Re: Solar powered lasers in space

On Sep 21, 11:21 am, Ian Parker <ianpark...@xxxxxxxxx> wrote:
On 20 Sep, 22:35, Willie.Moo...@xxxxxxxxx wrote:



I doubt that. If you are going to travel at 0.03c you want an ion
drive that accelerates to 0.05c (say)

Why do you doubt it? What's the basis? You state a conclusion and
provide absolutely no technical data to back it up. I find that
maddening! lol. The way to look at this is propellant fraction,
power level and thrust.

A 1 gee acceleration is convenient for interplanetary travel for a
variety of reasons. First off, you have gravity aboard ship during
the transit. Secondly, you get to where you're going pretty fast.
You fly halfway to your destination, carry out a powered pitchover,
and arrive at zero altitude and zero speed at your destination. With
clever programming the gee forces slide linearly from your start to
your finish so that you are acclimated to the destinations gravity by
the time you get there.

So, lets look at a 1 gee spaceships performance. You may remember
these from elementary physics

d = 1/2 a t^2
v = a t

So,

t = v / a

and so

d = v^2 /(2a)

Where d= distance travelled
a = acceleration
t = time
v = velocity

So, the velocity needed to attain the halfway point is

v = sqrt(d * a)

And the total delta vee to achieve the trip is

V = 2 * v

and the time in hours needed to make the trip is

t = V / 9.82 / 3600

So, we can construct the following chart

m kps hours LSD
Earth to d sqrt(d*a) t u
Moon 3.86E+08 61.60 1.74 2.03%
Mars-close 7.50E+10 858.20 24.28 24.88%
Mars-far 3.75E+11 1,918.98 54.28 47.25%
Venus-close 4.50E+10 664.76 18.80 19.88%
Venus-far 2.55E+11 1,582.43 44.76 40.99%
Ceres-close 2.70E+11 1,628.31 46.06 41.89%
Ceres-far 5.70E+11 2,365.88 66.92 54.55%
Mercury-close 9.00E+10 940.11 26.59 26.90%
Mercury-far 2.10E+11 1,436.04 40.62 38.04%
Jovia 8.55E+11 2,897.60 81.96 61.93%

So sailing the inner solar system in a 1 gee spaceship would be like
sailing the Pacific in a cruise ship. You'd have islands like the
moon, that are only hours a way. You'd have nearby territories like
Mars and Venus that are only a day or two away. Then you'd have the
outer planets that are weeks away.

The factor u is the propellant fraction for a Laser Sustained
Detonation (LSD) rocket operating with an exhaust velocity of 3000 km/
sec (kps)

Now, lets look at the power levels needed to achieve that, and the
propellant fractions. The first thing we realize is that to have
reasonable propellant fractions we need exhaust speeds to match the
flight speeds.

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

so

u = 1 - 1/EXP(Vf/Ve)

So, Vf/Ve must be less than or equal to 1 to have reasonable u.
3,000 kps - is a specific impulse of 30,000 - which is damned
difficult to achieve. And power to weight of the engine must be
tremendous. So, an ion rocket with 5,000 sec Isp - won't cut it for
this application. (it would do fine for 1/10th gee or 1/100th gee
operation) some sort of laser sustained detonation of inert working
fluids would be needed. This is nearly an exact analogue of nuclear
pulse propulsion, but the energizing force comes from pulses of laser
energy accurately directed at a thrust structure.

The power level to produce 1 kgf is

F = mdot * Ve
P = 1/2 mdot * Ve ^2

So,

mdot = 2 * P / Ve^2

and so

F = 2 * P / Ve

This is in newtons and 1 kgf = Newton / 9.82

so..

F(kgf) = P / (4.91 * Ve)

And with Ve = 3,000 kps = 3e6 m/sec we need 14.73 MW per kgf of
thrust. That's 7.37 quadrilion watts of power. And this is the low
power solution!!! haha.. The world has a few thousand super
tankers. To operate a few thousand interplanetary freighters with
this capacity requires tapping into 1e20 watts of power. Such
capacity would tie humanity together across the interplanetary
frontier.

At 1MW per sq meter - the sun centered laser array would have to cover
1e8 sq km of area. A disk 11,283 km in diameter - about the size of
the Earth - a very small fraction of the sun's total surface. Four
disks 6,000 km in diameter equally spaced around the plane of the
ecliptic would provide adequate power for a fleet of such spacecraft.

I am afraid I was simply thinking about energy. To go to 0.03c with
minimal energy you need ion drive. If you have plenty of energy you
indeed don't need to worry about that.

You are right that when you reduce Isp you are reducing power levels
and energy use. But you won't be travelling at 1 gee though.
Something far less.

Check it out

first,
The sun puts out lots of energy and capturing it and transmitting it
efficiently provides all the energy we need. So, this is part and
parcel of a space program. Not only is sunlight the first extra-
terrestrial resource humanity has used, making efficient use of it
along the lines I have describe is part of a rational commercial space
program.

second,
Lets review a little rocket basics shall we? haha..

Ion rockets have specific impulses of around 5,000 sec - that's
approximately 50 km/sec exhaust speed. .Awesome performance. Thrust
to weight is low, and so, accelerations of 1 gee won't be achieved.
1/100th gees are doable. Which is fine, because it reduces top speeds
from 0.03c - or 9,000 km/sec to 1/100th this value or 90 km/sec.

So, you won't be reaching 3% light speed with ion rockets we can build
today. Because this is 180x the exhaust speed. A rocket man has to
keep this in mine. Look at the rocket equation. If your delta vee,
or final velocity is 0.03c or 9,000 km/sec and your exhaust speed if
50 km/sec then your propellant fraction will be;

u = 1 - 1/EXP(9000/50) = 1 - 6.7e-79 ~ 1 = 100%

Which doesn't leave much for ion rockets, tanks, and payload!

But what you need to do is reduce accelerations to about 1/2% of 1 gee
- 5 milligees - which can be done by an ion rocket. So, the equations
I gave above are linear, so that means your acceleration times and top
speeds will be 1/2% of those given. That means you'll have a constant
gee ion spaceship, but it will accelerate at 5 milligees, instead of
1,000 milligees - and its top speed will be around 45 km/sec not 9,000
km/sec - and it will take hundreds of days to navigate the inner solar
system, instead of days, and hundreds of weeks to navigate the outer
solar system, instead of weeks. Power levels are 1% too - and so
total energy per kg is 1% - if energy is costly, and payloads are
easily stored and not time critical, then this approach is perfectly
doable. Either as an early stage,or at later stages for low value
freight - like building materials..

Lets check out the propellant fraction of an ion rocket whose delta
vee is 45 km/sec and exhaust speed is 50 km/sec

u = 1 - 1/EXP(45/50) = 0.5934 = 59.34%

So, now you have a doable vehicle. You've got 60% propellant, say 25%
structure (mostly ion engine and power supply) and 15% payload. Not
as nifty as an ultra high powerful laser light wing - but doable and
very similar to the interplanetary probes being launched by NASA today
- toward asteroids and jupiter.



There is one other point. If you have polulated the ecliptic with
lasers, you will (presumably) be able to phase lock them. You will
have a telescope 300million kilometers in diameter. In earlier
postings I have said how a number of problems are all tied together.
Now let us see what sizes you can see 10 parsecs distance.

Well we were talking about power beaming...

You're now talking about two large aperture telescopes exchanging
laser information and doing aperture synthesis like we now do with
radio telescopes - but in the optical realm. Of course with a very
accurate time reference you can record information at two telescopes
and combine the recordings to do your synthesis off line. What would
it take for two optical telescopes to achieve that? lol. I've often
wondered about that. All they need is a common reference. A quasar
or something might do - or a seed laser as you mentioned earlier. So
these are uses of space laser technology certainly.

Of course Bob Forward's idea of using laser light sails for
interstellar distances requires filled in apertures to get sufficient
energy to the light sails efficently.


Now a
parsec is by definition 1sec of arc with observations separated by 6
months. Therefore 10 parsecs means that viewed from the target the
telescope is subtending 1/10 sec of arc. 50 deg per radian 3600sec per
degree. That is 1/180000 radian. This means we see objects some 7cm
across. Clearly the gas in interstellar space + gravitational lensing
will prevent us getting anywhere near that figure. We should have
little difficulty though in seeing any planets.

Once the gas and dust and gravity lenses are mapped, we can use
computers to adjust the distortions - and once we have clear maps and
images and so forth, compare new images with old to look at changes in
the dust gas and gravity lenses!


We will know fairly quickly whether or not we are in a race, another
possible explanation of the Fermi paradox.

Yes, I read a science fiction book that had that as a premise. When
you look out into the cosmos you are looking back and time. So, if
the universe looks like its devoid of technical life, all that means
is that is was devoid before NOW. Think of the universe as a desolate
desert plain - in the Kalahari say. But it could be like the Kalahari
desert, these is a season - and in that season all the seeds of living
things that are not apparent other times of the year - spring to life
filling the entire plain with a riot of flowers, grasses, insects,
small animals of all types.. and then they all die out when the
season passes.

How like life to be this way. We wouldn't see evidence of technical
life in such a cosmos by looking at the Andromeda Galaxy which is 2
million light years away. Because 2 million years ago - using our
Earth as a reference - no one had technology.

It is only by looking at nearby stars that we will get an idea of how
common life is, and how common technology is in the cosmos.

We may get a very surprising result!

So, its worth doing certainly.

Of course what would synchronize evolution so closely across the
cosmos? We don't know. We don't know if it is. But it might be.
And its worth figuring out.

Haha.. if we found that there were dozens of technical civilizations
within 50 light years of earth among the 10,000s stars in that
distance, military planners would go ape***. If some were slightly
more advanced, some would argue we'd have to figure out how to deal
with them. If others were less advanced - but growing rapidly - some
would argue we'd have to try to figure out how to infiltrate them and
slow them down. The interesting thing is that this is what the crazy
UFO folks are saying is happening right now! lol. So, the universe is
stranger than we can imagine if this is true.


Humanity today consumes 10 TW of power - 1/10,000,000th the power
level postulated here. Needless to day, any industrial activity we
wanted to carry out on the planets or in free flying space colonies,
could easily be provided as well. This might also form the basis of
maintaining government control over this far flung array of humans, to
keep them from using high tech to attack one another - and stopping
the possibility of interplanetary war.

You are assuming that war is the result of competition for resources.
I say it is saying your prayers the wrong way.

No, I'm saying something else entirely. Just as police routinely cut
off power to a home where there is a hostage situation, power may be
cut off to a region or for an activity that threatens folks. This is
possible. Not inevitable.

A common device used widely to provide limitless amounts of energy
across the entire span of human activity - more cheaply than any other
form of energy - provides a means, a hook, by which that activity can
be controlled. Whether it is used that way, and the degree of control
exercised is a matter for people to decide. And if abused, can be
counted as a cost of use. Which means those being abused will find
other approaches.





A laser light sail requires no propellant, but power levels go way up
for both high thrusts, small sail size, and for ton of payload
moved.

There are two ways to do this. One is to heat a body to very high
temperatures and use the black body radiation for propulsion - this
requires some sort of plasma containment system that can't be built.
The other (if we are to have high gee forces) is to use a mirror to
reflect nearly all the energy incident on it. In order to limit the
size of the mirror.

To make logistics simple, it would be nice to have the mirrors operate
like wings do on aircraft - exerting 100 kg/m2 or more. A disk like
spacecraft that had multi-mode capabilities would be interesting.
That is, a spherical payload encircled by a mirror disk, that might
also operate as a radiator propulsor short term - for landing and
operating out of sight of the sun...

A laser is monochromatic.

One laser is monochromatic. A collection of lasers is not.

One laser pumped by a thermal light source is profoundly inefficient.
20 lasers pumped by a thermal light source, segmente into 20 bands of
color, is not.


This means you can use simple dielectric
sails which do not heat up.

Or a collection of dielectrics... Yes, this is the basis of GBO film.
But dielectrics, are not 100% efficient. But they can be nearly so.
One part per billion absorbed - for example, means that 1 billion
watts per square meter on the film will have only 1 watt of heating.
(but don't get in the way of the beam!) haha..


A metal sail is hopeless at anything like
a high energy.

Agreed. This is what I'm saying.


Black body thrustor

Pr = 1/3 ar T^4

Where ar = radiation constant = 7.57e-16 J/m3/K4
T = temp K
Pr = radiation pressure (watts/m2)

To exert 100 kg/m2 requires a temperature of 45,000 K - and the power
output of that square meter is

j = sigma * T^4

Where j = watts/m2
T = temp K
sigma = stephan boltzman constant = c * ar / 4
= 5.67e-8

j = 5.67e-8 * 45,000^4 = 232.5e9 W/m2

So, 1/100th of a meter squared, would be a square 10cm on a side,
would produce 1 kg of thrust, and consume 2.32 GW of power!!! Compare
this to 14.7 MW of power needed for 1 kgf in the LSD rocket!! But
the great advantage here is that no propellant is needed.

This is a black body radiator, some sort of cavity containing a
magnetically stabilized plasma - that efficiently absorbs powerful
laser energy. By encasing the cavity in a reflective parabolid that
transmits the laser energy but reflects the bulk of the plasma
radiation in a desired direction, the surface can absorb laser energy
from one direction and emit radiation in another direction. By having
a certain amount of plasma that stores a goodly amount of energy -
thrust can be maintained for a period of time - without direct
illumination. Excellent for landing and takeoffs, where laser energy
may be a hazard (the plasma itself would be a hazard if the cavity
developed any leaks! - and the exhaust itself would also be a hazard -
it may be possible to use the plasma energy directly by venting it for
landing and take off.

A mirror based system has the following relation;

Pressure = 2 U / c

Where U = power per unit area, c = speed of light, Pressure = N/m2

So, 100 kg /m2 = 982 N/m2 implies U = 3e8 m/sec * 982 N/m2 / 2 = 147.3
GW/m2 - which is less than that required for the radiation pressure
mode - and actually, the radiation pressure mode above, doesn't
include the momentum that is obtained from absorbing the light energy
in the first place.. which can help or hinder the thrust effects
depending on how the cavity is arrayed relative to the energy being
beamed in.

Of course to keep temperatures under control requires VERY VERY highly
reflective mirrors. Before the invention of GBO films I would say
such things would be impossible. But the advent of GBO films that
have reflectivities in excess of aluminized coatings - suggest that
continued development along this path would allow mirrors that absorb
less than 1 part per million of the incident energy. This means that
the wings need only

...

I think I missed the rest of your commentary. What is happening to
Google? They really suck these days. Ah well.

.