Shuttle Discovery in the repair shop (photos)

In late December, the space shuttle Discovery trundled from the launch pad back to NASA's Vehicle Assembly Building at the Kennedy Space Center in Florida amid tests and inspections after cracks were found in its massive external fuel tank--the big orange cylinder to which the shuttle clings. Just this week, engineers at the space agency said they believe they've determined what caused the cracks, and space shuttle management gave the go-ahead for repairs.

The goal is to get Discovery ready for a launch now set for February 24, on a mission to the International Space Station. The shuttle has been delayed since an initial expected launch date in the latter half of 2010.

Scaffolding surrounds the space shuttle's external tank for the repairs, which are centered on the intertank section between the liquid oxygen tank--the conical top portion of the big external tank assembly--and the larger liquid hydrogen tank below. More specifically, the work is being done on the "stringers," the support beams that show here in the ribbed portion of the external fuel tank. Additional support structures called radius blocks are being added to the stringers, which lie lengthwise around the circumference of the tank.

The upcoming flight will be Discovery's final mission (STS-133) after 25 years of service, as NASA puts an end to the shuttle program.

This photo from December shows wires and sensors--a total of 39 strain gauges and 50 thermal sensors--in place on the intertank section for a "tanking test" designed to help technicians assess the status of the stringers. (NASA said it was expecting the sensors to generate more than 6 terabytes of data.) At that time, four cracks were discovered near the top of three stringers. Cracks had first appeared at the top of a pair of stringers during fueling operations in early November for a launch planned at that time. (A separate problem, with the "ground umbilical carrier plate," actually was what caused NASA to scrub that launch.)

NASA expects to complete the repairs by the last week of January.

Here's a closer look at the stringers with their outer coating removed. The radius blocks are small by comparison--just six inches long and about one fifth of an inch thick. But those small pieces of aluminum, NASA says, are enough to keep the stringers flat. Without the radius blocks in place, the danger is that the stringers could flex as the liquid oxygen tank shrinks as it's filled, causing more cracks.

That outer layer of foam serves as insulation to help keep the chill in the super-cold propellants. It also protects against damage by what NASA delicately calls "aerodynamic environments" during lift-off.

The external fuel tank delivers fuel to the three main engines in the space shuttle itself during launch. Empty, it weighs 78,100 pounds, and it carries about 1,585,000 pounds of propellant. It's a little over 150 feet long, and has a diameter of nearly 28 feet. (Click here for a larger, more readable view.)

The big external tank also provides structural support for both the shuttle and the two separate solid rocket boosters. Its work is done at about 8 to 9 minutes into the flight, at an altitude of about 70 miles, when the shuttle has reached near orbital velocity; the external tank is then jettisoned and whatever parts survive disintegration fall into the ocean. (The solid rocket boosters, which provide about 80 percent of launch thrust, drop off earlier, at about 28 miles above the Earth.)

(The shuttle seen lifting off here is the Atlantis.)

This view of the external fuel tank and solid rocket boosters amid the scaffolding in the Vehicle Assembly Building gives a sense of the tank's generous proportions. That bottom section, again, holds the liquid hydrogen fuel. Just behind the tanks, you can make out part of Discovery's wings and the bottom rear of its fuselage, at the main engine exhausts.

The stringers come under scrutiny from a pair of NASA technicians.

Discovery has had a notable career. NASA says that it has flown more missions than any other spacecraft--38, not counting the upcoming flight. It has gone around the Earth 5,600 times and carried 174 people into space, and its payloads have included the Hubble Space Telescope.

At first glance, this looks like a circular saw, but it is in fact a backscatter device used to acquire data in a very exacting process. The backscatter devices, which bounce radiation off the tank and pick up the reflected energy, take about 90 minutes to survey a single 21-foot-long stringer. There are a total of 108 of the metal stringers on the tank.

Technicians also used computing radiography scanners which move faster, scanning as many as five stringers per hour.

Here, some repaired stringers get a new spray coating of foam insulation. This is from earlier repairs done in November on the initial set of stringers in which cracks were discovered.

Those November repairs involved the use of shaped metal pieces known as doublers.