More than two years after the Challenger disaster, NASA is preparing to return the shuttle to space. Hundreds of changes have been made in key shuttle systems, and a profound change in attitude has taken place at NASA. A new conservatism-an overriding concern with safety-underlies the most exhaustive rebuilding effort in the space agency`s history. Primary changes to the spacecraft, including the most critical elements of the overall redesign, are shown below.
External tank.
Primary changes include strengthening of fuel line housings and modification of the gaseous oxygen (”GOX”) valve, located in a pressurization line leading from main engines to the liquid oxygen tank.
– Factory joints: Insulation has been thickened, pins have been lengthened and moisture seals have been added.
Main engines.
Potentially catastrophic problems with the shuttle`s three main engines have occurred during past flights and test firings. Primary malfunctions and modifications designed to prevent them include:
– Dampers have been added to oxygen turbopumps to cut vibrations, reducing wear and decreasing the risk of damage to engine components.
– Outlet necks of main combustion chambers have been nickel-plated for added strength. A main engine explosion during test firing was attributed to a weld failure in the combustion chamber.
– Turbine blades in high-pressure fuel turbopumps, which have developed cracks on past flights, have been strengthened. A blade rupture during launch could lead to engine explosion.
– Temperature sensors in fuel turbopumps have been modified to prevent moisture buildup. Failure of two sensors caused premature shutdown of main engine during a past launch. The shuttle reached orbit on the two remaining main engines.
Solid rocket boosters.
Original design. Field joints.
– Destruction of the shuttle Challenger was caused by failure of the right solid rocket booster`s aft field joint. Ignition pressure deformed the joint, and multiple seal failures led to fatal rupture in the booster casing. The redesigned joint has been stiffened, sealed and virtually rebuilt to limit flexibility and eliminate potential leak paths.
How the Challenger field join failed.
1. Subfreezing prelaunch temperatures and a buildup of ice on the shuttle reduced O-ring resiliency. A flaw in putty may have created a leak path to O- rings.
2. At booster ignition, the surge in motor pressure forced rocket casings to rotate, or flex slightly, creating a gap in the joint. Ignition pressure forced a blowhole through putty; O-rings, stiffened by the cold, failed to seal the gap. Superhot gases burned past O-rings and escaped through the joint.
3. As the shuttle climbed through ”Max Q,” the period of greatest aerodynamic stress, burning fuel seared through the ruptured joint, triggering explosion of the external tank and structural breakup of the orbiter.
Redesign Modifications:
– Capture latch, a metal lip forged to the upper member of the joint, forms an interlocking fit with the lower member and greatly reduces the amount the joint can flex open at booster ignition.
– Putty has been eliminated; the insulation gap between segments has been sealed with adhesive.
– J-shaped insulation slot, also known as a ”stress-relief flap,” routes burning fuel away from the joint and uses the force of gas pressure to tighten the insulation gap, below. The slot also allows insulation to expand and contract safely during temperature extremes.
– Third O-ring provides a thermal barrier between burning fuel and the primary and secondary O-rings.
– O-ring diameter and groove size have been modified for improved ”squeeze” fit.
– Steel pins have been lengthened by 1/4-inch.
– Second leak-check port has been added to check integrity of O-rings after booster assembly.
– Joint heater keeps joint temperature at 75 degrees Fahrenheit or above.
– Weather seal encapsulates joint to keep moisture out.
– Two new retention bands have been added for extra strength.
– Joint fairing has been added for improved aerodynamics.
Nozzle: The carbon/resin liner which protects the nozzle shell from rocket exhaust, has been upgraded and thickened, and application procedures have been changed to prevent the liner from unraveling or detaching from the shell.
Original design. Case-to-nozzle joints.
– After repeated instances of O-ring erosion and near-failure in the past, the case-to-nozzle joint has been a longstanding concern to NASA engineers. Changes to the joint, which connects the booster`s aft segment to the rocket nozzle, are designed to decrease rotation and enhance O-ring protection.
Redesign – Modifications:
– 100 radial bolts encircle the joint to reduce the rotation gap.
– Putty has been eliminated; insulation gap has been sealed with adhesive.
– Third O-ring provides a thermal barrier between hot gases and primary and secondary O-rings.
– Insulation slot, or stress-relief flap, routes motor pressure and gases away from the joint.
Orbiter
Primary trouble spots on the orbiter are landing gear, which has often sustained impact damage, and insulator tiles, which have suffered heat damage on re-entry. Modifications are designed to beef up braking systems, thermal protection and structural supports, and to correct faults in power and maneuvering systems.
– Fuel lines: Safety latches have been added to valves in fuel lines that carry liquid propellants from the external tank to the main engines. The latches prevent the valves from closing prematurely, which would lead to main engine shutdown.
– Orbital maneuvering system: Valves improved.
– Wings: Internal metal-and-graphite skeleton and attachments between skeleton and outer skin reinforced.
– Fuel cells: Modified to provide backup water-removal system.
– Forward reaction control system: Primary thruster modified.
– Auxiliary power units: Electrical systems improved.
– Main landing gear: Thickened carbon brake linings, computerized antiskid system and tire-pressure sensors added; axles strengthened; hydraulics improved.
– Insulation: Mid-fuselage and chin panel (beneath nose) reinforced with carbon to prevent heat damage on re-entry.
Crew escape.
NASA has improved systems for crew escape on the ground and in controlled flight, after booster separation or during descent.
– Explosives have been fitted to the orbiter side hatch, so the hatch can be jettisoned in flight.
– An escape slide has been installed in the side hatch, enabling the crew to flee the orbiter quickly after landing.
– Escape baskets suspended by cable, which ferry astronauts from the launch tower to concrete bunkers near the pad, have been improved.
– A 12-foot aluminum rod, extended through the hatch opening, is a new bailout option for use in controlled flight. Astronauts slide down the rod, which carries them clear of the orbiter, and descend to Earth by parachute.
Chicago Tribune Graphic by Megan Jaegerman; Sources: National Aeronautics and Space Administration, Morton Thiokol, Rockwell International, ”The Report of the Presidential Commission on the Space Shuttle Challenger Accident,”
Aviation Week and Space Technology.
