GRAHAM WARWICK / WASHINGTON DC
DARPA research programme aimed at refuelling rather than replacing satellites
Development of a reversible high-pressure fuel cell is the key to a space transportation architecture based around using water for fuel, says the US Defense Advanced Research Projects Agency (DARPA). Under its Water Rocket programme, the agency has awarded Hamilton Sundstrand and Proton Energy Systems contracts to develop fuel cell technology.
The motivation behind the concept is the desire to refuel satellites rather than replace them, says programme manager Dr Robert Rosenfeld. It is also cheaper to boost fuel than satellites.
Water can be used in electric thrusters for slow manoeuvring or converted to hydrogen and oxygen and burned in larger thrusters for rapid manoeuvring. When the satellite is in daylight, electrical power from its solar arrays can be used to electrolyse the water into hydrogen and oxygen, which can then be recombined in the fuel cell to produce water and electricity to power the spacecraft at night.
The Water Rocket concept requires development of a regenerative fuel cell/electrolyser that can operate at high pressures and reverse its operation every 90min as the satellite orbits the Earth. High pressure is required to minimise the size of the hydrogen and oxygen gas storage tanks and keep down the size and weight of the satellite. "We have not built a reversible high-pressure fuel cell before," says Rosenfeld. As a result, the two-year, $10 million programme is focused on demonstrating reversible fuel cells, although smaller contracts have been awarded to study very high specific-impulse electric thrusters.
Challenges include building a regenerative fuel-cell stack able to operate at 2030lb/in2 (140bar) or more, with low corrosion and bidirectional catalysts, and keeping water and vapour separate.
Control of the transition between modes is critical, as electrolysers operate flooded with water and fuel cells need vapour. While the water and vapour have to be separated, the membrane that allows hydrogen and oxygen to combine in the fuel cell must remain wet. Minimum leakage and voltage loss is also required.
Under the year-long first phase, Proton plans to demonstrate a 140bar electrolyser, followed by a brassboard reversible high-pressure fuel cell in phase two. In phase one, Hamilton Sundstrand will demonstrate a 140bar fuel cell that is reversible, but uses vapour feed.
Source: Flight International
