Had the hostage rescue plan worked, the key step called for heavily armed U.S. troops to board vehicles outside Tehran in the dead of night and then race through the city's streets to take the embassy compound by surprise.

The helicopters would not have swooped into the compound under the aborted plan until after the hostages were safely in the hands of the rescue party.

Pentagon and White House officials broadly hinted yesterday that the United States had help inside Tehran in determining the exact moment to strike.

Presumably, the hostages at the moment would have been guarded only lightly by the militants. The planning and repeated rehearsals over the last several months convinced the would-be rescuers that once inside the embassy compound their work would be easy.

"The team itself was convinced that that was the part of the mission of which they were the most confident," Defense Secretary Harold Brown said at his Pentagon press conference yesterday.

Although Brown did not specify the basis of thie confidence, other military sources said it rested on absolutely reliable intelligence information on when to hit the embassy and what the situation would be there at the time.

The 90 troops in the specially trained U.S. "Blue Light" rescue team were armed with nonlethal chemical agents to incapacitate the militants but they were not depending on their use.

Under the rescue plan, the hostages would have been assembled by the troopers, picked up by helicopter and deposited at an undisclosed airstrip outside Tehran where six C130 transport planes would be waiting for them, propellers turning.

The rescue team, the helicopter crews and the 53 hostages would have climbed into the C130s and flown out of Iran, leaving the helicopters on the ground behind them.

Three separate bases in Iran were mapped out for the operation. One was near Tabas for refueling the helicopters; a second far enough outside Tehran so the helicopters approaching the city would not be detected, and a third for the escape by C130 transports.

Months ago, the Joint Chiefs of Staff had studied a plan calling for the rescue team to land directly inside the compound by helicopter. But as Air Force Secretary Hans M. Mark disclosed publicly last month, this idea "just wasn't feasible."

The revised plan called for the troops to sneak up on the embassy in vehicles; rather than crash directly into the compound, as was the case in the raid against North Vietnam's prison camp at Sontay in 1970.

Government officials indicated that the vehicles for the raid had been covertly deployed near Bast Two outside Tehran. U.S. troops would have stayed there the first night of the operation and then mounted the raid on the second night -- which would have been last night in Washington.

There were also suggestions that the 90-member Blue Light unit Brown talked about publicly yesterday had some backup. It appears that others -- numbers and identities unknown -- were deployed ahead of time at Base Two. They were said to have been in the process of being extracted when the White House announced the raid early Friday morning.

Sen. John Glenn (D-Ohio) raised the possibility of Iranians secretly helping the would-be American rescuers, stating: "There might be a fifth column that we have developed in or around the embassy that was going to help us."

White House and Pentagon officials said that the hardest part of the operation was flying the raiding party by helicopter the 500 miles from the Nimitz to Desert One, the refueling site.

"I'm not sure how many of your have flown 500 nautical miles on a helicopter in one trip," Brown said, "but not many people have. It's a difficult task, and no other country, in my judgment, could have attempted anything like this." a

Inside the Pentagon, several highranking officers shook their heads in disbelief upon learning the mission was called off after progressing so far. "They should have gone in with what they had," said one Army combat veteran, in a typical comment.

It looked for a while as if the rescue operation, which had been rehearsed at an undisclosed site in the southwestern United States was going to be a stunning success.

The six C130 transports had been flown without a hitch from the United States to an airfield in southern Egypt. The planes were crammed with electronic equipment to jam Iranian communications and confuse any Iranian air force planes that might rise to combat the rescue aircraft.

Sometime Thursday afternoon, Iran time, the C130s left their Egyptian airfield, refueling in midair. Presumably they took the long way to Iran, down the Red Sea and along the Arabian Sea coast to avoid flying over Saudi Arabia.

The C130s then made a brief rest stop at an undisclosed airfield on the western side of the Persian Gulf opposite the Iranian coast, sources said. They did not land at Behrain as some reports indicated.

Meanwhile, the nuclear aircraft carrier Nimitz, which had been sailing in the Arabian Sea, moved into the Gulf of Oman closer to the Iranian coast.

As twilight engulfed the region, the C130s took off again. About the same time, eight RH53 helicopters carrying the 90-man raiding party dressed in camouflage uniforms lifted off the Nimitz's deck.

Brown said the helicopters and the C130s entered Iranian airspace shortly after dark.

It was on the way toward the first staging base in the Dasht e Kavir salt desert that the misfortune that would ultimately doom the mission began to unfold.

One helicopter got caught in a severe sandstorm, lost its way, and eventually had to turn back and return to the Nimitz. Brown reportedly told congressmen at a private meeting that the chopper's flight crew become disoriented and suffered from vertigo, a condition where a pilot loses his sense of up and down.

A second helicopter ran into mechanical problems and landed in the desert short of the refueling spot. Its crew was picked up by another chopper, however, and brought to the first base -- Desert One.

Thus, six of the original eight helicopters arrived at the refueling base. But one of them discovered its hydraulic system was crippled, meaning it could not take off for the next leg of the journey to the outskirts of Tehran.

The loss was devastating. The operation plan called for termination of the rescue effort if there were fewer than six helicopters operational. The reason, sources say, is that it would take more than four helicopters to get everybody out of Tehran, including the raiding party, the 50 hostages and the three Americans in the separate foreign ministry quarters.

Thus, if the team headed for Tehran with only five choppers, and lost only one more at the next stop, it would have meant leaving people behind.

The team members in the desert, administration sources said, argued about the pros and cons of going ahead anyway with only five helicopters. But the team commander, an Army colonel, decided to stick with the original plan.

His no-go recommendation was probably influenced by the fact that a busload of Iranian civilians drove past the refueling site and would have had to be detained there to keep the rescue operation secret.

White House officials said the team's commander recommended against going through with the mission and that Carter and Brown concurred.

"We believed we could get by with six" helicopters, Brown said, and added a seventh in the early plan in case there was a breakdown. Later, an eighth helicopter was added just to be safe. "That was really quite considerable spares," Brown asserted.

Once Washington had approved aborting the mission, the five operating helicopters and six C130s prepared to take off from Desert One.

"It was complete darkness," said Gen. David C. Jones, chairman of the Joint Chiefs of Staff. "They had night vision devices" in the helicopters and C130s. "But it was unfortunate on the lift-off. It appeared that the helicopter struck the C130, and they were both engulfed in flames."

Eight servicemen died in the collision and four others suffered burns. The dead wewre left, apparently in the burning wreckage; the burning C130 and the helicopters were abandoned and the rescue party, including the wounded, board the remaining five C130s and left Iran.

Rep. Robert Bauman (R-Md.) said yesterday that he had been told that one of the abandoned helicopters contained classified documents detailing the rescue plan.

Said Brown yesterday: "I'll take responsibility within the Defense Department" for what happened in the abortive raid. "I'm not looking for goats.

"We can all be proud of the brave men who undertook the mission. They conducted themselves with skill and courage. And we are filled with sorrow for the eight men who gave their lives trying to rescue their fellow Americans."