So Zubrin's estimates improved the cost effectiveness of SPS by a factor
of 16 in just a few years? I suppose that's encouraging...

For current communications satellites, launch costs, even to GEO, are a
small fraction of total expenses, and time is money. The numbers you
have posted suggest for SPS getting to GEO vs just launching to LEO
would constitute about 90% of total costs; if it took more time but
saved much of that cost, it would clearly be worth it on a business
basis that is not there for commsats.

Comm sats also do not have anywhere near the huge solar power to mass
ratio we'd hope for from an SPS component, so the relative electric
thrust available would be much less, and the transit would take much
longer for a typical communications satellite than for an SPS.

The fact that you can only thrust for 50% of the time in LEO doesn't
seem a huge disadvantage - the more serious one would be whether SPS
components would need sufficient additional mass to withstand the
temperature variations that launch to GTO saves costs overall.

I don't think a reusable tug makes any sense for SPS construction; the
components should be self-propelled with a small engine and propellant
store, once in orbit.

The cost differential between LEO and GEO with chemical propulsion may
also drop if on-orbit propellant is available, as has been proposed
elsewhere. It could come from the Moon or from ultra-cheap Earth rocket
launches (big dumb boosters that are just putting up rocket fuel, so we
don't care if they blow up 20% of the time). The primary reason it is
usually estimated that getting to GEO costs 5 times as much per kg is
the mass of the fuel required for the orbit change; having that up there
cheaply already would eliminate that problem.

I don't particularly care what technology is used - I'm just looking at
the physics end of it to understand what's possible, and it seems
Zubrin's and your estimates are ignoring some pretty obvious ways to
save money by taking advantage of the physics available. Remember, if we
actually do this on a reasonable scale, we're talking about hundreds of
GW-scale satellites that will each have masses of hundreds to thousands
of tons, total expenses will be at least in the hundreds of billions of
dollars, so such a system could afford to do all sorts of custom things
that might not work on a smaller scale. Whatever's been done up to now
for communications satellites, ISS, etc. is not terribly relevant on
that scale.

Arthur

solarpowersatelliteplace@yahoogroups.com wrote:
> [...]
>
>
>
> I'm not sure who you're talking about. It is obvious that if you
> compare
> the performance of a booster to LEO with the same model to GTO or, if
> applicable to GEO, realise that the missions cost about the
> same...satellite
> customers today are "wasting" that much of their investment. I can
> get you
> some fairly reliable numbers for ASTRA 1KR, which launched in April.
>

>