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US rocket makers have suggested that plans for interplanetary exploration - imperilled by a forecast lack of funds following recent economic problems and a new administration in Washington - could be saved by the use of orbital "propellant depots" or space fuelling stations.

Aviation Week, reporting from an aerospace conference in Denver last week, covers the plans as proposed by Bernard Kutter of United Launch Alliance (ULA). ULA is the space-launcher effort run by US aerospace behemoths Boeing and Lockheed.

The idea would be to use modified, new upper stages on launch rocket stacks, which would go into low Earth orbit with substantial supplies of liquid fuel aboard. Later launches could put Moon or Mars spacecraft and their astronauts into orbit, but crucially the long-ranging ships could then dock and fill up with fuel from the orbiting "gas stations".

One technical problem to be overcome would be the issue of "boil off", where cryogenic liquid fuels gradually turn to gas as they warm up. The gas must then escape through safety valves, lest the whole storage tank rupture, but after a while this means that all the fuel is gone.

However it seems that Kutter and ULA have a plan which would reduce boil-off to manageable levels by deployment of a cone-shaped reflective sun-shield from the fuel tank module after reaching orbit. In the absence of any atmosphere, radiant heating from the sun's rays is the most serious problem.

“We can build a near-term depot without resorting to extreme, zero boil-off designs,” Kutter told Av Week.

The benefits of using the "gas stations" plan would be significant, according to ULA's analysis. A pair of launches by NASA's planned, massive new Ares V lifter could despatch four times as much mass towards Mars as a single Ares if orbital refuelling were used. Two Ares Vs using orbital refuelling would deliver 10 tonnes more to the Moon than a pair carrying out conventional missions.

Under current plans, Ares V heavy lifters would carry Orion Moon ships into space, with the astronauts travelling up separately aboard smaller Ares I launchers - in large part to avoid the expense of certifying the big haulers to carry people.

However, if as some suspect the Ares V gets axed following the ongoing Augustine review of NASA's Bush-era plans, ULA engineers believe that their refuelling scheme could still deliver big benefits using existing rockets like the Atlas V and Delta IV. The company has proposed a trial orbital experiment to confirm the feasibility of cryogenic fuel transfer in orbit, though NASA has not yet decided to go ahead.

Science-fiction aficionados will note the ideas without surprise, recalling that Robert Heinlein calculated that unaided chemical rockets flying to the Moon would need to use orbital refuelling to be practical. He wrote as much in his 1950 story The Man Who Sold The Moon.

In the event it turned out to be feasible to get men to the Moon using a single chemical launch stack, as the Apollo programme showed - incidentally a decade sooner than Heinlein predicted. But such operations turned out to be more or less an unsustainable dead end.

Av Week reports that ULA's proposals have "caught the attention of the Augustine panel", with four of seven possible ways ahead for NASA featuring in-space refuelling in some form. ®

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