NASA now has full shuttle fleet, settles into `routine' operations. After successful Atlantis flight, shuttles ready to fulfill promise as space workhorses for science, industry, military

With Atlantis's successful maiden flight, and little chance of getting additional space shuttles, NASA is settling down to get the most from its four-orbiter fleet. For the National Aeronautics and Space Administration, that means refurbishing and improving the individual spacecraft, and refining day-to-day operations. This includes significant upgrades such as a new generation of on-board computers from IBM and more durable hydraulic power units. Eventually, NASA may order major spacecraft retrofits such as the addition of docking equipment to permit a shuttle to hook up to a future space station.

Arnold D. Aldrich, manager of the Space Shuttle Projects Office at the Johnson Space Center in Houston, points out that NASA has more than a fleet of fine spacecraft. It also has vehicles that can be modified to meet the evolving needs of the nation's space program.

Shuttle prime contractor Rockwell International has shut down the main shuttle assembly line. Several thousand production workers have left. Yet, as Mr. Aldrich explains, the capability to repair and improve shuttle orbiters is still there.

Rockwell and its subcontractors are still producing spare parts. These include major components such as wing and tail sections. Indeed, were NASA to need a fifth orbiter, Rockwell has retained the basic capability to assemble one, using some of the spare parts.

Rocco A. Petrone, president of Rockwell's Shuttle Orbiter Division, explains that the key factors in gearing up to build a complex spaceship such as the shuttle are a solid design, an experienced management team, and a well-honed set of production procedures. All those still are available to him, Mr. Petrone says. Moreover, he expects that his key subcontractors will also maintain their capability for at least several more years because of the ongoing spare-parts production. Given this basic competence,

Petrone says that building a fifth orbiter would not be a major hurdle. Meanwhile, his team has the capability to make significant improvements in the existing orbiters when these are desirable, plus supporting continuing shuttle maintenance and, if need be, major repairs.

The fleet already shows its evolutionary capacity to a small extent. There are subtle differences between the look-alike orbiters. Atlantis and Discovery, for example, are structurally stronger than Challenger and Columbia so they can be launched from Vandenberg Air Force Base in California.

Vandenberg will launch shuttles toward the south, into orbits inclined at angles between 68 and 105 degrees to the equator. It takes more rocket thrust to inject a shuttle into these near-polar orbits than into the lower inclination orbits available from Cape Canaveral, Fla. (where Earth's rotation gives the spacecraft a boost). The higher thrust puts more stress on a shuttle.

Atlantis and Discovery also have improved thermal protection. A lighter-weight quilting replaces the tile insulation on their upper wings, fuselage, payload bay doors, and vertical stabilizer (tail).

Replacing the insulation and use of lighter structural materials such as graphite epoxy, have progressively reduced shuttle weight. Orbiter empty weights, listed in decreasing order, are: Columbia, 176,361 pounds; Challenger, 170,868 pounds; Discovery, 169,857 pounds; and Atlantis, 169,680 pounds.

Atlantis is also equipped to fuel and service the Centaur hydrogen-oxygen-powered rocket that will boost interplanetary spacecraft out of Earth orbit.

But such improvements are not as visible to the casual observer as is the infrared sensor pod that now bulges on top of the Columbia's vertical tail. From this perch, it will monitor heating of the upper port wing and fuselage surfaces during reentry. This is only one of several instruments built into the body of the refurbished spacecraft to record critical engineering data during launch and reentry.

Columbia ``is a flying test bed,'' says Rockwell's Petrone. It will provide new insights into shuttle aerodynamics, which could lead to further spacecraft improvements.

The biggest improvement that NASA officials would like to have is the addition of a fifth orbiter to the fleet.

Four orbiters are the minimum needed to provide a fully effective shuttle system. As Aldrich notes, a major mishap that took one orbiter out of service would seriously cripple NASA's ability to maintain its launch schedule.

At this point, however, it seems unlikely that the Reagan administration will seek funding for a fifth orbiter.