NASA Is Considering Fuel Depots in the Skies
Washington, Oct 22, 2011 - By considering a proposal to put filling stations in the sky, NASA is looking to accelerate plans to send astronauts to distant destinations.
The filling stations — NASA calls them propellant depots — would refuel a spacecraft in orbit before it headed out to the moon, an asteroid or eventually Mars. Currently, all of the fuel needed for a mission is carried up with the rocket, and the weight of the fuel limits the size of the spacecraft.
Next month, engineers will meet at NASA headquarters in Washington to discuss how propellant depots could be used to reach farther into space and make possible more ambitious missions using the heavy-lift rocket that NASA is planning to build. The discussions grow out of a six-month NASA study of propellant depots, completed in July.
However, the space agency has rejected the study’s most radical conclusion: that NASA could forgo the heavy-lift and use existing smaller rockets, combined with fuel depots, to reach its targets more quickly and less expensively. Those targets, for the next two decades at least, include a return to the moon or a visit to an asteroid. (A trip to Mars is unlikely until at least the 2030s.)
“This study highlights some interesting benefits of depots, but it is too singularly focused,” William H. Gerstenmaier, the associate administrator for NASA’s human exploration and operations directorate, said in a statement. “NASA is actively studying depots and how they can be used with other proposed elements to provide the lowest cost, sustainable exploration plan.”
Under the plan outlined in the document, the propellant depot would be launched first, and then other rockets would carry fuel to the depot before a spacecraft arrived to fill up. That would increase the complexity for an asteroid mission — 11 to 17 launchings instead of four — but could get NASA astronauts to an asteroid by 2024, the study said. The total budget needed for the project from 2012 through 2030 would be $60 billion to $86 billion, the study said.
By contrast, a study last year that designed an asteroid mission around a heavy-lift rocket estimated that it would cost $143 billion and that the trip could not happen until 2029. The earlier study briefly considered propellant depots but quickly dismissed them.
Last month, NASA announced the design of the Space Launch System, the new heavy-lift rocket. The goal is for it to lift 70 metric tons on its first unmanned test flight in 2017, and to be developed into a version capable of lifting 130 tons. The blueprint for NASA’s direction for the coming years, passed by Congress last year and signed by President Obama, calls on the agency to develop just that rocket.
Critics say the expense of developing and operating the massive new rocket, particularly in an era of tight federal budgets, would doom the project.
At a Congressional hearing in July, Representative Dana Rohrabacher, Republican of California, asked Maj. Gen. Charles F. Bolden Jr., the NASA administrator, about the possibility of depots as an alternative to the Space Launch System. General Bolden said that he did not know details about any propellant depot study, but that his agency had looked at alternatives to building a heavy-lift. “It turned out that was not as economical, nor as reliable,” he said.
Although General Bolden promised to provide the information, Mr. Rohrabacher said he had obtained the study about propellant depots only through unofficial channels.
“I’m shocked that the leadership in NASA would try to keep a report as significant as this away from decision makers of the legislative branch,” Mr. Rohrabacher said, adding that the study gave him “the ammunition to make a case” to revisit NASA’s plans for human spaceflight.
Propellant depots carry risks, too. Fuels like liquid hydrogen and liquid oxygen must be kept at ultracold temperatures and, unless the depots were heavily insulated, would boil away over time. And transferring fuel in the weightlessness of space is not straightforward, although perhaps simply setting the depot and spacecraft into a slow spin would generate enough force to push the fuel into the spacecraft.
“It’s not a simple thing to transfer cryogenic propellant, on the ground, much less in space,” said Eugene M. Henderson, an engineer at the Johnson Space Center in Houston, who is one of about 20 people who worked on the study. “It’s a big variable.”
Still, he described the technical challenges as “fairly trivial” and said demonstration projects could show that the technology is feasible.