Shuttle launch on ice while NASA ponders tank changes
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: April 29, 2005
The shuttle Discovery's launch on the first post-Columbia mission is on ice until at least July 13, officials said today, primarily because of recent tests showing ice buildups pose more of a potential impact threat than previously thought.
Because of time needed to complete a thorough debris analysis, to implement possible fixes and to address a variety of other late-breaking problems, NASA Administrator Mike Griffin today announced that Discovery's launch would be delayed from May 22 to no earlier than July 13.
"No one thing but the sum of all those things together necessitates we
move out six or seven weeks into the July window," Griffin said. "This is
consistent with our overall approach to return to flight, which is we're
going to return to flight, we're not going to rush to flight. We want it to
be right, so we're doing what we need to do to ensure that."
The shuttle Columbia was destroyed during re-entry Feb. 1, 2003, because of a hole in the left wing's leading edge that was caused by the impact of external tank foam insulation that broke off during launch 16 days earlier.
The foam responsible for Columbia's demise was intended to prevent ice from building up around the fittings that attach struts holding the nose of the shuttle to the tank. The so-called bipod foam has been eliminated in favor of small heaters.
Foam application techniques were changed to minimize the chances for foam shedding in general. Engineers believe the largest piece of foam that can come off the tank today is less than a half ounce. The piece that hit Columbia weighed some 1.67 pounds.
But recent testing shows ice buildups in two areas of the tank still pose a threat. One of those areas is where an oxygen feedline bellows is located. The bellows allows the propellant line to respond to the effects of supercold liquid oxygen and to flex slightly during launch.
The testing shows ice can build up on the bellows or on a bracket holding the line in place. Another ice problem area is near the tip of the tank around brackets that hold a pressurization line in place.
During a detailed review of past launches, engineers were able to identify signs of past ice damage on the belly of the shuttle. The review showed a clear bias for damage on the right side of the belly, which is consistent with ice falling from the feedline bellows area. Recent testing shows the impact energy of ice debris does not depend on whether it is rock solid or somewhat slushy.
The question is, when does the ice break off? And how does the airflow between the shuttle and the external tank affect the trajectory of any such debris? Ice and foam behave very differently when they enter the airstream around a space shuttle. That airflow tends to stop a piece of lightweight foam in its tracks, which increases the relative velocity of an impact as the shuttle, in effect, runs into the debris at high speed. Ice does not decelerate as rapidly, which lowers the relative impact velocity and results in a very different transport mechanism.
"This is a very complex problem," said shuttle program manager Bill Parsons. "Ice can come off early in the flight and it doesn't have a transport mechanism to ever get to the vehicle. There's a small region in there when you're at a particular Mach number that you have a transport mechanism that gets this ice to the vehicle. That's what we've learned.
"We've learned that through a lot of research and through previous damage the vehicle received. We've had hits on (solid rocket booster) cork that we attribute to the LOX (liquid oxygen) feedline bellows ice, we've had some hits in the tile that we attribute at this point in time to the LOX feedline bellows ice.
"That was the information that said, hey wait a minute, maybe we've been a little lucky (in past flights), maybe we don't understand this problem as well as we should and therefore we need to go and understand this problem and understand if we've been lucky or the design of this vehicle, and the way this ice comes off, won't transport it towards the vehicle."
While testing showed a low probability of ice damage, there is "still a probability that pieces of ice can come off and hit the vehicle and cause damage," Parsons said. "Because of that, it became important to eliminate that."
NASA plans to install a heater on both of the external tanks currently at the Kennedy Space Center, including Discovery's, to eliminate the problem once and for all. After an already scheduled practice countdown next week with Discovery's crew, the shuttle eventually will be rolled back to the Vehicle Assembly Building where engineers will attach a feedline bellows heater.
The shuttle then will be hauled back out to launch complex 39B for work to ready the ship for takeoff July 13. Engineers believe they have about 20 days of contingency time in the current schedule to handle unexpected problems.
Managers might even opt to move Discovery to a tank and boosters being prepared for the second post-Columbia mission, although as of this writing, most insiders believe that's a remote possibility.
Engineers also plan to either clean or replace 18 insulation blanks on Discovery's rear orbital maneuvering system rocket pods that were contaminated with hydraulic fluid during earlier pad processing. Engineers were initially concerned that normal ascent heating could cause the residual fluid to ignite, but Parsons said additional testing has shown the contamination is not as extensive as initially believed and that cleaning alone may be sufficient. Either way, the work can be done in parallel with other processing.
Of more concern to the launch team are two problems that cropped up April 14 during a key test in which the external tank was loaded with a half-million gallons of supercold liquid oxygen and hydrogen.
Two of four sensors in the hydrogen portion of the tank operated intermittently. The sensors detect low fuel levels when the shuttle's engines must be commanded to shut down. Running an engine out of hydrogen, resulting in an oxygen-rich shutdown, could be catastrophic and all four fuel depletion sensors must be operational for a countdown to proceed.
Engineers believe the sensors are not to blame but so far, they have been unable to find a wiring problem or any other explanation for their behavior during the tanking test.
Another vexing problem is the performance of a hydrogen pressurization relief valve. The regulator operates periodically when the tank is loaded to bleed off pressure that builds up as liquid hydrogen turns into a gas. During the tanking test, the valve operated, or cycled, much more than normal and engineers are not yet sure why.
Wayne Hale, deputy manager of the shuttle program, said he does not believe the new heater used to keep ice from forming on the forward attachment struts caused the problem, but engineers have not yet ruled out the possibility.
"I personally think that's low priority but ... every day intuition can mislead you," he said. "We've got to be rigorous and make sure that's not the problem. It's a little bit of a puzzle to us and we're going to have to do some troubleshooting.
"It is possible one of the changes we made to the tank contributed to this situation. There were other things that were done to the tank that had nothing to do with the Columbia accident. ... We're in the process of playing all those things out and we'll work through all of them."
Parsons said engineers are well aware of the "law of unintended consequences" and that as you make changes to this vehicle you better be very careful to understand what those changes do to the performance of this vehicle."
"One of the things we will be looking very closely at is when we put this LOX feedline bellows heater in that we do no harm. That's the first rule, do no harm."
As for why engineers just now realized ice formation around the feedline bellows posed a serious threat, Hale said "we concentrated on the foam because that was the cause of the (Columbia) accident."
"But we knew we had to do an exhaustive search through everything that could be a potential problem," he said. "After a great deal of testing and analysis, we've been able to take some 175 potential debris sources off our worry list. We believe we've mitigated those. And we have the engineering evidence to prove they're not a concern.
"We knew we had three or four more items to work on and we also knew there was this ice that forms in certain places on the external tank, which we thought was probably not a major concern but we needed to ensure that. So what you've seen here is the diligence and rigor of going through every piece of the process to ensure that we've eliminated at least to the best of our ability the hazard from ascent debris.
"We've come to the conclusion we really need to do something about this ice. We have several options to deal with it and it's going to take us just a few more weeks to deal with that problem. We certainly cannot fly until we've convinced ourselves that it's safe to fly."
The threat of ice forming around the liquid oxygen feedline bellows prompted NASA to modify the tank in the wake of Columbia to minimize its formation. Engineers thought that would be sufficient and indeed, they believe the so-called "drip lip" modification eliminates 70 percent of the ice that otherwise might form in that area."
But the recent testing suggests that is not enough.
"This is a 17-inch-diameter pipe that's got a small area that is exposed, cryogenic temperatures are exposed to the outside air and water can condense and form ice inside that cavity," Hale said. "In some of that testing, we liberated pieces as large as 5 inches long ... by probably a couple of inches of ice. That's a pretty sizeable piece of ice."
He said engineers plan to release ice in a wind tunnel capable of velocities three times the speed of sound "and see if ice will hold together" in a bid to better understand the threat.
"So there's testing going on," Hale said. "But clearly a piece of ice that big, going three times the speed of sound, can do some serious damage. So we need to go understand a little bit about the dynamics. But the bottom line is, if we can eliminate it that would be the best."
The proposed heater modification should do just that, assuming, of course, it doesn't cause any "unintended consequences." Other options include the use of infrared spotlights near the pad to warm the outer surface of the tank just enough to preclude ice formation.