[net.space] space elevators

Lynn.es@PARC-MAXC.ARPA@sri-unix.UUCP (10/19/83)

The chief advantage of the space elevator, or most rocket alternatives, 
is that you don't try to take your fuel with you.  Almost all the fuel 
in a launch rocket is used to lift itself, very little to directly raise 
the payload.  Avoid this and you get an incredible reduction in energy use.
/Don Lynn

REM%MIT-MC@sri-unix.UUCP (10/20/83)

From:  Robert Elton Maas <REM @ MIT-MC>

Yes the energy actually applied to the rocket payload would be the
same with an elevator. There are two major differences:
 (1) Rockets are very inefficient, most of the fuel is wasted in the
exhaust, very little is applied to knetic energy of the rising rocket.
 (2) Much of the energy actually applied to the body of a rocket is
spent lifting fuel and rocket engines rather than payload. Typically
the whole rocket at launch weighs an order of magnitude more
than the payload, so even with a perfect rocket you'd get only about
10% overall efficiency.

The combination of these two inefficiencies means very very little of
the energy in the rocket fuel actually gets applied to the payload.
With electric elevators you get nearly 100% efficiency from
electricity to payload-motion, and the way you make the electricity in
the first place is highly efficient too, much more efficient than the
burning of rocket fuel is.

eder@ssc-vax.UUCP (Dani Eder) (10/26/83)

                                         25 October 1983

     A correction to the comment on rocket efficiencies.  While it is true that the fact that most of the lift-off weight of current rockets not being payload introduces an inefficiency, current rocket engines are quite efficient at converting chemical energy into thrust. Typical figures are 75% for engines at sea 
level to 97% in vacuum, the difference due to back pressure from the atmosphere
at the engine nozzle exit.

                                                       Dani Eder
                                                       Boeing Aerospace

SASW%MIT-MC@sri-unix.UUCP (10/27/83)

From:  Steven A. Swernofsky <SASW @ MIT-MC>

Seth,

I think your physical description of "space elevators" leaves out some
important facts, but I am not sure if this is reflected in your energy
equations.

1.  The energy expended is NOT the same as a rocket launch.  A rocket
launch must expend energy to carry the payload to orbit and more energy
to carry the energy-to-carry-the-payload along with the payload.  An
elevator can carry the latter in the elevator shaft.  Thus, the space
elevator is more efficient in carrying things to orbit.

2.  The energy expended is "stored" in the payload's new position in the
Earth's gravity well, and can be recovered.  The object can be driven
DOWN the elevator, generating energy via flywheels or electromagnetic
generators.  The structure of the elevator makes this possible, where a
rocket-based means of propulsion would not.  In fact, a space elevator
can be (ideally) driven completely by energy recovered from objects
"dropped" from deep space or even lunar orbit.

-- Steve