On Wed, 19 Feb 2003 16:56:26 +0000, Ian Woollard <[EMAIL PROTECTED]> wrote:
>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%? > >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%. OK, that assumes laser thermal, which I didn't consider because I've never seen a design for it. Laser thermal does have the potential to be quite efficient. I was assuming pulse detonation, using water ice. Hit the ice with a light pulse to sublimate it and ionize it, then hit the plasma with a hard pulse to blow the stuffing out of it. Half the expansion will be against the ice: voila, poor man's Orion. It's much simpler than laser thermal, and cheaper to build, but efficiency suffers and steering is a bear. >No, under $100/kg; mainly the cost is the hydrogen; the electricity is >almost negligible. > >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. Yes you do. And taking Pierce's argument that industrial users pay about 1/3 as much as residential users, so $0.06/kW-hr, 1 MW/kg for 6 minutes is 100 kW-hr/kg, or $6.00/kg, or $2.73/lb. Hm, that's negligible, all right. >> - 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. No, it's hairier than spysats. With ice pulse detonation, you steer by moving the laser around on the ice pack. You have to do this fast enough to prevent the vehicle from flipping over. A modest power plant requires a modest vehicle, which is small, so has very high attitude rates. And as you get toward orbit, you're several hundred km downrange, and light propagation time becomes an issue. The example Jordin gave a few years ago was a vehicle less than a meter across, and it could tumble in about a millisecond. Round trip laser propagation in a millisecond is 300 km, so they couldn't control the vehicle more than 150 km away from the laser. They struggled with this quite a bit, and one of Jordin's colleagues complained, "There must be some way to beat this propagation delay." Jordin replied, "If there was, we wouldn't be doing this!" You can beat the propagation delay by having an smart fast mirror in orbit close behind the vehicle's insertion point. When the vehicle gets far away, you switch attitude control to the mirror. -R -- Every complex, difficult problem has a simple, easy solution - which is wrong. [EMAIL PROTECTED] _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
