[extropy-chat] Suicide the Green way

Tue Nov 21 07:47:54 UTC 2006

On Mon, Nov 20, 2006 at 05:13:01PM -0500, Robert Bradbury wrote:

>      Design of a high performance solar sail system.
>      Drexler, Kim Eric
>      1979, MIT Masters Thesis

Things have progressed a bit since, at least in theory.

>    Offhand, does anyone know why boron doesn't reflect light?  Or whether
>    we can have hydrogen mirrors?  Why do we have to constrain ourselves
>    to aluminum?

http://science.nasa.gov/NEWHOME/headlines/prop08apr99_1.htm

...

Right: A "conventional" solar sail, fully deployed and cruising into interstellar space. Innovative ideas for "gray" and electromagnetic sails may leave this concept in the interstellar dust. (NASA)

"A propellant-free system is very attractive because the main problem with interstellar travel is the weight of the propellant," said Geoffrey Landis of the Ohio Aerospace Institute at NASA's Glenn Research Center. He spoke Wednesday morning to the 10th annual Advanced Propulsion Research Workshop held by NASA, Marshall, the Jet Propulsion Laboratory, and the American Institute of Aeronautics and Astronautics being held Tuesday-Thursday at the University of Alabama in Huntsville.

Recent Headlines
December 3: Mars Polar Lander nears touchdown

December 2: What next, Leonids?

November 30: Polar Lander Mission Overview

November 30: Learning how to make a clean sweep in space
The original notion of space sails is to unfold a large aluminum coated Mylar blanket, face it to the sun, and let sunlight and the solar wind push the craft deeper into space. But that takes a long time to get anywhere. In 1984, Dr. Robert Forward, , vice president of Tethers Unlimited in Seattle and an advocate of sails and tethers for space propulsion, proposed giving Mother Nature a hand by using high-power laser or microwave transmitters that would beam for a few days or weeks to speed the probe on its way.

Gray sails could provide a better ride

Forward's Starwisp concept would have used a mesh of superconducting aluminum wires to receive its "push" from microwave photons, and then reflect to produce an equal magnitude thrust. This would propel the craft from Earth orbit past Neptune, at 1/20th the speed of light, in just a week. Since then, Forward and others have been rethinking the concept.

"My major message is, that's wrong, don't use it" said Forward as he pointed at the equation he used in his initial studies. Since 1984, he has determined that the sail material would absorb a significant amount of the energy, weakening the structure and possibly letting it collapse.

Forward now proposes putting that absorption to work in a "gray sail" made of carbon. The sail would absorb the light, getting a push from it, and reradiate it as infrared energy. With the sail oriented properly to the source, this would generate a significant amount of thrust in the desired direction.

A mission to interstellar space could be accomplished with a combination sail. An aluminum coating - just 70 atoms thick - would serve as a traditional reflective solar sail to boost the spacecraft out of Earth orbit, then cancel its solar orbital velocity so it plunges on a near-miss trajectory towards the sun.

Right: The sunshade for the Next Generation Space Telescope is not as large as a sail for space propulsion, but will provide valuable technical lessons on how to build one. (NASA)

As it passes just 3 solar diameters from the sun's visible surface, the aluminum would evaporate, exposing the carbon structure underneath. The carbon would absorb sunlight and heat to 2,000 K (almost 3,600 deg. F). Radiating infrared light would accelerate the craft at 14 times Earth's gravity (the Space Shuttle reaches a maximum of 3 G during launch).

"The trajectory is nearly a straight line" away from the sun, Forward said. He is proposing a laboratory demonstration using a 1 kilowatt microwave beam to levitate a 2.5 cm (1 in.) square, 02.5 micron-thick carbon film in a vacuum chamber.

A new use for radio

Landis also finds carbon sails attractive in a reworked approach to Forward's Starwisp concept. Landis proposes using millimeter-wave radio to push a carbon sail. Millimeter-wave transmitters are more efficient than lasers, so less power would be needed to run the system.

"If you're pushing terawatts into space," Landis said of the beaming system, "it's expensive."

A lens to focus the millimeter waves (using techniques similar to those that steer phased-array radar beams) would only have to be 185 km wide, as compared to a 50,000 km fresnel lens that would be required for a system.

The sail itself would be made of carbon fibers, or possibly with variants of the high-temperature superconductors that have been in development since the early 1990s. The transmitter technology already is becoming available through megawatt-power, 1,110 gigahertz (0.78 mm wavelength) gyrotrons developed for fusion power experiments.

Landis suggested a laboratory demonstration using a 2 cm (4/5th inch) diameter cone. Shaping it so it would stay on the beam "is a tricky design problem, but it's a design problem with a solution," he said.

A precursor space mission, carrying a 1 kg (2.2 lb) payload on a 10x10-meter sail would take 20 hours to accelerate. In three weeks, it would pass the orbit of Pluto and continue outward to the Oort cloud of comets surrounding the solar system. Reaching a star would take 400 years, so it's only good as a demonstration.

"It's still science fiction," Landis said, "but it's near-term science fiction."

Even closer at hand is a concept to sail without a deploying a sail, but throwing a switch and generating one around the spacecraft. In an approach called Mini-Magnetospheric Plasma Propulsion - or M2P2 - a probe would imitate nature to get the solar wind to push it into deep-space.

"The enabling technology is pretty much available today," said Dr. R.M. Winglee of the University of Washington Winglee works in the geophysics program which studies the magnetosphere, the region of space around the Earth where the solar wind is deflected by the Earth's magnetic field.

...

-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
______________________________________________________________
ICBM: 48.07100, 11.36820            http://www.ativel.com
8B29F6BE: 099D 78BA 2FD3 B014 B08A  7779 75B0 2443 8B29 F6BE
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 191 bytes
Desc: Digital signature
URL: <http://lists.extropy.org/pipermail/extropy-chat/attachments/20061121/248056db/attachment.bin>