Citing a cutback in one missile project and changes in two others, Hercules Inc. officials have announced a combination layoff and hiring that will result in a net gain of between 50 and 100 jobs.

Jack DeMann, spokesman for the rocket manufacturer, confirmed the changes Monday, two days after Morton Thiokol, manufacturer of space shuttle boosters, announced that about 60 people will lose their jobs.DeMann said exact numbers were not available nor was it known how many people will lose their jobs. However, many of the people laid off will be engineers and others involved in the research and development of new rockets, DeMann said. Most of the people hired will be involved in rocket production.

Hercules currently employs about 4,000 people at its plant near Magna, making it one of the largest employers along the Wasatch Front.

Although production workers generally need less skill than researchers, DeMann declined to compare the new jobs with the ones lost. "They are different types of jobs," he said.

The changes will begin immediately and will continue through April, he said.

DeMann said Hercules needed to make the changes because the company is ready to move from the development stages of the Titan IV and Delta II rocket motors to actual production. The federal government also has cut its budget for Hercules-manufactured intercontinental ballistic missiles.

The shift is a further indication of at least temporary hard economic times for Utah's rocket manufacturers. Hercules officials announced a pay freeze for most of its highest paid employees earlier this month, citing earnings that were below expectations.

The company's third-quarter profit in 1988 was $29.8 million compared to $48.8 million during the same quarter in 1987. An increase in costs for raw materials has plagued the company, officials said.

DeMann said there is less need for researchers and technicians now than when the company was more heavily involved in developing strategic defense systems. "We don't know what's on the horizon," he said.

He said some people will be moved to new jobs rather than laid off, particularly workers who were producing the ICBMs.

"If we can use someone somewhere else, we'll not show them the door," he said.

Thiokol, based in Brigham City, decided to lay off 60 people because it had completed tests on redesigned space shuttle booster rockets, officials said. The layoffs were announced in a memo posted Saturday to alleviate rumors circulating through the company.