HEADLINE: CHANGES IN ROCKET STRAINED BOOSTER'S SEALS, EXPERTS SAY
BYLINE: By WILLIAM J. BROAD, Special to the New York Times
DATELINE: CAPE CANAVERAL, Fla., Feb. 16
Critical points in the space shuttle booster rockets were put under increased stress because of structural changes in the boosters ordered by the National Aeronautics and Space Administration in 1983, according to public documents and former officials of the space agency.
The structural changes, which were made so that the shuttles could lift heavier payloads into space at lower costs, included using thinner casings on the boosters, each of which was made lighter by 4,000 pounds, while making the booster engines more powerful.
The changes put greater stress on the joints between the segments of the booster rockets, which already had a history of problems, aerospace experts said today.
''With the high-performance motor you have greater stresses, heats and pressures,'' said a former NASA official at the Kennedy Space Center who spoke on the condition he not be identified. ''The agency should have looked at how that related to the shuttle system as a whole, but somehow they missed it.''
''NASA is famous for its systems engineering,'' he added. ''But somehow, something broke down.''
No Comment From Space Agency
Charles Redmond, a spokesman at NASA headquarters in Washington, said: ''That's exactly the sort of thing the Presidential commission was set up to look at. It's not right for the space agency to speculate on such matters.'' NASA officials have said they knew there were problems with the joints but that they believed they would not threaten the safety of the shuttle.
The joints that connect the four main booster segments are sealed by a set of rubbery rings that are meant to keep hot gases and flames confined within the booster's casing. The booster joints have long been regarded as one of the most vulnerable parts of the rocket propulsion system, and investigators looking into the fiery destruction of the space shuttle Challenger Jan. 28 have said that failure of the joints is one of the leading theories for the explosion, which killed all seven astronauts aboard.
The decision to use a higher powered rocket was made at a time when officials of the space agency had waived shuttle safety standards for the booster joints. The agency nonetheless stayed with the joint design, while plans were made to go ahead with the new engines.
Past Problems of Joints
Documents made public last week by the Presidential commission investigating the Challenger disaster show a history of concern with the joints and link their problems to the more powerful engines.
''Frequency of O-ring damage has increased since incorporation of higher performance motors,'' noted a report presented at NASA headquarters on Aug. 19, 1985, by officials of Morton Thiokol Inc., which designs and makes the booster rockets, and the Marshall Space Flight Center in Huntsville, Ala., which oversees NASA's work on space shuttle engines.
The O rings are essentially large doughnut-shaped pieces of synthetic rubber that fill the tiny gap where the cylindrical booster segments are bolted together, much like rubber washers on a faucet. The rings are themselves protected from heat and flame damage by an initial barrier of putty.
By 1982, however, NASA knew that the O rings and their associated structures were not working as designed. The problem, according to a 1982 space agency memorandum, was traced to ''a phenomenon called joint rotation.''
Danger of Rotational Force
The rotation occurs because explosive energies inside the rocket force the relatively thin steel skins of the booster segments outward. The joints between the segments resist this outward motion becase they are made of thicker metal. The result is a very small rotation between elements of the joints.
The danger is that the rotation can unseat the O rings and create a slight gap in the booster's side just seconds after liftoff, allowing hot gases and flames to escape.
Photographs made public by the space agency show the Challenger's right-side booster apparently leaking in such a manner.
According to space agency officials, problems with O rings became worse after the switch in 1983 to light-weight steel casings and higher powered motors.
The lightweight casings for the solid-fuel booster rockets were first used in April 1983, on the sixth space shuttle mission. According to documents published by Rockwell International, the main builder of the shuttle, each of the shuttle's two boosters was made 4,000 pounds lighter by shaving away two- to four-hundredths of an inch of the steel casing. This reduction amounted to about 2 percent of the overall mass of the booster's metal parts.
Beginning with the eighth shuttle flight in August 1983, a new, higher powered booster engine was used. According to the Rockwell documents, thrust was increased by about 5 percent. The power was gained by narrowing the exit nozzle of the booster and making the fuel burn faster.
Engine Link Cited
In a document presented to the Presidential commission, Lawrence B. Mulloy, project manager for solid-fuel booster rockets at the Marshall Space Flight Center, noted that the frequency of O-ring damage, errosion and indications of problems increased after the introduction of the new engines.
He said there had been indications of problems on only one flight before the new engines, but six afterwards.
The former NASA official said today that one possible reason for the problems was that greater stress on the thinner skins of the booster rocket would have created more rotation in the booster's joints, increasing the chances that the O rings would be unseated and damaged by flames.
''As the pressure increased, they could open wider,'' he said. ''It's almost a linear function.''
He added that another factor could be the effect that the new engine's higher temperatures and pressures had on the putty meant to protect the O rings.
An engineer at Morton Thiokol in Brigham City, Utah, said today that the company had tested joint rotation before and after the introduction of the higher powered engines and found that rotation did not increase significantly.
''There's no difference to speak of,'' said the engineer, who spoke on the condition he not be identified. He added that he could release no details of the tests.
Another possible factor was mentioned in the NASA document of Aug. 19, 1985, that was made public by the Presidential commission. Engineers imply that a new type of putty had failed to properly protect the O rings.
The engineers recommended finding another type of putty in the short term. As a long-term solution, they recommended creating a new type of booster case with a ''capture feature'' that would reduce joint rotation by holding down the inner flange that supports the O rings.
Morton Thiokol engineers, in a document dated Aug. 26, 1985 and made public last week by the Presidential commission, listed 43 different ways to deal with the problems posed by O rings.
None of the proposed solutions dealt with new types of putty. Instead,
the vast majority of the 43 solutions called for new arrangements of O
rings and new types of booster casings that would reduce joint rotation.