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November 11, 2009 > TechKnow Talk: The Space Shuttle: End of an Era

TechKnow Talk: The Space Shuttle: End of an Era

The shuttle rises, slowly at first, then rapidly gathering speed as it mounts high into the sky on a long column of fire and smoke. It's an image to which we have become accustomed. But soon the space shuttle will make its final ascent into the heavens and the fleet will be retired.

Following the frenetic development and breathtaking successes of the Apollo moon landings in the late 1960s and early 70s, NASA struggled to chart the path forward for humans in space, to define another quest capable of capturing the imagination of the American people, as well as Congressional funding.

NASA ultimately settled on the concept of a reusable space vehicle to shuttle satellites, science payloads, and astronauts into Earth orbit. Most importantly, it would be the transportation needed to deliver construction materials, crew, and supplies to build and maintain an International Space Station.

Development began in the mid-1970s, and engineers immediately ran into some sticky technical issues. The shuttle is powered into orbit by a combination of three on-board engines fed by an external tank of liquid hydrogen and oxygen, supplemented by two external solid-fuel rockets. One of the major challenges faced by designers was the requirement for these solid rocket boosters to be re-usable. They must generate tremendous thrust then drop off the shuttle before it leaves the atmosphere and fall back into the ocean to be recovered and re-furbished.

The solution was a segmented rocket, with rubber O-rings between segments. The active ingredients in the solid propellant are ammonium perchlorate and powdered aluminum. Much like a Roman candle, once lit, the rocket burns ferociously until the fuel is consumed, starting with a hollow center core and burning radially outwards. Damage to a rubber O-ring, probably exacerbated by freezing temperatures, resulted in fuel burning explosively through one of the joints and caused the 1986 loss of shuttle Challenger and its crew, including the first "civilian astronaut," teacher Christa McAuliffe.

The mandate of re-use led indirectly to the loss of a second shuttle as well. All spacecraft prior to the shuttle had withstood the tremendous temperatures of re-entering the atmosphere by employing a heat shield that wore away during re-entry. This "ablative" heat shield did the job, but was destroyed in the process, rendering spacecraft suitable for only one trip to space.

The biggest challenge in developing the shuttle was finding a material that could withstand the heat and stress of re-entry over and over, allowing the shuttle to be re-used. This vexing problem was eventually solved by creating a silica-based fibrous material that was glued onto the shuttle in tiles of various shapes and sizes.

This approach yielded only limited success however, as the tiles were extremely fragile and many had to be replaced after each flight. Tile damage during launch was responsible for the loss of the shuttle Columbia and its crew in 2003, allowing the heat of re-entry to penetrate to the shuttle's aluminum skin, melting it and causing the shuttle to break apart.

These tragic reminders that space travel remains a dangerous pioneering adventure should not overshadow the incredible accomplishments of designing and building the space shuttle, billed by NASA as "the world's most complex machine." One simple yet very significant enhancement is credited to a worker manufacturing the external tank. He suggested simply eliminating the white paint used on the tank, saving about 600 pounds of weight at launch, thus increasing the payload capacity of the shuttles.

This enormous external tank, standing 154 feet tall and 28 feet in diameter, is the only major component that is not reusable. After exhausting its fuel it is detached, and burns as it falls back into the atmosphere.

NASA eventually built a total of five space shuttles (plus one non-spaceworthy test version), of which three remain in service. Each is 122 feet long with a wingspan of 78 feet. The entire assembly of shuttle, external fuel tank, and solid rockets weighs an astounding 2200 tons at launch, though most of that is fuel. The shuttle itself weighs about 86 tons empty.

With the solid rockets providing more than 70% of the thrust at launch, the shuttle can deliver about 50,000 pounds (25 tons) of payload into Earth orbit. The three on-board engines produce the remainder of the thrust and take over completely after the solid rockets have burned out and been jettisoned.

The shuttle's main engines are not operable after launch. Designed for unpowered landings, the wings allow the shuttle to "glide" to a runway following re-entry. But its 86 tons would not be confused with a glider airplane in flight. Once the shuttle has returned to the atmosphere, it loses altitude rapidly. It has only one chance to land.

Shuttles are capable of achieving orbits of 200-350 miles above the Earth. The International Space Station orbits at about 220 miles high, the Hubble Space Telescope at 350 miles. To date, shuttles have flown 128 missions. A shuttle logs several million miles on a typical flight, carrying a crew of 5-7 astronauts and specialists.

In addition to their crucial role in building and supplying the International Space Station, shuttles deployed the world's most amazing astronomical observatory, the Hubble Space Telescope, and visited it four times for servicing. They have delivered scientific instruments, communications satellites, and classified DoD payloads to orbit, supported hundreds of scientific experiments, and significantly advanced knowledge of the effects on humans of living and working in space.

But the end of the shuttle program is looming. Only a handful of flights remain, all focused on completing construction of the International Space Station. The last flights for each of the three shuttles will take place over the next few months, with the final shuttle launch currently scheduled for the fall of 2010.

The next generation of manned spaceflight vehicles is currently under design. Known as Orion, these craft will not only ferry humans to Earth orbit, but eventually to the Moon and Mars as well. But Orion is not expected to fly until at least 2015, and recent estimates place the date several years later.

This will leave the U.S. with no capability to place humans in space for many years. In the meantime, U.S. astronauts will be reliant on Russian launch facilities and the Russian Soyuz spacecraft, designed in the 1960s, to reach the International Space Station. Though talk of extending the shuttle program beyond 2010 continues, Congress has made no commitment to do so.

The final launch will truly mark the end of an era in spaceflight, the final act of a drama spanning more than 30 years. Hopefully, the remaining space shuttles will be put on public display where we can all get a close look at these well-traveled workhorses.

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