Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/spacedig/v11_3/v11_379.txt

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Subject: SPACE Digest V11 #379

SPACE Digest                                     Volume 11 : Issue 379

Today's Topics:
                Re: Apollo 12
            Re: Re: Dyson spheres?
         NASA Headline News for 05/08/90 (Forwarded)
----------------------------------------------------------------------

Date: 9 May 90 04:59:23 GMT
From: cs.utexas.edu!helios!wfsc4![e-mail]  (Hal Mueller)
Subject: Re: Apollo 12

In article <[e-mail]> [e-mail] (Gregory N. Bond) writes:
>For things like planetary probes I can understand how the window opens
>and closes, depending on the relative motions of planets.
>
>But for moon shots, well, it seems the moon is always in the same
>relative orbit, so a direct earth-moon window would happen once per
>day (as the earth rotated to the correct spot for launch).  So why is
>the window only open a few days per month?
>
>And, given that the launch was to LEO and then boost into lunar
>transfer orbit, then why have a window at all, seeing the eject point
>from LEO will come around once every orbit (a couple hours, max).
>

What you say is correct, but there are 2 other factors to consider:
landing site lighting conditions, and orbital planes.  It's desirable
to land in lunar day rather than night; the lighting requirements were
actually constrained even more than that in order to have decent
terrain relief.  That limits trans-lunar injection (TLI) to a short period
of the month.

As to why there must be a launch window shorter than a few days,
consider the problem in 3 dimensions.  There is some point in
the lunar orbital plane that the rocket must pass through with
a certain velocity; at that point the TLI burn can be performed
(this is actually a region, not a point).  Now the problem is
to put the rocket into an orbit that will hit that point with the
right velocity.  But payload to LEO is maximized by a launch into
an orbit with inclination equal to the launch site's latitude
(that is, launch due east).  It may be that, for any time (or for
a very large window), there exists an orbit which intersects the TLI point
and could be reached from your launch site.  But that orbit, if it
exists, will not be as efficient as the minimum inclination orbit.
(For that matter, you could simply maneuver once you're in orbit, but
that becomes VERY expensive).

The short answer is that to maximize payload to the moon, choose
a launch time such that your minimum inclination orbital plane
intersects the TLI point.

--
Hal Mueller            [e-mail]          n270ca@tamunix (Bitnet)
Graduate Student, Department of Computer Science
Research Assistant, Department of Wildlife and Fisheries Sciences
Texas A&M University, College Station, TX 77843

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

Date: 9 May 90 03:39:21 GMT
From: usc!cs.utexas.edu!news-server.csri.toronto.edu!helios.physics.utoronto.ca!physics.utoronto.ca![e-mail]  (Christopher Neufeld)
Subject: Re: Re: Dyson spheres?

In article <[e-mail]> [e-mail] (Russ Cage) writes:
>
>Needless worry.  Here is a back-of-the-envelope analysis:
>
>Assume inner surface of sphere is at 500 K and radiates like a
>blackbody.  Radiation is 3540 W/m^2 (uniformly inside the sphere).
>
>Surface of Sol is at roughly 5000 K, and is close to a blackbody.
>Radiation from Sol is rougly 35,400,000 W/m^2.
>
>Sol will get back ~1e-4 of its own radiated output from a Dyson
>sphere radiating uniformly at 500K, less from a cooler one.  You
>can probably ignore this; at worst, it will be a small problem.
>
   Nope, it doesn't work that way. Your conclusion is correct, though.
   Consider this back of the envelope calculation: take the sphere as a
solid object, thin skinned compared to the diameter of the sphere, with
equal temperatures on the outer and inner surfaces. Now, assuming that
the total potential energy of the sphere isn't increasing, it must
radiate as much energy to the outer surface as the Sun delivers to the
inner surface. If the skin is thin, the area of the inside is roughly
equal to the area of the outside. Since the two surfaces are at the
same temperatures, the inside surface radiates as much energy as the
outer surface, so it radiates one solar power of energy back into the
sphere. The Sun subtends 6.8E-5 steradians of solid angle from the inner
surface of the sphere (at 1AU). Assuming that the radiation from any
element of the sphere is isotropic (I don't know how good an assumption
this is), the Sun catches 1.1E-5 of the reradiation, which raises the
surface temperature of the Sun by much less than 1 degree.

   Anyway, that isn't how you would want to build a Dyson sphere. The
idea is to capture as much of the usable energy output of the Sun as
possible. To do this you need a large temperature differential between
hot and cold sinks. A possible design would be to put a solar furnace on
the inside surface, with most of the surface covered in parabolic
reflectors, with boilers at the foci. The outer surface would be
completely covered in radiator coils, and would behave as a cold sink at
about 400K. The hot sink could easily be made to stay around 3000K
before serious materials problems appear. The limiting efficiency in
this system is 87%, so useful work to the amount of 3.5 million tons
mass energy equivalent per second (3.15E+26 Watts or 68 billion megatons
of TNT per second) is possible.

>Re-reflecting the waste heat inside the sphere several times could
>increase its lifting capacity in direct proportion to the energy
>being reflected (photon momentum P = E/c).  Put that in your
>equation and crank it.

   Yes, but that makes the sphere less useful as a heat engine. BTW, in
the scenario I described in the first section, each surface would see
about three times the solar momentum flux (twice on absorption and once
on emission) or about 1.4E-5 kg / m s. In real units, that's one
kilogram for 425 m^2 of exposed surface, assuming solar momentum flux is
the only force which opposes the gravitational attraction.

>Russ Cage, Robust Software Inc.            [e-mail]


--
 Christopher Neufeld....Just a graduate student  |  "Spock, comment?"
 [e-mail]              |  "Very bad poetry
 [e-mail]          Ad astra! | captain."
 "Don't edit reality for the sake of simplicity" |

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

Date: 9 May 90 05:35:13 GMT
From: trident.arc.nasa.gov![e-mail]  (Peter E. Yee)
Subject: NASA Headline News for 05/08/90 (Forwarded)

-----------------------------------------------------------------
Tuesday, May 8, 1990                  Audio Service: 202/755-1788
-----------------------------------------------------------------

This is NASA Headline News for Tuesday, May 8...



The Kennedy Space Center mission management team announced a
possible problem today with the orbiter Columbia cooling system
found while preparing for the May 17 launch of STS-35 Astro-1.  A
valve change on two Freon cooling loops is required as the
shuttle's electronic gear generates tremendous amounts of heat in
space that must be carried away.  The system is now operating
with a reduced flow and managers will discuss conclusions at the
Flight Readiness Review scheduled tomorrow.

                            ********

Jet Propulsion Laboratory controllers completed a successful
three-day readiness review to prepare Magellan for orbit around
Venus in August.  Now 17 million miles from Venus, the Magellan
spacecraft continues in cruise with all systems performing
well.  Last Friday, the flight team celebrated the first
anniversary of the May 4, 1989 launch from the space shuttle
Atlantis.  Today marks the 96th day before Magellan reaches
Venus.

                            ********

Tonight, Hubble Space Telescope operators will begin preparations
leading to the first pictures.  The hardware is operating as it
should and calibrations continue to ready the telescope to focus
on the first image.  A 36-hour sequence of "bootstrap" commands
to position the telescope and lock the fine guidance sensors onto
guide and target stars will begin at 6:00 P.M. -- preparing for
the first photos possibly on Friday.

                            ********

NASA Administrator Richard H. Truly met yesterday with Canadian
representatives to discuss ongoing cooperative space and science
projects.  While in Ottawa, he visited students from the "Highly
Gifted and Talented Student" program in Ontario, Canada.  They
spent the last year studying the development of NASA.  The
students built scale models of rockets, Space Shuttles and Space
Stations as well as a Skylab model that was presented to Truly.

                            ********









----------------------------------------------------------------
Here's the broadcast schedule for Public Affairs events on NASA
Select TV.  All times are Eastern.


Wednesday, May 9....

                   12:00 Noon     STS-31 crew post flight
                                  news conference at Johnson
                                  Space Center.

                    1:49 P.M.     DARPA launch of Scout/MACSAT's.

Thursday, May 10....

                   11:30 A.M.     NASA Update will be
                                  transmitted.

Friday, May 11......

                   11:00 A.M.     Dr. Clifford Stoll, author of
                                  "The Cuckoo's Egg" will lecture
                                  on computer security.

Monday, May 14-16...

                   9-11:00 A.M.   STS-35 Pre-Launch Briefing.


All events and times are subject to change without notice.
-----------------------------------------------------------------
NASA Select TV: Satcom F2R, Transponder 13, 72 Degrees West
Longitude, Audio 6.8, Frequency 3960 MHz.
-----------------------------------------------------------------
These reports are filed daily, Monday through Friday, at 12:00
Noon, EDT.  This is a service of the Internal Communications
Branch, NASA Headquarters.

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

End of SPACE Digest V11 #379
*******************