From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech,sci.space.policy,sci.space.science
Subject: Re: Mission to Mars in 2015?
Date: Thu, 5 Sep 1996 14:14:29 GMT

In article <50ird4$fnn@lace.colorado.edu> fcrary@rintintin.Colorado.EDU (Frank Crary) writes:
>...Let's see... Cross section area goes
>as sin(th) where th is the angle between the Sun and the plane
>of the sail. The force from a non-normal reflection goes as
>2*sin(th) and the fraction of the force in the direction of
>orbital motion (for a circular or nearly circular orbit)
>goes as cos(th). So the optimal pointing would be that which
>maximizes 2*sin(th)^2*cos(th). That's at th = 45 deg...

Not quite right.  There are *three* trig functions involved:  one for
effective area (since the sail is tilted and doesn't intercept as much
sunlight that way), one for momentum transfer (since you get to use only
the perpendicular component of the photon's momentum), and one for angled
thrust (since the tangential component of thrust is what's being maximized).
The optimum angle turns out to be 35.3deg (measured between sail normal
and sun axis; that's 54.7deg measured to the sail plane). 

Actually, there are further complications because sails are not perfectly
reflective, but that's a real can of worms, and it doesn't affect the
optimum angle much.
-- 
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |   henry@zoo.toronto.edu

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Solar sair for Mir?
Date: Mon, 1 Jun 1998 01:38:54 GMT

In article <35729056.1804582943@enews.newsguy.com>,
 <Starman@microsoft.borg> wrote:
>Would it be possible to attach a solar sail to the Mir station after it is
>abandoned to boot it into a higher orbit.

No.  Unless you get into seriously exotic materials, solar sails are
useless below about 1000km altitude, because air drag dominates sail
thrust up to about that point.  There are other problems which might
move the boundary considerably higher, depending on detailed design.
--
Being the last man on the Moon is a |  Henry Spencer   henry@spsystems.net
very dubious honor. -- Gene Cernan  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Solar sair for Mir?
Date: Tue, 2 Jun 1998 13:17:46 GMT

In article <m1u364hdm3.fsf@flight.uchicago.edu>,
Alexey Goldin  <goldin@flight.uchicago.edu> wrote:
>> No.  Unless you get into seriously exotic materials, solar sails are
>> useless below about 1000km altitude, because air drag dominates sail
>> thrust up to about that point...
>
>Henry, how exotic materials can reduce drag? I wish I knew what you
>are referring to...

Perforated sails.  If the performations are considerably smaller than the
relevant wavelengths of light, the sail can remain fully reflective, but
air molecules can still go straight through.  (This wouldn't work in dense
air, where a boundary layer would form and effectively fill in the holes,
but in a region of molecular flow -- like LEO -- it's viable.)
--
Being the last man on the Moon is a |  Henry Spencer   henry@spsystems.net
very dubious honor. -- Gene Cernan  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Solar sair for Mir?
Date: Tue, 2 Jun 1998 13:27:43 GMT

In article <357948a3.1982896794@enews.newsguy.com>,
 <Starman@microsoft.borg> wrote:
>Bummer, I doubt the Mir solar cells collect enough energy to run an ion
>engine either.

Mir's solar arrays are quite large and there's plenty of power available
if you mostly power down Mir itself.  However, there is a drag issue too.

>How much energy was generated with the tether experiment
>on the recent shuttle flight before the cable failed?

Doesn't matter -- you can't raise an orbit using energy obtained by
lowering the orbit!  (That's where the power from tethers comes from.)

>Just out of curiosity, is there a breakeven point for solar panel drag vs
>ion engine thrust for LEO?

Yes, but it varies a lot with the details of the system and how it is
used, so you won't see a single number quoted.  It *is* a serious issue
for using solar-ion systems in LEO.

As an example of the sort of variation I'm talking about, you can nearly
eliminate drag, at the price of reduced power and possibly constrained
thrust directions, by "feathering" the solar arrays, keeping them edge-on
to the orbital velocity.  (I've seen a short paper, by Geoff Landis I
think, seriously examining the idea that the space station would be better
off to keep its solar arrays feathered at all times.  You need bigger
arrays, because you can rarely get the best Sun angle that way, but you
also reduce fuel use for reboost, and you get better microgravity.)
--
Being the last man on the Moon is a |  Henry Spencer   henry@spsystems.net
very dubious honor. -- Gene Cernan  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Solar Thistledown?
Date: Sat, 25 Mar 2000 15:53:42 GMT

In article <slrn8dj6j8.k9b.roystgnrNO@mycroft.jones.rice.edu>,
Roy Stogner <roystgnrNO@SPAMiname.com> wrote:
>>There have been proposals for perforated sails, with perforations
>>considerably smaller than the average wavelength of sunlight.  They would
>>have the same thrust per unit area as a solid sail
>
>Would it?  I read QED once, but don't remember much of the fraction I
>understood.  I suppose this is the same principle that lets a metal
>screen on your microwave window keep the microwaves inside...

Exactly.  There are some problems that need careful attention -- notably,
you may need to add other microstructures for efficient radiation of heat
to keep the thing cool -- but the basic idea definitely does work.

>...but it
>seems to me that such screens tend to use much more metal (i.e. have
>more widely separated perforations) than you would expect if the only
>design constraint is a maximum perforation size.

The microwave-oven screens are also designed to be mechanically durable,
nigh unto indestructible, since they are an important safety feature of a
consumer product which may be used for many years in an unpredictable
environment.

>>, but would be substantially lighter,
>
>That depends: is the binding contraint on solar sail weight the
>difficulty of manufacturing a sufficiently thin sail, or the
>difficulty of manufacturing a sufficiently thin sail that won't tear
>under the tension given it by light pressure (and whatever rotation is
>necessary for it to keep it's shape)?

Depends on how ambitious you are being.  Today's solar-sail technology is
dominated by the stresses involved in unfolding the sail from a small
package.  Really lightweight sails have to be built in space.  For that
case, the dominant constraint is simply that you have to make the stuff
thick enough to be reflective!  Even aluminum becomes transparent if you
make it thin enough.  The perforated sail pushes that limit somewhat
farther.

Structural design needs to be done with care on such sails; they can't be
just a simple flat sheet with a few cables tied to it.  Generally you'd
need a hierarchy of structural members in the sail itself, starting with a
fairly dense grid of very fine wires.  That separates structural load-
carrying from reflection, eliminating any major strength constraints on
the sail material itself.

>>and would also see less air drag ...
>
>I hadn't thought about the air drag on a solar sail before.  I suppose
>that would put a lower limit on the orbital altitude they could be
>launched from?

Yes indeed.  With solid sails, you can almost* forget about sail
operations below 1000km or so... which is an awkward limit, because that's
also about the upper limit of LEO operations -- the Van Allen belts start
getting serious at around that altitude.

* There is one exception -- in a near-polar orbit you can set up a
situation where the sail is almost edge-on to the orbital motion and yet
is yielding useful thrust.  The conditions needed are constraining enough
that this may not be very useful for general space transportation.

>What about non-photons in the solar wind?  Making a perforation small
>enough to block sunlight is one thing; making it small enough to block
>alpha and beta radiation would be something else.  I don't remember
>the composition of solar wind; how much momentum is normally carried
>by non-photons?

Essentially zero; it's several orders of magnitude less than the photon
momentum.  Moreover, it is much less manageable:  it's bursty and
irregular, and there is no good way (for a solid structure, anyway) to
reflect it.  (Absorbing incoming radiation -- a "black" sail -- does not
work nearly as well as reflecting it.)

Note that the solar wind, as usually defined, *is* the particles, not the
sunlight.  The appealing analogy notwithstanding, solar sails do not use
the solar wind.

>>There are some tricky practical problems, mostly notably how you
>>*make* them, but the idea has potential.
>
>Making them might be easy: just use UV or better light, or electron
>beams to burn the holes through the sail.  I don't suppose lithography
>would work on such a huge target...

Exactly.  The big problem is sorting out a production process that will
work well enough on the huge scale involved.  You really want to do it on
the material during production, not on the finished sail, but that doesn't
avoid the requirement for very high processing speed, to make square
kilometers of material in a reasonable amount of time with a reasonable
investment in machinery.  In particular, when you need *trillions* of
holes per square meter, punching them one at a time just isn't attractive.
--
Computer disaster in February?  Oh, you |  Henry Spencer   henry@spsystems.net
must mean the release of Windows 2000.  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Solar Thistledown?
Date: Sun, 26 Mar 2000 19:32:48 GMT

In article <8bj3n3$bja$4@newshost.accu.uu.nl>,
Martijn Faassen <m.faassen@vet.uu.nl> wrote:
>> [perforated sails] would
>> have the same thrust per unit area as a solid sail, but would be
>> substantially lighter, and would also see less air drag...
>
>Air drag in space? Do you mean the particles of the interplanetary medium?

Not in space in general, but in LEO, where there is enough left of Earth's
atmosphere to make big problems for sails.

>I imagine that drag could get pretty big if your sail is big enough.

The interplanetary medium is too thin to be much of an issue; solar light
pressure dominates it thoroughly.
--
Computer disaster in February?  Oh, you |  Henry Spencer   henry@spsystems.net
must mean the release of Windows 2000.  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Solar Thistledown?
Date: Mon, 27 Mar 2000 16:26:50 GMT

In article <slrn8dttn5.s6.roystgnrNO@mycroft.jones.rice.edu>,
Roy Stogner <roystgnrNO@SPAMiname.com> wrote:
>>Structural design needs to be done with care on such sails; they can't be
>>just a simple flat sheet with a few cables tied to it.  Generally you'd
>>need a hierarchy of structural members in the sail itself, starting with a
>>fairly dense grid of very fine wires...
>
>Cool - fractal sails.  Where would you attach cables?  One cable in
>the center to prevent the sail from bowing forward and reduce the
>required rotation rate, then n cables at the corners of an n-gon?

I would guess that you'd end up with shroud attach points in a
more-or-less uniform grid some distance above the surface, with thinner
cables fanning out from there to a finer grid closer in, and so forth
until the finest wires meet the ones reinforcing the surface (possibly
just take right-angle turns and *become* the ones reinforcing the surface).
But I haven't seen, or don't remember, the detailed proposals.

As you've inferred, the sail spins for rigidity.

>That sounds ok for getting constant acceleration away from the sun,
>but could be difficult to control while spinning.  It seems like there
>would be a limit to how fast you could reel and unreel cables.

It is potentially a problem, but I think the spin is supposed to be fairly
slow, and it might not be that big a deal.

>Could a hierarchical design still be physically modeled as a
>homogenous sheet, or (I fear) would the load bearing need to be done
>heterogenously on the scale of the entire scale, too?

Good question; not sure.  I'd guess the latter.

>If I'm asking too many questions (or too many questions with available
>answers I've missed), just a URL or literature reference would be
>appreciated.

I don't know how much has been published on this.  Ah, there's a brief
discussion of it (as the "lattice sail") in Jerome Wright's "Space Sailing".
Unfortunately, his reference for more detail is to Eric Drexler's MIT thesis
dissertation, which may not be readily available...

>>>...the composition of solar wind; how much momentum is normally carried
>>>by non-photons?
>>Essentially zero; it's several orders of magnitude less than the photon
>>momentum.  Moreover, it is much less manageable:  it's bursty and
>>irregular, and there is no good way (for a solid structure, anyway) to
>>reflect it...
>
>What about magnetic effects?  I have only sketchy knowledge there...

Interacting with the solar wind magnetically is a very promising idea; it
gets around the big headache of solar sails, the need to *fill* a huge
interaction region with something solid.  That means you can maybe make
the region big enough that the much smaller momentum flux is still useful.

There are still a lot of unknowns, though.  Aside from the minor :-)
engineering problems of *building* such a device, fundamental problems
remain.  The solar wind *is* quite irregular.  Its behavior changes in
fundamental ways as it expands and thins out -- plasmas are much more
complicated than gases -- and that may be a problem.  How the solar wind
would interact with a *small* magnetosphere is not well understood:  the
well-studied natural examples are all large, and the scaling laws are
uncertain.  And the simplest sorts of magsails are basically drag devices,
like the black solar sail, and turning them into lifting devices -- much
more versatile and useful -- adds a new level of complication and
uncertainty.

>What would the payload fraction of a solar sail be?  It seems like the
>only advantage of a magnetic sail is the fact that one doesn't require
>bulky reflective material... but unless a solar sail is 99.9% sail,
>ditching the reflective sail in order to pick up 1/1000 as much thrust
>from the solar wind wouldn't be an acceleration win.

Depends on the design, but usually the sail is a significant fraction of
the total mass of the system, because it's so hard to fill that huge area.
As noted above, not having to fill the area opens up the possibility of
using much larger areas... in addition to potentially making the thing
easier to build.

>>...when you need *trillions* of
>>holes per square meter, punching them one at a time just isn't attractive.
>
>Diffraction grating?  I haven't studied optics; would it be possible
>to break a laser beam into a large number of separate beams without
>losing too much focus?

Yes, that can be done, but there are some awkward problems like needing a
very-short-wavelength laser (to punch holes substantially smaller than the
wavelength of visible light), and where all the vaporized material goes.
It doesn't sound easy to me.
--
Computer disaster in February?  Oh, you |  Henry Spencer   henry@spsystems.net
must mean the release of Windows 2000.  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Mars Solar Sail Direct
Date: Wed, 12 Apr 2000 19:46:37 GMT

In article <38F4A6BD.4B8DF6F0@peterlynnkites.com>,
Robert Lynn  <robert@peterlynnkites.com> wrote:
>I'd be very impressed if a solar sail could get 1 km/s per day delta V...
>However I'd believe 1 m/s per day for a good solar sail.

The reality is in between.  JPL's fairly conservative sail design for the
1986 Halley rendezvous mission got about 90m/s/day (in open space with
continuous sunlight).  Aggressive use of current technology might double
or triple that.  Assembly in high orbit (so the sail doesn't have to
unfold) and considerably improved materials might give 500-700m/s/day.
Several km/s/day is conceivable with advanced space-assembled designs, but
that's well beyond what we can build now.
--
"Be careful not to step                 |  Henry Spencer   henry@spsystems.net
in the Microsoft."  -- John Denker      |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Mars Solar Sail Direct
Date: Thu, 13 Apr 2000 20:18:33 GMT

In article <8d3ipt$bgp$1@inf6serv.rug.ac.be>,
Filip De Vos <fidevos@eduserv1.rug.ac.be> wrote:
>: ...about 90m/s/day (in open space with
>: continuous sunlight).  Aggressive use of current technology might double
>: or triple that.  Assembly in high orbit (so the sail doesn't have to
>                               ^^^^^^^^^^
>Do you mean a 'high' LEO (say 1000 km circular) or 'high' orbit, like GEO
>or beyond? The problem seems to be (if human precense is required, which I
>think it is) the van Allen radiation belts.

Probably seriously high, GSO or higher.  Barring highly advanced sail
materials (perforated microstructures), 1000km or so is the absolute
minimum because of air drag, and gravity-gradient effects would make at
least some sail designs difficult to control much below GSO.  There just
isn't any happy altitude between roughly 1000km and GSO, in fact, because
of the belts.  Indeed, somewhat above GSO would be better.

You might perhaps be able to assemble advanced sails in LEO -- keeping
them edge-on to the air drag -- and then boost them to high altitude using
some higher-thrust propulsion system.  The details could be tricky.
--
"Be careful not to step                 |  Henry Spencer   henry@spsystems.net
in the Microsoft."  -- John Denker      |      (aka henry@zoo.toronto.edu)

From: jtkare@ibm.net (Jordin Kare)
Newsgroups: sci.space.tech
Subject: Re: Solar Sailing Below 1000 km: Radarsat
Date: Sat, 20 Jun 1998 23:28:56 -0700

In article <6mer8s$ofr$1@nnrp1.crl.com>, beanstalkr@aol.com (Beanstalkr) wrote:

> >The trick is to align the solar sail so that it is almost edge-on to the
> velocity vector, reducing atmospheric drag to almost zero, while still
> maintaining a significant thrust from solar radiation pressure.>
> ...one
> should be careful of assuming "almost zero" drag when the sail has low
> angle of attack. I know it's hard to calc the drag of a few air
> molecules but the "skin effect" will be dragging the sail no matter what
> the attack angle.
>          At 17.000 mph orbital velocity, there could be quite a few high-speed
> molecules scrubbing against the sail's boundary layer of captured air
> molecules.  I wish this effect could be quantified here!

You're in the "collisionless" or "free molecular flow" regime at these
altitudes; gas molecules essentially don't collide with each other over
distances large compared to your satellite size.  To first order
(definitely not worrying about factors of 2), the drag on your edge-on
object is just

rho*A*v*V

where rho is the gas density
   A is the surface area (both sides!)
   v is the mean molecular thermal velocity = sqrt(kT/m) ~ sound speed at
temp. T
   V is your orbital velocity.

rho*A*v is the mass of gas molecules bouncing off the side of your object
per unit time.  Depending on the local effective temperature, a reasonable
estimate is 1/10 of the drag you'd get "face on".

>        A good topic!  I would like to point out that if there are enough air
> molecules at 1000 km to produce drag, then there are enough to produce ...
> lift!

It's very hard to get much lift in a free molecular flow regime, alas. I
suspect the best you can do is L/D ~ 1.

Jordin Kare
Kare Technical Consulting