Message-ID: <AZpuco-00VcJ0MYk5a@andrew.cmu.edu>
Reply-To: space+@Andrew.CMU.EDU
From: space-request+@Andrew.CMU.EDU
To: space+@Andrew.CMU.EDU
Date: Tue, 13 Feb 90 01:33:25 -0500 (EST)
Subject: SPACE Digest V11 #39

SPACE Digest                                      Volume 11 : Issue 39

Today's Topics:
			   Re: Spacewarps?
		  measurement standards (aerospace)
		     Laser/solar sail propulsion
----------------------------------------------------------------------

Date: 13 Feb 90 03:44:10 GMT
From: jarvis.csri.toronto.edu!helios.physics.utoronto.ca!physics.utoronto.ca!neufeld@rutgers.edu  (Christopher Neufeld)
Subject: Re: Spacewarps?

In article <10208@hoptoad.uucp> tim@hoptoad.UUCP (Tim Maroney) writes:
>In article <10180@hoptoad.uucp> tim@hoptoad.UUCP (Tim Maroney) writes:
>>>Why do you say containment wouldn't require energy?  The treatments
>>>I've seen indicated that degenerate matter or neutronium would be
>>>required, and these are not gravitationally stable at these sizes....
>>>The centrifugal force from spinning at near c would tear
>>>the thing apart in nanoseconds unless some sort of opposing force were
>>>used to contain it.
>
>In article <1990Feb12.001825.27799@helios.physics.utoronto.ca>
>neufeld@physics.utoronto.ca (Christopher Neufeld) writes:
>>   Containment doesn't require energy. A force is not energy. Energy
>>results when a force acts across a distance (the integral of the scalar
>>product of F with dx). Consider: a compressed gas canister contains the
>>gas, but it doesn't need batteries to do it, and it won't fail after some
>>well-defined period of time because it has run out of energy.
>
>A compressed gas canister is not containing a torus of collapsed matter
>rotating at near the speed of light.  The problems are rather different.
>
   No they aren't. I used a gas canister as an example. It requires no
energy to exert an arbitarily large force so long as the object against
which the force is being exerted does not recoil under the force. I can
list several other examples: a support column in a building does not do
work on the building, the sun does not do work on a body in a circular
orbit (ignoring quadrupole and higher order effects), a refrigerator magnet
does no work in order to remain attached to the refrigerator, super glue
does not do work when hanging an object from the ceiling. Simply put, the
matter of *containment* by a force, no matter how large, does not require
the expenditure of energy. Whether it is necessary for the mechanism which
is exerting the force to consume energy is an entirely different matter
than that of containment. A standard electromagnet is dissipative, a
superconducting magnet is not (in the proper regime).

>>   Remember, if it consumes energy, it must be radiating energy, or raising
>>the potential energy of the combined system. So, if your spinning torus is
>>held together with the thirtieth century equivalent of elastic bands and
>>chewing gum, they wouldn't consume energy. If it's held together by
>>"tractor fields", well, then the elements generating your fields might
>>themselves be dissipative, but that is a separate problem from the matter
>>of containment.
>
>Let's get real, Chris.  What kind of material are you proposing as a
>passive containment system?  You know and I know that molecular bonds
>are about as strong as tissue paper compared to the centrifugal force
>we're talking about.  I was thinking of some form of magnetic
>containment, personally; but it's pretty silly to talk about passive
>containment, except by some completely new form of matter.
>
   Well, I wouldn't say this is necessarily any less likely than the idea
of the space warp in the first place. Normal materials are bound by
gravitational or electromagnetic forces. If we're considering making the
torus out of degenerate nuclear material, you might allow for the
possibility of a nuclear binding mechanism rather than an electromagnetic
one. Anyway, magnetic containment is not intrinsically dissipative either.
While the actual implementation may be dissipative, there is no theoretical
reason to require it. The original question was "what sort of energies
would be required to contain such a system". The answer is that, in
principle, no energy would be required beyond startup energy, though your
mileage may vary depending on how you decide to do it. Without a concrete
suggestion as to how the containment is to be done it is not possible to
put any numbers on the energy consumption.

>(This isn't the first time you've made some truly astonishing
>assumptions on materials science here, either.  First it was your
>statement that we could never develop high-temperature superconductors
>lighter than today's low-temperature superconductors; 
>
   Whoa! I didn't say this. I said that today's low-temperature
superconductors could be made into an energy storage device of a certain
mass and volume per unit energy stored. I then stated that it would be
unlikely that you could do much better than that for high-temperature
superconductors. This was not because they wouldn't be lighter. Most of the
weight I mentioned in that posting was for mechanical support against the
magnetic explosive force. This is independent of what material makes up the
conducting elements. The magnet I listed had to fight a thousand
atmospheres of pressure. How light the superconductors become isn't the
question, it's a matter of ductility and tensile strength. High Tc
superconductors are ceramics, and are brittle. Their mechanical properties
are poorer than those of conventional superconductors. This is not likely
to change in the forseeable future, because the superconducting state seems
closely linked to the presence of coupled two dimensional conducting sheets
with a non-conductive spacing material. Non-conducting materials usually
have poor tensile strength because the electron orbits are highly localized
so that any stretching of the lattice effectively breaks the bonds. While
my research isn't into the mechanical properties of high-Tc
superconductors, my master's degree came from a theoretical examination of
the electrical properties of them, and my PhD work now is associated with
some of their other properties.

>now it's this
>assertion that we will be able to develop materials orders of magnitude
>stronger than the theoretical maximum for molecular-bonded matter.
>
   You said molecular bonds, I didn't.

>Not
>only are they both highly strange assumptions, they seem to conflict
>directly; finding the common denominator is left as an exercise for the
>reader.)
>
   Actually, I think your position is closer to conflicting. You say that
molecular bonds can't be made very much stronger, and yet believe that
ceramics can be designed with higher ductilities than some metal alloys
designed specifically for good mechanical properties (and flux pinning and
a few other useful things, like high BCS coupling strength).

>-- 
>Tim Maroney, Mac Software Consultant, sun!hoptoad!tim, tim@toad.com
>
>"I see little divinity about them or you.  You talk to me of Christianity
> when you are in the act of hanging your enemies.  Was there ever such
> blasphemous nonsense!" - Shaw, "The Devil's Disciple"


-- 
 Christopher Neufeld....Just a graduate student  |
 neufeld@helios.physics.utoronto.ca              | "Vulcan has no moon."
 cneufeld@pro-generic.cts.com                    | "I'm not surprised!"
 "Don't edit reality for the sake of simplicity" |

------------------------------

Date: Mon, 12 Feb 90 17:58:32 EST
From: John Roberts <roberts@cmr.ncsl.nist.gov>
Disclaimer: Opinions expressed are those of the sender
	and do not reflect NIST policy or agreement.
Subject: measurement standards (aerospace)


>From: cs.utexas.edu!samsung!munnari.oz.au!cluster!metro!otc!gregw@tut.cis.ohio-state.edu  (Greg Wilkins)
>Subject: Re: metric vs. imperial units

>You guys just keep making excuses!!!!   Yes an entire country can convert
>from imperial to metric -> Eg Australia and we have the safest airline in
>the world.  If any airline or spaceflight relies on the pilots ability to
>do mental arithmetic in his native units, then it aint safe!!
Other people have posted comments on this. Keeping space flight standards
consistent with aviation standards at least for the time being sounds
reasonable.

>The whole point is that metric has many big points in its favour:

> - The rest of the world is using it
Agreed.

> - To some extent, when designing physical systems, recurring decimal
>   places can be avoided, hence round off errors can be reduced.
For shame! That's one of the few areas in which the "standard" system really
shines, since it was a major design criterion. For instance, express 1/3
of a foot in inches (4). Now try 1/3 of a meter in mm (333.33333333333...).
As another example, the number of feet in a mile (5280) is evenly divisible
by 2, 3, 4, 5, 6, 8, 10, 11, 12, 15, 16, 20, 22, 24, 30, 32, 33, 40, 44, 48,
55, 60, 66, 80, 88, 96, 110, 120, 132, 160, 165, 176, 220, 240, 264, 330,
352, 440, 480, 528, 660, 880, 1056, 1320, 1760, and 2640, a total of 46
integer factors! In contrast, the number of meters in a kilometer (1000)
is evenly divisible by 2, 4, 5, 8, 10, 20, 25, 40, 50, 100, 125, 200, 250,
and 500 - only 14 factors. In either case, the most useful factors are
probably those <= 20, still a ratio of 12 to 6.

> - To do any sort of calculation by computer, metric is by far better than
>   imperial, thus reducing the possibility of program error.
That depends on how you express the terms. If metric distances are given
as "3 kilometers, 768 meters, 122 millimeters", and "standard" distances
are given as "11256.3827165 feet", then the metric system will lose out.
(Of course you wouldn't use metric measures in this fashion, but "standard"
measures are used both ways, depending on the application.) I would presume
that the traditional measures were developed for mental calculation, while
the metric system was developed for use with hand calculations and slide
rules. Digital computers can handle both, if you're careful.

>I am told that America is the most inwardly looking nation on the planet,
>and boy does it show!!!   
Please convince the Central American nations of this fact - it would be very
helpful to our diplomatic relations! :-) Actually, I suppose this is partly
correct and partly incorrect, and attitudes vary over time. Since "inwardly
looking" has no clear definition, commenting on this is sort of like trying
to respond to the statement, "Men! You're all alike!" You could also say
that the US is the only nation sufficiently democratic not to have forced
its citizens to give up their traditional measures. :-) :-) :-)
The government is willing to go fully metric any time the public agrees to
it. I believe the military has already gone mostly to metric (except aviation,
I suppose).

>If you guys are serious about international
>cooperation you have to pick a set of common units (for more cost and
>saftey reasons), and when it comes to the choice metric wins hands down.
I think things are headed in that direction - it's just slower in some fields
than in others. Also remember that the US is not the only country to stick to
"peculiar" standards. Europe has been blocking an international standard on
HDTV television broadcast format unless it's PAL-compatible (or SECAM?) while
the US and Japan are willing to settle on an extension of NTSC. 

>Anyway, to sum up, stop making excuses, start making changes, put a bit
>of effort in , and maybe, just maybe you will be able to talk technical
>with the rest of the world in twenty years or so!
Or maybe we'll be able to use subterfuge to get you to talk technical with
us. Haven't you folks in metric countries wondered why your integrated
circuits have a pin spacing of 2.54mm? :-) :-) :-) :-)
                       John Roberts
                       roberts@cmr.ncsl.nist.gov

------------------------------

Date: Mon, 12 Feb 90 09:55:29 EST
From: John Roberts <roberts@cmr.ncsl.nist.gov>
Disclaimer: Opinions expressed are those of the sender
	and do not reflect NIST policy or agreement.
Subject: Laser/solar sail propulsion


>From: portal!portal!cup.portal.com!hkhenson@apple.com  (H Keith Henson)
>Subject: Re: Spacecraft drives and fuel efficiency

>A civilization actually able to travel between the stars it seem likely to
>me is going to be tapping stars for energy.  We already know of one way to
>do that, build a bunch of monster solar power plants, and convert the
>energy to laser light.  Lasers will levitate against 1g, and as someone
>(Henry?) recently said, a year at one gee is the speed of light.  
>(How to stop is left as an exercise)  Keith Henson

Lasers can (theoretically) lift a payload against 1g in a launch from Earth
(the Livermore proposal) because the laser is heating a reaction mass (i.e.
a block of ice) which is several times the mass of the payload, and which is
consumed in a few minutes. (Another option involves heating the air under
the payload as reaction mass.) Driving a manned spacecraft and its sail
in open space at an acceleration of 1g by pure photon pressure without blasting
it into plasma in a fraction of a second would be a much greater challenge.
(It might be done with a *really* huge thin sail (to reduce required power
density), optimized for reflection at the wavelength of the laser (to reduce
heating).) In practice, I think much lower accelerations are likely to be
preferable.
                     John Roberts
                     roberts@cmr.ncsl.nist.gov

------------------------------

End of SPACE Digest V11 #39
*******************