NASA 'cautiously optimistic' shuttle won't need repairs
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 14, 2007
Sophisticated computer analysis indicates the aluminum skin directly below a small gash in the shuttle Endeavour's heat shield will not exceed NASA's 350-degree safety limit during re-entry, a top NASA manager said late today, despite temperatures of up to 2,100 degrees just outside the gouge. If overnight tests in a high-temperature furnace show the computer models are accurate - and if independent analysts agree - NASA managers may decide there's no need for a spacewalk repair job.
"We've completed the initial thermal analysis," said John Shannon, chairman of NASA's Mission Management Team. "It is going through an independent quality check and that's to take people who were not involved in the process to look at the assumptions that were made, the math, to make sure everything was done correctly. We've also completed the computational fluid dynamics that tells you what the flow inside that small cavity would be. Ames Research Center did that work, it's currently being verified by the Langley Research Center.
"The results of that, we're cautiously optimistic that we can fly as is, that's what the results were today. However, that's without any of the arc jet testing that we've talked about. Last night, we ran a baseline case of non-damaged tiles in the arc jet facility and they got a baseline temperature measurement on the backside of the tiles. At 7 o'clock, in about 40 minutes, we're going to start the arc jet tests with the exact damage we have on orbit. That will play back into the analysis."
The computational fluid dynamics calculations indicate the temperature of the shuttle's aluminum skin under the tile gouge would not exceed 325 degrees Fahrenheit. Shannon said the 350-degree limit is conservative and intended primarily to ensure the glue-like material used to hold the tiles and their support pads in place does not break down.
"We should get roughly the same temperature measurements that were predicted by the analysis," he said. "If we do, we'll know we have a good analysis. That is approximately a 24-hour cycle. ... So I would think that by late tomorrow afternoon, maybe in the early evening, we will have all our quality checks done, we will have our verification of the computational fluid dynamics model and we'll have the arc jet data to say that that modeling was correct and then we can make a decision on whether we need to repair or not."
For his part, shuttle Commander Scott Kelly said today he would be comfortable flying Endeavour back to Earth "as is" if mission managers decide the gouge doesn't require repairs. But he would be equally at ease overseeing a repair spacewalk.
"I feel comfortable," Kelly told CBS News in a space-to-ground interview. "My understanding is this tile damage is not an issue of the safety of the crew, It's more an issue of the ability to reuse the orbiter and damaging the orbiter. So we still have analysis ongoing, we still might choose to repair it, but I'm not concerned with our safety."
A team of astronauts, flight controllers and engineers is studying repair options in case the ongoing computer modeling and arc jet tests indicate a repair is needed. If so, the preferred approach would be to defer a station spacewalk planned for Friday and instead stage a repair EVA on Saturday.
Astronaut Rick Mastracchio, anchored to the end of a 50-foot extension attached to Endeavour's 50-foot robot arm, would be maneuvered under the shuttle to reach the damaged tile just behind the ship's right-side main landing gear door. Canadian astronaut Dave Williams would assist, tethered to the orbiter boom sensor system extension.
Two repair options are on the table: application of emittance wash, a black paint-like material that would be dabbed into the gouge to improve the heat rejection capability of the damaged tiles and/or injection of a heat-resistant putty-like material called STA-54.
It's not yet clear who would operate the robot arm if a repair is ordered. Astronaut Tracy Caldwell and educator-astronaut Barbara Morgan have both operated the arm for other tasks and Kelly is qualified as well.
"Anytime you've got someone on the end of the arm, it's a tricky thing," Caldwell, celebrating her 38th birthday today, told CBS News. "So we need to be careful about all of the clearances. The shuttle attached to the space station is a really complex vehicle and we've got a lot of places we've got to look out for. But I would say the trickiest part is looking out for, coming underneath the orbiter. The payload bay doors on the port side where we have the shoulder of the shuttle arm based, just making sure we clear that, and the wing. And then just having really good coordination between those of us inside the cockpit operating the arm and our guys outside who may perhaps be on the end of it."
Under the Saturday repair scenario, the astronauts would inspect the damage site again on Sunday, using a laser scanner and high-resolution camera on the end of the orbiter boom sensor system arm extension, and then stage an additional spacewalk Monday to make up space station assembly tasks deferred from Friday. That spacewalk, if approved, would be carried out by Williams and space station astronaut Clay Anderson.
NASA managers are enjoying an unfamiliar luxury during Endeavour's flight - more time than usual to assess their options. Endeavour's mission is the first to include use of a new station-to-shuttle power transfer system, or SSPTS, which lets the orbiter tap into the station's solar power grid. That, in turn, helps conserve the hydrogen and oxygen used by the shuttle's electricity producing fuel cells.
Based on the successful operation of the SSPTS, mission managers on Sunday extended Endeavour's mission by three days, from 11 to 14 days, and added the fourth spacewalk Friday. As of late Monday, engineers were predicting the shuttle would have enough power to remain docked to the space station an additional three days beyond that, if necessary, while still holding two days in reserve for landing-day weather issues or other problems.
Adding a fifth spacewalk to the mission likely would require a two-day mission extension, pushing Endeavour's landing from Aug. 22 to Aug. 24. But that is just one possible repair scenario and mission managers have not yet made any final decisions one way or the other.
The tile damage in question was caused by a piece of foam insulation, possibly mixed with ice, that broke away from a propellant feedline bracket on the side of Endeavour's external tank 58 seconds after blastoff Wednesday. Shannon said today analysts believe the debris originally measured 4 inches by 3.8 inches by 1.8 inches and weighed just 0.336 ounces.
But the debris hit an aft strut at a relative velocity of about 205 mph. Surprisingly, the debris bounced off the strut and a large piece hit the belly of the orbiter at a relative velocity of about 150 mph. Engineers were surprised by the ricochet, believing a chunk of foam or ice would break up into numerous small pieces rather than bounce off a strut as the debris in question actually did.
In any case, the debris gouged out an irregular cone-like pit that tapers to a gash measuring 1-inch long and 0.2 inches wide at the base of the 1.12-inch thick tile. Laser scans of the gouge were used to fabricate precise three-dimensional mockups using tiles identical to those on Endeavour. Engineers are running complex computational fluid dynamics calculations to model the flow of re-entry heat inside the damage zone. Those models will be verified by subjecting the deliberately damaged tiles to an arc jet furnace capable of duplicating re-entry heating and flow fields.
If the tests show the underlying aluminum skin of the shuttle will not be damaged by re-entry heating, mission managers could opt to forego any repair work and bring Endeavour home on Aug. 22 as is. If the test results are ambiguous, or if they indicate a higher likelihood of damage, a repair spacewalk will be ordered.
"Through the preliminary thermal assessment, which was based on the computational fluid dynamics, we did not exceed the structural temperature constraints of the underlying aluminum," Shannon said today. "And of course, we have to go through the quality assurance checks and the independent verifications but if that holds, it'll be real good news to us."
He said the computer models were conservative and assumed, once the temperature in the cavity reached 1,600 degrees, that an underlying layer of material that provides some insulation was suddenly gone.
"They just completely took it away, and we still did not reach the limit on the underlying aluminum structure," he said.
Interestingly, Shannon said a review of past foam loss from external feedline support brackets indicates more instances of debris in the wake of the Columbia accident than before. One of NASA's post-Columbia improvements was to add an hour of "hold" time to the shuttle's countdown, after the external tank is loaded with supercold rocket fuel, for a thorough launch pad inspection.
It is possible, Shannon said, that the additional hold time contributes to the formation of additional ice. It is believed that ice near the feedline brackets plays a role in the liberation of foam insulation. In any case, Shannon said engineers and managers will discuss possible additional inspections and analysis before the shuttle Discovery is attached to its fuel tank early next month in preparation for launch in late October.
Speaking to reporters today, Morgan said the ride to orbit was thrilling and added that it took several days to fully adapt to life in space. Named as backup to Christa McAuliffe in the original Teacher in Space program, Morgan waited 21 years for a chance to ride the shuttle and continue McAuliffe's legacy.
"It's remarkable," she said of launch last Wednesday, "it's a lot noisier than I expected and it actually did not shake as much as I'd expected the shaking to be. But you feel the thrust and it keeps going and going and going. Towards the very end, before MECO, before main engine cutoff, it gets a little bit difficult to breathe. And then as soon as those main engines cut off, you pretty much pitch forward and if you unstrap, of course, you're floating free. But you can definitely feel that floating feeling right away."
After "three or four days on orbit, the floating is fantastic," she said, "it's something you get used to and have a lot of fun with. At first, it's a little disconcerting. That very first day on orbit, the entire day I felt like I was upside down the whole time."
Morgan has downplayed her role as the first teacher in space and instead has focused on the astronaut part of her title, overseeing the transfer of supplies and equipment from the shuttle to the station and operating the robot arm today to help install a 7,000-pound storage platform on the lab complex.
"Teachers and astronauts really do the same thing, it's all about doing work, it's about exploring, it's about experimenting, it's about discovering and sharing it," she told CBS News "And that's what we're here to do."
Later in the day, Morgan fielded questions from kids at a Discovery Center in Boise, Idaho, her home state, the first of three modest educational events planned for Endeavour's mission. Joined by Williams, Anderson and Al Drew, Morgan took delight in demonstrating the effects of weightlessness, turning somersaults, slurping balls of liquid from a drink bag and demonstrating exercise techniques by picking up Drew and Williams with one hand each.
Surprisingly, Morgan handed the microphone to her crewmates more often than not and the event resembled a typical astronaut question-and-answer exercise more than an educational event. But the kids clearly enjoyed the demonstrations, frequently laughing and applauding the astronauts' antics.