Shuttle boss hopes for quick fix to fuel sensor woes
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 21, 2007
Engineers have been provisionally cleared to remove a suspect feed-through plug and an external connector from the shuttle Atlantis' external fuel tank for laboratory testing and a possible fix to eliminate intermittent electrical glitches with low-level engine-cutoff sensors.
In an interview today with CBS News, shuttle Program Manager Wayne Hale said no final decisions have been made, but one leading candidate for a near-term repair is to possibly solder the external socket to the male pins in the pass-through connector to eliminate any open circuits in that part of the system when it is chilled to cryogenic temperatuures.
This scenario assumes the internal connector is sound and is not contributing to the problem. The internal connector cannot be modified unless repair crews enter the external tank, work that likely would require a roll back to the Vehicle Assembly Building.
"The leading contender today is we will, starting next week, pull the through plug with the external connector, all in one piece, off (the tank), clip the wires and wire harness and send that whole assembly off to the lab for teardown and evaluation," he said.
"Then in parallel, we've got a (qualification) process that's going through a solder of the wires directly to the male pins of the pass through. And of course, you close all that out, splice your wires into the wire harness and plug it back in."
But that is simply one option under consideration. How the agency will actually proceed - and what that might mean in terms of an eventual launch date - will depend on what engineers find when they remove and examine the hardware.
The current "no-earlier-than" launch date is Jan. 10, but that target was announced before engineers had a good idea of what might be needed to fix the problem.
Of seven options presented to shuttle managers Wednesday, only two - simply replacing the external connector or swapping out the connector and the pass-through plug - lead to launch attempts in early to mid January. Replacing the external connector with a soldered pin-and-plug arrangement likely would result in additional delay.
But again, it's not yet clear how NASA will proceed.
Going into the weekend before the Christmas holiday, Hale said he had given the team "kind of a two-part guidance. If they can come to a consensus that we don't have to go into the tank taking these pieces out, then I basically told them to go ahead and do that. If they can't get to that and they need to go into the tank, then we need to have a management review before they do that because ... getting into a tank includes a fair amount of technical risk. So they're off working on those two things."
Engineers also are reviewing low-level engine cutoff - ECO - sensor problems in other rocket systems to learn about other possible approaches to the problem.
Lockheed Martin's Atlas-Centaur rockets "had some problems, Centaur (second stage) did, back in the early '90s and they have gone to not using the kind of pin-and-socket connectors that we use and actually soldering the wires through this kind of system," Hale said. "And then I've got a huge dump here from the NESC (NASA Engineering and Safety Center) on every other anomaly report they could find in the history of mankind, almost, on connector failures, particularly in the cryogenic experience.
"We've got problem reports with connectors from Los Alamos National Labs, the Ames Research Center, some ELV (unmanned rocket) problems, just all over the place, some of which sound like ours, some of which don't. So we're using that in our team to evaluate these things and say what can we learn from this history that's out there? So, that's going into the team that's working very hard to come up with a change."
Atlantis was grounded Dec. 6 and 9 when intermittent failures of ECO sensors at the base of the hydrogen tank occurred during fueling and later, draining. A fifth sensor, which indicates when the tank is 5 percent full, also malfunctioned when the tank was drained.
The wires that carry signals from all five sensors pass through the same connector in the wall of the external tank. The three-part connector features a pass-through fitting with male pins, embedded in glass, on both sides. Wires from the sensors inside the tank terminate in a female connector inside the tank that is plugged into the male pins of the pass-through. A similar female socket plugs into the pass-through on the outside of the tank.
Based on data collected during a fueling test Dec. 18, engineers believe the problem involves gaps in the external part of the connector that are occurring when the system is chilled to ultra-low temperatures. They believe the sensors themselves are healthy and that only two circuits are actually experiencing problems: ECO sensors 1 and 3.
"The feed-through plug is a male-male plug," Hale said. "If you think about it, each of the pins goes through this vitreous glass (in the wall of the tank) and then the connectors the wire harnesses terminate in are the female sockets.
"The guys reported yesterday ... they came back and said we're pretty sure that the ECO sensor 1 (problem) was on the outside but ECO sensor 3, we're kind of indeterminate on. I haven't heard the results. It would really be nice if we could say with a high degree of confidence this is on the outside."
If the problem is, in fact, in the plug or external connector, repairs could be made at the launch pad. But if the problem involves the internal connector, engineers likely would have to go inside the tank and that, almost certainly, would require a rollback to the Vehicle Assembly Building.
"If we go inside, that's going to complicate our lives," Hale said. "You've got to go into the tank and that probably is going to require a rollback.
"But there are a multiplicity of options. The thing I've got to do as a manager is kind of sit on my hands for a couple of days, let the technical guys slug it out and come forward with a recommendation and not try to keep pulling this plan up by the roots to see how it's doing."
The goal of Atlantis' flight is to deliver the European Space Agency's Columbus research module to the international space station. But problems with the systems used to keep the station's solar arrays properly pointed at the sun could force station planners to schedule a spacewalk to swap out a so-called beta gimbal assembly motor.
Depending on when such a spacewalk might be scheduled, Atlantis could face additional delays not related to the ECO sensor work. Asked if NASA might take advantage of any such delay to implement a more extensive ECO sensor fix, Hale said the near-term goal is to get Atlantis ready to fly as soon as possible.
"I don't think that's going to factor at all," Hale said. "My goal is to get back to a highly reliable system as soon as we can to be ready to fly as soon as we practically can and then if station has other considerations, then we'll talked about what makes sense. And I know they're working their problems."
But engineers at the Marshall Space Flight Center in Huntsville, Ala., are assessing potential long-term fixes that might be implemented several flights downstream. But for the two tanks at the Kennedy Space Center - Atlantis' and a tank scheduled for launch with the shuttle Endeavour in February - some sort of interim fix appears more likely.
"We've got the short-term team, which is looking on the near-term flights, the two tanks effectively that we have at the Kennedy Space Center, what are we going to do near term in which you would go off and do some kind of quick change that would mitigate if not everything, most things.
"Then we've put together what we call the long-term team under one of the chief engineer people at Marshall Space Flight Center, who's looking at a three- to four-month longer term investigation and come back with what we should do for the long term. I'd like to eliminate it as an issue."
As for whether NASA must find and address the "root cause" of the problem before Atlantis flies, Hale said "it depends on what our fix is."
"If you go to a completely different class of fix, which is why I think everybody is so enamored of the solder (technique), then you could probably say you don't need to go to root cause, I just need to know where the problem was occurring and we took it out.
"If we find out, for example, soldering doesn't work - and there's some discussion about what happens to solder joints in cryogenic temperatures, do they break or degrade - if that doesn't work and we need to go back with a plug-type socket connector again, but a different strength or different assembly method, something of that nature, then you really need to go to root cause. And that takes a bit of time."