At 11:00 AM 4/1/2009, John Williams wrote:
Keith wrote:
>
> I don't know who on this list is up to understanding the technical
> parts . . . .
>
I think I am. Or I was. Probably now I switched from being
one of the "good guys" (working in the space industry) to
become one "evil minion" (working in the oil industry) :-)
> The root problem is the same space flight has had all along--the
> rocket equation. All sins flow from the fact that at best one part
> in 60 of the liftoff mass gets to GEO or lunar orbit with
> chemical fuels. Here it is in graphical form.
>
If you want to play with the rocket equation, just use
this javascript:
http://www.geocities.com/albmont/relroket.htm
It's a relativistic rocket equation, but it works (obviously)
for v << c.
The whole problem is that you need energy/power/speed/name-it
to get the rocket away from Earth's athmosphere. Right now,
the only way to do it is by chemical rockets.
There are other ways that would almost certainly work. Laser
ablation, which takes a GW/ton of payload, and various methods that
accelerate a vehicle to escape plus enough to get through the
atmosphere. But your point is correct in that rockets or something
closely related seem to be the current and possibly the best way to
get above the atmosphere.
Though in the long run (and assuming we can get the cable) you can't
beat a moving cable space elevator for efficiency. 15 cents of
electric power per kg to GEO.
Now comes the second problem. Suppose you get to LEO.
Ah, but you didn't read the specifications. The first stage in this
design does not go to LEO, and the second (laser) stage doesn't
either. It heads directly to GEO on one continuous burn. Amazing
what you can do with 12-17 km/sec exhaust velocity and over a g of
thrust. The energy in the laser beam is equal to a ton of TNT per second.
Theoretically,
it's possible to use "more efficient" ways to transfer to GEO. One way
is to continously thrust with a high-specific-impulse engine. But this
would make the transfer take eons - and now economy plays a very
important part in the equation:
It's not as bad as you think. Ion engines will take a power sat
constructed in LEO to GEO in a few months. Unfortunately by the time
it got there it would be full of holes and in dire need of
repair. They are big enough to intercept a *lot* of space junk.
you don't want to _wait_! Time is money.
If you put another batch of lasers on the ground or build a set at
GEO, then lift off to GEO is 5 hours. Initially, with only one set
of bounce mirrors, we let the laser stage go around the Hohmann
transfer orbit one and a half times. This puts the laser and bounce
mirrors in the right place to circularize the laser stage to GEO.
The time is money is certainly true. The "design to cost" criteria
is to have parts delivered to GEO be incorporated into a finished
satellite in a week or less. Starting at GW of power sat every day
or two, ramping up over time to 2 GW/day or more. The intent is to
displace fossil fuel entirely by mid century.
So, the pretty little mathematical and physics of transfer bows
to the implacable and ruthless laws of economics, and we use
chemical rockets.
They are ok for the first step, but using high exhaust velocity laser
propulsion for the second stage reduces the lift off mass by a factor
of 5 and the cost by a factor of 6. It's the difference between 5
cent per kWH which won't really compete with nuclear and 1 cent,
which takes over even the oil market.
Keith
_______________________________________________
http://mccmedia.com/mailman/listinfo/brin-l_mccmedia.com
