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From: markh@csd4.csd.uwm.edu (Mark William Hopkins)
Newsgroups: sci.space.shuttle
Subject: Re: Propellant velocity
Message-ID: <12468@uwm.edu>
Date: 27 May 91 17:58:39 GMT
Article-I.D.: uwm.12468
References: <1991May25.215849.15606@zoo.toronto.edu> <12463@uwm.edu> <1991May27.022456.2921@agate.berkeley.edu>
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Organization: University of Wisconsin - Milwaukee
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(I wrote):
>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 :))...

In article <1991May27.022456.2921@agate.berkeley.edu> fcrary@lightning.Berkeley.EDU (Frank Crary) writes:
>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.

That's my point.  The right fuel makes all the difference.  The variation
between required mass and exhaust velocity is exponential!  So the difference
between 50 km/s and 100 km/s exhaust velocity is correspondingly significant.
With 50 km/s fuel there's no practical way to get much above a 50 km/s change
of velocity.

What's near term and what's long term is unsayable.  Knowledge, creativity and
inventiveness cannot be predicted, and breakthroughs may literally happen
tomorrow.  But it all needs support, and it makes sense to put top priority on
it since it is so fundamental.

Incidentally, 50 km/sec exhaust velocity would STILL enable you to reach the
moon in 4 hours, ignoring the effects of having to pull against Earth's
gravity, if you could accelerate at 1G to 50 km/sec and decelerate likewise.
You'd get to Mars even with this fuel in a few weeks, and not a whole year.