Re: Re: Re: starship-design: Re: Perihelion Maneuver

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

KellySt@aol.com wrote:
>In a message dated 12/5/97 3:44:11 AM, kuo@bit.csc.lsu.edu wrote:

>>KellySt@aol.com wrote:

>>>Agreed.  Best we can do is pick likely or unlikely advances, and keep to
>>>systems that seem to have a good mix of simplicity and performance.

>>I would argue we pick only _very_ likely advances.  Either that or
>>admit that there's no particular reason to think the design will
>>_ever_ work.

>>For instance, any design which requires fusion power other than from
>>H-bombs is speculation.

>Actually we do have working fusion systems.  Some need more efficent systems
>like lasers and such.  Others aren't commercially usable to produce
>electricity.  But we do have them.

We do _not_ have any working fusion power.

So far, every fusion reactor even built requires more energy to
start/maintain fusion than it returns.  If you've got something
to the contrary, I would very much appreciate a reference.

Right now, we have working fusion power the same way we have working
perpetual motion devices.

>Beyond that plannig on such systems in the next 50 years is highly
>conservative even by the standrads of commercial investors.

Commercial investors take amazing risks all the time.  They need to
do so in order to make their overall profits.  That is the nature
of investment.  Given the current profitability of the stock market
compared to bonds, I would have thought that obvious.

>>>For example:

>>>If we wait for physics to come up with major changes (zero-point energy,
>>>inertia damping, gravity control, etc..) we could get fantasic increases in
>>>performance, speed, etc.  But we haven't a clue what might be discovered and
>>>perfected in the next 50 years.

>>I seriously doubt any of these will be a factor, ever.  I'm in the
>>majority camp which figures that the density of zero point energy
>>is uselessly small.

>>Something _might_ be discovered in the next millenia which will lead
>>to fantastic increases in space propulsion beyond the theoretical
>>anti-matter rocket.  If so, I'll bet it won't look anything like
>>anything we've imagined.

>We only figured out mass conversion and fission theories in the last hundred
>years.  Expecting we woun't find a few such stagering things in the next
>hundred is really better against the odds and history.

So what?

I know it's against the odds to expect currently foreseeable technology
to remain the "best" for the next century.

However, I also know it's against the odds to correctly guess any
new technology _any_ amount in the future.  Any successful guesses
in the past are merely lucky guesses, with no predictive value.

Therefore, anything we would work out a rough design for must come
from known technology.  Even if we wanted to speculate on unknown
technology, we'd only be guessing at its performance.

>>>Fuel/sail avoids a 160,000 to 1 fuel ratio of a pure fusion rocket, and uses
>>>cheap and plentifull fuel.  But it requers a massive array of solar powered
>>>microwave sats in our solar system.

>>You were looking to avoid a mere 160,000-1 fuel ratio?  In favor
>>of a 400-1 fuel ratio?  Just how lightweight did you think the
>>microwave satellites were going to be?  Show me numbers.  Power-weight
>>ratios.  Desired output thrust.  I'll bet that given any reasonable
>>numbers, you'll find that the mass of the microwave emitter satellites
>>will end up weighing more than 400 times the sailship.

>Don't care about the weight of the sats since we don't need to carry them.

But you _do_ have to build them.  That's going to cost--and by my
estimate cost a hell of a lot more than the fuel you're "saving".

I make that estimate using mass comparisons, because it's hard to
say what the actual monetary costs may be in the future.  I make
the assumption that at any given time, the cost of a ton of fusion
rocket fuel will be less than the cost of a ton of beam emitter
array.

>>For every kilogram of sailship, you need 1,500,000,000 watts to
>>push 1 gee.  Assuming your emitter satellites were 500 times the
>>mass of the sailship, that means they have a power/weight ratio
>>of at least 3,000,000 watts/kg (this includes the collectors,
>>emitters, and lenses)!

>>If you can't even do that, then you're massive array of satellites
>>is going to weigh more than the huge amount of fuel you're trying
>>to avoid using.  And I'll bet you that fuel costs less per kg
>>than microwave beam satellite.

>The cost is at least a good question.  As a rough guess the 160000 to 1 ration
>ships fuel would cost 1,600 trillion dollars ($1.6 E 15) at current comercial
>rates.  

>if you assume 1,500,000,000 watts (I wasn't assuming a 1 g thrust by the way)
>and a 40,000,000,000 kilogram fuel sail ship.  Thats about  60 E 18 watts.  At
>current power plant costs on earth that would run about E 18 dollars.

These costs aren't so useful, because they are terrestrial costs.
It should go without saying that current costs of items on Earth
probably won't reflect future costs of items in space.  That's why
I chose the mass based approach.

>So assuming you only want to launch 1 ship, one time (and assuming
>no changes in costs) the staged fussion ship would  cost less.
>Thou the microwave platforms could pay their way selling power
>comercially, which could complicate the economics.

The only saving grace of the laser sail vs. increased fuel would be
that the beam emitters may already be built and/or they may be reused.

For a first interstellar mission (which is what we should be discussing,
since it's so hard already), it's unlikely they would already be
built.  There's no way to justify the expense of making them such
long range other than being meant for an interstellar mission.

The possible reuse of the lasers is particularly notable if it is
reused in a single mission (e.g. sequencially launching multiple
modules which provide deceleration fuel).

However, the possible reuse of lasers for marketable power generation
is, IMO, dubious.  First, there has to be a market for that amount
of power.  Second, the introduction of that much extra power generation
into the market will devalue itself.  Third, in this example we
assume fusion power is available for the deceleration leg.  If that's
the case, then who's going to bother buying beam power?  All of these
issues are debatable using speculative numbers, of course.

>>>All in all we are down to designs that seems extreamly expensive, but
>>>possible.  Or ones the requre unknown physics or technology.

>>Positive feedback fusion technology, other than H-bombs, is unknown
>>technology.

>?  No we have had runing fusion reactions that produced more power then they
>took to run, and have other systms that could work efficently at those scales.

Huh?  Give me a hard reference!  This would be very exciting news!
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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