The latest rung on the shuttle's ladder.

The latest rung on the shuttle's ladder

"Everything still looks clean," said a NASA spokesman after listening to the latest in a series of daily conference calls. "That certainly is nice to hear." The topic was the ongoing analysis of the April 20 test-firing of one of the space shuttle's redesigned solid-propellant rocket boosters (SRBs). The original SRB design has been widely cited as the culprit in the Challenger explosion 27 months ago that killed seven astronauts and grounded the entire shuttle program.

The booster used in the test at contractor Morton Thiokol's facility near Brigham City, Utah, included deliberate flaws to test whether the redesign would successfully prevent the leakage of hot exhaust gses. A narrow gap termed a "blowhole" was put in the sealant between the rearmost segment of the boost er's casing and the rocket nozzle; another defect was placed between two segments midway along the casing.

The first few days of inspection after the test revealed no "anomalies" (unanticipated problems) at all, according to a Thiokol official, based on preliminary appraisals that ranged from reading instruments to sending an engineer crawling inside for a look. As the booster was progressively taken apart, subsequent study was expected to show whether there were any trace of soot or other signs that the hot gases might have gotten through a protective flange called a "J-seal," a key element in the new design.

If test results continue to be as favorable as the first look, only two more tests are planned with the full-scale booster firing for the full 2 minutes that a pair of SRBs need to power the space shuttle's liftoff. Five days after the test, NASA continued readying the shuttle Discovery for an Aug. 4 launch.

Barring unforeseen difficulties, engineers will conduct the first of the two remaining test-firings in June, without the built-in flaws but using a new test stand at Thiokol that simulates structural stresses induced by the SRB's interaction with other parts of the shuttle. The final test is aimed for July.

Meanwhile, even before the redesigned SRBs carry the shuttle on its return to space, NASA plans to seek proposals from industry for the development of an improved SRB called the Advanced Solid Rocket Motor. Envisioned as offering increased payload weight, reliability and safety, the new booster is planned for service in the mid-1990s.