PROMONTORY, UTAH, DEC. 30 -- The Morton Thiokol employes at this bustling space-age workshop, brightly colored prefab buildings scattered about a hilly wasteland of snow and sagebrush, thought their luck had changed after the January 1986 space shuttle disaster.

The rocket boosters they built had failed, apparently because of cold weather, and seven astronauts had died. More than 200 employes were laid off, and others transferred or put on four-day weeks.

But the plant here went to work on the booster redesign and picked up other projects. By this month, more than 8,000 people were working here, a company record.

Then came Tuesday, probably the worst day in the company's history since Challenger blew up nearly two years ago. The first blow was the huge, violent fire at dawn in an MX missile fuel-casting building, leaving five men dead -- the first Thiokol fatalities in 20 years -- and the building nothing but naked girders.

Second, in midafternoon, the National Aeronautics and Space Administration announced that last week's second test of the redesigned shuttle booster, initially labeled a success, had uncovered a flaw that might delay the program for months.

Morton Thiokol vice president for space operations John Thirkill summed up the impact of two such misfortunes during a holiday week: "How would you feel if your wife left you and you wrecked your car, on the same day?"

"People here take it very personal," said Bill McGaha, a Thiokol production supervisor and member of the city council in Brigham City, the town of 16,000 located 25 miles east of here where many Thiokol employes live. "It's like losing a member of your family. It hits hard."

The flag in front of the plant administration building flew at half-mast today as separate teams of investigators pored over clues at two widely separated sites -- the plain where the shuttle booster still lies, horizontal, after last week's flawed 122-second test firing, and the hill where 100,000 pounds of rocket fuel made of aluminum, ammonium perchlorate and synthetic rubber ignited and destroyed an Air Force-owned MX production building.

This desert near the north shore of the Great Salt Lake became a missile plant site because its remoteness promised safety and security. Many of the operations here are done automatically or by remote control to avoid injuries and save lives. But the local residents seem to recognize and accept that there are still dangers.

"It's like working in a firecracker factory," said Max Muir, who once worked in quality control at the plant as an Air Force civilian and was recently elected to the Brigham City council. "They know something is going to blow up sometime . . . . They've instituted many safety measures, but they're still dealing with very hazardous material."

Thirkill appeared at work today in a ski sweater, a sign that he, like many other Thiokol employes, had interrupted a vacation to meet the demands of a double misfortune. A joint Thiokol-Air Force panel is being formed to study the MX missile fuel fire. If it discovers a serious flaw in the design of the casting equipment that was being removed at the time of the fire, the plant's MX assembly line could be severely disrupted.

Thiokol spokesman Rocky Raab said the only way the solid fuel could have ignited was from "heat, friction or impact," but the cause is undetermined.

NASA and Thiokol investigators are examining the booster, but they will be unable to separate the rocket nozzle -- where a protective plastic ring broke off during the test -- for close inspection until Friday or Saturday. First must come a close look at the joints that caused the shuttle disaster and seemed to pass last week's test in their new design.

NASA officials said today that they did not know how long the shuttle program will be delayed.

"Sure we're down," Thirkill said. "No one likes to see a program interrupted and see problems occur, but the thing we have to do is set about to solve them."

A tiny part of the community here, including some people who had close ties to the shuttle project, wonder whether the fixation Thiokol and NASA have on deadlines might be interfering with solving recurring problems. "When NASA needs to meet a schedule, they'll do anything to meet that schedule, come hell or high water," said Roger Boisjoly, a former Thiokol engineer who warned against launching Challenger in weather below the 50-degree tolerance of engine seals on the boosters.

Boisjoly, who lives 10 miles south of Brigham City in Willard, discovered that his was a very unpopular position, particularly after he provided investigators with documents showing the shuttle's potential problem with low-temperature launches and sued the company for $1 billion. He said emotional stress, stemming in part from a chilly reception from other Thiokol employes, led him to leave on a disability plan that pays 60 percent of his salary for two years.

But most of the community strongly supports Thiokol and wants the work to continue with as little delay as possible. "A lot of people are very proud of what we're doing," McGaha said, "and they think it is important to the country as a whole."