The American flag in the courtyard of the main gate at Martin Marietta Corp.'s Michoud Assembly Facility here is flying at half staff.

A tribute to the seven astronauts who went down in flames Tuesday, the flag is also a symbol of the mood inside the plant that builds the external propellant tanks for the space shuttle program.

Among the 4,700 employes, the mood is as somber as the overcast skies outside, with grief, anger and fear overshadowing the production of the 27.6-foot-wide, 154-foot-long tanks that deliver liquid hydrogen and oxygen propellants to the space shuttle's engines.

The explosion also focuses the spotlight on Martin Marietta, the diversified aerospace and high-technology powerhouse. With headquarters in Bethesda, it is the largest company in the Washington area, with $4.4 billion in sales and $249.4 million in profits for 1985.

with $4.4 billion in sales and $249.4 million in profits for 1985.

Although Marietta is far from new to the space business, Tuesday's disaster was new to the company, which year after year has praised itself in its annual report for its "continued flawless performances of the company-built external fuel tank" on the shuttle missions.

Proud of the work they have done in the 13 years they have been building such tanks for the shuttle program, the Michoud employes' red eyes and tears make it clear they are dismayed that their work could be involved in Tuesday's disaster.

But their shock quickly gives way to anger as they hear reports that the tank they built could have been the cause of the crash. "There is no doubt that the propellants inside the tank were involved in the explosion," said a Michoud spokesman. "But that doesn't say that something was wrong with the tank. . . . There's little that can go wrong with the tank."

With the space shuttle program now halted, pending an investigation into the crash, the workers are also fearful that their jobs could be lost if the shutdown lasts a long time or the investigation points to the external fuel tank as the problem.

Today, Marietta officials said it was responding to NASA's request for help in its investigation of the fatal launch. Not only is it sending a participant to NASA's investigative team, but the company also has set up its own investigative unit to gather up and review all data on the particular fuel tank that was used in Tuesday's flight.

The external fuel tank represents big business for Martin Marietta. Overall, financial analysts figure that the external-fuel tank program contributes about $400 million to the company's sales, making it account for about 10 percent of Marietta's 1985 sales of $4.4 billion.

Even more significant is the program's annual profit -- about $40 million, according to Wolfgang H. Demisch, defense industry analyst with First Boston Corp. That means that in 1985, the program accounted for about 16 percent of the firm's $249.4 million net earnings announced last week.

Overall, financial analysts calculated that about 15 percent of the company's business dealt with building and developing launch vehicles, spacecraft and instruments for space.

Yet, analysts said that Tuesday's crash should have little effect on Marietta's bottom line in the long term.

"In terms of having an impact, it's not that significant," said Robert G. Maloney of Wood Gundy Ltd.

However, "they do stand to lose something in the near term, in 1986 and 1987, from some loss of business from a slowdown" in the space-shuttle program, said Robert Kugel of Morgan Stanley Co.

At a minimum, industry experts expect the program will be halted for at least three months and perhaps as long as two years -- the same amount of time it took NASA to regroup from the 1967 disaster when three astronauts were killed during a training session.

The exact time will in part depend on whether the tank was the cause, added Demisch. If it is and it is a minor flaw, it can be corrected quickly; but it will take significantly longer if it means a redesign of the entire fuel tank.

Even so, analysts unanimously note that, whatever the cause, Marietta will benefit from the large role the company is playing in building unmanned rockets to send satellites into space.

"The military and commercial sector cannot accept an indefinite delay in the space shuttle program," said Demisch. "This will increase the requirements for buying expanding unmanned rockets -- the type that Martin Marietta was selected to make for the Air Force last year." They are slated to build 10 for $2 billion.

The accident "virtually ensures" the execution and expansion of the program, Kugel added.

Marietta has a number of other space programs in the works. Under a $24 million contract, it is competing with Boeing Co. to design part of the permanent space station. The winner of the competition will design a pressurized module in which astronauts manning the space station will work and live. It also has NASA contracts to study two space vehicles, one to move satellites from one orbit to another; another to ferry satellites and possibly people back and forth between a space station and higher orbits.

It also worked on key instruments for the space telescope and the Galileo research satellite that were slated to be launched later this year.

This space work will continue in full force, Marietta said today. As for the continued production of the external fuel tanks, Marietta will also continue to complete its contract for 60 tanks; 36 have been delivered and 16 are in various stages of development. "We have not received any word from NASA to suspend production," a spokesman said. "We are continung to meet our production schedule. We have a contract and we have to meet the obligations of the contract."