VISITORS here to Air Force Plant 42, site one, are greeted by a desert of prickly Joshua trees and a row of signs: ``The time spent now ... to build it right ... will keep them safe ... on every flight.'' As if to ward off recommendations made last month that national reliance on manned shuttle flights be shifted to unmanned rockets, the slogan concludes: ``America's best ever space shuttle, Endeavour.'' Written for an employee jingle contest held to encapsulate Rockwell Space Systems Division goals, the words underline the national focus on human safety in the wake of the 1986 Challenger disaster. As with the construction of orbiters Columbia, Discovery, Challenger, and Atlantis before it, desert-site assembly of the fifth shuttle is proceeding apace.
Unlike the previous four, that pace is slower - four years of assembly versus the usual 18 months - partly in the name of safety, partly because of staggered funding. Under budget ($1.88 billion, not including main engines) and on schedule, Endeavour is close to 90 percent complete. Rollout is April 1991, and barring changes from recent recommendations, the first flight is scheduled for May 1992.
Although it is nearly identical to others in the fleet, two main differences will distinguish Endeavour: a drag chute to aid deceleration and reduce loads on the landing gear and brakes, and design features that may later accommodate extended orbits of up to 28 days at a time. Longer orbital time was envisioned for use in the decade-long construction of an orbiting space station, which is also under reconsideration.
Endeavour was authorized by Congress in 1987 as a replacement for Challenger, whose right rocket booster exploded on liftoff in 1986, killing all seven crew members and stalling the nation's space program for two years. Since initial launch in 1981, the space-shuttle orbiter fleet (so-called ``space transportation system'' or STS) has logged 36 missions. It has launched 31 communications satellites, three interplanetary spacecraft, and the Hubble Space Telescope; repaired two satellites in orbit; flown four Spacelab missions, and conducted more than 642 civilian scientific experiments.
Stung by a decade of criticism over design, technology, and construction flaws - computer glitches, as well as hydraulic and fuel leaks leading to delayed or shortened flights - site director Ted Clauss greets visitors armed with a list of operational improvements. Besides the drag chute, there is a carbon braking system, a quick-release escape hatch, and nose-wheel steering.
``After building four others, the learning curve is in our favor as well,'' he says. For example, one crew of six assembles both wings instead of three crews of six. Two men handle all hydraulics instead of six on three different shifts. The result is a better shuttle - 40 percent fewer errors in first-time inspections of the previous shuttle, Atlantis - and more accountability.
Now suspended about 10 feet above a cement floor in the hangar across from Clauss's office, Endeavour is surrounded by scaffolding and is minus a set of wheels. Makeshift plastic anterooms guard each entrance to the interior, flapping like desert tents from air forced outward to ward off lint. Excess clothing fluff that blocked cooling ducts has been blamed for the overheated computers that crippled telescopes on the most recent flight of Columbia.
While there have been modifications and improvements, Mr. Clauss warns guests that the technology in the command console is ``vintage 1970.''
``We're still dealing with antiquated gauges and meters, toggle switches and circuit-breakers whose design is 20 years old,'' he says. All must be consistent with the three other orbiters - Columbia, Discovery, Atlantis - and flight simulators where astronauts train at the Johnson Space Center in Houston. Ditto for Endeavour's cockpit computers.
If and when Congress were to appropriate enough money to upgrade the fleet, so-called ``glass cockpits'' would be installed - edge-lighted displays, state-of-the-art computers.
``Those are political considerations,'' Clauss demures. ``No additional money for improvements is likely in the wake of recession and budget cuts,'' says Robert G. Melton, an aerospace professor at Pennsylvania State University in University Park, Pa.
More money would allow replacement of auxiliary power units that drive hydraulics in favor of electro-mechanical advances now used on 747 jet airplanes. Lighter composite materials, used in the B-2 Stealth bomber, could be used in wings, tail, elevons, and fuselage.
Another political consideration is how often, and for what duration, the Endeavour will fly. Only the capacity of 28-day flights is being designed into the module - plumbing, wire harnesses, and brackets that will accommodate extra provisions, if and when appropriated. That means cryogenic tanks of oxygen for breathing, hydrogen to run hydraulics, and larger storage facilities.
``Now the [Norman] Augustine report [calling for an overhaul of NASA] really puts things on hold,'' says Alex Bessler, chairman of the Space Physics Department at Rice University.
Even without further funding, advances in thermal protection since the first shuttle launch in 1981 will benefit Endeavour: lighter carbon for nose cap and wing edges; a lighter and cheaper flexible surface insulation, which has replaced 8,000 tiles around the upper fuselage; more customized tiling to eliminate hot spots around nose cap and landing gear doors that caused hydraulic problems in previous shuttles.