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starship-design: Re: debate

KellySt@aol.com wrote:
>In a message dated 12/6/97 12:05:58 PM, kuo@bit.csc.lsu.edu wrote:

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

>>So far, every fusion reactor ever 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.

>?  We have had several fusion systems where more energy was taped
>out from the reaction then took to start it, thou the inefficency
>of the systems was too great to get positive payback from the
>whole system (my favority being the comercial laser fusion tests
>in the '80's).

Please, name one.  Just one, at least.

>If the systems used better
>systems or were scaled up, they would have been able to produce power.  And a
>few years ago the DOE annouced one of their systems had reached and exceeded

Which one?

>>>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.

>Commercial investors never take amazing risks.  Their entire focus
>is to avoid amazing risks.

Untrue.  They regularly invest large sums of money on speculations
which may fail, and when it's other people's money it can make big
news when the bet fails.

The larger the sums of money, the more risk investors take.  On one
end of the spectrum are personal savings, which require a low risk
strategy in order to save up retirement funds safely.  On the other
end of the spectrum is Bill Gates, who has so much money to spare
that he can afford to risk most of his eggs in one basket--his own
company--because it is the most profitable place to put it.  There's
maybe a 5% chance that a some "disaster" will cripple Microsoft
stock--that's too much of a risk for anyone to put their entire
retirement savings into Microsoft stock alone, but Bill Gates can
afford to put the vast majority of his money in Microsoft stock
because even in the worst case what remains is plenty.

>>>>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?

>So your comment that "Something _might_ be discovered in the next millenia
>which will lead to fantastic increases in space propulsion..." is
>statistacally far to conservative.

No, I don't think so.

Sure, something will lead to fantastic increases in space propulsion
compared to TODAY's technology.  But look carefully at what I say.

I say, "Something _might_ be discovered ... which will lead to
fantastic increases ... beyond the theoretical anti-matter rocket."

Beyond the theoretical anti-matter rocket.

I have great confidence that for interstellar travel something on
the level of a theoretical anti-matter rocket or less will remain
the best we can hope for in the next millenia.  The physics of
relativity and conservation of energy strongly suggest this.

>>>>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

>Mass comparisons are rather irrelavent.  The cost of an ore, vrs a
>manufactured systems vary wildly, and are not liniarly related to mass.

Yes, they vary wildly--but in all cases the cost of the manufactured
system is more than the cost of the raw materials used to manufacture
it.  This should be obvious.

The costs of various raw materials may differ, but fusion rocket
fuel (Deuterium and possibly Hydrogen; D-D fusion is trivial if
we have Li6-H fusion) will very likely always costs less than
the metals/composites used to manufacture beam satellites.

>>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.

>Which is another advatage of a dispered phased array system that could be
>adapted to longer range without significant modification.  (Even thou the
>efficence would decline.)

Huh?  Advantage over what?  A dispersed phased array system can't
be adapted to longer range without significant modification.  What
_can_ be done is to increase it's efficiency by bunching it up
together as tightly as possible, ideally shoulder-to-shoulder.

The only thing you gain by dispersing them over a wide area is...
you don't gain anything, actually.  At every range, the beam
produced by the tightly bunched up array is superior to the
beam produced by the widely dispersed array.

>>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.  

>Presumably for large scale industrial operations in space, such as non near
>earth asteropid work and transport.  But agreed, this is speculative.

The only serious use for them I can imagine is for laser powered
rocket transport.  Assuming nuclear reactors remain expensive and/or
fission materials remain restricted, laser powered rockets offer
great potential savings in rocket costs.

For any sort of heavy industrial work where it's worth putting a
high power refinery on site, it's also worth putting its power
source on site.  Beamed power really only offers a potential
advantage in cases where the power is only needed a small fraction
of the time (which is the case for rockets).

>>Second, the introduction of that much extra power generation
>>into the market will devalue itself.  

>Quit likely.

>>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?  

>I can't follow this bit.

The affordability of the powerful deceleration leg fusion rocket
(that we can afford it at all) suggests we have similar fusion
power generation capability which is relatively affordable.
Given the plentiful inexpensive energy everywhere, who's going
to need beam power?
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi