Flying missions puts tremendous stress on the Shuttle orbiter, the white and black spacecraft that takes off like a rocket and lands like a plane. So every few years, each orbiter goes through a routine but invasive series of inspections and special tests called the Orbiter Maintenance Down Period (OMDP).

"It's basically an overhaul of the whole vehicle," said Stephanie Stilson, NASA vehicle manager for Discovery.

Modifications and upgrades to orbiter systems are made during the Orbiter Major Modification (OMM) portion of OMDP. Modifications range from the simple -- such as changing a part's label -- to something as complicated as the first-time changeout of the orbiter's rudder speed brake's operating mechanism. This time around, Discovery underwent 99 upgrades and 88 special tests, including new return-to-flight changes.

"During the typical sequence of preparations for launch, called the 'flow,' there are about 4,000 requirements to meet in about 250,000 hours of work," said Carol Scott, NASA lead project engineer and chief engineer for Discovery. "But during an OMDP, like this one, there are more than 8,000 requirements. It takes about a million hours of work, because of the amount of detail in the work."

To allow thorough structural inspections, nearly all accessible parts were removed, exposing the orbiter's airframe. Constructed mostly of high-grade aluminum, the airframe is inspected for corrosion and wear and tear. Corrosion is often not visible to the naked eye, occurring in patches so tiny that it sometimes requires being magnified up to 10 times its original size.

Additionally, the orbiter endures painstaking wiring inspections. It is crucial that any damaged wires or cables are found and fixed. This may sound simple, but it's a pretty tall order: Each orbiter houses about 150 miles of wiring!

Discovery's overhaul, which began in September 2002, marks the first time an OMM was performed at KSC. Previous overhauls occurred at the California plant where the orbiters were built.

"As the Shuttle program progressed, the KSC team spent so much time with their hands on the vehicles -- from landing to launch, every mission -- that we became the experts," said Scott.

Into the Orbiter
Anyone entering Discovery's crew compartment must first go through a series of preparations, from emptying pockets and tethering eyeglasses to putting on a "cleanroom suit" and booties over regular clothes. These precautions keep the area extra-clean and prevent anything from getting loose inside, even objects as tiny as a stray hair. In the zero-gravity environment, loose items can impede astronauts' work, get caught in the orbiter's ventilation system or interfere with other orbiter systems.

Immediately below the flight deck is the middeck, where astronauts sleep, eat, exercise or work with small payloads. But inside the middeck today, suited technicians are working on wiring.

In the cockpit, or "flight deck," a series of flat-panel displays called the Multifunction Electronics Display System replaced Discovery's original monochrome screens and tape meters. Nicknamed the "glass cockpit," this flexible system displays one of several instrument or status displays on any screen.

"It's a state of the art system," said Laurel Patrick, NASA digital processing system engineer. "It keeps the crew better informed, increases redundancy, and improves in-flight maintenance and problem management. It even weighs less and consumes less power."

Discovery is the third orbiter upgraded with the glass cockpit. Only Endeavour, the youngest of the orbiters, is still awaiting that improvement.

A hatch leads from the middeck into the payload bay, where school bus-sized payloads can travel into space. The silvery radiator panels, which reflect excess heat out of the payload bay in orbit, were removed and the payload bay doors were stripped down to the bare frame. A variety of tests and inspections were performed, from work on the insulating blankets that line the bay to testing the latches on the doors. Special equipment was needed to open the payload bay doors on Earth, since they are designed to operate in space.

Another important payload bay component -- the airlock, with the Orbiter Docking System attached -- is in a nearby workstand. The Russian Space Agency, which designed and still owns the Orbiter Docking System, sent representatives to KSC to perform maintenance and review of the system. During space missions, the airlock is the crew's gateway to the International Space Station.

From Nose Cap to Body Flap
Have you ever wondered where the Shuttle orbiter gets its black and white coloring?

That comes from the black and white tiles, gray reinforced carbon-carbon (RCC) panels and white thermal blankets that make up its Thermal Protection System, which protects the orbiter from the extreme temperatures of launch, space and entry.

"On each orbiter, there are about 24,000 tiles," said Stilson. "Because every single tile is unique, they are manufactured with a serial number that identifies that tile's size, shape and location on the orbiter."

During a standard flow, about 100 are repaired and replaced. For Discovery's Orbiter Maintenance Down Period (OMDP), that number is up to 1,400 so far -- about twice what was expected.

While the RCC panels are removed, the bare metal of Discovery's wing leading edges are exposed for corrosion repair. When every inch and rivet of the leading edge has been inspected and repaired, two coats of an anticorrosive compound and one coat of gleaming white paint are evenly applied with a spray gun.

The orbiter's airframe, including the wing leading edges, are made of high-grade aluminum. In the image at left, the wing leading edge has been stripped bare for inspection. The image at right shows the wing leading edge after anticorrosive compound and paint are applied.

A "first" is happening beyond Discovery's wings. For the first time in Shuttle history, work is underway to remove the parts that drive the rudder speed brake's movement. Located in the back of the tail, the rudder speed brake comprises four panels that open outward during landing, creating drag and slowing the vehicle. Four unique rotary mechanisms, called actuators, control the RSB panels, and workers are carefully lifting them onto twin scales joined together, in order to determine the actuators' center of gravity.

"Because Discovery's main engine compartment is so crowded with hardware, workers have to get into some pretty creative positions to open and inspect bundles of wires encased in protective black tubing," Scott said. The lines that feed Discovery's three main engines are color-coded to indicate whether they carry gaseous or liquid hydrogen or oxygen.

During standard Orbiter Major Modification inspections, technicians found a crack in a metal ball that is part of a 17-inch pipeline that delivers liquid oxygen to the Space Shuttle Main Engines. The finding led to a fleet-wide inspection, ensuring no other cracks existed. Later, a new 17-inch line was installed in Discovery -- work that was never expected to be required and had never been done before. Several NASA Centers contributed Shuttle expertise to resolve the problem successfully.

During re-entry, Discovery's main engines are partially protected by the body flap, which also helps control the up-and-down motion of the orbiter. Because the aft end of the orbiter is a very corrosive environment during launch, the body flap also endured a thorough structural inspection before it was repainted.

From nose cap to body flap, Shuttle Discovery's overhaul is no small task. But the amount of care and attention to detail required during OMDP ensures that when the Shuttle fleet returns to safe flight, Discovery will be healthy and ready to take on her next assignment.

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