Once the plane-like orbiters are mothballed in 2010, boosters from the shuttle's propulsion systems might be used to power lunar missions as well as flights to the international space station.
In the Apollo era, a single launch of the giant Saturn 5 rocket carried to orbit everything needed for a moon expedition: a lunar lander, crew module and service module that provided propulsion and power. A similar approach today would require another vehicle capable of lifting huge amounts of hardware.
The likely solution: launchers created from parts of the so-called shuttle stack that includes the external fuel tank and twin solid rocket boosters.
"We talk about retiring the space shuttle. What is really meant is that we need to retire the space-shuttle orbiter," NASA Administrator Michael Griffin told Congress recently. "If I remove the orbiter and put on a cargo module, I have a heavy lifter."
A shuttle-derived booster would have lots of advantages besides its ability as a heavy lifter.
The shuttle's components are considered safe for human missions. Many of the facilities to build, process and launch a new vehicle already are in place. The time required to develop a shuttle-based booster would be a relatively short four to five years. Many shuttle workers at places such as Kennedy Space Center could continue on in the new effort.
"The shuttle-derived answer is significantly better in terms of the work force at the Cape [Canaveral]," said Steve Oswald, a former astronaut and Boeing vice president who heads the company's shuttle efforts.
Other possible solutions include upgraded versions of Atlas and Delta expendable rockets built for the Air Force, or a completely new design. NASA is expected to complete a study by mid-July assessing options for new launchers, spacecraft and ways of getting to the moon.
Current versions of the Atlas 5 and Delta 4 don't have the potential lift of some shuttle offshoots, but the rockets could possibly be used to carry crews to the space station. For missions to the moon and beyond, engineers estimate shuttle-derived vehicles could hurl up to 120 tons into a circular orbit 250 miles above Earth. That's close to the lift provided by the Saturn 5, the most powerful rocket ever built.
"If the requirement is for great big pieces to go to orbit, the shuttle-derived vehicle is it," said Mike McCulley, president of shuttle prime contractor United Space Alliance and a former astronaut. "I'm not criticizing either the Atlas or the Delta, but they have their limits. . . . If you start with a new rocket, it's years and billions [of dollars]."
Cost is an issue
The idea of using pieces of the shuttle as a powerful cargo launcher dates to the 1970s. The most serious effort was in 1987, when engineers at NASA's Marshall Space Flight Center in Huntsville, Ala., started developing a concept called Shuttle C [cargo] in the wake of the 1986 Challenger accident.
The plan would have replaced the shuttle orbiter on some launches with an unmanned carrier to haul satellites and other items to space. The notion was abandoned in 1990, however, when it became clear that launch costs would be cheaper on expendable rockets.
A new study by NASA and shuttle contractors made public during an Orlando space-exploration conference in January identified several basic designs that could put people and cargo into orbit. One design would use one of the shuttle's twin pencil-shaped solid rocket boosters as a first stage, with a new manned vehicle or cargo carrier and a second stage perched on top.
The new manned ship, dubbed the Crew Exploration Vehicle, or CEV, will replace the shuttle orbiter as America's primary means of putting people in space after 2010. The CEV initially will be used to carry astronauts to the space station. NASA plans to select a design for the crew vehicle in early 2006 from two competing industry teams.
A single, modified shuttle booster rocket could carry an estimated 24 tons, enough to launch the CEV to the station. However, NASA is getting pressure from the Pentagon to use the Delta and Atlas fleets for some of its future missions. The financially struggling rocket lines badly need another government customer in addition to the Air Force.
Current versions of the Atlas and Delta rockets also could handle CEV missions weighing 22 tons or less without major modifications. It's possible that Atlas, Delta and the shuttle's booster rocket all will be used to launch the CEV. No decision has been made.
Griffin has said cost would be the biggest factor. However, he and others continue to hail the safety record of the shuttle's solid rocket booster.
After being redesigned in the wake of the Challenger accident, two of the boosters have performed safely on each of the 88 shuttle launches since. It's unclear how costly and time-consuming it would be to qualify the Atlas and Delta rockets as safe for humans.
"Atlas [5] and Delta [4] are still relatively new vehicles," Oswald said. "The [shuttle] boosters are just incredibly reliable. We haven't seen anything that has scared us in the solid rocket motors since Challenger."
Heavy missions
Atlas and Delta rockets also are being looked at for weightier missions to the moon and beyond. However, two other shuttle-derived proposals appear to be far more likely alternatives.
A design similar to the old Shuttle C concept would swap the space plane for a cargo carrier mounted on the side of the external fuel tank. Unlike the orbiter, the carrier would not be reusable.
Cheaper, expendable main engines would be developed to replace the costly reusable versions on the shuttle. Engineers estimate the rocket would be capable of hauling a hefty 100 tons to an orbit 250 miles high.
Another, more powerful shuttle-derived design would modify the tank to put four main engines beneath it and a cargo carrier with a second-stage engine on top. The colossal 36-story launcher, which would look similar to traditional expendable rockets, also would be equipped with larger solid rocket boosters than those presently used on the shuttle.
This so-called in-line heavy-lift rocket could launch an estimated 120 tons to orbit. It would rival the Saturn 5 as the most powerful booster ever.
"If you did the in-line heavy, you could just duplicate Apollo," said Oswald, explaining how a manned moon mission might work. "You could do it all in one launch."
"If you end up using the side-mount [design], you probably are going to need to launch some piece of that with something else," he added. "If you were going to go to Mars, you might end up needing two or three [launches] to get going, but that's better than 10 or 12."
Other factors
There are benefits to using existing shuttle hardware, besides its ability to lift heavy loads. The cost of developing entirely new rockets would almost certainly be far greater.
Much of the needed infrastructure to support shuttle-derived launchers already is in place at Cape Canaveral. Existing launch pads could be modified, depending upon the rocket's design. The fuel tank and solid rocket boosters would likely continue to be processed in current facilities.
"I think shuttle-derived is a win-win for the nation and NASA and for us," said McCulley, whose company would be the leading contender to operate any new shuttle-derived rocket. "There is a lot of investment, a lot of capital, that the United States has got in this infrastructure that is designed to operate the existing system."
The costs of operating such a system would almost certainly be cheaper than the current shuttle program. About 40 percent of the present expense goes toward maintaining the three reusable orbiters and preparing them for flight. Discovery, Atlantis and Endeavour would be retired.
As a result, the orbiters' processing hangars could be closed or used for the new Crew Exploration Vehicles. Shops that work on the orbiters' thermal-protection systems would no longer be needed. And shuttle-landing facilities at KSC, Edwards Air Force Base in Southern California and White Sands Missile Range in New Mexico, as well as emergency runways in Europe, might be shut down.
"We think that the operations costs would be on the order of half of what they are today," Oswald said. "Certainly, the shuttle-derived vehicle's number of folks would be smaller. We think that number is about half."
The direction NASA chooses will affect not only the shuttle work force at KSC and other agency field centers, but also the direction of human space exploration for years to come. The challenge is to quickly find the safest, most effective alternative at the lowest possible price.
The result could be that parts of the shuttle still will be flying decades from now.
"When you get into the 100-metric-ton [110-U.S.-ton] range, it's clear that the nation owns one and only one vehicle in that class," NASA Administrator Griffin told reporters last month. "That is, broadly speaking, the shuttle stack."
