NASA analysis of shuttle fuel tank enters final stages
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: April 28, 2006
NASA managers have decided not to make any major, last-minute changes to the foam insulation that prevents ice from forming around fittings that hold pressurization lines and a cable tray to the space shuttle Discovery's external fuel tank. Work to minimize the amount of foam used on the fittings will continue but in the near term, wind tunnel testing shows the so-called ice/frost ramps should stand up to the aerodynamic buffeting of launch as is, without shedding large, potentially dangerous pieces of insulation.
But with just two months to go before the July 1 opening of Discovery's launch window, engineers do not yet know whether the tank is safe to fly without larger foam air deflectors that were removed after a big chunk of insulation fell off during the first post-Columbia flight last July.
A decision is not expected until NASA's formal flight readiness review, a two-day discussion that begins June 16. At that point, shuttle program manager Wayne Hale said today, the engineers studying the issue will say "yes, this structure will hold together or no, it won't. And if the answer is 'no, it won't,' well, we're not going anywhere. We have some confidence that it will, but you don't know until you do the math and get the numbers."
The air dams, known as protuberance air-load - PAL - ramps, were in place primarily to protect the pressurization lines and cable tray from buffeting during the region of maximum aerodynamic pressure, which occurs during the first minute of flight.
NASA managers decided to remove the long ramps late last year - eliminating 37 pounds of foam that could be a source of potentially dangerous launch debris - based on computer modeling that indicated the external pipes and fittings were tough enough to stand up to launch turbulence on their own. The decision was made on the assumption wind tunnel testing this spring would confirm the computer modeling and because engineers were unable to come up with reliable techniques to ensure the ramp foam stayed in place.
"That change constitutes the largest aerodynamic change that we have made to the space shuttle launch system since it first flew," Hale said. "And we're approaching that with a great deal of care, doing the work necessary to prove the aerodynamics will still be good, that we have not introduced an aerodynamic loads problem that will cause the structure underlying to come to grief. That's an intricate process."
He said a final analysis of the aerodynamic forces acting on the tank in the absence of PAL ramps is due for completion May 5. That data will be folded into an on-going structural analysis intended to determine whether or not the tank can meet NASA's standard safety factor of 1.4, where 1 represents worst-case conditions. The results will be presented at the flight readiness review in June.
As of today, however, Hale would not say whether preliminary indications are favorable or not.
"The aerodynamics loads are higher without the PAL ramp than they were before with the PAL ramp," Hale told reporters at the Kennedy Space Center. "It's not an absolute number they talk about, I don't have a nice number I can give you that says it's, you know, 1.3 times higher than it was before. I don't have a number like that.
"I will tell you at the end of the day, the structural analysis folks, when they look across all the scenarios, all the Mach numbers, all the angles of attack, all the potential vibratory cases that they look at, (after) thousands of computer runs, will have to come back and tell me that we have the space-shuttle-standard 1.4 factor of safety or better for us to be able to launch. So we'll have to be able to withstand 140 percent of whatever the maximum load case is."
The shuttle program faces retirement in 2010 as NASA transitions to a new spacecraft and a new exploration mandate that calls for a return to the moon by 2018 or so.
Some in the shuttle program worry the program could be terminated early if NASA fails to get Discovery off the ground this summer or fall at the latest. But Hale dismissed those concerns, saying "look, if we go fly and have another accident, THAT will be the end of the program."
"I would rather not fly and say we couldn't get our act together and if people in high places think that's fine for us to quit, that's OK, than to rush to some ill-advised launch where we have a catastrophe. That would be worse. So I don't think about those kind of things. I'm going to keep working on this program until they tell me to quit working on this program and I'm not worried about whether July is the end of the program or not."
Here is the latest launch processing schedule (a more detailed schedule that runs through the end of the year and includes critical space station launches and milestones is available on the CBS News STS-121 Quick-Look page):
While initial test data were positive, "when we came right down to it, the recommendation I came to is we're in a flight test program, classical flight test," Hale said. "When you make a major change (like removing the PAL ramps), you should fly that major change without other major changes to see how it performed. And if you have subsequent changes to be made, you make those in subsequent flights."
The current ice-frost ramp design performed well during recent wind tunnel testing, but Hale said small pieces of foam almost certainly will come off during Discovery's launch.
"It is not without risk to fly these ice/frost ramps as they exist," he said. "There was a strong, concerted opinion from several folks that we should wait until we have a good design on these pieces of foam and then change them as well before we go fly. That not without merit and we considered it very strongly.
"However, at the end of the day, we came back to the fact that it is more appropriate to make one change at a time to take care of the biggest problem we have and then work our way to the next situation we'd like to improve.
Bill Gerstenmaier, NASA's chief of spaceflight at headquarters in Washington, agreed with that approach, saying the agency is "really kind of pushing the state of the art of our analysis and wind tunnel capabilities throughout the country."
"There's not really one wind tunnel where you can simulate all the proper conditions that are going on with the tank," he said. "There's not really one test facility where you can simulate all these things that come together in a shuttle launch.
"The tank expands when it's pressurized, it contracts when it's cooled down, the vibration from the solid rocket motors cause vibration through the tank structure, which goes through this bracketry, those press lines have gases flowing through them, they're moving up and down, they're dynamically moving in and out. All of that is tremendously difficult to simulate in our test facilities and to put together in computational flight dynamics.
"So at some point, you really need to go to flight ... with some instrumentation so you can monitor that performance and see how the design you put together with the best of your engineering capabilities actually performs in flight. And that's exactly what we're doing here."
In other recent developments, NASA managers have decided to replace one of two electronics boxes in Discovery's aft compartment - a maneuvering jet control unit - because of concern about so-called "tin whiskers." in recent weeks, engineers have been concerned about a metallurgical phenomenon in which thin, hair-like whiskers of tin can extrude from circuit card guides in a certain type of chassis. Should a whisker break off and fall on an exposed electrical component on a circuit card, a short could result.
The boxes in question are used in a variety of critical shuttle systems to control the movement of the vehicle's main engine nozzles, wing flaps and to fire maneuvering jets. But the components are redundant - there are four for each system - and managers decided the chances of multiple whisker-related shorts was negligible. One reaction jet driver box, however, was replaced with a pristine unit.
Engineers also have replaced main engine No. 2 because of concern about solder defects in an attached control computer and completed the replacement of critical low-level fuel sensors in the hydrogen section of Discovery's external tank. The engine cutoff - ECO - sensors were replaced because of electrical indications of a potential failure mode in one of the four. The sensors are part of a backup system used to make sure the engines don't shut down early or run too long because of other problems.
NASA managers will meet next Thursday to decide whether or not to carry out a tanking test June 1 to verify the performance of the ECO sensors. Some engineers have questioned the value of a tanking test given that thermal stresses caused by the super-cold rocket propellent apparently contribute to foam cracking and, potentially, to debris during launch.
Shuttle tanks are certified for 13 fueling cycles, Hale said today. A fill-and-drain without tank pressurization counts as one cycle but countdowns in which the tank is pressurized for launch count as two. A tanking test, in which the countdown would be taken down to its final minutes to test the ECO sensors and the pressurization system, would count as two cycles on the tank.