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From: fcrary@lightning.Berkeley.EDU (Frank Crary)
Newsgroups: sci.space.shuttle
Subject: Re: Propellant velocity
Message-ID: <1991May27.022456.2921@agate.berkeley.edu>
Date: 27 May 91 02:24:56 GMT
Article-I.D.: agate.1991May27.022456.2921
References: <12430@uwm.edu> <1991May25.215849.15606@zoo.toronto.edu> <12463@uwm.edu>
Sender: root@agate.berkeley.edu (Charlie Root)
Organization: ucb
Lines: 44

In article <12463@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes:
>It sounds like to me, that we private sector people have a wonderful opportunity
>to upstart the governmental space agencies of the world by applying our own
>personal funds towards such a project.
>
>The reality of the matter ... and irony ... is that relative speed of fuel
>expulsion is the determining factor for everything: how much fuel you need to
>bring on board, the size of the booster to accomodate it, how far you can get
>under sustained acceleration, (and thus) how fast you can get to the moon and
>Mars, (and thus) how much time you're forced to spend in weightlessness, as
>opposed to 1 G artificial gravity, and a whole lot more.
>
Largely true, but it is very important to note that the non-chemical, e.g.
all the high exhaust velocity, rockets tend to have low accelerations. There
are many very good propulsion ideas that can offer order of magnitude 
improvements in this velocity, BUT they lack the acceleration to lift off
from the Earth's surface under their own power, or even make a minimum 
energy transfer orbit injection to the Moon or Mars. Also, at least with
chemical rockets, high exhaust velocity results in more expensive rockets
and fuels which are awkard to handle.

>The right fuel makes all the difference between having to take several whole
>days to float to the moon on inertia, and a few hours to ram it in full gear
>(and ram it to full stop when halfway there :)).  It makes all the difference
>between a years' traversal to Mars on gravitation, and a week because of a few
>hours sustained thrust and deceleration.
>
No. Baring the use of nuclear explosives (e.g. like the Orion rocket) none
of the current designs are capable of the high acceleration this requires.
Even were the acceleration possible, the continued thrust would not be.

To produce a constant 1-g (9,8 m/s^2) acceleration for 4 hours would require
a 141.1 km/s change in velocity. Baring the nuclear explosives, the highest
exhaust velocity I have seen in ANY published paper on advanced propulsion
was about 50 km/s. With this exhaust velocity, the above delta-v would
require a spacecraft that was 94% fuel. The remaining 6% would need to be 
fuel tanks, not payload. High, constant acceleration for long periods is just
NOT possible with near term (e.g. the next 25 years) technology.

By the way, the private sector is already working on this... OCS is looking
at electric propulsion for use in upper stages (e.g. for transfers from low
Earth orbit to Geostationary...)

					    Frank Crary