Space Shuttle Discovery's return-to-flight mission marks the beginning of the end for a program whose design evolution exemplifies the pitfalls NASA is seeking to avoid as it embarks on a new direction in space exploration.

Thirty-five years ago, when NASA was struggling to make the case for a new space transportation system, it promised to build a reusable vehicle that would haul all of the nation's civil, military and even commercial satellites into orbit and eventually help construct a space station.

The melding of civil and military requirements--coupled with the budgetary and political pressures that affect all large aerospace programs--produced the engineering compromises that haunt the space shuttle program to this day. These include the use of an external fuel tank whose foam-shedding problems doomed Space Shuttle Columbia in 2003 and cast a cloud over Discovery's mission.

"The decision in 1972 to build the future of the space program around the shuttle has had consequences that still constrain NASA a third of a century later," said John Logsdon, director of the Space Policy Institute at George Washington University here and a member of the Columbia Accident Investigation Board. "[NASA Administrator] Mike Griffin and his associates are working hard to make sure that the choices they are making now enable a productive exploration program, not limit their successors' flexibility."

The beginning of the end

The February 2003 loss of the Space Shuttle Columbia set in motion a change in U.S. space policy that few would have predicted in the immediate aftermath of NASA's second shuttle disaster in about 17 years. By going beyond simply finding the technical root cause of the accident and recommending engineering remedies, the Columbia Accident Investigation Board laid important groundwork for the White House to announce the following year a new space exploration vision that called for replacing the shuttle and returning to the Moon.

While NASA continues to struggle with the Columbia Accident Investigation Board's top return-to-flight recommendation--modify the space shuttle external tank to prevent it from shedding insulating foam--the agency has taken to heart the board's advice that the shuttle be replaced.

NASA now plans to retire its shuttle fleet in 2010 or earlier and build a replacement system that, in accordance with the Columbia Accident Investigation Board's advice, will be based on existing technology and designed to launch crew and cargo separately.

Today, even as it celebrates the space shuttle's return to flight after a two-and-a-half-year hiatus, NASA is once again trying to sell a wary public on a new space transportation system.

In the world of aerospace engineering, tradeoffs and compromises are inevitable. And the budgetary and political pressures that attended the birth of the shuttle remain on hand today. This time, however, NASA is not proposing a vehicle that will be all things to all people, but rather a set of vehicles to suit its own unique needs.

Mistakes of the past

Before Project Apollo had achieved its first lunar landing, NASA began charting a future that included orbital outposts--space stations--stretching from low Earth orbit to the Moon. The massive Saturn 5 rocket was to launch the space stations, and a reusable space plane was to transport the astronauts back and forth.

NASA's early space shuttle concepts envisioned a two-stage fully reusable vehicle capable of taking off and landing like an airplane. "That's a far cry from what we got," Logsdon said.

By 1970, the White House had lost its appetite for large space programs, Logsdon said. Production of the Saturn 5 was ended, and NASA was told to forget about a space station for the time being.

That forced NASA to seek allies to justify building the shuttle. "The key ally was the national security community," Logsdon said.

The Pentagon agreed to get behind the shuttle provided it had certain characteristics, Logsdon said.

"One of those characteristics was the ability to launch classified payloads that could be up to 60 feet (18 meters) in length" and weigh up to 18,200 kilograms, Logsdon said. "The width of the payload bay was driven by NASA's desire to eventually build a space station."

Another Defense Department-driven requirement, Logsdon said, was the ability to take off and return to a West Coast launch site after a single polar orbit. Because of the Earth's rotation, a single polar orbit would not bring the shuttle back directly over its launch site, meaning it would have to glide farther through the atmosphere to land than otherwise would be the case. That drove NASA to add large delta-shaped wings and a more robust--not to mention heavier-- thermal protection system to its space shuttle design.

The space shuttle had evolved from a dedicated crew transport to a brawny, all-purpose vehicle that would be so busy hauling the nation's civil, military and commercial payloads that it would have to fly some 50 times a year. At the start of 1971, NASA told the White House Office of Management and Budget (OMB) that it could build such a fully reusable two-stage workhorse for $10 billion. OMB told NASA it could have $5 billion.

Thus began a six-month effort to find design alternatives that could be built for the available budget. The first cost-cutting design change, Logsdon said, was to move the fuel tanks to the outside of the vehicle. The second was to augment the liquid-fueled main engines with solid-propellant strap-on boosters, which generally are cheaper to develop but more expensive to operate.

NASA ended up with a shuttle design that fit within the $5 billion ceiling for development, but would prove far more costly to operate and entail greater risks than initially promised.

Logsdon said the White House made a policy mistake in 1972 by "putting NASA in a position where it had to promise more than it could achieve" in order to sell the space shuttle program and ensure a post-Apollo future for human space flight.

"The consequences of that mistake, Logsdon said, "still constrain today's NASA leaders."

Rather than repeat the mistakes of the past, Logsdon said NASA today appears "determined to propose an approach to the next-generation system for carrying people to space that learns from shuttle's history."

Lessons learned

NASA has yet to formally unveil plans for its next space transportation system, but the agency has said it intends to build a Crew Exploration Vehicle for transporting astronauts to and from orbit and a second unmanned system for launching cargo.

In addition, NASA and the Pentagon have no plans to meld their requirements into a single system, a point made clear in an Aug. 5 letter the two agencies sent to the White House Office of Science and Technology Policy. That letter, signed by Griffin and U.S. Air Force Undersecretary Ronald Sega, the Pentagon's top space official, says the "[Defense Department] and NASA believe that separating human-rated space exploration from unmanned payload launch will best achieve reliable and affordable assured access to space while maintaining our industrial base in both liquid and solid propulsion systems."

The letter goes on to say that while the Defense Department would consider using the 100-metric-ton-class heavy-lift launcher that NASA says it needs to hurl cargo toward the Moon, it has no interest in the smaller rocket the agency intends to use to launch the Crew Exploration Vehicle.

NASA's intends to use the space shuttle's major components for both vehicles.

Three decades ago, NASA envisioned a shuttle so robust and cheap to operate it would eventually launch 50 times a year. In reality, the shuttle has proven costly and difficult to operate, and took more than 10 years to mark its 50th launch in September 1992.

Griffin, in a television interview several days after Discovery's less-than-perfect liftoff, acknowledged the shuttle's unfulfilled promise. "The shuttle has been a step along the road to allowing humans routine access to space, but it did not reach that goal," he said. "We need to keep at it."