NASA said Monday it will ask contractors this summer to submit proposals for a $1.2 billion program to design and build an advanced space shuttle booster rocket in a government-owned plant. Under the plan, the winning contractor will design, build and operate the plant. The rockets to be built there will be used by the shuttle program well into the next century. The National Aeronautics and Space Administration gets its shuttle rockets from a single private source, Morton Thiokol Inc., whose manufacturing facilities are situated in Utah. A government-owned plant will enable NASA to change contractors if it becomes dissatisfied. The request for proposals will be issued in June or July, NASA said, so the advanced solid rocket motor will be available for launching after Oct. 1, 1993. The new booster will enable the shuttle to carry 12,000 more pounds of cargo to orbit than the current 65,000 pounds. That increase will be equivalent to 2.4 extra shuttle missions a year on a 14-flight schedule. ``To achieve the level of process control and automation needed for high quality and reproducibility, NASA has concluded that a substantially new facility is required,'' the space agency announcement said. The overall cost for development and testing the booster is $1 bilion. The facilities will be budgeted at $200 million to $300 million. The site has not been announced, but NASA officials have told Congress the prime candidates are Bay St. Louis and Yellow Creek in Mississippi and an area near Cape Canaveral, Fla. NASA says the government plant must be accessible by rail and water. J.R. Thompson, director of the Marshall Space Flight Center in Alabama, said five firms have expressed an interest in making the rockets. United Technologies, Atlantic Research, Aerojet and Hercules were interested in operating a government-owned plant, he said, while Morton Thiokol wanted to improve its Utah facilities. Contractors bidding for the plant will be required to propose a private-financing option for the facility, both for a government site or a location of the contractor's choosing.