Re: [ERPS] Turnaround Crew

[Ian Woollard]

Randall Clague wrote:

A BOTE: Kinetic energy in LEO is about 30 MJ/kg, a kW-hr costs about
18 cents, and a kW-hr is 3.6 MJ.  So electricity, the cheapest form of
energy (and least storable, what a coincidence) costs about 5
cents/MJ.  That puts LEO kinetic energy cost at $1.50/kg, or about 68
cents/lb.  The question is, how efficiently can you convert
electricity into kinetic energy with a laser and a launch vehicle.  If
your system efficiency can reach 1%, which I doubt (laser efficiency *
propagation efficiency * propellant coupling efficiency * vehicle
coupling efficiency), you can make orbit for $68.00/lb.  If your
system efficiency is 0.1% - more believable - it'll cost $680/lb.

Actually, no, way more efficient than that.

Semiconductor lasers >30+% efficient (wall plug to light)
Transmission losses 50% (guesstimate, sounds reasonable using reasonably big telescopes)
Coupling ? (how efficient can you absorb laser light and turn it into heat) 50%?

Overall 0.3*0.5*0.5=0.07 = 7%

And that's pretty conservative... Some of the semi lasers are nearer 70%; the absorption coefficient may very well be much above 50% and your transmission losses are diffraction and aiming limited; neither hits a fundamental limit, so it can in principle be nearly 100%.

That assumes a mass fraction of 0%, of course.  A mass fraction of 90%
- more believable, though optimistic IMVHO (I don't have a good handle
on propellant Isp for laser launch)

Jordin Kare was talking about using hydrogen for fuel; he said he could easily get 600 seconds out of it. That means you have a mass fraction of 80%.

 - drives your cost to between
$680/lb and $6800/lb.

No, under $100/kg; mainly the cost is the hydrogen; the electricity is almost negligible.

  Then you have the cost of developing a laser
powerful enough to launch something - again, I don't know how to
calculate the power requirement

The rule of thumb is 1MW/kg. Actually that's one problem, probably one of the bigger problems; is that you need a reasonably huge power plant to do this.

 - and the non-trivial problem of
steering the beam quickly enough to steer the vehicle when it gets a
few hundred km downrange.

It's a similar problem as spy sats, pretty much. In fact you collimate the light by pointing the lasers the wrong way through a telescope.

-R
  

-Ian

Motto: "You're Not Authorized to Know Our Motto." -Nova Express