New solar sail proposal might win one of NASA's Centennial Challenges near term.
Robert Clark
17:15 09 February 2006.
"The Station-Keeping Solar Sail Challenge A solar sail pushed through
space by the force of the Sun's photons to a target could earn $2.5
million with an equal amount available for keeping a solar sail for 90
days at a fixed point in space."
http://www.newscientistspace.com/channel/human-spaceflight/dn8701-nasa-unveils-its-toughest-challenges-yet.html
Via Solar Array to the Outer Planets.
"New Scientist is covering the work of Rudolph Meyer (UCLA), who
envisions a vehicle that sounds for all the world like a cross between
a solar sail and an ion engine. And in a way, it is: Imagine a flexible
solar panel a solid 3125 square meters in size, and imagine this
'solar-electric membrane' weighing no more than 16 grams per square
meter, far lighter than today's technology allows."
...
"Update: Geoffrey Landis was kind enough to forward the complete text
of his comments to New Scientist (the magazine quoted only the last
sentence). Landis wrote: "Professor Meyer suggests an interesting
thought-experiment about what may be possible in the future. The solar
array needed for his mission requires reducing the mass of solar arrays
by several orders of magnitude from existing technology."
http://www.centauri-dreams.org/?p=638
The solar sail is required to only weigh 50 kg for 3125 square meters.
But a recent advance involving carbon nanotubes can produce thin sheets
of arbitrary size for which a 1 kilometer square sail, or 1000m x 1000m
= 1,000,000 square meters, would only weigh 30 kg:
Researchers produce strong, transparent carbon nanotube sheets.
"Strength normalized to weight is important for many applications,
especially in space and aerospace, and this property of the nanotube
sheets already exceeds that of the strongest steel sheets and the Mylar
and Kapton sheets used for ultralight air vehicles and proposed for
solar sails for space applications, according to the researchers. The
nanotube sheets can be made so thin that a square kilometer of solar
sail would weigh only 30 kilograms. While sheets normally have much
lower strength than fibers or yarns, the strength of the nanotube
sheets in the nanotube alignment direction already approaches the
highest reported values for polymer-free nanotube yarns."
http://www.physorg.com/news5890.html
The sheets do not have the strength of individual carbon nanotubes but
are stronger than steel on a per weight basis.
To be made into solar cells will require the sheets to made in both
p-type and n-type semiconductors. The sheets appear to be formable into
both types:
Strong, Transparent, Multifunctional, Carbon Nanotube Sheets.
SCIENCE,VOL 309, 19 AUGUST 2005, p. 1215-1219.
"However, black sheets of solution spun
MWNTs have been used as nontransmissive
hole-collecting electrodes in solar
cells (10), and transparent p-type SWNT
sheets have been used as hole-injection electrodes
in inorganic LEDs based on gallium
nitride (32)."
http://www.cnrs-imn.fr/GDRE_NanoE/Textes/transparentsheets.pdf
Transparent, Conductive Carbon Nanotube Films.
Science, 27 August 2004:Vol. 305. no. 5688, pp. 1273 - 1276.
"For the nanotubes, the ease of chemical charge-transfer doping to
obtain such transparency-versus-conductivity optimization (via exposure
of the nanotubes to vapors of appropriate chemicals) provides an
additional advantage for the t-SWNTs. Moreover, charge transport in
these t-SWNTs is p-type, unlike the far more common transparent
conducting oxides [e.g., indium tin oxide (ITO)], which are n-type.
This should permit new complementary applications and alternative
photonic coupling schemes (3)."
http://www.sciencemag.org/cgi/content/full/305/5688/1273
And there are various ways now available for making general nanotubes
of either type:
CARBON NANOTUBE COMPUTER CIRCUITS
Novel processing and microfabrication lead to first single-molecule
logic gate.
"The principal challenge in constructing NOT gates out of nanotubes,
Avouris explained, is that invariably, without special processing,
transistors fashioned from nanotubes are p-type--that is, they conduct
positive charge carriers (holes). But NOT gates require n-type
transistors, the type that conduct negative charge carriers
(electrons), as well as p-type.
"Some researchers have demonstrated recently that doping nanotubes with
electropositive elements such as potassium is a viable method for
preparing n-type nanotube transistors. Now the IBM team has discovered
another way to do it. Simply heating (annealing) p-type nanotube
transistors in vacuum converts p-type into n-type, they reported."
http://pubs.acs.org/cen/topstory/7936/7936notw1.html
Nanotubes: Surprising Sensitivity To Oxygen Creates New Possibilities.
March 29, 2000
"We've demonstrated that carbon nanotubes can behave as both n-type and
p-type semiconductors. Until now, all nanotube measurements had
suggested p-type conducting behavior only."
"In their paper, the Berkeley researchers found that the degree of
oxygen exposure is the determining factor as to whether a carbon
nanotube functions as an n-type or p-type semiconductor. The ability to
function as either type is critical if nanotubes are to ever replace
conventional silicon devices."
http://www.lbl.gov/Science-Articles/Archive/zettl-nanotubes.html
And another research team may have already created thin-flim nanotube
solar cells of comparable lightness to the transparent nanotube films:
Sensational Materials: Cheap solar cells based on nanotubes.
Future Materials News - February, 2006
"Professor Nunzio Motta of QUT's School of Engineering Systems and Dr
Eric Waclawik from the School of Physical and Chemical Sciences are
developing the new renewable energy source using nanotechnology.
Weighing only 10 micrograms per square centimetre, it's possible to
generate additional power by linking up the polymer tiles in a
patchwork that increases the size of the device to suit the user's
needs."
http://www.future.org.au/news_2006/feb/cheap.html
Bob Clark
Robert Clark
propulsion and the solar cell powered ion engines discussed by Rudolf
Meyer. For Meyer's proposal you need not only a very thin light weight
material to act as a sail, but in fact this material would also have to
consist of solar cells, a much more stringent requirement.
NASA's Centenial Challenge only mentions a solar sail. But Meyer's
proposal should probably qualify at least for the transport part if not
the station-keeping part - especially when you consider a remarkable
new advance in the fuel efficiency of ion engines:
Super-powerful new ion engine revealed.
11:21 18 January 2006
NewScientist.com news service
Emma Young
"Tests on a prototype called the Dual-Stage 4-Grid (DS4G) thruster, at
ESA's Electric Propulsion Laboratory in the Netherlands showed that
DS4G's two-step process produces an ion exhaust plume that travelled
at 210 kilometres per second - more than 10 times faster than
possible with the engine in SMART-1, and four times faster than the
latest prototype ion engine designs. This would mean a spacecraft could
carry much more weight for a given amount of fuel, or it could go
further, faster.
"Crewed or heavyweight robotic missions to Mars become a distinct
possibility. And there's even talk of interstellar missions [beyond
the solar system]," says Orson Sutherland of the Australian National
University in Canberra, who led the team that built the engine, in a
project coordinated by Roger Walker of ESA's advanced concepts team in
the Netherlands."
http://www.newscientistspace.com/article.ns?id=dn8599
Robert Zubrin and Dean Spieth have discussed pure solar sail
propulsion in this report:
Ultra-Thin Solar Sails for Interstellar Travel
Phase I Final Report
December 1999
http://www.niac.usra.edu/files/studies/final_report/333Christensen.pdf
The new transparent nanotube films are comparable to the weight
requirements for the nanometer thin aluminum future sails they mention
and so might make the future sails they discuss *currently* feasible.
Also the new transparent nanotube films' low weight density surpasses
that of the micron thick metal-coated mylar sheets they discuss as the
current technology by a factor of 100. So the acceleration would be
larger by a factor of 100.
Zubrin and Spieth discuss that the metal-coated mylar sheets have an
areal density of about 7g/m^2 and the acceleration of this material at
1 AU, excluding payload mass, will only be about 1.2 mm/s^2. A trip to
Mars using solely solar sail propulsion would take about a year.
However, using the new transparent nanotube films, the acceleration
would be 100 times larger so a trip to Mars might be smaller by a
factor of the square root of this (using the relationship for distance
travelled at constant acceleration s = (1/2)at^2). That is, the time
required would be smaller by a factor of 10, or about a month.
Bob Clark
George Dishman
"Robert Clark" <rgrego...@yahoo.com> wrote in message
news:1147106335....@g10g2000cwb.googlegroups.com...
> I should distinguish more clearly the use of pure solar sail
> propulsion and the solar cell powered ion engines discussed by Rudolf
> Meyer. For Meyer's proposal you need not only a very thin light weight
> material to act as a sail, but in fact this material would also have to
> consist of solar cells, a much more stringent requirement.
> NASA's Centenial Challenge only mentions a solar sail. But Meyer's
> proposal should probably qualify at least for the transport part if not
> the station-keeping part - especially when you consider a remarkable
> new advance in the fuel efficiency of ion engines:
You need to think more carefully about the aims Robert.
There are key differences, notably ion engines need
reaction mass and the engine has a significant mass too.
For long term station keeping, which is where a sail
would excel, the need for reaction mass defeats the
object.
George
Robert Clark
The Zubrin and Spieth proposal may provide currently feasible solar
sail propulsion within the *entire* solar system and perhaps even
interstellar travel near term:
Ultra-Thin Solar Sails for Interstellar Travel.
Phase I Final Report
December 1999
http://www.niac.usra.edu/files/studies/final_report/333Christensen.pdf
With their proposal you could use solar light alone to power the craft
because of the ultra-light sails. So you would not need the gigawatt to
terawatt lasers proposed for beamed propulsion of solar sails.
The new transparent, nanotube films are comparable to the weight
requirements for the nanometer thin aluminum future sails they mention
and so might make the future sails they discuss *currently* feasible.
On page 16 of their report, they summarize trip times for various sail
materials and suggest a mesh nanotube material as the ultimate in solar
sail material which might allow a solar sail to reach Pluto in 1.7
days, the Oort cloud in 1.2 years and Alpha Centauri in 32 years.
I'm not sure if this type of material is already feasible. But the
recent single atomic layer graphene sheets might do the trick:
Discovery Of Two-Dimensional Fabric Denotes Dawn Of New Materials Era.
Chernogolovka, Russia (SPX) Oct 22, 2004
"Researchers at The University of Manchester and Chernogolovka, Russia
have discovered the world's first single-atom-thick fabric, which
reveals the existence of a new class of materials and may lead to
transistors made from a single molecule. The research is to be
published in Science on 22 October."
http://www.spacedaily.com/news/materials-04zzt.html
Radical fabric is one atom thick.
Friday, 22 October, 2004, 13:18 GMT 14:18 UK
"Professor Andre Geim, who leads the research team, explained that the
material they have discovered could be thought of as millions of
unrolled carbon nanotubes which have been stuck together to make an
infinitely large sheet, an atom thick.
They showed that electrons could travel sub-micron distances without
being scattered, which means fast-switching transistors."
http://news.bbc.co.uk/1/hi/sci/tech/3944651.stm
Electric Field Effect in Atomically Thin Carbon Films.
Science, Vol 306, Issue 5696, 666-669 , 22 October 2004
http://www.sciencemag.org/cgi/content/abstract/306/5696/666
The atomically thin sheets made so far are only a few ten's of microns
across. But it might already be possible to link up the smaller sheets
into arbitrarily large sheets.
Nanolithography at the 100's of nanometers scale which would be
requied to form the mesh is already available.
Bob Clark
Robert Clark
may make possible a suggestion I had for adaptive optics: to keep a
satellite above a large ground-based scope and even following along
with it as it tracks its target. Then shine laser light down from the
satellite to the scope to measure atmospheric distortion:
From: Robert Clark
Date: Sun, Dec 23 2001 5:41 am
Subject: Laser Enhanced Optics.
Email: rgregorycl...@yahoo.com (Robert Clark)
Groups: sci.astro, sci.space.policy, sci.physics
http://groups.google.com/group/sci.physics/msg/b0f2e0f0b8edafc3
From: Robert Clark
Date: Tues, Jan 1 2002 2:50 am
Subject: Re: Laser Enhanced Optics.
Email: rgregorycl...@yahoo.com (Robert Clark)
Groups: sci.astro, sci.space.policy, sci.physics
http://groups.google.com/group/sci.physics/msg/5bde2c745f456e95
Bob Clark
Henry Spencer
Robert Clark <rgrego...@yahoo.com> wrote:
> If solar sails for satellite station-keeping do become available it
>may make possible a suggestion I had for adaptive optics: to keep a
>satellite above a large ground-based scope and even following along
>with it as it tracks its target. Then shine laser light down from the
Trouble is, the "laser guide star" technique -- using a ground-based laser
to induce fluorescence high in the atmosphere -- has already pretty much
solved this problem, without requiring the satellite.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | he...@spsystems.net