On 7/11/06, Arthur Smith <apsmith@...> wrote:
>
> I apologize for not including your text in my response, but Yahoo!'s new
> html formatting makes that somewhat difficult.


Gmail is excellent for managing group mail, and is pretty good at annotated
responses like this one. I sent you an invitation email...if you got good
spam filters it might wind up in your spam folder.

I find your pessimism about solar electric propulsion surprising - it's
> been used at least twice with quite remarkable success already - the
> EU's SMART-1, and Deep Space 1.


It's not a general pessimism about SEP per se. I don't think SEP is
appropriate for ascent maneuvers. HS 601HP commercial communication
satellites are happily using SEP for stationkeeping. Commercial customers
are generally a bit more skiddish about new technology than planetary
explorers (or in the case of Deep Space 1, technology + planetary
explorers.) I think SEP would be appropriate for ascent from GTO to GEO, in
which case the maneuvers happen in contiuous sunlight, and the impulse
losses are greatly mitigated. Reusable tugs would rather not go past GTO
because it is easier for them to aerobrake from GEO than to propulsively
deorbit from GEO after delivering the GEO payload. Don't launch in August
or May, just before the equinox GEO eclipses start screwing you up. It's
best not to spend too long on a low GTO because of the the threat of orbital
debris.

It should at most add a few months to
> get from LEO to GEO. Claiming it will take over ten times the cost to
> deploy to GEO as to LEO for something with as much power available as
> SPS components seems very unlikely.


LEO ascent related cost savings dissipate as you go to higher energies. The
jumbo jet will take you to Nepal faster and cheaper than a camel, but it
doesn't help as much when you are actually climbing Mt. Everest. The
Bluestar/Yellowtug/Yellowfly model I was basing my numbers on has three
major phases of the total GEO ascent:

1. LEO; the cheap 9000m/s delta-v maneuver and the one that wants the most
improvement in reality. Bluestar, the fully reusable two stage runway
operated spaceliner does this maneuver.
2. GTO; the more expensive 2750m/s delta-v maneuvers from LEO to GTO.
Yellowtug receives fuel from Bluestar at a space station, and after its
maneuvers, it recovers to LEO by aerobraking.
3. GEO; the most expnesive 1550m/s. This is executed by the battery powered
Yellowfly upper stage, which was Bluestar's original GTO kickstage. Because
the required return maneuver would be very onerous to design into Yellowtug,
the expendable Yellowfly is used.

My estimate included amortizing Yellowtug's hardware cost over twenty
ascents in a straight-line fashion.

I'm not going to get into
> engineering details, but any business working those angles would be
> clearly malfeasant if they wasted that much of their investment on such
> an orbit transfer.


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.

Mass requirements for rocket launch and deployment shouldn't be any
> worse than for the solar sail, which was why I mentioned that example.
> Something that's in thin foldable or rollable sheets can surely be
> easily compacted and made safe for launch without adding hugely to mass
> requirements. I'm not trying to account for 10% effects, really just
> orders of magnitude at this point. A Japanese group recently launched a
> suborbital rocket looking at rapid deployment of a triangular thin sheet
> of this sort with robotic assistance.


The rigging is the major problem. For most real satellites the rigging is
the backing the solar cells are glued to. For solar sails, it's the framing
their connected to. The solar wings of the ISS have a deal of "flexibility"
to them, and are rigged mostly by the little skeleton column in the middle.
They are very weak when deployed. If the ISS suddenly found itself sitting
on the Moon, they would collapse, as the moon has about 10 times as much
gravity as these things can stand (0.015g).

The problem with any form of solar concentrator is the pointing
> requirements are very strict, which limits what you can do as far as
> maneuverability via sailing. That's part of why I believe PV is a better
> choice - but a concentrator might be the right choice anyway depending
> on engineering details - if it's possible to do it economically with one
> and the other is better, then it's certainly possible to do it
> economically with the other.


A combination is probably best. At the least, an SPS needs enough PVA
generated power to meet its own contingency needs with a considrable safety
factor. With ion engines for primary control on a crewless TDG SPS designed
to maintain attitude mainly through balancing of the solar pressure on its
generating elements, it could easily have enough fuel to last ten years, and
then be visited on occasion (every 5 to 10 years) by a crewed service craft
that would service its turbine bearings and refill its ion propellant
supply.

Yes, certainly, the farther you go out in the solar system, the less
> effective solar energy becomes. Inverse square law etc. The lowest
> material requirements for space solar power would, conversely, be in
> orbits much closer to the Sun. There's 90 million miles of room
> sun-ward; if we ever do choose to build space colonies and space
> manufacturing facilities, heading in would be much more effective than
> heading out.


To an extent...I don't think the ideal location for space colonies is much
closer than Earth's orbit. If a UV bandpass filter style of photovoltaic
cell could be built...one that used UV rays to generate electricity but let
visible and infrared pass through...it could be built into the colony's
windows to provide both UV protection and electricity. Incorporating
current assumptions into Island drawings leaves one wondering where the
solar wings are.

I wasn't referring to a human-inhabited lunar colony, in talking about
> Criswell's lunar solar power project. Rather, it is a robotically run
> system that should require little or no oxygen or hydrogen for
> construction or operations, other than for occasional human visits to
> replace or tend equipment. The moon is ideal for the construction
> proposed; solar panels have already been made from the equivalent of
> lunar regolith in experiments (only 1% efficient so far though).


Oh. That still doesn't change the situation all that much though; the same
technology could still be deployed on Earth, and still save enough on
transmission and tracking losses to make up for the atmosphere. Basically,
no matter how you slice it, it doesn't stand a chance against earth-based
solar power, and as many of the variables of GEO based SPS and Criswell type
station have the same effect on both, it stands even less chance against GEO
based SPS.

Finally, if you're going to take over the universe, go in person! I read
Mike Wolverton's The Depths of Space, excellent book until the last couple
pages. The conclusion made me literally nauseous.


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