Can a solar sail take the shape of a giant parachute, or will it
collapse under photon pressure?
The seemingly obvious answer is yes it
is possible; while photons are not a fluid like air or water,
nevertheless the impinging photons should exert force on the mirrored
structure similar to that which a fluid exerts (ignoring fluid flow
around the parachute, of course!)
HOWEVER, I read a single-line
contention (on www.planetary.org) that this will not work, "because
photon flow is not a fluid." (this is the entire reference). I also
note that there are few schematics of parachute-like solar sails,
within articles published on the web. What I require is a fairly
simple physical explanation (I am an M.E. with a minor in
aerodynamics, but many years ago!) of why this is either possible or
not possible.
Please be prepared to defend your answer (max of 200
word rebuttal, if at all)
Hi,
According to the article on Wikipedia regarding Solar Sails :"
"Parachutes" would have very low mass, but theoretical studies show
that they will collapse from the forces placed by shrouds. Radiation
pressure does not behave like aerodynamic pressure."
"The highest thrust-to-mass designs known were developed by Eric
Drexler, in an MIT master's thesis, He designed a sail using
reflective panels of thin aluminum film (30 to 100 nanometers thick)
supported by a purely tensile structure."
http://en.wikipedia.org/wiki/Solar_sail
I found an article on solar sails on NASA's website:
http://www.nas.nasa.gov/Services/Education/SpaceSettlement/CoEvolutionBook/EDUC.HTML#Solar%20Sailing
quotation from that article : "If I had to draw a sail today, it would
be a hexagon about six miles across, and weighing 20 tons. This is
somewhere between the size of Manhattan and San Francisco, but the
metal of the sail could be wadded up to the size of a Volkswagen bug.
They could be made both much larger and much smaller. The sail itself
would be a spinning (to keep it taut) metal mesh with long, parallel
strips of very thin metal foil glued to it. At regular intervals
across the front, wires would come up, and be bundled to form groups,
with each group having a wire coming from it, with these wires, in
turn, bundled to form groups still farther in front of the sail. After
this bundling and re-bundling has concentrated the load of light
pressure on the sail enough (that's what the wires are for), shroud
lines take the concentrated force to the payload"
My research into this showed that solar sail designs mainly consist of
a huge, extremely lightweight sail and some kind of payload (science
instrument, guidance sensors, place to have coffee... etc).
Without any kind of support, as with the parachute design, when
sunlight pushes on the sail, it will collapse and flow around the
payload. There are two main ideas designers have come up with to work
around this problem: support the sail in 3 dimensions with a
structure, or to spin the sail. Both methods work well to hold the
sail out flat.
Cosmos 1 and the sails NASA are currently testing are not Parachute in
design (far from it actually), because of the "collapsing problem".
http://planetary.org/solarsail/spacecraft_design.html
Source Links
Solar Sail - Wikipedia
http://en.wikipedia.org/wiki/Solar_sail
Tech ISP - Solar Sails Fundamental Physics
http://www.inspacepropulsion.com/tech/sails_physics.html
News On Solar Sails
http://www.solarsails.info/news/
Cosmos 1 The Solar Sail (to be launched very soon)
http://planetary.org/solarsail/update_20041223.html
ISAS Deployed Solar Sail Film in Space
http://www.isas.jaxa.jp/e/snews/2004/0809.shtml
NASA team successfully deploys two solar sail systems
http://www.nasa.gov/centers/marshall/news/news/releases/2004/04-208.html
Links of Interest
Solar Sail Technology Development
http://solarsail.jpl.nasa.gov/introduction/how-sails-work.html
Union pour la Promotion de la Proplulsion Photonique
(Solar Sail Site -- there is an english version)
http://www.u3p.net/
ISAS Deployed Solar Sail Film in Space
http://www.isas.jaxa.jp/e/snews/2004/0809.shtml
Far-out Pathways to Space:Solar Sails
http://www-spof.gsfc.nasa.gov/stargaze/Solsail.htm
Google Searches
Parachute +"Solar Wind"
Parachute +"Solar Sail" +Design
"Eric Drexler"+ MIT +"Solar Sail"
thanks,
webadept-ga CNN.com - Russian sub launches solar sail - July 20, 2001::
The prototype of a spacecraft that will coast on solar winds successfully launched Thursday night from a submarine in shaped version of the solar sail
http://archives.cnn.com/2001/TECH/space/07/20/solar.sail.launched/index.htmlHOME |
Thaks for your input, but except for the Wikipedia article (which I've
seen), you offer no support for the "collapsing problem." I don't
find, anywhere in your source material, except for the Wikipedia
article (sorry I'd remembered it as being from Planetary.org), a
specific reference to this "problem." Perhaps I've missed something?
Then again, my question asked for a physics definition, not simply web
references (no offense meant, it's just that I've spent time on the
web over this, too)
Drexler's work was/is the seminal concept for spinning sails. I am
aware of a great deal of work done with spinning, and also
boom-mounted sails; I'm looking for parachute-type sails. FWIW, I have
found several references to parachute-type sails, but these are not
referenced--or even commented on--by their proposers (typically they
are listed under "other types" of sails in an author's argument for
his pet--non-parachute--project.)
You state, "Cosmos 1 and the sails NASA are currently testing are not Parachute in
design (far from it actually), because of the "collapsing problem"."
Do you have referenced support for this statement, or are you drawing
the same conclusion I did--Wikepedia says is can't be done, and I
can't find anybody doing it, therefore it cannot be done? You
understand the problem here, of course--Wikipedia is not peer
reviewed, nor is this specific statement referenced, footnoted or
backed up with any physics or mathematics.
Thanks for your input, but I'm looking for a physics definition,
describing the issue. Can you help?
Sorry I didn't include this earlier; I would happily pay a tip for the
source material for this statement (from Wikipedia):
"Parachutes" would have very low mass, but theoretical studies show
that they will collapse from the forces placed by shrouds. Radiation
pressure does not behave like aerodynamic pressure."
No problem, I'll see what I can find there.
webadept-ga
Hi again, I think I've found what we are looking for here. It appears
that the "parachute" idea hasn't been around for awhile, and it is
difficult to find Web information on it "proving it false", but I
believe I have enough information for us now to see why we have
stepped away from the design in favor of newer and better designs.
A note at this point is that most of these links point to PDF files,
so you would probably want to right click and "save link as" rather
than just clinking on those, so that your browser doesn't slow down as
much.
In the paper Propulsion for Interstellar Space Exploration,
G. Genta says this
"Canopy sails, in the form of parachute or pillow sails, are very
simple and easily deployed and hence seem to be the most convenient
alternative; however they are not stable and must be kept unfurled by
some structural device. A parachute sail in space tends to collapse
toward a closed position as the only force which keeps it unfurled is
the radial component of the light pressure, which vanishes if the sail
is flat and reaches sufficient values only if the sail is much curved.
As a result, a parachute sail must be very slack, with the twofold
drawback of giving way to internal reflections of the light and to
increase the actual area of the sail with respect to the effective
area seen by the light: in the case of a spherical shape, the actual
area is twice the effective one, doubling of the mass of the
reflecting surface. Even worse is the situation for the pillow sail,
for which a stable configuration does not exist. To keep a parachute
or a pillow sail unfurled it is possible to use a small inflatable
torus at its periphery or, as an alternative, to rotate them in such a
way centrifugal acceleration keeps the surface reasonably flat."
http://www.giancarlogenta.it/cospar.pdf
On the Arizona State University website in the Solar Sail Research Group area:
http://www.eas.asu.edu/~sunsail/reports.html
Dynamic Modeling and Attitude Control of Solar Sail Spacecraft (July
2001, PDF 2.10 MB) by Dr. Bong Wie lays out the foundation of our
groups's research at ASU. At 35 pages, it describes the leading issues
and possibilities in sailcraft design and control. This paper was
prepared for the NASA Solar Sail Technology Working Group.
http://www.eas.asu.edu/~sunsail/files/final_report.pdf
That one is very good, addressing the physics of the problem. He also
goes through each of the main designs for solar sails, and shows how
they work, the drawbacks, solutions and benefits.
The last paragraph of this paper is most relevant to us,
An Introduction to Solar Sail Technology (July 2004, PDF 14.3 KB) by
Nalin Ratnayake provides an informal overview of our field.
http://www.eas.asu.edu/~sunsail/files/Intro_to_Solar.pdf
Basically in the last paragraph of that paper a few things started
clicking for me on why I wasn't finding the information and studies we
are seeking on this, and that is the design of a "parachute" type sail
rests on the misconception that "solar wind' has anything to do with
wind. What we are dealing with here is light.
quote:
"I would like to address a common misconception among those who have
heard of this technology: that the force of the solar wind propels
solar sail craft. This is not strictly true. ?Wind? implies that there
are physical objects flying through some medium, which the sail
captures. The kinetic
energy is transferred from the objects to the sail, propelling it as
well as anything to which it is attached. In the nautical sense, these
flying objects are packets of moving air. And while it is true that
high- velocity particles
are indeed flying through space (the solar wind), these particles are
too few and far between to generate significant pressure for
spacecraft propulsion. The propulsive medium is actually light itself,
NOT the solar wind."
The "parachute" design has not been seriously considered as feasible
since the 1960's, and this is why I'm not finding information on the
design.
As a side note, I did find this database out there though, and thought
that you would probably enjoy at least knowing about it.
Langley Technical Reports Server
http://techreports.larc.nasa.gov/ltrs/
I believe the paper "Dynamic Modeling and Attitude Control of Solar
Sail Spacecraft" which I linked to above, covers everything you were
looking to know about this. The others cover similar ground, or
generalize enough to keep us on track. If you don't feel that these
covered the question fully, just let me know.
thanks,
webadept-ga
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