Re: Solar powered lasers in space

On 18 Sep, 05:14, Willie.Moo...@xxxxxxxxx wrote:

True. But the energy budget of the Earth is;

50,000 TW - direct solar
320 TW - currents, winds, tides, rivers
40 TW - photosynthesis (total)
10 TW - human industry (today)
110 TW - human industry 2060AD

Space solar of course is potentially far larger than that intercepted
by the Earth's land area under clouds.

2) Raise steam & drive a turbine- Here wavelength is unimportant.

Don't forget atmospheric transmittance

http://www.profc.udec.cl/~gabriel/tutoriales/rsnote/cp1/1-11-1.gif

This is true. You are now thalking about GEO. Some earlier postings
mentioned LEO. At GEO utilisation is indeed 24/7 (almost) there is an
eclipse season. At the equinoxes this is about 1hr 10min per night
around midnight. Away from equinoxes you have 24/7.


This is quite interesting. I feel that this concept should be extended
further. A set of lasers such as you describe can be made much more
capable with a few changes. These changes are quite compliocated and
perhaps a little bit difficule to understand but they would not add
greatly to the cost. What you weant on your window is a piezoelectric
system capable of putting a delay of half a wavelength in or out.

If you take a pattern at infinity (in fact the Earth will still be in
the Fresnel region - not quite infinity) and take a Fourier Transform
you get the pattern that has to transmitted. One thing - The radiation
intensity is real (we are not worried about the phase (angle in Argand
diagram). Our laser outlets are only capable of varying phase angle
not intensity. However by giving freedom to phase we can achieve a
general pattern by varying phases alone. This means.

1) The system is capable of being focussed either into a very small
region or into a more diffuse region.

2) The system will focus on a number of spots simultanously some
diffuse some points.

when the phase is controlled by a pilot beam arriving from the target
- as in 4 wave mixing - the beam automatically adjusts for atmospheric
distortion and movement of the window...

Indeed unlike SDI the "targets" are cooperative. That does not mean
that a simple pilot beam with phase mirror will work. There is 1/3 sec
required for a double journey to GEO, so the phase we require is the
phase 1/3 sec into the future. Simple extrapolation will do this. In
fact we can say that our fastest speed is goiing to be 8km sec (see
below). 8km/s will be at a grazing angle so that there will be a cross
motion not exceeding 1km/s. This is 300m on a return journey.
Fundamentally therefore we can say that the present phase is OK where
we cannot resolve 300m. This implies that we can have simple phase
models on individual lasers with an overall angle added. A computer
calculates this overall angle. In 1/3sec at 1g (10 ms^-2) we move
about 55cm so the second order correction (for acceleration) casn be
quite course.

This is something that needs thinking about but is not a show stopper.
If you want simply to transmit to the Earth's surface you don't need
to worry about movement. One red herring which needs to be disposed
off. Looking UP from the Earth you need adaptive optics. Looking down
you don't, as the departures from rectalinear proagation are all in
the last 5km or so.

Thought - Could an asteroid be moved by concentrating laser light onto
it?

Yes.

http://groups.google.co.uk/group/sci.space.policy/browse_frm/thread/4...

Lasers were not mentioned in the NASA report, perhaps they should have
been. I think too it was a great pity that Rand Simberg saw fit to
hijack the discussion.

Personally I am not sure I like the idea of nuclear bombs, where there
is an alternative.

Personallly I'd much rather that a series of small engineered nuclear
blasts go off in deep space far from Earth's biosphere,than in my
hometown as a few large blasts that kill everyone I know and love.

Actually the concepts of LISA, the concept of a large space
(fragmented) telescope and the concept of laser arrays are very much
bound up. With active phase control you can always reach the
diffraction limit and you can work out with 1.22lambda/d just what
that is.

Correct. But the point is with 4-wave mixing, is that a pilot beam
arriving from the target panels (we have cooperative targets unlike
SDI) provides a safe reliable means of control.


Launch costs is again an interesting one. If you have an energy system
it will (I presume) go from LEO to GEO using ion drive.

That's a possibility. Solar sailing is another possibility. However,
I merely postulate a chemically powered kick stage to apply two
impulsive burns.

The first is 2.43 km/sec to ascend to GEO and the second is 1.47 km/
sec to circularize at GEO.

Assuming the kick stage is hydrogen/oxygen propellant operating at 4.5
km/sec exhaust spee, we can estimate the propellant fraction;

Vf = Ve * LN(1/(1-u)) ---> u = 1 - 1/exp(Vf/Ve)

u = 1 - 1/exp(4.0/4.5) = 0.589 = 58.9% propellant fraction

If structure is 3.1% total mass, then that leaves 38% of the payload
in LEO at GEO.

In other words,2.53 tons of payload in LEO means 1.00 tons of payload
arrive at GEO -

If you wish to recover the rocket for reuse, then lyou've got to drop
the stage from GEO to LEO or Earth Surface.

Lets say that 6.1% of the structure is left behind after the second
impulsive burn. This reduces the mass at GEO to 35% - now lets do the
same calculation for the deorbit burn from GEO - that burn is 1.47 km/
sec so,

u = 1 - 1/exp(1.47/4.5) = 0.279 = 27.9% propellant fraction

So, of the 6.1% budgeted to remain after the second burn and the
release of the payload, - 1.7% of that total is propellant - leaving
4.4% of the original payload at LEO - structure (with re-entry TPS and
landing hardware)

So, if we have a 500 metric ton lift capacity to LEO - this would be a
reusable stage that would have the following mass budgetes

175 metric tons GEO based powersat
22 metric tons structure
303 metric tons propellant
500 metric tons LEO

Partially deploying the powersat at LEO and concentrating sunlight
onto a solar thermal rocket that heated liquid hydrogen to high temps,
would produce a jet with 10 km/sec exhaust velocity. Lets say that
nominally 1 GW of thermal energy can be used this way.

Since F = P/2Ve = 1e9 /(2*10,000 m/sec) = 50 kN = 5,091 kgf

Which is adequate for space boost. In this case, the solar thermal
rocket would require a propellant fraction of;

u = 1 - 1/exp(1.47/10.00) = 0.137 = 13.7%

With the same structural fraction (and hydrogen oxygen propellant for
deorbit without the solar collector) this increases the payload to
80.2%

80.2% - payload
13.7% - propellant (solar thermal (hydrogen))
1.7% - propellant (hydrogen/oxygen)
4.4% - structure

So the same 500 metric tons to LEO would result in

401 metric tons - powersat
69 metric tons - hydrogen propellant
8 metric tons - hydrogen oxygen
22 metric tons structure

So, this is how it would work. You'd develop an off-the-shelf kick
stage with conventional engines - and start orbiting 175 ton
powersats. Then, you'd develop solar thermal capability - and double
the size of your powersat (or loft two at a time)

That is only for building the first one! Suppose insead of Hydogen and
Oxygen you just had Hydogen with a fine (250nm quarter wave is optimal
- 300 to allow for evaporation) slurry of Carbon particles to allow
the "fuel" to absorb light. We now have something a bit like a
hypersonic plane. We burn hydrogen in a tubojet? engine to get up to
20km or so where the atmosphere is always clear. Now as atmospheric
pressure falls we use the laser to heat up a basically fuel rich
system. Our rocket motors will be capable of heating Hydrogen to over
3000C and giving an exhaust velocity/specific impulse of 8-10km/s.

For LEO-GEO I would envisage ion propulsion (exhaust velocity 50km/s).
Tug would use photovoltaics!

From my earlier calculations you can see that each kilowatt of

capacity (3.3 sq m of surface area0 requires 200 milligrams of payload
- add 25% - that's 25 milligrams per kilowatt. Very light weight
which makes the whole thiing pay!

Anyway, 1 gram = 5 kW, 1 kg = 5 MW, 1 ton = 5 GW,
1 gram = 4.6 m diam, 1 kg = 145 m diam, 1 ton = 4.58 km

This is with two sheets of polymer 50 microns thick. 85 mm thick and
the 5 GW payload would be 175 metric tons.> - Ian Parker- Hide quoted text -

OK. One thing you may not have taken into account. Under continuous
operation your laser is going to get very hot. How is the heat
dissipated? You may simply say OK it will work at 1000C or cooling may
have to be built in.


- Ian Parker

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