J. Kare's laser launch

I recently heard Jordin Kare talk about laser powered craft. The craft
would have a large heat exchanging surface for a laser to hit. Heated
gas would propel the ship.

He said that not having to carry the energy source aboard the ship is a
major advantage. A variety of things can be used as reaction mass
including hydrogen. Some very good ISPs are supposedly possible.

A megawatt per kilogram is required to achieve orbit. He noted big
enough lasers to do the job are difficult and expensive.

Then he mentioned 3 different methods have been discovered to take beams
from many small, cheap lasers (such as the pointer he was using) and
combine them into one large, coherent beam. Channeling the beams
together via fiber optics was one method, IIRC.

Are huge, powerful lasers about to become more doable and less expensive?

--
Hop David
http://clowder.net/hop/index.html

Hop David wrote in
:


Are huge, powerful lasers about to become more doable and less
expensive?

A chemically-powered laser in the megawatt class is being developed
to fly on a modified Boeing 747 to zap short/medium range ballistic
missles. But the chemistry makes it doubtful to operate in large
arrays (oxygen-iodine?).

The main gotcha is the low efficiency of the lasers; for a serious
payload the wasted energy would be well up in the gigawatt--if not
terawatt--range. Short of a dedicated power plant, that almost
rules out the commercial power grid and points to a chemically
powered laser array.

I think there's still an awful lot of work to be done on very
high powered lasers, with obvious weapons potential. But the concept
seems valid. A demonstrator system capable of launching a few
kilograms could be possible with existing technology.

--Damon

In article ,
Hop David wrote:
Then he mentioned 3 different methods have been discovered to take beams
from many small, cheap lasers (such as the pointer he was using) and
combine them into one large, coherent beam. Channeling the beams
together via fiber optics was one method, IIRC.

Not exactly, although the net effect is the same.

Only one of the three techniques (spectral combining, which uses a
diffraction grating) actually directly puts diode-laser beams together.
The other two use diode lasers -- much the most efficient light sources
ever built -- to efficiently pump other lasers which have good properties.
One uses a specially-made optical fiber as the laser element; this
technique was long thought to be limited to low power output, but not so.
The other uses a metal-vapor gas cell with characteristics matched to the
diode output.

Are huge, powerful lasers about to become more doable and less expensive?

Maybe not huge, but it looks like it's rapidly getting easier to build
sizable lasers with good optical properties, long life, and low cost.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article ,
Damon Hill wrote:

The main gotcha is the low efficiency of the lasers; for a serious
payload the wasted energy would be well up in the gigawatt--if not
terawatt--range. Short of a dedicated power plant, that almost
rules out the commercial power grid and points to a chemically
powered laser array.

But compared to the cost of even a single launch with conventional
methods, a dedicated power plant is a pretty small requirement. So is
chemical power, for that matter -- in either case, you get huge gains by
leaving that infrastructure on the ground instead of having to haul it
up by its own bootstraps.

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

Since the launch mission is on the order of 10^2 to 10^3 seconds it
would be preferable to NOT have a gigawatt powerplant sitting idle most
of the time. Two possibilities spring to mind

1) operate a plant that sells power to the grid most of the time,
except during launch.

2) Turn the problem around; figure out how to store 1 gigawatt x 10^3
seconds of electrical power. When the launcher is not working, the
storage facility can time shift power for the grid and make money that
way.

In rec.arts.sf.science, wrote:
Since the launch mission is on the order of 10^2 to 10^3 seconds it
would be preferable to NOT have a gigawatt powerplant sitting idle most
of the time. Two possibilities spring to mind

Launch a load every minute. That's the whole point of the model, isn't
it?

--Z

"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the borogoves..."
*
I'm still thinking about what to put in this space.

On 4 May 2005 14:49:12 -0700, "
wrote:

2) Turn the problem around; figure out how to store 1 gigawatt x 10^3
seconds of electrical power. When the launcher is not working, the
storage facility can time shift power for the grid and make money that
way.

That's easy - Jerry Pournelle came up with the solution decades
ago... :-)

Take a bunch of rocket motors, and lay them on their side. Run
the exhaust through an MHD tube, and you get a LOT of electricity for
a short period. In between launchings, you use the 'conventional'
power plant to manufacture fuel for the rocket motors.

Launch a load every minute. That's the whole point of the model, isn't
it?

Honestly, this is where almost all the models break down - they use a
build it and they will come mentality. The real challenge is to
provide a cost effective space access technology that is cost effective
with only 5-10 launches per year.

(Of course, personally I do believe that if you build it they will
come. But to put this in perspective, would you take your parent's
life savings and invest in it? Neither will most investors...)

David Summers wrote:
Launch a load every minute. That's the whole point of the model,
isn't
it?

Honestly, this is where almost all the models break down - they use a
build it and they will come mentality. The real challenge is to
provide a cost effective space access technology that is cost
effective
with only 5-10 launches per year.

But if you look at what actually gets launched into LEO, a huge chunk
of it is propellant for sending satellites to GTO. In a world with
lunar or martian exploration, propellant will be an even higher
percentage of what gets put into LEO. Propellant subdivides pretty
easily, so it's at least theoretically possible to size your laser
launch system such that it can handle 10 GTO missions a year, 50 kg of
LOX to orbit at a time.

Not something you'd want to put your grandmother's life savings into,
but not "build it and they will come" either.

-jake

To expand on this, if you use separate 1 KW lasers pointed at the
target you need 1,000,000 of them to get 1 MW on target. If you use
100 KW lasers, you only need 100 of them. So increasing the power of
the individual lasers makes it exponentially more feasible.

So you really want high power lasers.