Date: Mon, 1 Aug 88 19:05:46 PDT
From: Ted Anderson <ota@angband.s1.gov>
Message-Id: <8808020205.AA08893@angband.s1.gov>
To: Space@angband.s1.gov
Reply-To: Space@angband.s1.gov
Subject: SPACE Digest V8 #309

SPACE Digest                                      Volume 8 : Issue 309

Today's Topics:
			   Re: Solar Sails
			 Re: Ramscoop engine
			   Re: Solar Sails
	       Re: Space Station Alternatives Required
			 Re: Von Braun quote
			   Re: Solar Sails
			   Re: Solar Sails
Re: Special communications system prepared for Neptune encounter (Forwarded)
			   Re: Solar Sails
			  Shuttle-C details
			Re: Shuttle-C details
----------------------------------------------------------------------

Date: 21 Jul 88 16:58:49 GMT
From: att!lzaz!lznv!psc@ucbvax.berkeley.edu  (Paul S. R. Chisholm)
Subject: Re: Solar Sails

< "Would you buy a used operating system from these guys?" >

In article <10922@oberon.USC.EDU>, robiner@ganelon.usc.edu (Steve) writes:
> If the light bounces off the sail, how does it impart momentum.  What 
> energy of the photon is now reduced?  I think the photons must be
> ABSORBED by the sail for this to work.

Imagine (if you will:-) a photon travelling from left to right.  It has
momentum of p kg-m/sec (x component; y and z components are zero).

It strikes the light sail and is momentarily absorbed.  An electron in
the light sail goes to a higher energy level, and the light sail's
momentum is increased by p (in the x direction).

A short while later, the photon is re-emitted in the direction from
which it came.  The excited electron settles down, the photon now has
momentum -p, and the light sail's momentum has increased by by 2p.

-Paul S. R. Chisholm, {ihnp4,cbosgd,allegra,rutgers}!mtune!lznv!psc
AT&T Mail !psrchisholm, Internet psc@lznv.att.com
I'm not speaking for my employer, I'm just speaking my mind.

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

Date: 21 Jul 88 14:05:42 GMT
From: aplcen!aplcomm!stdc.jhuapl.edu!jwm@mimsy.umd.edu  (Jim Meritt)
Subject: Re: Ramscoop engine

In article <1988Jul19.045507.25185@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
}Agreed that there is a real problem with doing something with the
}interstellar mass once you've got it; fusing ordinary hyrdogen is not
}easy.  If you're willing to sacrifice the fuelless nature of the Bussard
}ramjet, one way around the problem is to react the interstellar gas with
}antimatter carried on board.  *That* reaction is fast!


"How about quantum fluctuations?" he said, dodging the bricks. "I read a
read interesting article in JBIS a year or so ago...."

Comments on vaccum energy, anyone?


Disclaimer: Individuals have opinions, organizations have policy.
            Therefore, these opinions are mine and not any organizations!
Q.E.D.
jwm@aplvax.jhuapl.edu 128.244.65.5  (James W. Meritt)

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

Date: Thu, 21 Jul 88 23:59 EDT
From: "OK, IS THIS BETTER?" <HINSOND%UNCG.BITNET@cunyvm.cuny.edu>

+ ------------------------------------------------------------------------------

Shari Landes (mind!shari@princeton.edu):

>Is it possible that if you travel faster than the speed of light in space,
>and come back to earth, you would be able to see yourself in the past?
>
>I think it is possible because when you approach the speed of light, time slows
>down, and if you travel the speed of light, time stops.
>But, if you return to earth, after traveling in space faster than the speed
>of light, would you be able to see yourself before you left?

This is a very intriguing question but I'm afraid the answer is disappointingly
simple:  If time travel were possible, wouldn't someone have told us by now?
Think about it..

q:)

----------------------------------------------------------
######################  Dave Hinson ######################
             T I M E  L O R D  A T  L A R G E
Disclaimer:  I make no excuses, I am totally irresponsible
----------------------------------------------------------

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

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

Date: 21 Jul 88 15:39:02 GMT
From: sgi!daisy!wooding@ucbvax.berkeley.edu  (Mike Wooding)
Subject: Re: Solar Sails

In article <1988Jul19.235426.15443@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> >... My question is this: just how does the sun 'push'
> >against the sail? Is it the solar wind of charged particles (protons and elec-
> >trons), or the the actual photonic flux, or both?
> 
> It's mostly the light; as I recall it, the solar wind contributes very little.

 An earlier poster suggested tacking (sailing up wind - er is that
 up light?) might be possible. How's that work. Doesn't a sail boat
 depend on keel and aerodynamic effects on sail? Would "solar" wind
 be "channeled" to produce high and low pressure areas? 
> -- 
> Anyone who buys Wisconsin cheese is|  Henry Spencer at U of Toronto Zoology
> a traitor to mankind.  --Pournelle |uunet!mnetor!utzoo! henry @zoo.toronto.edu

 m wooding

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

Date: 22 Jul 88 01:49:52 GMT
From: al@eos.arc.nasa.gov  (Al Globus)
Subject: Re: Space Station Alternatives Required

From article <8807160300.AA07099@crash.cts.com>, by jim@pnet01.cts.COM (Jim Bowery):
> This congressman, who has supported full funding for
> the space station every time it came up, is certain that, for political
> reasons, the space station WILL BE CANCELLED.  He shares our concern.
> 
> We ask the help of all pro-space people in ensuring that there are
> alternatives to the space station just in case.

My favorite alternatives are broken up into diciplines:

* Manned presence - extend shuttle stay time.  There are a number of
ways to do this.  I think you can get up to about a month.

* Microgravity - use the Industrial Space Facility.  Essentially a man tended
space station under development by private industry in (I think) Texas.
Most of the materials people are unhappy about astronauts moving about
and degrading the gravity environment anyway.

* Life Science - use Mir.  Life science research has little technology
transfer problem and the long lead times make dependence on the Soviet's
a minor issue.  If they cut us off we'd have plenty of time (and incentive!)
to get a facility together before any serious problems developed.

* Astronomy - dedicated facilities.  Launch with one of the bevy of
commercial launchers.  Space Station is lousy place to
put most astronomy instruments anyway.

* Earth viewing - same as astronomy.

* Orbital construction - design a new space station for this purpose only.
Since it is a single purpose facility it should be MUCH cheaper -
the integration problems will be much less (believe me, I know).

A big advantage of breaking the Space Station up into a lot of small
facilities is robustness.  Any single failure will not take out the
whole ball of wax.  Just think if we ever really built our $20 billion
station then had a major accident ala Challeger.  It would be the
end of the manned space program (and maybe NASA).  Putting all our
eggs into a single facility is a very bad idea.  And that's what
Space Station is, a single facility.

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

Date: 21 Jul 88 18:18:43 GMT
From: att!whuts!homxb!mtuxo!tee@ucbvax.berkeley.edu  (54317-T.EBERSOLE)
Subject: Re: Von Braun quote

In article <1219@thumper.bellcore.com>, karn@thumper.bellcore.com (Phil R. Karn) writes:
> 
> I've never understood why it's so necessary to put humans into space in
> order to benefit from their ability to react to unforseen circumstances.
> In many situations, it makes far more sense to keep such people on the
> ground instead of sending them along with the payload, especially since
> the state of the communications art has gotten so good. 

But it would'nt be near as much fun!

-- 
Tim Ebersole ...!{allegra,ulysses,ihnp4,mtune,...}!mtuxo!tee

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

Date: 21 Jul 88 07:42:14 GMT
From: unisoft!gethen!abostick@ucbvax.berkeley.edu  (Alan Bostick)
Subject: Re: Solar Sails

In article <1748@puff.cs.wisc.edu> eric@puff.cs.wisc.edu (Eric "TheBoo" Bazan) writes:
>I can under-
>stand how actual particles could exert a push against physical matter, but
>not photonics energy. If so, how does photonic energy 'push' matter in a vac-
>uum. I have a poor physics background, so this may be a stupid question, but 
>I'm curious anyway.
>
>			thanks,
>				-Eric(eric@shorty.cs.wisc.edu)
>				      eric@cs.wisc.edu

Photons _are_ actual particles, that happen not to have rest mass.

The energy of a photon is related to its momentum by the formula

			E = c*p

E = energy, p= magnitude of momentum, and c = speed of light.  This
comes (take your pick) out of relativity, or out of classical
electromagnetic theory.

When light is reflected off a lightsail (or any mirror) it changes
direction of propagation, and hence its momentum changes.  When it is
normally incident (i.e. direction of propagation is perpendicular to the
reflecting surface) the momentum changes direction by 180 degrees; or,
if you will, the momentum changes sign.  The photon changes its momentum
in this case by 2*p, where p was the magnitude of its original momentum.
Since overall momentum must be conserved, that means that the reflecting
surface was given a kick of momentum of magnitude 2*p in the direction
that the photon was originally traveling.  Expressed in terms of the
photon's energy, the kick of momentum given to the reflector is 2*E/c.
Now, the Solar constant is a measure of power flux, the amount of energy
passing through a given area in a given time.  So a reflector will be
given a steady momentum push over that unit time equal to the total
energy reflected in that time.  That is to say, the _pressure_ on the
reflector due to the incident light which is reflected will be given by

		P = 2*S/c

where P is the pressure (force per unit area), S is the solar constant,
and c is the speed of light.  This means that, in the neighborhood of
the earth (above the atmosphere) the pressure exerted on a reflector is
about 4.0e-6 Nt/(m**2).  That is to say, it will take a reflecting
surface of rather more than 2 million square meters (or, roughly, a
circular mirror over one and a half kilometers in diameter) to be able
to accelerate a mass of one kilogram at a rate of one gravity, and that
kilogram has to include the weight of the reflecting surface!  Clearly
this is not the way to go if you are in a hurry; but if you are willing
to travel at a more leisurely pace, solar sails have promising
properties.

					Alan Bostick
					ucbvax!unisoft!gethen!abostick

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

Date: 20 Jul 88 04:45:39 GMT
From: unmvax!charon!geinah.unm.edu!ee2131ac@ucbvax.berkeley.edu  (SEDS-UNM)
Subject: Re: Solar Sails


Members of the solar sails discussion might be interested in the work    
done by Steve Abrams at the Univ. of Texas at Austin.  He will be
presenting a paper on solar sails this August during the SEDS 1988
International Conference in Houston.

Steve can be reached at:  sedspace@doc.cc.utexas.edu

-Ollie

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

Date: 21 Jul 88 18:17:50 GMT
From: ulysses!thumper!karn@ucbvax.berkeley.edu  (Phil R. Karn)
Subject: Re: Special communications system prepared for Neptune encounter (Forwarded)

> ... Linked
> electronically, the two systems -- 23 VLA antennas that now have 
> their X-Band receivers, and the 112-foot and 230-foot dishes at 
> Goldstone -- will function as a single receiving system.   

I'm curious to know just what this means. Are the received signals from
the two sites being added coherently, as in VLBI?

Phil

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

Date: 21 Jul 88 18:40:30 GMT
From: ulysses!thumper!karn@ucbvax.berkeley.edu  (Phil R. Karn)
Subject: Re: Solar Sails

> [...] My recollection is that
> people who run orbiting satellites have to take it into consideration as
> a minor source of orbit perturbations.

The effect of solar radiation pressure on a satellite's orbit is usually
negligible. Of course, it can become significant for an unusually large
and light satellite like Echo, or a satellite with a solar sail.

The effect of solar radiation pressure on a satellite's ATTITUDE, on the
other hand, is major -- in most cases it is the single most significant
perturbing force. Voyager was able to save a considerable amount of
attitude control fuel early in its mission by slowly rolling around its
antenna boresight axis during the cruise phase. This had the effect of
averaging out the effect of the solar radiation torque on the
magnetometer boom, greatly reducing the angular momentum imparted to the
spacecraft that had to be removed with the attitude control thrusters. I
am hoping that we can use a similar trick with AMSAT Phase IV, since the
antennas present an assymetrical cross section to the sun.

On AMSAT-Oscar-7, each of the 145/432 MHz turnstile antenna elements
(made out of ordinary metal carpenter's rule from the local hardware
store!) was painted white on one side and black on the other.  The
radiation pressure of light on a reflecting surface is twice that of
light on an absorbing surface, so the resulting torque produced a nice
slow spin.  A permanent bar magnet along the spin axis kept the spin
axis in line with the earth's magnetic field, and the eddy current drag
of the earth's magnetic field cutting across the metallic spacecraft
kept the spin rate from ramping up too high. Note that the direction of
rotation is OPPOSITE that of the toy "radiometers" one can find in
science museum gift shops. They contain air and work by the reaction of
the heated air on the black surface; this force exceeds the imbalance in
photon pressure.

Phil

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

Date:     Fri, 22 Jul 88 10:49:14 CDT
From: Will Martin -- AMXAL-RI <wmartin@almsa-1.arpa>
Subject:  Shuttle-C details

From the recently-Digested Canopus summaries:
> SHUTTLE-C COULD HELP SPACE STATION - can880507.txt - 5/10/88
> 
> Shuttle-C looks much the current Space Shuttle but for wings and
> vertical stabilizer which are lacking, and windows on the forward
> fuselage. It would use the same boosters and tank, and would carry
> its cargo in a strongback sitting above an engine module identical to
> the Shuttle's boattail section.

I think this was mentioned earlier, but I just cannot recall -- is this
shuttle variant recoverable or a one-time-use unit? Without the wings or
tail it certainly cannot fly back, even under remote control, but
perhaps it is designed for parachute recovery?

If it is one-time use, how can the engine module be identical to the
regular shuttle? I thought the only justification for the very expensive
SSMEs was that they would be re-used "n" times to make them economically
viable. Or is the thing supposed to stay in orbit for use as a tug or
something? 

If it is recovered by 'chute, is the basic structure (the "strongback"
mentioned above) stronger than the regular shuttle? There have been
numerous postings about the fact that the shuttle would break up if
ditched, so either sea- or land-based 'chute recovery would depend on
the structure being strong enough to withstand rough touchdowns.

Regards, Will Martin

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

Date: 24 Jul 88 21:38:31 GMT
From: cfa!cfa250!willner@husc6.harvard.edu  (Steve Willner P-316 x57123)
Subject: Re: Shuttle-C details

From article <8807221618.AA24068@angband.s1.gov>, by
wmartin@ALMSA-1.ARPA (Will Martin -- AMXAL-RI): 
> I think this was mentioned earlier, but I just cannot recall -- is this
> shuttle variant recoverable or a one-time-use unit? 
> If it is one-time use, how can the engine module be identical to the
> regular shuttle? 

Here's some more information from CANOPUS that was omitted from the
condensation: 

With a two-engine module, Shuttle-C would be able to place 
100,000 pounds in the Space Station orbit (253 miles). A three-
engine module would raise that to 150,000 pounds. The manned 
Shuttle at present is rated for 43,000 pounds to that altitude.

For the Space Station scenario, an Orbital Maneuvering Vehicle 
(an unmanned, short-range space tug now in development) would be 
atop the payload. Its guidance system would be tied into the 
Shuttle-C maneuvering system and would direct it to the Space 
Station. After the payload was off-loaded, the OMV would send the 
empty Shuttle-C on a re-entry course and then return itself to 
the Station.

Eudy said that disposing of the boattail and its main engines 
was found to be more economical than recycling. The main engines 
would be drawn from the manned Shuttles after 10 missions rather 
than being overhauled. A 50 percent discount in engine costs 
could be realized by the increased production rate this would 
demand.  

Development cost would be up to $1.5 billion. Current definition 
studies are to run through 1989. If the project is approved as a 
"new start," the first launch could come as before 1994, early 
enough to support Space Station.
-- 
Steve Willner            Phone 617-495-7123         Bitnet:   willner@cfa
60 Garden St.            FTS:      830-7123           UUCP:   willner@cfa
Cambridge, MA 02138 USA                 Internet: willner@cfa.harvard.edu

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

End of SPACE Digest V8 #309
*******************