Cost increases announced last week by the National Aeronautics and Space Administration for the manned space shuttle threaten four unmanned projects the space shuttle is being built to carry in the next five years.

The projects include a spacecraft to fly over the north and south poles of the sun and another to orbit the planet Venus and map its surface, using radar to penetrate its sulfuric acid clouds. The biggest projects threatened by the shuttle increase are the Galileo project to orbit Jupiter and the Large Space Telescope to scan the heavens from orbit around Earth.

"Are we in a situation where this country finally achieves an operational space transportation system, only to have lost the capability to use it for science?" asked Sen. Adlai E. Stevenson, chairman of the subcommittee on space and science.

"That's a serious concern," NASA Administrator Robert A. Frosch admitted to Stevenson, "but I will do everything I can to make sure that doesn't happen."

Frosch told Stevenson's subcommittee last week that NASA needed $270 million to pay what he described as sudden extra costs in producing the carbon and silica tiles that will cover the shuttle's metal skin as protection against the heat of reentry from space. Each shuttlecraft will be covered by 34,000 of these tiles.

Frosch proposed the space agency take the money from production funds for the fourth and fifth space shuttle, a move that could delay their production and will almost surely add $300 million to $400 million to their production costs.

In less time than it took to cross town, the Pentagon was warning Congress that any delays in production of the fourth and fifth shuttles could have an impact on verification of what the Soviet Union is doing with its strategic arms.

The Air Force plans to use no fewer than three of the first 27 shuttle flights to place into orbit satellites to watch Soviet missile deployment. The 14th, 20th and 26th operational flights of the space shuttle Columbia have been turned over to the Air Force. The Mission Control Center in Houston is being rebuilt to accommodate those three missions.

Besides those planned missions, the Air Force wants the option of placing its surveillance satellites aboard the shuttle on a moment's notice. The outbreak of war or even the threat of war somewhere in the world might mean the Pentagon would want a quick pair of eyes in space.

Without a fourth and fifth shuttle, the Air Force could never be guaranteed of quick response time: Suppose one shuttle were already in orbit and the other two were being repaired when the Air Force suddenly needed a flight. Congress has been warned of this possibility, and is reluctant to delay production of the last two shuttles.

That means the money to pay for the cost increases in the heat shield tiles may have to come from somewhere else. At NASA, the "somewhere else" in space science, which always takes a back seat to manned space and military space projects.

There are only four science programs now in the works that require the big money needed to pay for the shuttle's cost increases. One is a project to orbit Venue with a radar mapping device, another a mission to fly two spacecraft around the north and south poles of the sun.

The two biggest are Galileo and the Large Space Telescope, both of which are about to move into their peak funding years. The telescope can be delayed but it's farther along than Galileo, which cannot be delayed because its launch date of January 1982 depends on the positions of the planets.

On thing is clear: NASA has no plans to delay the space shuttle to accommodate either the telescope or Galileo. The only defenders of those projects are the scientists whose instruments will fly on the two unmanned spacecraft. The shuttle's defenders are legion in the halls of Congress and the Pentagon.