "some risks" but adjudged the plan "militarily feasible" and recommended it to Brown, who then approved it and sent it on to President Carter.

Casualties expected. Asked about a radio report that the Central Intelligence Agency had estimated 60 percent of the hostages would be killed under Carter's rescue plan, Brown reported: "There is no such study to my knowledge."

Command decision.Brown said "all" of the commanders of the rescue mission agreed that it should be called off once they were down to five helicopters instead of the six considered a minimum for success. "None of them wished to proceed with less than six helicopters," the secretary added.

"There wasn't any question about the wisdom of extraction," said Jones in backing up Brown.

There have been reports that there was a hot argument among commanders during the first night of the mission about whether to proceed to phase two or call off the rescue attempt for lack of the sixth helicopter.

Security. Brown said that there was no evidence that either the American or Israeli press had published anything that alerted Iran to the rescue mission.

Destroying documents and helicopters. Brown said "the fierce burning and ammunition in both the helicopter and C130 cooking off" after they collided in the night at the refueling spot kept the rescue team from destroying their choppers and removing secret equipment from them before leaving Iran.

Jones, after he and Brown had addressed some of the specific criticism of the planning and execution of the mission, said "the Joint Chiefs of Staff are very disturbed about the implications that on the scene people didn't act properly." He said this criticism "by people who were not there does a disservice to some very valiant men."