As part of a continuing effort to increase the cargo payload in its four shuttles, America`s space agency three years ago reduced the bulk of the attached fuel tanks by 13 percent, leaving them with thinner walls and fewer structural supports.
With the huge, aluminum tanks seen as a likely key to explaining Tuesday`s shuttle explosion, experts are now asking whether making them lighter might have also made them weaker.
Officials of the National Aeronautics and Space Administration have thus far refused to comment on possible causes for the tragedy, but a spokesman for the firm manufacturing the tanks denied that the modifications reduced their strength.
”That is absolutely not true,” said Evan McCollum, spokesman for Martin Marietta Corp.`s external tank manufacturing division in New Orleans. ”NASA asked us to look at ways to reduce the weight of the external tank, obviously without sacrificing any strength, and we did that.”
Despite the shuttle catastrophe, McCollum said, the company has not been asked by NASA to suspend production of the tanks.
Around the time that the tanks were made lighter, NASA also ordered changes to reduce the weight of the solid fuel rocket boosters that give the shuttle its initial thrust on liftoff.
By using a lighter-weight steel in casings, NASA reduced booster weight by 8,000 pounds, and there were plans to soon replace much of the steel with wound filaments of plastic reinforced with graphite, which would have resulted in a further weight reduction of 56,000 pounds.
A spokesman for the booster manufacturer, Chicago-based Morton Thiokol Inc., acknowledged Thursday that the company`s plant in Utah is conducting an internal investigation into whether the thinner casing might have played a role in the explosion.
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”We are not going to say that we are to blame; but we are not going to presume to say that we are not to blame either,” said spokesman Rocky Raab.
The major tank and booster modifications, made in 1983, were part of a continuing push by NASA to reduce shuttle weight to allow for more goods and passengers on board. After 25 flights, the shuttle has yet to come closer to achieving the cost-saving goals predicted for the reusable craft when it was first proposed.
Critics of the shuttle program claim that it costs two to four times as much per pound of payload compared to the expendable rockets NASA used prior to the first shuttle flight in 1981.
In the past, NASA press releases announcing cutbacks in weight and material have welcomed the changes as increasing payload capacity beyond the 65,000-pound limit in the shuttle`s original design. But some critics have questioned whether such constant modifications are in the best interests of safety.
Challenger exploded 74 seconds after liftoff at an altitude of about 9 miles, when the aerodynamic stresses on the craft are at their peak.
The 150-foot-long external tank, 27.5 feet in diameter, carries the shuttle in piggy-back fashion and supports the two solid fuel rocket boosters during launch. It empties its load of liquid hydrogen and oxygen, two of the most volatile substances known to man, into the shuttle`s main engines in 8.5 minutes. Then it is jettisoned.
The hydrogen fuel tank is located in the bottom portion of the external tank where TV pictures indicate Tuesday`s explosion started.
The lighter tanks were first used on the sixth shuttle mission in April, 1983. They have been used on 18 of the 24 previous shuttle flights.
Marietta has a NASA contract worth $2.6 billion, which includes design and development, to make 59 tanks.
McCollum said the decision to reduce the tank`s weight was made by NASA in 1979, about the time his company was delivering a heavier version for use on the shuttle`s maiden voyage in the spring of 1981.
The original tanks weighed slightly more than 76,000 pounds, McCollum said. He said the company was able to reduce the weight–and increase the shuttle`s cargo capacity–by 10,000 pounds through a number of relatively minor changes.
Weight reduction was accomplished by a ”lot of little things,” he said. The one thing that accounted for the biggest single weight loss was eliminating the white paint that was previously used to coat the 154-foot-high tank, he said, which saved 1,000 pounds.
Other measures to reduce the weight include the elimination of portions of ”stringers,” which are used to provide structural strength along the length of the tank, using fewer stiffener rings employed to provide circular strength, and by modifying major frames in the hydrogen tank.
Some weight was reduced by making the walls of the tank slightly thinner and by using lighter-weight titanium alloy in the struts that attach the solid rockets to the side of the external tank.
Several hundred pounds of weight were reduced with the elimination of the antigeyser line that was used to prevent the buildup of gaseous oxygen in the liquid fuel tank.
All changes were approved by NASA, McCollum added.
But he could not immediately provide stress-test reports indicating whether the changes had led to any reductions in safety margins.
After the seventh shuttle flight, the metal casings surrounding the solid fuel in the two booster motors were lightened as part of an overall effort to increase the spacecraft`s payloads, Thiokol`s Raab said.
Raab said that after the first seven missions, project officials decided to make numerous changes to put the shuttle in ”high-performance mode”
including making the casings that protect the booster fuel thinner.
Raab said figures were not readily available showing just how much thinner the casings were on the shuttles that flew between mission eight and the 25th when Challenger exploded.
But he said that he believed the difference in thickness was ”relatively minor.”
He explained that in addition to lightening the weight of the protective casing, the company removed numerous sensing devices and radio transmitters from the booster rockets that had been used to test for possible flaws during the first seven flights.
Further, said the Thiokol spokesman, ”significant” changes were made to reduce the size of the opening of the rocket outtake nozzle to increase performance on flights 8 through 25.
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All combined, these changes had a significant impact on the shuttle`s ability to orbit ever-heavier payloads, the company official said.
Thiokol estimated that an additional 3,000 pounds could be carried aloft by the Challenger because of the changes it made in the booster rockets after mission seven.
The spokesman emphasized that once all the changes–thinner shell, removal of sensors and radios and improvements on the nozzles–were made, Thiokol used ”virtually identical” boosters in each of the 17 remaining flights. ”No two shuttles are ever precisely alike, but our equipment was virutally identical each time after SDS 7,” he said.