New Scientist is covering Gregory and James Benford’s intriguing sail concept that would get a spacecraft up to 60 kilometers per second. That’s faster than any spacecraft we’ve ever launched; by comparison, the fastest vehicle out there is Voyager 1, now pushing toward the heliopause at some 17.5 kilometers per second. The brothers Benford (Gregory from the University of California — Irvine and James of Microwave Sciences in Lafayette, CA), talk about beamed microwaves driving a sail design with a difference.
At play here is an effect James Benford discovered when testing a thin, carbon-mesh sail with beamed microwaves. The forces exerted on the sail turned out to be stronger than expected, because the heat from the microwave beam was causing outgassing from material in the sail itself. It was the push from these unexpected gas molecules that gave the sail the extra push. You can read the New Scientist story here.
I haven’t talked to James Benford since 2003, but even then he was zeroing in on this phenomenon, which is called ‘desorption’. It seemed clear that desorption could be used purposefully.
“You want to use desorption as a tool,” Benford said then. “You could paint on materials that will come off at the right temperature, and plan a mission to use that effect… The engine is basically the beam from earth. The fuel is the material that’s on the sail.
“All of this comes from our experiments, where we found materials coming out of the sail. We expected this from some materials; if you just have an ordinary surface and put it in vacuum, the surface will have weakly absorbed molecules of air and water, the sort of stuff that clings to any surface. Water is very clingy in vacuum, but if you heat it up to a few hundred degrees for a little while, you burn all that off.
“But when you go up to much higher temperatures, you start to bring out things that are actually embedded in the lattice of the carbon, things that have been in there like carbon dioxide. It takes high temperatures to get those out. We found we were getting a lot of gas out of the material, and that gas, being very hot when it comes out, gives you quite a bit of thrust.”
The Benfords’ concept will be described in an upcoming issue of Acta Astronautica. In it, they speculate that such a sail could be covered with a paint designed to emit gas to harness the desorption effect. They believe a one-hour microwave burst could bring the vehicle up to speeds that would take a spacecraft to Mars in a month’s time. Pushing the vehicle would be a 60-megawatt microwave beam, currently beyond our capabilities, but surely not for long.
Sources: Interview with James Benford (16 February 2003); Celeste Biever Boston, “Solar super-sail could reach Mars in a month,” New Scientist 29 January 2005. Details on the Acta Astronautica article when they become available.
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This might also be an economic launch method:
The (may be manned) spaceship is launched straight up
e.g. with relatively modest rockets, to reach a height of
100km, where it starts falling back to earth.
Then the sail is unfolded like a parachute,
and a strong beam of microwave is used to accelerate it
upward probably to an other Planet.
A nice target would be Titan , after a one year high speed journey
the high and thick atmosphere of Titan allows deceleration
without killing the passengers.
Microwave beaming offers numerous benefits in comparison to straight solar sail propulsion by photons. It’s interesting to speculate on what sort of deceleration methods might be used to establish an orbit at Titan — here, the image of the Jupiter arrival in the movie 2010 comes to mind, with the thick atmosphere allowing aerobraking and capture at the destination.
It should be possible to decelerate in Titan’s atmosphere.
There is no need to go into an orbit , Titan’s gravitiy is low,
and all the propellant for the journey back has to be produced
on the surface of Titan anyway. Liquid Oxygen and liquid methane
could be the propellant which need no cryogenics to store
on the surface of titan.
On the arrival at earth, where the ship will be very fast, it could
brake with a sail and microwave before entering earth’s
atmosphere . Such kind of braking should also be possible
for a spaceship that comes from th moon, and may save a
lot of wheight for heatshield…