Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V13_0/V13_070.TXT

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Subject: SPACE Digest V13 #070

SPACE Digest                                      Volume 13 : Issue 70

Today's Topics:
              Russian Attack on Tel-Aviv
          Re: What is cosmological constant?
              Re: SPACE Digest V13 #044
             Re: The Plains of Abraham II
              Re: SPACE Digest V13 #050
              Re: Interplanetary travel

Administrivia:

    Submissions to the SPACE Digest/sci.space should be mailed to
  [e-mail].  Other mail, esp. [un]subscription requests,
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----------------------------------------------------------------------

Date: 18 Jan 91 23:07:51 GMT
From: usc!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!netnews.upenn.edu!grad1.cis.upenn.edu![e-mail]  (Michael Gilbert)
Subject: Russian Attack on Tel-Aviv


Did anyone catch the newscast on friday at about 5:30 pm (Eastern US
time)?  The air raid sirens went off in Tel-Aviv, only to have it
discovered that what they saw streaking across the sky was not an
iraqi missle, but a Russian booster rocket re-entering!

Sure picked a good time and place for it, eh? :)

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Date: 19 Jan 91 16:49:41 GMT
From: coplex!disk![e-mail]  (joefish)
Subject: Re: What is cosmological constant?

In article <[e-mail]> [e-mail] (Daniel E. Platt) writes:
>In article <[e-mail]>, [e-mail] writes:
>picture.  I've heard it suggested by Big-Bang supporters in the '70's (before
>things seemed so strongly in their favor) that Einstein ended up regretting
>that he suggested it at all... that he had intended to submit it as a conjecture
>as one possible way that the universe could be steady state.  The idea seemed
>to take off with a life of its own, and he regretted the choice.  However,
>the person I heard it from had also said that E/M fields don't contribute to
>the curvature since its stress-energy tensor is traceless.... (if you remember
>that faux pas that I made when I asserted it so strongly).
>

    It is difficult for me to understand why Einstein's Cosmological
constant is discussed other than in the interest of the history of
the General Theory of Relativity.
      When Einstein introduced it, it was assumed that our galaxy
represented the entire universe, and to be stable, a finite universe
needed the constant.
      The type of universe could only have been steady state at the
time, so Einstein did not regret the choice, he merely acknowledged
that if he had known that there were countless galaxies and they
were apparently receding, he would never suggested (or considered)
his cosmological constant.
     There is no reason to consider applying his constant at
the present time, but it is useful to remember that the Big Bang
was never thought of until Hubble's discoveries in the 20's and
30's.

     It is also not certain that present concepts of the big bang
are correct.   While the general idea that the universe is expanding
is well accepted, the idea that it happened x years ago is flawed.
     It can pretty well be assumed that x years is the minimum
time since creation-:-), but a maximum time is impossible to
prove, since the universe will always appear pretty much as it
does now.
     The Cosmological Constant offered by Einstein was important
as long as it was unknown that the universe is expanding, and
before the number of stars in all the other galaxies were known,
but the constant would make no sense with the present known
size of the universe with the present estimate of the number
of stars in all the galaxies, regardless of the type of universe,
steady state, big bang, or whatever else there may turn out to be.

Joe Fischer      [e-mail]

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

Date: Sat, 19 Jan 91 22:12:59 EST
From: Tommy Mac <[e-mail]>
Subject: Re: SPACE Digest V13 #044

Shuttle was the answer:

Question -
     What can I do to follow up that neato thing that Kennedy started?
                                           -Nixon



Tommy Mac      "What Spices?  -  First words spoken to Columbus in America
Acknowledge-To: <18084TM@MSU>

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

Date: 18 Jan 91 19:58:34 GMT
From: zephyr.ens.tek.com!wrgate!mtdoom![e-mail]  (Dan Tilque)
Subject: Re: The Plains of Abraham II

[e-mail] (S Schaper) writes:
>Well, Henry, this is your big chance to invade the U.S. Liberate the Northwest
>third of the country...

The Ontarions are coming, the Ontarions are coming...

You realize that you'll have to fight off the Californians if you want
this particular neck of the woods, don't you?

---
Dan Tilque    --    [e-mail]

Free the Upper Left Coast!

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

Date: Sat, 19 Jan 91 22:27:05 EST
From: Tommy Mac <[e-mail]>
Subject: Re: SPACE Digest V13 #050

Nick Szabo:  I've seen you flaming people left and right since I got on this
             server, but I will try to be civil while flaming you:

You seem to feel that space stations are a no-no, or perhaps evil, because:

-They would be a manned presence in space
-They are *OUTDATED*
-They would cost too much for what they deliver.

Nick, maybe in fifty years, what you say would be true.  But right now, the
MIR *space station* is the pinnacle in terms of what we have learned (as a
species) re; Humans in space. And why send anything into space, if you don't go
into space to reap the benefits?

Maybe you should find some way of expressing that itty-bitty bit of irritation
that you feel toward the Soviets better-planned space program, or our non-plann
ed program, rather than raging on people who have innocently concluded that a
space-station would be good, including me, many strangers from this list, and
even some soviets who know more about it than all of us put together.

Hope you don't think I'm a ----head, but you have been putting the screws to
anyone who advocates anything you don't agree with.

-Tommy Mac.  18084tm@msu   Graffitti of the week-
                                  Illiterate?  Write for free pamphlet.
Acknowledge-To: <18084TM@MSU>

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

Date: Fri, 18 Jan 91 22:31:45 EST
From: John Roberts <[e-mail]>
Disclaimer: Opinions expressed are those of the sender
    and do not reflect NIST policy or agreement.
Subject: Re: Interplanetary travel


>Date: 7 Jan 91 03:12:51 GMT
>From: usc!cs.utexas.edu!news-server.csri.toronto.edu!utgpu!cunews!cognos!geovision![e-mail]  (Gord Deinstadt)
>Subject: Re: Interplanetary travel

>[e-mail] (John Roberts) writes:
>>[I said]
>> Since the ideal reaction mass is 4 times the (payload+
>>       ------------------------------------------------
>>>structure) mass, there is no point in having a higher-density fuel.
>> ---------------
>[Mr. Roberts asks]
>>I don't understand why that should be considered an ideal ratio. By the
>>classic equations, this should allow a total change in velocity of about
>>1.6 times the exhaust velocity, but why is that a particularly good number?

>Alas, although I read about this a year or two ago, I have not seen
>anything which actually gives the derivation.  It was
>new at the time.  As I understand it this is the ratio that gives the
>minimum energy requirement for a given delta-vee.  If you increase
>exhaust velocity to decrease the reaction mass, the energy
>required goes up (in the limiting case the exhaust is photons and we
>know how bad that gets).  If you decrease the exhaust velocity, the
>savings per unit of reaction mass is more than eaten up by the increase
>in reaction mass you have to carry.  Apparently the minimum falls at
>the point where the reaction mass is 4 times the mass of everything
>else.

>Perhaps someone who knows more about it would be kind enough to
>post the math?  On the space-tech mailing list, perhaps.

>Gord Deinstadt  [e-mail]

The math I posted a week or so back seems to be plausible, or at least to
come up with the same number (about 3.922). As I mentioned then, the formula
does not take relativity into account. (Is there a basic rocket equation
applicable when exhaust and possibly the rocket are moving at relativistic
speeds? Someone must have considered it, but there may be no simple answer.)

For the formula to have any meaning, a number of assumptions must be made.
For instance, we must assume that engines can be built to convert the energy
supply to exhaust kinetic energy with equal ability and efficiency over a
range of exhaust velocities (though any particular engine might have only
one velocity). For any practical application, the physical properties of
rocket materials and propellants are likely to make this difficult.
It is also assumed that the cost of energy is the overriding concern, with
little or no regard to availability of propellant, etc. While there are
possible scenarios in which this would be the case, I suspect that in general
it would not. Within the orbit of Mars, at least, solar energy is available
in useful concentrations, and can be applied to solar sails or ion drives.
Nuclear-powered ion drives made using near-term technology are also based
on the premise that energy is more readily available than reaction mass in
interplanetary space.

Also, one should note that if energy is such a critical consideration, then it
is much more energy efficient to use a propellant mass many times that of the
rocket, all expelled at the moment of launch (i.e. a linear launcher on a
celestial body, with the celestial body as the "exhaust").

>Date: 12 Jan 91 03:26:40 GMT
>Subject: Re: Interplanetary travel

>Delta vee is the independent variable.  Knowing it, you will
>get the miminum energy consumption by applying the formula for that delta
>vee.  You will also get the highest vehicle performance for a given reactor
>output, even though other ratios yield a lower overall mass.
>(In other words, your high-energy ship would
>go both faster and farther if you added some tankage and changed the
>blend, without changing the ship in any other way.)  The only question
>is whether you also get the minimum cost.

I suspect there will almost always be more significant cost factors (see
statement above and table below.)

>Higher power means higher cooling requirements (conversely more propellant
>means more cooling capacity).  Also bigger engines and more fusion fuel
>(which will cost something).  Against this is the cost of more propellant
>(not highly refined, possibly just pumped out of a lake on some Jovian
>moon), and the extra cost of the tankage and plumbing.

Jovian moons (except possibly the outermost?) are deep within a large gravity
well, which increases the energy expense of extraction. It might be better
to find an asteroid in a convenient location. There is some speculation
that propellant might someday be extracted from Earth's moon for less than it
costs to lift it from Earth.

>(I imagine
>direct injection could be used to eliminate the need for a heat exchanger.)
>I suspect that, incorporating all these costs into the basic relationship,
>the lowest-cost point will fall near the minimum-energy point.  It is
>of course impossible to know for sure until such a ship is actually built,
>and even then the optimum point depends on externalities, eg. interest
>rates.

After poking at the computer a while, I got the following table showing the
performance at various mass ratios. The first column represents sample
mass ratios, where this is defined as the mass of the propellant divided by
the mass of everything else. The second column is a multiplier showing
relative delta-V as a function of mass ratio, where the non-propellant mass
and the energy available are regarded as constants. While it is true that
delta-V is maximized when the mass ratio is about 3.9-4.0, you should note
that the curve is very flat over a wide range of mass ratios. This means
that delta-V is relatively insensitive to mass ratio over that range, so it
is highly probable that some other factor will more important to costs than
a specific mass ratio. For instance, if you run a variable-velocity ion
drive rocket with a mass ratio of 1 instead of 4, you will get a 75%
savings in propellant use (and a 60% savings in total mass) with only a
14% loss of delta-V. This ignores the performance benefits you might gain
by using a smaller container to store the propellant.

The third column gives the relative delta-V for the specialized case in
which the rocket uses a "fuel", which serves as both energy source and
propellant. In this case the energy available is a linear function of
the amount of fuel on board. This description applies to almost all modern
rockets and perhaps someday to advanced fusion rockets. Note that the
possible delta-V continues to go up with increasing mass ratio, and that
there is no obvious optimum point. The ratio chosen would then be a complex
function of technological limits, relative costs, mission requirements,
and so on.

                     ideal
                     "thermal
 mass      relative  exchange"     "fueled"
 ratio     delta-V   (i.e. ion)    rocket
---------            ----------    --------
 0.100000     ->     0.426240      0.134789
 0.200000     ->     0.576551      0.257842
 0.400000     ->     0.752375      0.475844
 0.800000     ->     0.929372      0.831256
 1.000000     ->     0.980258      0.980258
 2.000000     ->     1.098612      1.553672
 3.000000     ->     1.131905      1.960516
 3.500000     ->     1.136976      2.127087
 3.800000     ->     1.137994      2.218358
 3.900000     ->     1.138075      2.247518
 4.000000     ->     1.138044      2.276089
 4.100000     ->     1.137911      2.304094
 4.500000     ->     1.136499      2.410878
 5.000000     ->     1.133208      2.533931
 6.000000     ->     1.123472      2.751933
 7.000000     ->     1.111508      2.940774
 8.000000     ->     1.098612      3.107345
 9.000000     ->     1.085449      3.256347
10.000000     ->     1.072371      3.391136
15.000000     ->     1.012406      3.921033
20.000000     ->     0.962763      4.305605

>There should be a tug based at each port to supply the
>delta vee when it is required.  The tugs should have separate throttles for
>reactor power and propellant flow, so the crew can optimize for different
>loads, costs, and deadlines.

That's a good idea if it can be done.

>Gord Deinstadt  [e-mail]
      John Roberts
      [e-mail]

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

End of SPACE Digest V13 #070
*******************