🚀 go-pugleaf

>From OTA@S1-A Wed Dec 16 03:59:33 1981

SPACE Digest                                      Volume 2 : Issue 61

Today's Topics:
		  Re: Linear accelerators as launchers. 
				 Skyhooks
		      Planetary Science in extremis
		    A few more comments on skyhooks   
		     multiple-laser launching systems
				Spaceports
			   Elevators into Space
			       Momentum   
----------------------------------------------------------------------

Date: 15 Dec 1981 0855-PST
From: Jim McGrath <JPM at SU-AI>
Subject: Re: Linear accelerators as launchers. 
To: ota at S1-A
cc: space at MIT-MC

My original message only assumed that accelerators be used as the
"second stage", ie you get them up to suborbital speed and then
accelerate them to orbital velocity.

I did a short linear search of my library, but could not find the
article.  Any pointers from out there?

I think the correct equation to use here is 2*A*L = Vf^2 - Vi^2 (Vf =
final velocity, Vi = initial velocity).  Then if we keep a= 100m/s^2
(10g), then we have for a final velociy of 8km/s (a couple of hundred
miles up), then we have

                  L                        Vi
                
                1 km                    7.93 km/sec
                10 km                   7.87 km/sec
                50 km                   7.32 km/sec
                80 km                   6.93 km/sec
                125 km                  6.24 km/sec
                200 km                  4.89 km/sec

If we let A = 100g, then we get 


                1 km                    7.87 km/sec
                5 km                    7.32 km/sec
                10 km                   6.63 km/sec
                20 km                   4.89 km/sec

So we would need on the order of tens of thousands rings in order
to accelerate from about 5km/sec.  Two conclusions can be drawn -
you want this system first for mass transport, not person transport,
and it is easier to build than a skyhook by a couple of orders of
magnitude.  Thus it does appear to be an interesting step between
rockets (or even lasers) and skyhooks.

Jim

-------

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

Date: 15 Dec 1981 0917-PST
From: Jim McGrath <CSD.MCGRATH at SU-SCORE>
Subject: Skyhooks
To: space at MIT-MC

Hans - thanks for the references.

Re skyhook stability - my comment on vertical stability was meant
as a comment on principle - ie that you do have to be careful
(ie anchor it, prevent the payload/structure mass from getting too high,
etc...), and that puts greater demands on the engineering (it also
sets you up for diasters - say you lose your space anchor - then
the beanstalk comes crashing down)

Once again, on the shear forces I was not so much thinking of
operational problems, but potential diasters - like someone slamming
into it in a passanger plane.  The problem is that such diasters
WILL happen, and that the potential loss of such an expensive
structure in one will deter people from building them unless the
engineering and safty precautions are VERY good.  Thus I do not
expect skyhooks within 20 years - more like 20 to 50 years (ie
after we have a lot of space engineering experience and experience
building VERY large structures).

Jim

-------

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

Date: 15 Dec 1981 1222-EST
From: MPH at MIT-XX
Subject: Planetary Science in extremis
To: space at mc

This is an excerpt from an article in the December 18 issue of Science,
"Planetary Science /in extremis/." Copyright AAAS, 1981.

[Start of excerpt]
The office of Management and Budget (OMB) wants the National Aeronautics
and Space Administration to virtually cease its planetary exploration
activities as of fiscal year 1983.  The order was given to the space agency
privately on 24 November.

Although NASA will undoubtedly appeal the decision and try to negotiate a
compromise, it has little time.  The FY1983 budget must be ready for
submission to Congress in January.

The OMB proposal, as it stands, includes the cancellation of the Galileo
orbiter/probe mission to Jupiter, which is already nearly built, and the
Venus orbiting imaging radar (VOIR), which had been penciled into budget
projections as a new start for 1984.  The only mission that will /not/ be
affected is Voyager 2, now on its way toward encounters with Uranus and
Neptune.
[End of excerpt]

-------

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

Date: 15 Dec 1981 1017-PST
From: Ted Anderson <OTA at S1-A>
Subject: A few more comments on skyhooks   
To:   space at MIT-MC  

Note that a skyhook that looses its earth surface anchor doesn't fall
down it falls UP.

As to safety, I would like to emphasize here that a structure that is some
100,000 Kilometers long and weighs millions of tons is not likely to be
inconvenienced by some thing as trivial as a C-5A running into it at 500
MPH.  Though I don't deny that it might be possible to design a
geosynchronous skyhook that would fail in this situation I think it would
be hard to do.  Keep in mind that it needs a strength reserve (strength in
excess of that needed to maintain static equilibrium) to support the
lifting of, say, a million tons per year to GEO or it wouldn't have been
built in the first place.  It is not likely to be a fragile object.

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

Date: 15 Dec 1981 1317-CST
From: Jonathan Slocum <LRC.SLOCUM at UTEXAS-20>
Subject: multiple-laser launching systems
To: space at MIT-MC

As for JP's comment, I wasn't trying to nastily dampen enthusiasm for
getting out there SOMEHOW, it just doesn't/didn't look like riding a
laser beam thru the atmosphere would be viable.  Question-raisers are
not always enemies: maybe some are just ignorant friends willing to
risk appearing like fools if it will stimulate discussion/learning.
Or maybe I AM a fool, but a flat-worlder I am not!

The multiple-beam answer to blooming is certainly thought-provoking.
Each beam runs into the same problems I raised before, only (it is
hoped) at a manageably small level.  Ohmic heating will still take
place, albeit to a smaller extent, and the air in the beam path will
still expand.  Argument: the beam will be off before the expansion
gets going good.  Counter-point: yeah, but the NEXT time the beam
comes on the air will be in its expanding/ed state, and the next, and
the next...  Argument: well, the beam path will have moved (following
the launch vehicle).  Counter-point: but not always significantly; at
sufficient distances and angles of incidence (beam path to vehicle
trajectory) the movement will be small, approaching zero -- especially
toward the laser end, where motion is minimized, hence ohmic heating
and consequent beam dispersion is maximized where its effects are
least desired (farthest from the target).

So we have the following variables: number of lasers (affecting the
pulse length and beam deflection between pulses, hence ohmic heating
potential); how widespread they are (affecting the extent to which
they tend to share a common beam path, i.e., appear in effect like a
single beam); how high they are (how much air they have to punch
thru); and of course their individual power output & frequency.
Without the expertise to run the numbers, I still suspect that
inefficiencies at the target will require a large total beam power,
which will require a LOT of (expensive?) lasers if the average per
beam is to be kept sufficiently low.  If high laser altitude and large
numbers of them are also necessary, it will take LOTS of mountains to
get around the path-sharing problem.  Dispersing the lasers also tends
to increase the laser-to-target distances (through the air).

By the way, the hull of this vehicle is gonna have to ba a damn good
mirror, lest it be subject to melting; what happens to the reflected
energy?  Does anybody REALLY want to watch this thing go up??

Cheers.
-Jonathan
-------

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

Date: 15 December 1981 2202-EST (Tuesday)
From: Hans Moravec at CMU-10A (R110HM60)
To: space at MIT-MC
Subject:  Spaceports
CC: Hans Moravec at CMU-10A


The orbiting linear accelerator article (I thought both the article
and the idea were extremely good) was

Roger D. Arnold and Donald Kingsbury,  The Spaceport,
Part 1: Analog v99 #11  November 1979  pp 48:67  and
Part 2: Analog v99 #12  December 1979  pp 61:77

They propose an accelerator length of 600 km subjecting payloads to
5g, with an active stiffening system on the structure.  Neither the
mass nor the complexity is obviously lower than a cable performing
the same task:  Imagine a cable in low earth orbit that spins in
the plane of the orbit so that the spin just cancels the orbital
velocity at the points where the cable tips come closest to the ground.
The cable is like two spokes of a giant wheel that is rolling on the
earth's surface at orbital speed.  A flying machine can now jump
up and grab the cable end at its lowest and slowest point (for a few
seconds the tip is actually stationary with respect to the ground,
just like the portion of the rim of a rolling wheel in contact with the
ground is momentarily stopped).  The cable can actually enter the
atmosphere (and with terminal guidance and high precision, it could
even kiss the ground), so the job of docking with it is simpler than for
the linear accelerator spaceport.  The payload then hangs on to the end,
and lets the cable flip it around to be flung off at high velocity
later.  At the top of the swing the cable tip is moving at twice
the (orbital) velocity of the cable's center of mass, and if the
payload lets go then, it is sent off with a factor of more that sqrt(2)
beyond escape velocity.  The cable loses some orbital momentum in the
process, wich it can regain from incoming payloads, or high specific
impulse engines at its middle, just like the orbiting linac.

Such a non-anchored skyhook can be build low and spinning fast,
or long and orbiting high and turning slow.  If you build one
to orbit at synchronous height, it has most of the properties of
the synchronous beanstalk.  It turns out that there is a lower
orbit which is optimum in the sense that it minimizes the taper
required by the cable.  The length of such an optimum cable is
one third the diameter of the earth (this is a general principle;
cute, huh?).  So we have the cable about 4000 km long, with its
center orbiting 2000 km above the surface.  With a material
that can make a beanstalk with a taper of 100, we can make an
optimum rolling cable like this with a taper of only 10, using
100 times less material for the same payload capacity.  The
rolling cable can hoist 1/50 of its own mass on each touchdown.
Such touchdowns happen every 20 minutes, in succession at six
equally spaced points around the orbit.  The cable is very long
relative to the depth of the atmosphere, and because of the scale
and the cycloidal shape of the tip trajectory, the cable ends
appear to descend from the sky vertically on each touchdown, with
a continuous upward acceleration of 1.4 g.  They stab downwards into the
atmosphere at a tame 2 km/sec, slow to a dead stop for an instant at
their lowest point, and accelerate gently upwards to leave in the same
way. The tip stays in the atmosphere five minutes each touchdown.

The material of the cable (if graphite) has a tensile strength of
at least 3 million pounds per square inch, so one or two square inches
at the cable ends is certainly sufficient for most tasks. The average
cross section would then be about five square inches.  This gives
the whole rolling skyhook somewhat the scale and geometry of a
typical transatlantic telephone cable, except that the graphite
is five times less massive than the copper and steel of the phone cable.

It seems at least possibly cheaper to me than the accelerator, but cost
analyses would have to decide.  The big advantage of the accelerator
is that it can be engineered entirely with known materials and techniques,
while the cable awaits the next increment in high strength materials.

Re: collisions with aircraft, I agree that most of the time a taut 
3 million psi, inch diameter, cable would be to a slow moving aluminum
plane much like a cheese cutter is to a piece of cheese.  Almost all
of the cable is above the atmosphere, however, and a collision at orbital
velocity would be another matter.  The hit probability is no greater
than for a big satellite. The rolling cable is 4000 km long and about
5 cm in diameter.  This gives it the same "frontal" surface area as
a 500 meter diameter sphere.  A collision would not be much of a disaster
on the ground, because the small cable diameter insures that the
cables burns up on reentry (though the sheet of flame across the sky as
several thousand kms burn simultaneously should be interesting).  Still
the cost to the owner (or insurance) and to the payload on the cable
at the time certainly make this event undesirable.  Some kind of
Norad (or coast guard) traffic control or monitoring would seem
worthwhile.  Given a few hours or days warning a skyhook can dodge
a few kilometers, but it will probably be the least maneuverable
object in earth orbit.  It will probably have to be given right of way
most of the time, just as law of the sea gives oil tankers right of way.

Here are a few more skyhook references:

Arthur C. Clarke, The Fountains of Paradise,
    Harcourt, Brace and Jovanovich, 1978.

Charles Sheffield, The Web Between the Worlds, Ace SF, 1979.

Charles Sheffield, How to Build a Beanstalk,
    Destinies Vol 1 #4, Aug-Sep 79, pp 41:68,  Ace books.

Charles Sheffield, Skystalk, Destinies Vol 1 #4, Aug-Sep 79, pp 7:39

Charles Sheffield, Summertide, Destinies Vol 3 #2, Aug 81, pp 16:84

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

Date: 15 Dec 1981 22:29:14-PST
From: decvax!watmath!bstempleton at Berkeley
To: decvax!ucbvax!space@Berkeley
Subject: Elevators into Space

Correct me if I'm wrong, but might there be a problem with the fact that
the Elevator is pulling a lot of mass into space against friction?

As you pull the mass up, you give it momentum, and you thus give some downward
momentum to the tower itself.  All the momentum should be conserved at the end,
but you keep pouring energy into the elevator which gets used up as friction,
and the tower is a little bit lower every time.  Perhaps the friction is very low,
but over the whole 35,000 km it could add up.

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

Date: 15 Dec 1981 2338-PST
From: HPM at S1-A
Subject: Momentum   
To:   space at MIT-MC  

No, such momentum loss is not a problem.  The anchored cable-ballast system
is ultra stable and self correcting - it is a weight at the end of a
long string attached to an unstoppable spinning turntable.  Any
momentum loss by the cable (which must be by dropping off or picking
up mass - as you say momentum is conserved in a closed system) causes
the cable to lean forward or lag backward behind its anchor position.
Its force vector is then no longer along a radius of the earth, but
has a tangential component.  This component taps the earth's
angular momentum, a tiny fraction of which transfers itself to the
cable until it straightens out again (this needs damping, otherwise
you just get a 100,000km pendulum!).  By riding loads up far beyond
synchronous orbit you can actually get net energy from the system -
centrifugal force will launch payloads for you, with energy that
is drawn from the rotational energy of the earth.

      The non-synchronous rolling cable is on its own, however,
and any momentum it loses in launching payloads must be
regained sooner or later by landing other ones, or operating thrusters
(or sails? what a kludge that would be), else the cable crashes.

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

End of SPACE Digest
*******************