Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Propellant velocity
Message-ID: <1991May27.170425.8459@zoo.toronto.edu>
Date: Mon, 27 May 1991 17:04:25 GMT
References: <1991May22.164754.22298@zoo.toronto.edu> <12430@uwm.edu> <1991May25.215849.15606@zoo.toronto.edu> <12463@uwm.edu>
Organization: U of Toronto Zoology

In article <12463@uwm.edu> markh@csd4.csd.uwm.edu (Mark William Hopkins) writes:
>... 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 problem with doing this in the private sector is, where's the market?
Remember that both the comsat people and the science people put a much
higher priority on avoiding risk than on better performance.  There is a
very high startup cost involved in building and test-flying the thing
before people will accept it as a credible system.

There is some private interest in the idea.  For example, OSC has a project
to build an ion-rocket orbital-maneuvering stage.  Don't expect it to fly
tomorrow, though.

>The reality of the matter ... and irony ... is that relative speed of fuel
>expulsion is the determining factor for everything...

Well, no, not quite.  While exhaust velocity is crucial, thrust is also
significant.  If engine thrust is not at least comparable to the local
force of gravity, very inefficient trajectories result.  The most notable
example of this is what happens if you try to build an Earth-to-orbit
launcher with inadequate thrust... :-)  Sometimes it is better to trade
off exhaust velocity for increased thrust.

One can also benefit from reducing exhaust velocity if it reduces the mass
of support systems enough.  Electric rockets suffer from the mass of their
power supply; high-energy nuclear rockets (e.g. fusion) suffer from the
mass of the cooling systems needed to get rid of waste heat (at very high
exhaust velocity, there is not enough mass flow through the rocket to use
the fuel as coolant).  Tradeoffs happen.
-- 
"We're thinking about upgrading from    | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 to SunOS 3.5."              |  henry@zoo.toronto.edu  utzoo!henry