Lightsail

OK, here is my attempt to revive things.
I recently did a simple calculation concerning lightsails and how fast
they could go. I used a graphene film coated with 100 nm of Au as my
sail. I was surprised that no matter how big you make the lightsail,
it cannot exceed solar escape speed using only solar photons. Of
course, the closer you start to the sun the faster you can go but
still cannot exceed suns escape velocity.
I do not have that calc in front of me now but does this seem right to
others? My conclusion is that your lightsail has to be initially
accelerated to escape speed before deploying

"Pat Flannery" skrev i meddelelsen
dakotatelephone...

On 5/8/2010 8:56 AM, Frogwatch wrote:

OK, here is my attempt to revive things.
I recently did a simple calculation concerning lightsails and how fast
they could go. I used a graphene film coated with 100 nm of Au as my
sail. I was surprised that no matter how big you make the lightsail,
it cannot exceed solar escape speed using only solar photons. Of
course, the closer you start to the sun the faster you can go but
still cannot exceed suns escape velocity.
I do not have that calc in front of me now but does this seem right to
others? My conclusion is that your lightsail has to be initially
accelerated to escape speed before deploying

Was this using a straight-out trajectory from the Sun, or an orbit that
was getting larger and larger in radius as the lightsail was picking up
speed?
One would think that by setting the lightsail at a 45 degree angle to the
incoming sunlight, its orbit would eventually get large enough to leave
the solar system, no matter how low the acceleration from the incoming
photons was at a great distance from the Sun.

Light pressure goes by inverse square same as gravity. If you unfurl
your solar sail and point it straight towards the Sun it will behave in the
same way as if the Sun's gravity had suddenly lessened. If you begin in a
circular orbit your solar sail will enter an elliptical one - unless the
sail is so light that the light pressure on it actually exceeds gravity;
this is the same no matter the distance from the Sun that you start in.
Certainly the orbital angular momentum of the light sail does not change if
you point the light sail towards the Sun.
What you want to do with your light sail if you want to achieve escape
velocity is precisely to point it at a 45 degree angle, reflecting sunlight
back the same way as the rocket plume if that were your method of
propulsion. Then the light pressure will have a component pointing
outwards, and that is fairly useless except to fine-tune orbital
calculations. There will also be a component that serves to increase your
orbital angular momentum. It's counter-intuitive, but it's how you want to
use your lightsail if you want to achieve escape velocity from the Sun.
Again, unless your sail is so light that the light pressure is stronger than
gravity.
And of course, analogously with tacking against the wind but not a
similar phenomenon, you can tilt your solar sail the other way and move
inwards.

Jon Lennart Beck.

On 5/8/2010 8:56 AM, Frogwatch wrote:
OK, here is my attempt to revive things.
I recently did a simple calculation concerning lightsails and how fast
they could go. I used a graphene film coated with 100 nm of Au as my
sail. I was surprised that no matter how big you make the lightsail,
it cannot exceed solar escape speed using only solar photons. Of
course, the closer you start to the sun the faster you can go but
still cannot exceed suns escape velocity.
I do not have that calc in front of me now but does this seem right to
others? My conclusion is that your lightsail has to be initially
accelerated to escape speed before deploying

Was this using a straight-out trajectory from the Sun, or an orbit that
was getting larger and larger in radius as the lightsail was picking up
speed?
One would think that by setting the lightsail at a 45 degree angle to
the incoming sunlight, its orbit would eventually get large enough to
leave the solar system, no matter how low the acceleration from the
incoming photons was at a great distance from the Sun.

Pat

Frogwatch wrote:
OK, here is my attempt to revive things.
I recently did a simple calculation concerning lightsails and how fast
they could go. I used a graphene film coated with 100 nm of Au as my
sail. I was surprised that no matter how big you make the lightsail,
it cannot exceed solar escape speed using only solar photons. Of
course, the closer you start to the sun the faster you can go but
still cannot exceed suns escape velocity.
I do not have that calc in front of me now but does this seem right to
others? My conclusion is that your lightsail has to be initially
accelerated to escape speed before deploying


If you neglect relativistic effects and dimming of the star over time then,
no matter how small the light pressure is, as long as it is there you can
always escape from any star orbit no matter how massive the star is. Of
course, it might take a long time. To see this, imagine the following flight
plan: At the perigee of the orbit, you put your lightsail at a 45 degree
angle for one minute, this gives you an epsilon increase in speed. Put the
light sail aligned with the light flux and wait one orbital period, you get
no acceleration during that time but you will come back to the same point,
and have the same light pressure, so you once again can put your lightsail
at a 45 degree for on minute and gain exactly the same epsilon increase
in orbital speed. Repeat N times with N large enough to have N*epsilon
to be equal to escape velocity. On your way out you can keep some acceleration
from the light sail to have a little extra push.

That is a ridiculous flight plan, it would take a lot of time to escape that
way. But it does show that you can reach escape velocity.


Alain Fournier

In sci.space.policy message 4fa7d3ad-a15c-48ff-803d-188bfdc24373@h9g200
0yqm.googlegroups.com, Sat, 8 May 2010 09:56:29, Frogwatch
posted:

OK, here is my attempt to revive things.
I recently did a simple calculation concerning lightsails and how fast
they could go. I used a graphene film coated with 100 nm of Au as my
sail. I was surprised that no matter how big you make the lightsail,
it cannot exceed solar escape speed using only solar photons. Of
course, the closer you start to the sun the faster you can go but
still cannot exceed suns escape velocity.
I do not have that calc in front of me now but does this seem right to
others? My conclusion is that your lightsail has to be initially
accelerated to escape speed before deploying

No.

Reflected light pressure equals solar gravity, at any distance, for a
film of about 1.8 gsm, ~ 2 um of paper.

100 nm of gold is rather thick - hundreds of atoms - and gold is dense.
Aluminium should be better.

However, one cannot usefully comment on a calculation for which full
details are not available.

See in URL:http://www.merlyn.demon.co.uk/astron-2.htm.

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