President Reagan has decided to rent launching pads and sell government rockets to private companies to help get commercial space ventures off the ground.

The president's directive, announced yesterday, would allow private firms to buy parts and government-owned plans for the rockets at cost, as well as pay for the use of government launch pads, equipment and engineers. The plan does not envision that entire rockets would be sold.

Delta, Atlas and Titan rockets--called expendable launch vehicles (ELV) because they can be used only once--have been sending up government and private payloads for years. The government decided a decade ago to phase out these expendable rockets in favor of reusable space shuttles.

Contracts with the companies building the expendable rockets are ending, and the rockets' launch pads at Cape Canaveral were due to be shut down. Turning over the older rockets and launch pads to private companies would "offer a domestic backup for the shuttle at essentially no cost to the U.S. government," according to a White House announcement of Reagan's decision.

"The private sector would assume all costs of ELV production now borne by the U.S. government," Reagan's announcement said. "There would also be a market for U.S. government facilities and equipment that would otherwise be underutilized or no longer required.

"In summary, partnership between the U.S. private sector and the U.S. government will strengthen the U.S. space launch capability, develop a major new industry, contribute favorably to the U.S. economy and maintain U.S. leadership in space transportation."

Pleas from several private companies prompted the government's action, said Isaac Gillam, an administrator in NASA's Office of Space Policy. The companies said they wanted a chance to launch rockets to compete with both the space shuttle and the European Space Agency's Ariane rocket, Gillam said.

Gilbert Rye, staff officer for the National Security Council, said that the cost of keeping the government facilities open and operating will be borne by the private companies using them.

"It's a good deal for both sides," he said, since the plan could save the government the money it would take to decomission facilities, and might prevent layoffs. For the companies, it could save start-up costs, and would allow them to buy the spare parts of some rockets instead of having to manufacture them, he said.

The companies believe there is a strong market for companies wanting to put satellites in space, and that the European Ariane system may get an edge if the U.S. government does not help American firms by offering the use of spare parts and facilities.

Companies now are simultaneously booking passage for the same satellite on both the American shuttle and the European Ariane to ensure that the firms are protected if a launch is delayed. Such a delay could cost a firm millions of dollars per month.

American companies wishing to carry payloads into space say they believe they can break into the market if reliable American rockets can be launched with government help from specially designed government launching pads such as those at Cape Canaveral.

One current competition is for the business of Intelsat, a consortium of more than 100 nations that is about to launch its sixth-generation communication satellites and is considering the shuttle, the European Ariane and two independent American ventures to transport them.

NASA's Gillam said that boosting commercial enterprises is only one reason for the new policy.

"People feel a little queasy about the shuttle still, because it is new and there is not a lot of confidence in its schedule reliability," he said. "They are also afraid" that Defense Department payloads "may bump them off the schedule."

The new American ventures would mean more competition for the shuttle, but NASA and State Department groups that studied the subject "concluded that competition would be healthy for the shuttle, to help keep it on schedule," said Rye.

The presidential directive orders government agencies to help develop a commercial industry in the ownership and operation of space boosters to serve as a backup to the government-run shuttle program.