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NASA's debris experts have been working on foam issue for years
Florida Today ^ | Feb. 3, 2003 | John Kelly
Posted on 02/03/2003 12:30 PM CST by McGruff
HOUSTON - From tiny fragments to bowling ball size chunks, NASA has known for years that pieces of the foam insulating its external tank would break off and sometimes strike the bottom of the orbiter during its climb to orbit.
A 1997 report by Greg Katnik, who among things monitors problems with ice and debris during shuttle launches, described a launch in which more than 300 tiles were damaged by the falling spray-on foam material.
Katnik’s report was part of an ongoing process of studying the issue. NASA has in recent years made slight adjustments in how it prepares the tank for launch to try to minimize the so-called “shedding” of foam.
On the particular launch Katnik was studying back in 1997, more than 100 of the dents were larger than an inch. Some were more like scrapes, measuring as long as 15 inches.
“Foam cause damage to a ceramic tile!” Katnik said in a questioning tone in a report on the status of his tile investigation, which appeared in a NASA newsletter-like Web site. “That seems unlikely. However, when that foam is combined with a flight velocity between speeds of MACH two to MACH four, it becomes a projectile with incredible damage potential.
The big question?: At what phase of the flight did it happen and what changes need to be made to correct this for future missions?”
The foam has now become one focus of the investigation into the loss of Columbia, which burned up during reentry through the Earth’s atmosphere on Saturday morning. When it launched Jan. 16, a large chunk of debris fell from the tank and struck the under side of the orbiter. A video obtained by Florida Today shows a flash of light upon impact.
Shuttle program manager Ron Dittemore has said that the impact, which NASA believed at the time to be “inconsequential,” could have damaged the tiles that protect the orbiter from the heat of reentry. Dittemore said the shuttle transmitted data in its final moments showing spiking temperatures near the left wing where the tile damage occurred on launch day.
The mission that Katnik was studying was STS-87, a science mission aboard the shuttle Columbia. All of the shuttles have sustained tile damage during launch and while enduring the rigors of flying in space and back through the atmosphere to get home. While extremely fragile, the tiles on the under side of the orbiter are very fragile and succeptible to damage, especially from objects moving at very high speeds. Columbia’s had particular trouble with its tiles, including losing hundreds of them during its delivery from the manufacturer to Kennedy Space Center aboard a modified 747 in preparation for its inaugural 1981 flight.
About three hours before every launch, a crew clad in orange jump suits walks up and down the entire length of the shuttle launch tower. They’re looking mostly for ice, which tends to form on the outside of the big orange external tank because of the supercold liquid hydrogen and oxygen stored inside. They’re also looking for anything else that could be considered dangerous foreign object debris, or FOD in NASA’s acronym-heavy lingo.
The damage on that particular Columbia mission was not normal, Katnik said in his report.
One suspected reason for the change in damage, according to reports by Katnik and outside organizations that helped Kennedy Space Center study the issue, is NASA changed the way it “foamed” the external tank sometime shortly before that mission in an effort to be more environmentally friendly by reducing the use of ozone-depleting materials.
“Freon was used in the production of the previous foam,” he reported. “This method was eliminated in favor of foam that did not require freon for its production. MSFC is investigating the consideration that some characteristics of the new foam may not be known for the ascent environment.”
Dittemore said the agency has been studying the shedding issue for years and has made improvements in processes. One of them includes basically sanding the material on the outside of the orbiter to reduce the rough part of the surface that is more likely to flake off during launch. Usually, the pieces in question are tiny. But with the shuttle orbiter, NASA has long said even the tiniest piece of debris can become a potential disaster on launch.
NASA enlisted the help of Dryden Flight Research Center, the U.S. Air Force and the Southwest Research Institute in recent years to study the foam debris issue. Those investigators used pictures, telemetry data transmitted from the orbiter, radar coverage during launch, aerodynamic modeling, lab analysis and other tools, according to the government reports.
“At this point, virtually every inch of the orbiter was inspected and all hits were documented and mapped to aid in visualizing the damage,” Katnik reported. “Maps were constructed of the lower surface, the left and right surfaces and the top surface of the orbiter. At this point, a "fault tree" was created.”
Katnik did not return telephone calls seeking comment on his report.
The theories he espoused in his summary report, according to the NASA documents, included:
— The shrinkage of the gigantic tank during the loading of the liquid hydrogen, at 400-plus degrees below zero, and oxygen, at almost 300 degrees below zero. Katnik wrote, “These extreme temperatures cause the external tank to shrink up to six (6) linear inches while it is on the pad prior to launch. Even though this may not seem much when compared to the circumference of the external tank, six inches of shrinkage is significant.”
— The environmentally-friendly material. The report said Marshall Space Flight Center was assigned to research whether the new freon-free process had previously unknown weaknesses that made it a bad choice for shuttle launch conditions. It’s not immediately known whether the material was changed back to the previous version after that or not. The new material, according to the report, was also used on the previous mission, STS-86, which also had a higher than normal level of tile damage. However, that was deemed an anomoly at the time and, because it was a night launch, NASA did not have photographs good enough to study what was going on in the space between the tank and orbiter during that ascent, the report said.
— The aerodynamics of a new roll maneuver called "heads up," which had not been completed before.
— The primer that bonds the foam to the metal tank did not hold, but that theory was dismissed early on because the areas of damage, which were decribed as “divots” in the report, were not deep enough to expose that primer.
A report by Dryden Flight Research Center tested the new foam using a special aircraft.
“Initial results of the flight tests at Dryden, which were designed to replicate the pressure environment the shuttle encounters in the first 65 seconds after launch, indicate the new foam survived the tests in perfect shape, with no evidence of flaking or erosion found,” according to a Dryden press release issued at the time to tout its assistance to the shuttle program.
According to the release, “Dryden research pilot Dana Purifoy flew the F-15B through a series of side-to-side yaw maneuvers beginning at 7,300 feet altitude. He then increased speed and altitude in a stair-step approach, finally zooming up to 61,000 feet at speeds of up to Mach 1.5 (1.5 times the speed of sound) before descending for landing.”
"It was important that the F-15B could match part of our (shuttle launch) profile, and it does a fantastic job of doing that," said aerodynamicist Roy Steinbock of Lockheed-Martin Michoud Space Systems, staff engineer on the experiment, according to comments reprinted in the center’s press summary at the time. "Our main goal was to try to match the dynamic pressure history (that the external tank encounters during a shuttle launch).
The Dryden F-15B can match the high-altitude, low-pressure environment that the shuttle encounters, and can test a multitude of Mach numbers in (one) flight. That's something we cannot do anywhere else--we can't replicate that in a wind tunnel."
On March 19, 1999, the U.S. Air Force issued a separate press release detailing its role in ongoing evaluations of the foam problem. That one noted that during a Nov. 29, 1998, launch, the orbiter experienced a significant loss of foam during ascent, causing damage to the termal protection systems on the under side of the orbiter.
The report said that is a “concern because it causes tile replacement costs to significantly increase, however, it is not a flight safety issue.”
Dittemore made a similar point on Sunday, saying that the ongoing problems with tile damage, including one flight where the “popcorning” external tank foam damaged more tiles than normal, was a cost and processing issue. He said that while the damage to the tiles never appeared to pose a threat to the orbiter, it did take a lot of time and money to replace the tiles. There are more than 20,000 of the insulating pieces under the orbiter.
In the Air Force release, its Materiel Command reported assisting NASA with supersonic tunnel tests on panels of the shuttle external tanks, which helped match some of the conditions of launch.
As of the time of that release, the Air Force said NASA’s efforts so far to discover the cause of the foam shedding were unsuccessful. The report said that the Air Force also was unable to replicate the damage being seen during flight in its tests in the supersonic tunnel. However, the report said the Air Force generated plentiful data for NASA to look over about the stresses on the material from heat, vibration, pressure and other forces being applied during launch
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