NASA ponders shuttle repair work during spacewalk
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 31, 2005
(UPDATED with quotes, details from afternoon briefing)
Engineers are considering what, if anything, to do about two protruding "gap fillers" on the belly of the space shuttle Discovery that could trigger increased re-entry turbulence and localized, potentially dangerous, heating if they are left as is.
This is not a new issue and it's not related to the foam debris that fell of Discovery's tank during launch. Engineers spotted the protruding gap fillers in photos shot by the space station's crew last week as commander Eileen Collins flipped the shuttle about before docking to permit a close examination of the shuttle's underside heat shield.
"We have a team of folks that are working aggressively at options to go and make that gap filler safe if we decide it's an issue," Hill said at a morning news briefing. "We expect to have final results on aero heating and a decision on whether we need to do anything about the gap fillers on Monday."
Engineers may opt to do nothing if the analysis shows Discovery can safely return to Earth as is. If not, "we have various options that range from pulling the gap filler out, to trimming the gap filler, to putting it back down into the gap," Hill said. "And those are actively being worked right now. I couldn't tell you yet where it's going."
Brent Jett, scheduled to command the third post-Columbia mission, told CBS News today astronaut troubleshooters are leaning toward recommending that one of Discovery's spacewalkers use pliers to simply pull the offending gap fillers out if a repair job ultimately is deemed necessary.
"There's a team off right now looking at that from an EVA perspective," Jett said. "There are three astronauts on that team - Joe Tanner, Jim Reilly and Dave Wolf. I spoke to Joe this morning and he said as of right now, the leading candidate is to remove it, with the second option being to trim it. But that could change between now and tomorrow."
NASA managers have said in recent days that Discovery appears to be in good shape and that engineers expected to be able to clear the shuttle for re-entry as is during a final meeting Monday. Minor dings to the shuttle's heat-shield tiles have, in fact, been cleared and engineers are expected to clear the ship's wing leading edge panels later today.
The protruding gap fillers are not associated with foam debris shed by Discovery's external fuel tank during launch. The ceramic cloth material is inserted between heat shield tiles on the belly of the shuttle to ensure smooth air flow and to prevent adjacent tiles from rubbing as the shuttle's aluminum skin flexes during temperature swings.
One gap filler protrudes a full inch above the surface of the surrounding tiles while the other extends 0.6 inches. The largest known case of a gap filler protrusion toward the front of the shuttle is 0.25 inches.
The concern is that one or both gap fillers in question could "trip the boundary layer" during re-entry, that is, disrupt the smooth, laminar flow of supersonic air across the belly of the shuttle and create eddies of turbulence that, in turn, would result in higher downstream heating.
Wayne Hale, chairman of NASA's mission management team, said late today a final decision on how to proceed will be made Monday, after a thorough review by aerodynamicsts and spacewalk planners.
"Initially, when we saw this problem I thought well, gee, gap fillers, we've seen that before, flown through assymetric boundary layer transitions before, very little damage on the bottom of the orbiter and the only real effect we've seen from these aero transitions in previous flights has been to tiles that were already damaged," Hale said.
"So my immediate knee-jerk reaction was that we can live with this. On the other hand, this is bigger than we've seen before, the EVA guys have gone out and are putting together a plan that we'll hear about tomorrow. And if it's relatively simple, I mean, why worry? Why would you not just go take care of it if you had a simple plan to deal with it. So it really depends on how the plan comes out tomorrow."
It is not unusual - or necessarily unsafe - for gap fillers to slip out of position and extend into the airflow, as long as they don't extend too far. If they do, they can trigger an early transition to turbulent airflow, exposing downstream tiles to higher heating for longer periods.
After a mission by the shuttle Columbia in 1995, technicians found an exposed gap filler roughly the same size as the larger gap filler seen protruding from Discovery's tiles. During Columbia's re-entry, a few downstream tiles were damaged by higher heat loads. Peak heating on the underside of the shuttle typically runs about 2,300 degrees Fahrenheit, but turbulent flow associated with a boundary layer transition can reach 2,800 degrees depending on where and when it occurs.
"Both of these gap fillers exceed our generic constraint, or kind of our conventional wisdom," Hill said. "This is about a quarter inch protrusion in a forward location. Both of these are well above a quarter of an inch. That's why we have to depend on site-specific aero analysis. ... Boundary layer transition at these Mach numbers is such a tricky thing, we can't speculate. We have to wait for the data."
At the risk of making more out of this development than it might warrant, here is a brief description of the boundary layer and how it works with the space shuttle from "Comm Check: The Final Flight of Shuttle Columbia" by this writer and Michael Cabbage of the Orlando Sentinel. This discussion is focused on the wing leading edges, but the idea is generally the same):
During all re-entries, the smooth flow of air in the boundary layers eventually transitions to turbulent flow. That transition normally occurs at velocities between eight and 12 times the speed of sound. Anything above Mach 13 is considered an early transition and results in 15 to 25 percent higher heating.
"We do worry in general about an early boundary layer transition, especially a far forward early boundary layer transition," Hill said. "The farther forward we go, the more severe the entry heating problem is if we trip that boundary layer early or if we start seeing turbulent flow at a far forward location.
"It increases the heating, both at the area where we tripped, say, where these gap fillers are protruding, and it increases the heating on all the areas downstream of it. So that's our general concern. Our biggest concern ... is going to be that exact location right around where we trip the boundary layer. We'll see a significant increase in heating there.
"I can tell you that if we have thin tiles, or we already had damage right there, it would be something we would be very worried about with a protrusion as far as an inch at a forward location like that," Hill said. "So I am very anxious to see this aero heating analysis tomorrow."
Aboard Discovery, the astronauts plan to stage the second of three planned spacewalks Monday, this one to replace one of the international space station's four stabilizing gyroscopes.
The mission management team already approved a one-day extension for Discovery's flight to let the shuttle crew assist in repairs to station exercise equipment as well as supply stowage and transfers. Discovery now is targeted for landing at the Kennedy Space Center at 4:37 a.m. on Aug. 8. Here are updated deorbit and landing times for Aug. 8 and 9 (in EDT):
EDT........HH...MM...SS...EVENT Monday, Aug. 8 03:35 AM...12...16...56...Deorbit Burn 04:37 AM...12...17...58...Landing at KSC (planned) 05:10 AM...12...18...31...Deorbit 06:12 AM...12...19...33...Landing at KSC Tuesday, Aug. 9 03:53 AM...13...17...14...Deorbit Burn 04:56 AM...13...18...17...Landing at KSC 05:25 AM...13...18...46...Deorbit Burn 06:31 AM...13...19...52...Landing at KSC
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