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Space Shuttle Moves Closer To Liftoff
Technical changes loom large on the Discovery
Lawrence D. Maloney -- Design News, May 16, 2005
After the Columbia crash over Texas and more than two years of agonizing investigations later, NASA is at last getting set for another space shuttle launch.
On April 29, NASA announced a launch window of July 12 to July 31 to give shuttle workers more time to fix fuel tank sensors and check systems for ice buildup. It will be the first shuttle launch since seven astronauts perished on Feb. 1, 2003, when the Columbia orbiter broke apart just 15 min. before its scheduled touchdown.
Mission planners have specified a daylight liftoff to ensure optimum photography of the orbiter and its mammoth external fuel tank. During the launch of Columbia, mission control failed to spot a suitcase-size piece of foam that ripped away from the fuel tank—causing a hole in the thermal protection layer of the orbiter's left wing. That breach became catastrophic when spacecraft encountered super hot atmospheric gases during reentry.
The added vigilance follows a raft of design changes aimed at sharply reducing the chances for falling debris at liftoff. In addition, Discovery will pioneer new methods to detect and repair damage to the heat shield, which must withstand temperatures of 3,000F during reentry.
Target #1: The external fuel tank
Following the report of the Columbia Accident Investigation Board (CAIB) in August 2003, attention has focused squarely on the external fuel tank (ET). The largest element of the shuttle system, which also includes the orbiter, the three main engines, and two solid rocket boosters, the ET measures 27.6 ft wide and 154 ft tall. Carrying 535,000 gallons of fuel in two tanks inside an aluminum skin just 0.125-inch thick, the ET must withstand 6.5 million lbs of thrust. An outer layer of polyurethane-like foam, with an average thickness of about 1 inch, keeps ice from forming on the exterior and protects the aluminum skin from aerodynamic heat. After liftoff, the ET delivers its liquid hydrogen and liquid oxygen propellants to the shuttle's main engines before being jettisoned about 8.5 min into the flight.
The CAIB probe determined that the heat shield was gouged by a large chunk of insulating foam from the bipod area where the ET attaches to the orbiter. "In four simple words, 'the foam did it,'" said Board Member Scott Hubbard when the CAIB issued its report in August 2003. Hubbard noted that post-accident testing of the reinforced carbon material in the heat shield revealed that "it was not tough enough to withstand an impact of this piece of foam at 500 miles an hour."
Among the techniques to assess the impact of foam debris are the lifting insulating foam trajectory tests at California's Dryden Flight Research Center. There, researchers use an F15B aircraft to test foam divots at speeds of up to Mach 2. The test bed features an in-flight system to eject divots from the plane and a high-speed video system to track and record the trajectory of the divots.
In response to such tests and the CAIB's recommendations, engineers at Marshall Space Flight Center did a top-to-bottom evaluation of the ET to pinpoint all potential areas where foam insulation could be lost. Result: Several design changes. For example, engineers have eliminated the wedge-shaped bipod ramps formerly used on the bipod fittings that connect the tank to the orbiter through the shuttle's two forward attachment struts. Designed to prevent ice buildup—another source of debris—the ramps were a prime area for shedding insulating foam during liftoff.
This cutaway view of Space Shuttle Discovery shows deployment of the manipulator arm and new sensor boom for inspecting the orbiter's protective heat shield.
To prevent ice buildup as the shuttle sits on the launch pad loaded with cryogenic fuel, the redesign adds four rod heaters below each fitting. The cartridge-style heaters, measuring 0.25 inches in diameter and 5 inches in length, can produce up to 300W of power. In addition, ground-based PLCs will control the heaters based on input from temperature sensors.
Although the new design eliminates most of the foam in the bipod area, technicians at Lockheed Martin's New Orleans assembly facility must still apply the insulation to other areas of the tank. The ET Project Office at Marshall has developed tougher control procedures for this spray-on process, including video recording of the work, improved spraying methods, and acquisition of all process parameter data. It also is employing new nondestructive evaluation techniques, such as backscatter radiography, to spot defects in the insulation. The goal is to insure that any foam debris from the tank weigh no more than 0.075 lb.
Among other changes, engineers have redesigned three bellows on the feed line that carries liquid oxygen fuel from the ET to the main engines—a modification aimed at preventing ice debris. They also have converted an important bolt catcher component from a two-piece welded design to a one-piece machined part for enhanced structural strength. The canister-like bolt catcher captures the part of the bolt that remains with the external fuel tank when the sold rocket boosters separate from the ET about 2 min after launch. And to make sure that the external tank is thoroughly monitored during liftoff, NASA will mount a video camera on the ET's liquid oxygen feed line.
"This will be the safest, most reliable tank that NASA has ever produced," Sandy Coleman, manager of the External Tank Project at Marshall, told Design News. "The changes we made are significant, and we picked those areas where we were most vulnerable."
Spotlight on the Orbiter
Optical Eyes:
Besides the ET, the Orbiter also has undergone scrutiny. There have been more than 100 design modifications, including the addition of a multi-function electronic display system. Engineers subjected the Discovery's reinforced carbon-carbon panels and nose caps to an unprecedented battery of tests, including ultrasound, CAT scan, and a new procedure called flash thermography. In this technique, technicians apply a burst of hot, intense light to the panel, then use a heat-detecting infrared camera to scan for flaws.
Discovery also will fly with a new impact-detection system, consisting of 22 temperature sensors and 66 accelerometers under each wing. Sensor data will flow from the wing to the crew compartment, where it will be transmitted to Mission Control. A new digital camera, mounted on the orbiter's underside, will capture and transmit images to Mission Control, where experts can analyze any damage at the very outset of the flight.
Inside its payload area, Discovery will carry a new 50-ft boom that attaches to the shuttle's existing robotic arm. Featuring a camera and laser system, the boom will enable the astronauts to inspect the heat-protecting tiles during flight. "The difficulty in working with the boom is that it is basically an extension of the arm," explains Michael Wright, the lead payload deployment officer for the mission. "Very precise maneuvers are required to put the sensor in the right place to get the right data."
If missing or damaged tiles are spotted by the boom, the astronauts may have the option of repairing the problem during space walks, using a sophisticated caulking gun and a heat resistant material known as STA-54. In the event of a large breach, as occurred with the Columbia, mission planners have fashioned a contingency plan that would temporarily house Discovery's crew in the Space Station for later rescue by another shuttle.
Needed: A clean sheet of paper
Central Focus:
Along with these design changes, NASA appointed an independent "Return to Flight Task Group" that has been monitoring the space agency's progress in meeting the technical and operational recommendations of the CAIB. The group was scheduled to issue its final report a month before the launch.
Despite these efforts, shuttle critics remain skeptical. "No doubt NASA will do everything it can to make the shuttle safe," notes Brookings Institution Scholar Gregg Easterbrook, "but you are still talking about a technology that is 30 years old. What is needed is a completely fresh design for the spacecraft and the launch vehicle."
While final designs are far from complete, NASA is stepping up its search for a successor to the shuttle, which it plans to retire in 2010. The thinking now: A "crew exploration vehicle," featuring a module and capsule design combined with an expendable launch system. Prototypes could be ready as early as 2008, with a manned flight by 2014.
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