On Feb 28, 2011, at 1:17 AM, Frank wrote:

> I don't think, that a pure HelioGyro as in Your paper would work as a
> GEO-SSPS.
> As You know, JAXA had problems to steer the Ikaros solar sail
> spacecraft
> because of the high momentum of the rotating foils, so they looked
> also
> into decreasing the rotation speed.
> I think, that because the blades of a HelioGyro SSPS have to point
> allways towards Sun,
> the Sun pressure would push them away of the Sun, if the rotating
> speed is
> not pretty high.

I've added the IR system to my design using fiber lasers. It turns
out that, with some reasonable assumptions, the fiber lasers provide
enough stiffness to get rid of the rotation. This simplifies the
system quite a bit.

I've submitted the paper to ISDC with this analysis and hope to
present it there. In any case, it will show up on space.alglobus.net
in the relatively near future.

>
> Also the Craft, which orbits around Earth but has to point towards
> Sun has
> to change it's attitude constantly.

It does not have to change it's attitude much. Just point towards the
sun and make sure you don't have major gravity gradient issues --
which is not a big deal at GEO anyway. You just need to rotate about
1 degree a day to maintain sun pointing (360 degrees/year).

If you want to point at Earth you have to change attitude 360 degrees
in a day. Of course, if the sail is sun pointing the power beam needs
to rotate to keep on target.

> So a pretty good attitude control System is so important, best a
> fuelless one,
> because a SSPS would stay years in orbit and need a lot of fuel
> delivered to
> GEO over the years.

It's not clear it needs much fuel. I've done simulations of 100 g/m^2
PowerSats using STK that suggest that you only need to correct for
inclination creep and a little bit of orbital decay. The
eccentricity goes to 0.2 but then back to zero in the course of a
year. There may be clever ways to use the sail to stay more-or-less
where needed, but I don't know what they are (yet). The sat does
wobble around initial location, but the beam needs a lot of pointing
capability anyway so I think one can just live with it and compensate
with beam pointing.

This will, hopefully, be the topic in a third paper on the system.

In a late 2004 talk, Rutan made the following predictions:

1) Within 5 years 3,000 tourists will have been to space -- this did
not happen.
2) Within 15 years sub-orbital tourism will be affordable, and 50,000
people will have flown.
3) Within 15 years the first, expensive orbital tourist flights will
have happened.
4) Within 25 years orbital tourism will be affordable.

There have been a lot of wild claims in this business, but Rutan may
know what he's talking about. His privately financed SpaceShip One was
piloted into space three times in 2004, and Rutan has a contract with
Virgin Galactic to build a true sub-orbital tourist vehicle.

Al Globus
http://alglobus.net

Views expressed in this email are only my opinions and are not the
position of any organization I'm familiar with.

On 01.03.2011 03:52, Al Globus wrote:
>
> On Feb 28, 2011, at 1:17 AM, Frank wrote:
>
> > Also the Craft, which orbits around Earth but has to point towards
> > Sun has
> > to change it's attitude constantly.
>
> It does not have to change it's attitude much. Just point towards the
> sun and make sure you don't have major gravity gradient issues --
> which is not a big deal at GEO anyway. You just need to rotate about
> 1 degree a day to maintain sun pointing (360 degrees/year).
>
In addition to the attitude change there are also Sun pressure forces
which are pushing the craft away of the Sun.
How can the craft stay in GEO with a large solar cell area and getting
regular SRP-forces of up to say 4.5 N per square km
of solar cell area which are also changing direction with each grade of
attitude change?
Couldn't this push the satellite evtl. out of orbit? How is station
keeping done?
If you need thrusters, you'll need fuel too.

>
> If you want to point at Earth you have to change attitude 360 degrees
> in a day. Of course, if the sail is sun pointing the power beam needs
> to rotate to keep on target.
>

My way to solve this problem was to use the mothership as the
collector unit which points towards Sun while a beaming
daughter unit (microwave or laser) points toward Earth.
Both units would be connected to each other through a
CNT tow, which serves as a power line.

I am not happy with this solution, since the pointing/attitude control
system of the beamer is not fuelless, but for a demonstrator it might
be o. k.
>
> > So a pretty good attitude control System is so important, best a
> > fuelless one,
> > because a SSPS would stay years in orbit and need a lot of fuel
> > delivered to
> > GEO over the years.
>
> It's not clear it needs much fuel. I've done simulations of 100 g/m^2
> PowerSats using STK that suggest that you only need to correct for
> inclination creep and a little bit of orbital decay.
>
Did those simulations bear in mind SRP forces on large
solar cell arrays of lightweight Power Sats too or are they based on
standard satellite data?

> The
> eccentricity goes to 0.2 but then back to zero in the course of a
> year. There may be clever ways to use the sail to stay more-or-less
> where needed, but I don't know what they are (yet). The sat does
> wobble around initial location, but the beam needs a lot of pointing
> capability anyway so I think one can just live with it and compensate
> with beam pointing.
>

How can you point the beam? Do you have to move optics?
In this case the center of mass of the spacecraft which is subject
to both - SRP-forces and gravitational forces - will change and this
would move the spacecraft attitude.

Best wishes

Frank

On Mar 1, 2011, at 7:00 AM, Frank wrote:

> On 01.03.2011 03:52, Al Globus wrote:
> >
> ow can the craft stay in GEO with a large solar cell area and getting
> regular SRP-forces of up to say 4.5 N per square km
> of solar cell area which are also changing direction with each grade
> of
> attitude change?
> Couldn't this push the satellite evtl. out of orbit? How is station
> keeping done?
>
> > It's not clear it needs much fuel. I've done simulations of 100 g/
> m^2
> > PowerSats using STK that suggest that you only need to correct for
> > inclination creep and a little bit of orbital decay.
> >
> Did those simulations bear in mind SRP forces on large
> solar cell arrays of lightweight Power Sats too or are they based on
> standard satellite data?
>
I modeled the system as a 1 m^2 satellite weighing 100g. The force of
sunlight is a function of area and the mass is (mostly) a function of
area, so this is a good model for any sized spacecraft that weighs
100g/m^2.

>
> > The
> > eccentricity goes to 0.2 but then back to zero in the course of a
> > year. There may be clever ways to use the sail to stay more-or-less
> > where needed, but I don't know what they are (yet). The sat does
> > wobble around initial location, but the beam needs a lot of pointing
> > capability anyway so I think one can just live with it and
> compensate
> > with beam pointing.
> >
> How can you point the beam?
>
The detailed design is scheduled for Part III of the paper series, but
basically it's a system of mirrors. We have a design that could work,
but there may be better ones.

> Do you have to move optics?
>
The current design has two mirrors that move to point the beam 360
degrees over the course of a day. A third mirror just moves a little
to turn the beam 90 degrees.

However, we haven't worked on this enough to be confident there isn't
a significantly better way. The basic problem is that the beams come
into the center in a 360 degree ring (which can have depth if
desired). This ring is in the plane of the sail, so it faces the sun
but the beams are normal to the sunlight vector. The beams must be
combined, turned 90 degrees to clear the sail when edge on to the
receiver, and directed to Earth. Any good ideas are welcome
(actually, bad ideas sometimes lead to good ones :-).

> In this case the center of mass of the spacecraft which is subject
> to both - SRP-forces and gravitational forces - will change and this
> would move the spacecraft attitude.
>
The Ikaros had an experiment to control the sail by LCDs that could
change their optical properties when stimulated electronically. I'm
hoping there will be a paper on the results, but I haven't seen it.
It's a nice way to go because there are no moving parts. In our case,
only very small control moments are needed so there is reason to
believe it will work. However, this another part that we have some
general ideas, but doing the numbers will have to wait for Part III
(at least).

>

The dinosaurs were destroyed by an asteroid because they weren't space-
faring. It's almost as if Gaia then thought "Well, dinosaurs worked
pretty well, but space-faring is necessary. Maybe I'll try mammals
this time." Humanity is now developing systems to detect and deflect
asteroids, and could build orbital space colonies to spread beyond
Earth to ensure life would survive a planetary catastrophe.

Al Globus
http://alglobus.net

Views expressed in this email are only my opinions and are not the
position of any organization I'm familiar with.