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could robert forwards theories come to fruition


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#1 meridian

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Posted 22 March 2003 - 10:54 PM

recently i was introduced, to the sadly departed robert forward's theories on space flight, i was just wondering if anybody that that the theories, had any practical applications for the future.  his solar sail idea although very good does not fully establish itself for me, so can any body enlighten me on the practicalities.

#2 Christopher

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Posted 23 March 2003 - 12:52 PM

meridian, on Mar 22 2003, 02:45 PM, said:

recently i was introduced, to the sadly departed robert forward's theories on space flight, i was just wondering if anybody that that the theories, had any practical applications for the future.  his solar sail idea although very good does not fully establish itself for me, so can any body enlighten me on the practicalities.
Of all Forward's radical concepts, the only one you have a problem with is solar sails??  That's the least implausible, easiest-to-achieve one of all!  There's already serious work underway on developing solar-sail technology.  Here's the JPL site on the subject, for instance.  And here's the Planetary Society's page on them.

And Forward didn't even originate this idea.  It's been theorized since the 1920s.  Both Arthur C. Clarke and Poul Anderson wrote seminal stories on the theme in the early '60s.
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#3 Ilphi

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Posted 24 March 2003 - 01:33 AM

What material is the current thinking to make the sail out of, incidently? I know optimumly we need something that can reflect over 99% of light but still be lighter than anything we have today...
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#4 tennyson

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Posted 24 March 2003 - 02:23 AM

Actually we've had material that could work for solar sails since the 1970s, with the main contender being good old fashioned aluminized mylar although various combinations of more exotic extruded polymers and such have been suggested. But as far as I know the current Planetary Society project as well as the other studies and the Russian light relection experiment which used similat technologies uses simple aluminized mylar. The main problem has not been light reflection but designing a "package" that can unfold after being launched into space via conventional rocket without significant chance of tearing.
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#5 meridian

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Posted 24 March 2003 - 01:45 PM

but how are they foing to be able store enough power so that thy can tavel at any decent rate when they exit the solar sysem?

#6 Christopher

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Posted 24 March 2003 - 04:10 PM

meridian, on Mar 24 2003, 05:36 AM, said:

but how are they foing to be able store enough power so that thy can tavel at any decent rate when they exit the solar sysem?
Umm, first of all, there's essentially no friction in space to slow anything down, so if a craft gets up to a decent speed in the early stages it can just coast the rest of the way, forever if necessary.  There's no need to "store" power, because no power is needed to keep moving.

But solar sails, craft which rely on sunlight, are for in-system travel.  They couldn't get up sufficient velocity to make an interstellar journey in less than a few thousand years.  You need a more powerful light source to accelerate a lightsail to a practical interstellar velocity.  If you're familiar with Forward's lightsail proposals, then you should already be aware that his plans involved the use of extremely powerful lasers or microwave beams.  Here's a summary:

http://ffden-2.phys....lightsails.html

Also, see if you can track down a book (probably out of print) called Project Solar Sail, edited by Arthur C. Clarke.  It should tell you all you need to know about lightsails and more.
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#7 tennyson

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Posted 25 March 2003 - 05:45 AM

I noticed some interesting supplementary information relating to solar sails and magsails in the January/February 2003 Ad Astra and The Planetary Report. A litttle later this year The Planetary Society will launch thier proof of concept solar sail Cosmos 1 onboard a Russian rocket. It is a 30 meter diameter aluminized mylar solar sail that will test propulsion concepts for even more advanced solar sails.
here is a quote from the Ad Astra article"Today we can build sails that are tens of meters square, made of plastic thinner than saran wrap, and sprayed with aluminum to make the material reflective. Someday we're going to need a light sail that's the size of Texas, and made of a film or aluminium maybe a hundred atoms thick."
Also several articles in both magazines mention new carbon fiber mesh materials that are much stronger than mylar and have already been pushed by lasers in experiments.
Also here is a quote about the Magsail concept, "A new propulsion concept called Mini-Magnetospheric Plasma Proplusion (M2P2) comes from Robert Winglee of the University of Washington. M2P2 uses a magnetic bubble of plasma particles in a field produced by a solenoid to interact with the charged particles in the solar wind and absorb some of their momentum. This magsail could allow a vehicle to ride the solar wind up to 100km/s. Such a system is intended for travel within the solar system but it could also provide an intial first stage boost for a ship with a secondary fusion or other propulsion system. Note that a magsail would also help in decelerating a ship as it approaches its desination star."
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#8 Woodmansee

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Posted 26 March 2003 - 03:19 AM

Solar sails don't store anything. They get their propulsion by reflecting light that is directed at them, just like the sails on a boat get their propulsion by diverting the air that is coming at them.

One big issue with making a good solar sail, really a light sail, for interplanetary travel (as opposed to solar system travel) is making the density of the sail small enough. Aluminized Mylar is way too heavy.

Mylar is fine for inner solar system travel, but if you want to get to another star in a reasonable amount of time (i.e. a century or so) then you need something much less dense. Then you also need huge arrays of multi-gigawatt lasers to push shine at the light sail to get you there. Light from the sun just doesn't have the concentrated energy you need as you get far away.

However, for the inner solar system (say Mars inward), solar sails would be very useful with just sunlight, and extremely light materials are not needed.

Paul


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