NASA's grand plan to revive human exploration beyond Earth orbit relies greatly on utilizing a heritage of hardware from the soon-to-be-scrapped space shuttle program.

The centerpiece of this system is a larger Apollo-like capsule. It would haul four astronauts to and from the Moon, six crewmembers on future missions to Mars, and deliver crew and supplies to the International Space Station.

But to get things off the ground, factually, NASA is eying astronaut-carrying and cargo-lofting launch systems that build upon space shuttle components.

In some quarters, NASA's vision of exploration is an anti-doldrums undertaking for the agency. Yet the plan is rife with technical issues that need resolution. Others suggest that the strategy is dead on arrival, or is sketchy at best.

Sporty slap

For the most part, editorial pundits have not been easy on NASA's newly announced strategy.

Gregg Easterbrook is a senior editor of The New Republic, a contributing editor of The Atlantic Monthly and a visiting fellow at the Brookings Institution. He took a sporty slap at NASA in a column he writes for, of all places, NFL.com - the official site of the National Football League.

"NASA says it will take $104 billion and 13 years to build the amazingly 1960s-like hardware. Let's see, that's a target of 2018 -- 49 years after the first Moon landing," Easterbrook wrote.

"So half a century after America was able to land people on the Moon, we'll be able to do it again. Imagine if you had declared in 1952, 49 years after Kitty Hawk, that for a mere $104 billion, you could build a wooden flyer that would remain in the air for 12 seconds," Easterbrook proclaimed. "Isn't this Moon announcement awfully similar?"

Image Gallery: NASA's New Spaceship

VIDEO: A New Era of Space Exploration
Video animation of how NASA plans to get back to the Moon by 2018.

Workable solution

Putting those barbs aside, the NASA vision as unveiled last month by NASA chief, Michael Griffin, is starting to undergo technical critique.

"I think Griffin's team has come up with a truly workable solution that really does make sense," said Jerry Grey, Director, Science and Technology Policy for the American Institute of Aeronautics and Astronautics (AIAA). Grey is also Visiting Professor of Aerospace Engineering at Princeton University.

"Certainly there will be technical issues," Grey told SPACE.com, "but in view of the current concerns over shuttle and station, the ever-present budget constraints, the political issues, and the lofty long-term goals -- which are indeed the right ones -- it would be hard to find a better approach."

Given that, Grey continued, there are a host of technical issues that must be dealt with.

Unproven features

For one, the use of a shuttle solid-rocket booster (SRB) as a main stage will require extensive engineering, modeling, and flight-testing, Grey said. "The structural, aerodynamic, vibration, and other environmental conditions for an SRB having an upper stage and a large, heavy, top-mounted payload are very different from those involved in the current usage of these motors," he said.

Furthermore, an SRB has never flown in this configuration but has always had the structural support of the external tank, Grey pointed out. Also, integration of the upper stage and the top-mounted payload, and providing the necessary electric power, guidance and control, communications, and other "housekeeping" functions to the upper stage and the Crew Exploration Vehicle (CEV), raise a whole host of technical requirements -- especially in validation and verification -- that have never been addressed for the SRB, he said.

"So although by itself the SRB is a well-proven piece of hardware, the new flight article will have many new and as yet unproven features that have yet to be human-rated," Grey added.

Vacuum restart

Another hardware hurdle to be overcome is use of the Space Shuttle Main Engine (SSME). Built for re-use, the SSMEs are in the NASA strategy as an upper stage motor, as well as clustering five of them for the heavy-lift booster.

Although well proven in its current usage, Grey said, both projected applications of the SSME involve new technical matters that must be addressed.

The SSME has never been used in an upper stage, which itself will require a whole new design, and as yet not human-rated, Grey said. "This will involve a new propellant feed system to the engine, validation of a totally different set of environmental conditions than the engine has experienced in the past, and almost certainly a re-start capability -- in vacuum -- which involves a whole new set of technical requirements."

"Remember, too, that although the engine has operated in vacuum during part of the shuttle launch trajectory, it will now require not only vacuum re-start, but also an initial chill-down and start under vacuum conditions, Grey noted, utilization that will demand human-rating, he said.

There's another issue of tasking the SSME to upper stage duty. The SSME is now optimized for the shuttle launch trajectory, so they are probably under-expanded for full vacuum operation. That isn't really a technical problem, Grey noted, but it would give the engines a lower specific impulse than that of a fully expanded liquid oxygen-liquid hydrogen upper-stage engine.

Specific impulse is a performance measure for rocket propellants that is equal to units of thrust per unit weight of propellant consumed per unit time.

Engineering attention

Then there's the business end of the shuttle-derived heavy-lift booster.

A shuttle orbiter is outfitted with a trio of SSMEs - a well-proven engine configuration.

"The new five-engine SSME cluster, mounted on a stage that will certainly be very different from the orbiter in its propellant feed and thrust-vector control characteristics and its structural, aerodynamic, and environmental behavior, will require considerable engineering attention," Grey explained.

Grey said that the upper, or side-mounted, stage use of the SSME -- or whatever other alternative engine may be selected -- will also require a new design approach.

"The payload characteristics are certain to be much different than those of the CEV," Grey said. Lastly, the SSME is expensive and was not intended to be expendable. "Keeping budget control on the expenditure of six units on each flight might turn out to be an issue."

Show me the money

Taking a step back from technical aspects of NASA's vision quest, there's the subject of money.

Grey said that while he is supportive of the stated doctrine of allowing the schedule of the whole program to be the "dependent variable", there is an obvious budgetary impact.

"The only valid corollary to this doctrine is not to fix the total budget, or even the budget for each element of the program," Grey said, "but to fix the annual budget cap for the program as a whole. That allows each mission date to move to the right as much as needed to stay within that annual cap." 

Grey offered a scorecard account for implementing the new NASA vision.

"I believe that all these technical issues, although perhaps more extensive than NASA has as yet acknowledged, are all solvable," Grey concluded. NASA's approach to focus vision into reality "is probably the best that can be devised under the given set of conditions," he said.

A missing piece

At a fundamental level, the NASA plan is "Apollo II: The Sequel", observed Roger Launius, Chair of the Division of Space History at the Smithsonian Institution's National Air and Space Museum in Washington, D.C

"This architecture for reaching the Moon is certainly one that makes possible a return, but it is also one among several approaches that could have been successful," Launius told SPACE.com. "There is no one right answer and I suspect that before the hardware is solidified NASA will have to make modifications to the approach in response to technical, schedule, political, or economic challenges," he said.

Launius said he sees a missing piece of the NASA blueprint. That is, what is the rationale for taking on the challenge in the first place? What are we going to do once we reach the Moon?

"If science is the driver, what scientific activities are going to dominate? Decisions on this will affect the structure of the science effort as a whole, the landing site selection, the nature of robotic predecessors, the types of experiments developed, and a host of other issues that require sustained thought and planning," Launius suggested.

Political will or won't

The space historian said he hoped these activities would not be an afterthought, as was too often the case during Apollo.

"The Apollo program was about flags and footprints, and it was effective in helping to win the Cold War with the Soviet Union. The Apollo leadership also managed to tack on some science activities, but they were definitely afterthoughts," Launius recalled. "But those times have passed, and we must move beyond the Apollo concept to embrace a more engaging and sustained approach. I hope this program is successful in doing this. I will be properly ecstatic if program officials are successful in doing so," he said.

Leaping back to the Moon in 2018 will demand sustained political will, Launius said. That will be a major challenge for NASA's current chief, Michael Griffin, and perhaps especially for his successors, he said.

"At a fundamental level, political will is the most critical challenge facing those who wish to venture into space in this century. It is even more significant than the technological issues that also present serious challenges to returning to the Moon," Launius advised.

Governmental decision-makers, supported by the taxpaying public that elects them, have to agree over the long haul that the expenditure of funds for this exploration agenda is in the best interest of the nation, Launius said. "Without that political will, discovery and exploration cannot take place at an aggressive rate."

Historical trends

How real is the $104 billion price tag for NASA's Moon, Mars and beyond manifesto?

While the funding profile for the initiative is modest, "historical trends for earlier projects suggest that despite efforts to contain costs, they will escalate in response to technical challenges encountered in the project," Launius added.

Now toss in the Iraq war and Hurricane Katrina rebuilding.

The NASA vision must compete for federal dollars with both those priorities, and a host of other Congressional agenda items.

"As a person excited by the prospect of returning to the Moon, I am thrilled that the United States is finally intent on moving beyond Earth orbit," Launius concluded. "Like everyone, I am curious to see how this plays out over the next few months as project definition becomes more solid. The devil will be in the details."