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Defining the debris threat: What can shuttles withstand?
Posted: March 22, 2005

NASA finally has test data indicating any foam debris falling from the space shuttle's redesigned external fuel tank almost certainly will be below the threshold expected to cause "entry critical" damage to wing leading edge panels, officials said today.

But with less than two months to go before Discovery's launch on the first post-Columbia shuttle mission, engineers are still assessing whether worst-case foam debris impacts could cause entry critical damage to the orbiter's heat-shield tiles.

The STS-114 astronauts pose in front of Discovery's redesigned external fuel tank. Credit: Lockheed Martin
In some areas, like certain places around the landing gear doors or the area where external tank propellant lines enter the belly of the shuttle, worst-case impact damage could be potentially catastrophic.

The results of ongoing tests and analyses will be discussed during a debris verification review in early April and a broader shuttle-wide design certification review before flights resume in mid May.

But NASA managers are optimistic test data and the results of sophisticated computer modeling will show the tank's insulation does not pose a significant safety threat to the wing leading edges.

"If you think back to what we knew before Columbia and what we thought our vulnerabilities were and where we are now, it is clear that before we were guessing and now we know, within a few fractions of a percent, exactly how vulnerable we are," deputy shuttle program manager Wayne Hale said in an interview late today. "We've come a long way."

Columbia was destroyed during re-entry Feb. 1, 2003, because of a hole in the leading edge of the ship's left wing that was caused by a major foam debris impact during launch 16 days earlier.

NASA's primary rationale for returning the shuttle to flight this May is the elimination of the foam in question, along with other design changes expected to minimize the amount - and size - of any foam debris that might shake loose or pop off other areas during launch.

The shuttle's redesigned fuel tank has been modified to ensure no pieces of foam heavier than 0.023 pounds - 0.37 ounces - can break away from the upper portions of the tank during launch.

Debris from the upper part of the tank poses the greatest threat to the ship's nose cap and reinforced carbon carbon leading edge panels, which experience the most intense heat during re-entry. The 0.023-pound limit is for foam covering the liquid oxygen tank at the very top of the structure.

Foam from the intertank area, where bipod struts attach the nose of the shuttle and where the foam originated that doomed Columbia, can be slightly more massive: 0.03 pounds, or 0.48 ounces. Pieces of foam from the bottom of the tank pose less of a threat and thus can be larger still: up to 0.75 pounds.

These artist's concepts show the previous bipod foam design (top) and the new configuration for the post-Columbia era (below). Credit: Lockheed Martin
A major question as the shuttle Discovery's May 15 launch target approaches has been whether foam in that range of sizes from the top of the tank could cause the kind of wing leading edge damage during ascent that could threaten a shuttle during re-entry.

To the surprise of many, recent testing showed major cracks or holes in leading edge panels are not required to bring down a space shuttle. Coating damage alone could do it, if the damage was in a particularly bad spot and if the impact that marred the coating also caused internal delamination. Small cracks and holes, obviously, also pose serious threats.

The problem was in the testing. Using a nitrogen gas-powered cannon and a wing leading edge mockup at the Southwest Research Institute in San Antonio, Texas, engineers were able to create entry critical damage from pieces of foam as small as 0.044 pounds - 0.7 ounces - depending on impact velocity and angle of incidence.

But they were not immediately able to test down to the range of sizes expected to be liberated from a redesigned fuel tank because smaller foam bullets tended to come apart before hitting the leading edge panels.

Steve Poulos, manager of the shuttle project office at the Johnson Space Center, said in an interview today that those issues appear to have been resolved and test data show the sort of debris that could be expected to fall off the redesigned tank would be below the critical damage threshold.

"We have done testing that exceeded the parameters to help us in the validation of our models: higher speeds, higher angles, different geometries," he said. "And in some of those instances we did do damage to the panels and quite honestly, we expected to do it because (the impact energy was) greater than 1,500 foot pounds.

"I'm not one to live in the world of assumptions," he said. "I'm a test person. I've got empirical data where we've impacted the panel and we haven't damaged it once below 1,250 foot pounds of kinetic energy."

That's a measure of the impact energies expected during an actual launch.

"Recently, they've done testing down to very small masses, I think the last number I heard, and I could be wrong, was about .02 pounds, .025, at velocities on the order of 2,500 feet per second. And zero damage (was seen)."

NASA has conducted extensive RCC impact testing. This file image was taken during the Columbia investigation in June 2003. Credit: NASA
Poulos discussed the impact testing immediately after attending a meeting to review what level of damage a new laser sensor will be able to detect in orbit if, despite the test results, an impact causes a crack or mars the surface coating of a wing leading edge panel.

The sensor, or laser dynamic range imager - LDRI - and a television camera, both mounted on the end of a 50-boot boom, will be used during the second day of Discovery's mission to inspect the shuttle's nose cap and wing leading edge panels.

"I saw the latest certification level test results from the LDRI and it shows clearly the ability to detect down to as low as a 15 thousandths (of an inch) crack," Poulos said. "Coating loss is easily detectable with these sensors as is a crack. So there's no concern from the sensor perspective to being able to detect coating loss or a crack."

All in all, he said, "there is nothing today that I have seen from the external tank data that gives me any pause or any concern about having any damage at all on our RCC panels."

He said NASA now has "sufficient capability, based on all the testing that's been done at Southwest Research and the Glenn Research Center and the Kennedy Space Center, to verify in my mind that we have sufficient capability for what the tank is expected to liberate."

But Hale cautioned that impact testing alone is not enough. Sophisticated computer modeling also is required to predict how debris might behave after it comes off the tank and moves into the airflow around the shuttle. As such, there is still room for debate in the upcoming design reviews.

"The transport mechanism, which is a key element in this, is a model, a computer model of aerodynamics," he said. "We've done a lot of wind tunnel work to try to make sure it's right, but it's a model. So we do have an ongoing discussion about the validity of models and usefulness of models.

"If you state it in absolutely black-and-white terms, I think you will get some discussion whether this is proven to within the last decimal place. ... I'm almost hesitant to say anything in black and white until that review. But you've got a sense of where we're going."

The debris verification review is planned for the first week in April, just a few days after the shuttle Discovery's planned April 4 rollout to the launch pad at the Kennedy Space Center. Launch is targeted for May 15.

Discovery is scheduled to be moved into the Vehicle Assembly Building next Monday, as depicted in this file photo, for mating to its external tank and solid rocket boosters. Rollout to the pad is slated for April 4. Credit: NASA
Rollout had been planned for late this month, but engineers and technicians needed more time to complete final processing, eating into the 12 days or so of on-pad contingency time managers had built into the schedule. An April 4 roll out will leave the team with just a few days of spare time to handle unexpected problems and still make the May 15 target.

NASA insiders say the launch date likely will slip up to a week or so when all is said and done, but shuttle program manager Bill Parsons said today no decisions will be made until after Discovery reaches the launch pad.

"We've slipped a few things (but) we feel very comfortable with where we're at in the processing and the milestones we still have in front of us," he said at a news briefing. "We probably need about 30 more days of processing before we can really say whether we should slip the May 15th launch date or not. But that is the opening of the window and currently, if everything was to go all our way, we feel we can make that."

In the wake of the Columbia disaster, an independent accident review board made 29 recommendations to improve safety. NASA tracks its progress meeting those recommendations in a formal document known as the shuttle return to flight implementation plan.

The latest version was released today and it included a new approach to shuttle re-entries based on the expected risk to the public below the orbiter's flight path if a shuttle has known damage or flight control issues.

When Columbia broke apart above Texas, debris rained down along its flight path stretching from just west of Fort Worth all the way into southwestern Louisiana. In what still ranks as a remarkable stroke of luck, no one on the ground was injured and property damage was minimal.

But new flight rules are being developed to minimize that risk. The crew of any shuttle with known damage that could affect structural integrity or any other problems that might affect flight control will be directed to land at White Sands, N.M., one of NASA's backup landing sites.

Only one previous shuttle mission has landed at White Sands. Columbia used the alternate site in 1982 for landing of STS-3. Credit: NASA
"If there is some kind of problem with the shuttle, a loss of its flight control redundancy, for example, or a threat to the structural integrity of the vehicle ... these flight rules will call for us to go to (White Sands)," said Bryan O'Connor, NASA's director of safety.

Descents to White Sands carry the shuttle over less densely populated areas than those to the Kennedy Space Center in Florida or Edwards Air Force Base in California's Mojave Desert.

"On average, entry opportunities into KSC are half the public risk level of entries into EAFB," according to the implementation plan. "On average, entry opportunities into WSMR (White Sands Missile Range) are one-seventh the public risk level of EAFB and one-third the public risk level of KSC."

But because White Sands does not have the infrastructure needed to support routine shuttle landings, the preferred landing site for undamaged orbiters will remain the Kennedy Space Center with Edwards as backup.

"The bottom line is that the orbiter will normally land at KSC," according to the implementation plan. "But if it is compromised in a way that poses a threat to the public, it will land at WSMR."