[ExI] Spacecraft (was MM)

[Eugen Leitl]

On Thu, Dec 30, 2010 at 11:30:10PM -0800, Samantha Atkins wrote:

> > And 20 years ago, that was the right conclusion.  Now we have a path,
> > even if it kind of expensive, to 9+km/sec exhaust velocity.  That
> > means mass ratio 3 rockets to LEO and even better LEO to GEO.
> 
> I must have missed it.  Please give details, links, etc.  How expensive?  How large a payload? What technologies?

He's probably talking about laser propulsion.

In general there are ways to reduce costs to LEO, e.g. by
using maglev launchers, use hypersonic air-breathing propulsion/
rocket hybrids to make it to LEO. Once you're there, you can
use electric propulsion (plasma thrusters) to climb higher.

Even with most efficient propulsion your dealing with a
highly assymetric situation: http://xkcd.com/681_large/

As you see, it is easier to fall down (and aerobrake,
which doesn't use up reaction mass, or use extremely
efficient plasma thrusters) than to climb up. For SPS
you probably need orbits above 1 Mm, orelse there would
be too much drag. Ideally, you would use mostly photonic
pressure for orbit adjustment, using the PV panel as your 
sail.