National Effort Mobilized to Access Shuttle Flaws

Craig Covault / Kennedy Space Center

Teams of materials, structural and cryogenic propulsion engineers mobilized across the U.S. are zeroing in on the cause and repair of hydrogen propellant line cracks that have grounded the space shuttle program. But more questions than answers remain.

Engineers are examining flow liner weld and other techniques that could return at least one orbiter to flight by about October or November, when requirements for International Space Station support will begin to rapidly mount.

An equally important aspect of analysis is to ensure that any repairs will not introduce materials stress or contamination that could create a safety problem worse than the tiny hydrogen liner crack., themselves.

NASA and United Space Alliance, the shuttle contractor, are also examining major changes to inspection techniques that Somehow allowed the hypersonic transports to fly two decades without the crack being detected in the first place. Although these particular crack may prove harmless, the inspection shortcomings raise concerns.

"I have questions about why we missed these in the past," said Ron Dittemore, NASA shuttle program manager. "Clearly, visual inspection techniques are not adequate or else we should have picked these up earlier."

The new emphasis on propulsion system inspections also comes as the NASA Office of Inspector General has completed a report criticizing Kennedy Space Center oversight of some United Space Alliance safety procedures and documentation (see p.57).

In addition to defining, testing and qualifying a repair technique, the objective of seven analysis teams is to determine why the cracks are there and whether there is the potential for them to grow or flake off metallic debris. Any such debris could cause an explosion in the cryogenic hydrogen/oxygen engine system.

The teams, which include nonaerospace experts, are divided into materials and processes; stress and loads analysis; propulsion system integration; design and test; vehicle inspections; logistics, and fault tree analysis.

Experts at United Space Alliance, Boeing/Rocketdyne and the Marshall Space Flight Center are leading the assessments, but NASA has also brought in specialists from many other materials-related disciplines. "We are drawing on a lot of different disciplines and a lot of different industries that may have something to offer," Dittemore said. The first major review involving all the teams was held July 17

A return to flight by the fall will be important to maintaining International Space Station assembly, resupply and crew changeout flights by Atlantis and Endeavour with minimal disruption. Columbia's 16-day STS-I07 Spacehab science mission, originally, set for this month, is likely to be moved out of sequence and delayed until December or early 2003.

Aside from the hydrogen line flow liner work, the nearly 2,000-person shuttle Kennedy workforce continues to process Columbia, Atlantis and Endeavour and their payloads through normal flows toward the next scheduled flights. Discovery is out of normal flight rotation anyway because it is beginning a year of preplanned modifications and inspections.

The potential for further delay is a concern for layoffs, however. Warnings for potential layoffs have already been issued to a handful of Kennedy contractor employees.

"When we first identified the crack in Atlantis we believed it was a larger safety issue than we do today," said Dittemore (AW&ST July 1, p. 34). "It is much less of a safety concern now than when we started," he said. "But I am still concerned, and have a lack of understanding [about the cracks] that prevents me from flying. Until we have a better understanding, we will not fly."

Eleven cracks have now been found across the fleet -- three each in the hydrogen lines of Atlantis, Discovery and Columbia and two in Endeavour. In addition, one crack has been detected in a similar feed line in the Main Propulsion Test Article (MPTA) used more than 20 years ago to cluster-fire shuttle main engines at the Stennis Space Center in Mississippi.

The fact that the cracks in hydrogen plumbing liners range from the MPTA built in the late 1970s and fired relatively few times to orbiters with diverse flight rates indicates they are not related to the utilization rate or the age of the hardware, Dittemore said. Discovery has launched 30 times; Columbia, 27; Atlantis, 25, and Endeavour, 18.

"It may be that when we welded the flow liners in the vehicle pipes we may have stressed the liner," Dittemore said. "The cracks are like stress-relief cracks and are very slow growing or not growing at all. We have been flying with them for a very long time."

One option in the analysis is to fly with them as is, if materials and safety analysis justify that. United Space Alliance managers doubt that will be the case, however, and welding over the cracks was the primary option being examined last week, especially by Marshall Space Flight Center engineers.

"We have some very skilled practitioners in welding techniques. We have talked to them, and they feel confident that if we desire they can make small welds [to seal the cracks]," Dittemore said.

Grinding out the cracks to smooth their surface is also an option, as is welding a doublet -- a bridging piece of metal-over the cracks. Part of these options is the possibility the fix could be made to one orbiter, which would be flown, then reinspected before approved for the other three.

Before that work is approved, managers have to understand how the cracks form-something that could take another 2-3 weeks. Once any repair is done, however, flights cannot begin for another 7-8 weeks from the time the order is given to reinstall engines.

To answer the questions, tests are underway to better define the stress environment inside the 12-in. diameter lines that provide each engine nearly 128,000 gal. of -423F liquid hydrogen at a rate of 254 gal. per sec. during 8.5 min. of powered flight.

The liners in Atlantis, Endeavour and Discovery are made of inconel, while Columbia's is stainless steel. Original materials samples saved from the manufacturing of these liners is being examined microscopically to see if there is evidence of flaws in the original materials.

Samples of the materials are also having flaws induced in them in the laboratory to see how cracks mayor may not propagate in the cryogenic and launch loads environment.

And the configuration of the plumbing further complicates the analysis.

All of the cracks are in liners covering flex bellows on the orbiter side of the interface where the hydrogen lines attach to each main engine immediately upstream from each engine's low-pressure fuel turbopump.

The cryogenic hydrogen crosses the liners at 30 psi., then enters the pump where its pressure is boosted to 276 psi. The potential that the flow transition at this point plays some role in the crack process is being examined. Also, there are more liners in each hydrogen pipe than the ones involved with cracks.

Each line actually has three bellows with both an upstream and downstream liner on each section. So far, cracks have been found only on the upstream and downstream sides of the last bellows section be- fore the line connects with each engine. The question is why.

To further complicate the issue, each of the three hydrogen lines in the back of the orbiters is snaked somewhat differently to route hydrogen from a central manifold to the top-mounted No.1 position and the port and starboard engine No.2 and 3 positions.

Seven of the cracks have been found in plumbing leading to No.1, while the remaining four have been found in the line leading to the No.2 bottom-left position. Why the cracks are at those two positions and not the No.3 spot has also raised questions. "There is a phenomenon going on we don't quite understand that we need to understand better," Dittemore said.

Another problem involves understanding why the cracks are in cryogenic hydrogen but not oxygen lines that operate at -297F. So far, hydrogen embrittlement has not come up as a major factor, managers said.

The cracks range in length from 0.3 in., like the initial one spotted visually in Atlantis, down to 0.15 in. and not apparent to the naked eye. Some cracks are superficial, but one penetrates 100% and another 90% through the sleeve. The cracks are about evenly split between axial and circumferential in direction.

Eddy current testing that highlights potential flaws due to changes in electrical potential has been most useful in spotting cracks, which are then verified by ultra sound tests. A combination of eddy current and ultrasound testing will be implemented to inspect the flow liners from now on.

Aloft in space on the ISS, the Expedition 5 crew is completing seven weeks in orbit on a planned four-month stay that could be extended to 5-6 months based on orbiter scheduling.

They originally were to be resupplied in mid-August by the Atlantis STS-112 crew, which is also scheduled to install the first large starboard section of the 1SS transverse truss. That flight is not likely until October.

Expedition 5 has also been scheduled to be relieved and replaced by Expedition 6, to be launched on the Endeavor STS-113 flight in early October. That flight is not likely until November. "We have been doing a lot of heavy thinking about the schedule, Dittemore said. But decisions are still a week or two away.