Discovery will return to assembly building next week
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 13, 2010
The shuttle Discovery's external tank will be loaded with supercold rocket propellants as early as Friday in a critical test to help engineers understand what might have caused cracks in two structural ribs, or stringers, during a Nov. 5 launch attempt. The test also will show whether the repaired stringers can withstand the thermal stresses of fueling and help NASA managers develop "flight rationale" before proceeding to another launch attempt around Feb. 3.
Hoping for the best, NASA managers and engineers met Monday and agreed that after the test, whenever it takes place, the shuttle "stack" will be hauled back to the Vehicle Assembly Building for additional inspections, and possible repairs, before returning to pad 39A in mid January for work to ready the ship for launch.
To make the fueling test as realistic as possible, the tank will be fully loaded with a half-million gallons of liquid oxygen, at minus 298 degrees Fahrenheit, and hydrogen rocket fuel, at minus 423 degrees, starting at 7 a.m. EST (GMT-5). The fueling timeline will be similar to the one used for an actual launch, including several built-in holds and an on-pad inspection. As the countdown ticks into its final minutes, the tanks will be pressurized as they would for a real launch, subjecting the hardware to additional stress.
The countdown clock will hold at the T-minus 31-second mark for five minutes before cutoff and detanking operations.
Thirty-nine strain gauges and 50 temperature sensors have been attached to the tank near the upper section of the ribbed "intertank" where vertical stringers meet a flange that supports the base of the liquid oxygen tank. Twenty-one of the strain gauges are mounted near the two cracked stringers to the left of the shuttle, along with numerous thermocouples. The rest are positioned in a similar location on the right side of the intertank flange facing the orbiter.
To attach the sensors directly to the tank's structure, technicians first had to remove foam insulation in the targeted areas. Fresh foam must be reapplied before the fueling test but that work has been slowed by cold weather and a requirement for ambient temperatures of at least 75 degrees in makeshift access platform tents to make sure the insulation cures properly.
Whenever the test takes place, the sensors should help engineers map out temperature gradients in critical components as well as the stresses imparted by thermal contraction of the aluminum-lithium alloy used in the stringers and panels making up the intertank. They also will provide insight into the health of the two repaired stringers that cracked during fueling Nov. 5.
At the same time, stereo cameras will be focused on thousands of small targets, or dots, on the outside of the tank to precisely measure deflections in the structure of the tank as it responds to cryogenic temperatures.
As a side benefit of the fueling test, engineers will confirm that a 7-inch gaseous hydrogen vent line attachment on the side of the external tank is functioning properly after repairs to correct a presumed alignment problem. The Nov. 5 launch attempt was called off because of a leak in the vent line quick-disconnect fitting. The cracks were discovered later, after engineers noticed a large fissure in the foam insulation near the top of the intertank.
When the damaged foam was removed, four cracks were found in two adjacent stringers. The damaged sections were replaced and doublers were installed to add additional strength. Sophisticated X-ray devices then were used to inspect other stringers around the circumference of the intertank on the side facing the shuttle. No other problems were found.
The lithium-aluminum alloy used in the so-called super-lightweight tanks is known to be relatively brittle and cracks are not unusual. But Discovery's are the first known cracks to develop under cryogenic conditions at the launch pad and engineers do not yet understand the root cause.
The instrumented fueling test, ongoing computer analysis and off-site testing with high-fidelity mockups are intended to collect the data needed to make sure the huge tank is structurally sound and that the repaired stringers can cope with the thermal stresses of fueling.
The tests also are designed to make sure the normal stresses and temperature extremes won't lead to potentially dangerous losses of foam insulation during Discovery's climb out of the dense lower atmosphere when debris poses the biggest threat to the ship's fragile heat shield.
Assuming the weather cooperates and the fueling test is carried out late this week, Discovery will be hauled back to the Vehicle Assembly Building next week to give engineers a chance to carry out non-destructive X-ray inspections of the backside of the huge tank, which cannot be done at the launch pad.
Some engineers have recommended adding additional doublers to other stringers where loads are most severe to provide additional strength, but it's not yet clear if that might be necessary.
In any case, strain gauges and temperature sensors installed as part of the tanking test will be removed and the tank's foam insulation will be repaired, operations that will be easier to carry out in the controlled environment of the VAB. Discovery will remain attached to the tank throughout the operation.
If the root cause of the cracks cannot be determined, the testing should provide the data needed to either justify making another launch attempt or to order additional tests and/or repairs.
The next shuttle launch window opens Feb. 3 and closes Feb. 10. Assuming a tanking test Friday, Discovery will be moved back to the VAB Dec. 21 or 22. If no problems are found in the wake of the fueling test, the inspections in the VAB and the testing underway at other NASA centers, and if Discovery is back at the launch pad by around Jan. 14, NASA should be able to make another launch attempt Feb. 3.
But if any significant problems or questions emerge from the testing and analysis, launch on a space station resupply mission, Discovery's 39th and final flight, could slip even further behind schedule.
NASA originally hoped to launch Discovery Nov. 1, but the flight was delayed one and then two days because of work to replace quick-disconnect fittings in the shuttle's right-side orbital maneuvering system rocket pod. Launch then was delayed 24 hours to Nov. 4 because of an electrical glitch in the circuitry associated with a main engine controller. Troubleshooting showed the likely cause was transient contamination in a cockpit circuit breaker.
A launch attempt Nov. 4 was called off before the start of fueling because of stormy weather at the Kennedy Space Center. The launch attempt on Nov. 5 was cancelled because of a gaseous hydrogen leak in a 7-inch vent line quick-disconnect fitting. Engineers then saw the cracked insulation on the side of the shuttle's external tank.
After the cracks were discovered, launch was delayed to no earlier than Nov. 30. Launch then slipped to no earlier than Dec. 3 and finally to NET Dec. 17. On Dec. 3, NASA managers said more time was needed to resolve the problem and launch was delayed to at least Feb. 3.