[ExI] Explaining technical matters

[Keith Henson]

I recently got a comment back from someone who is reading my draft
paper on laser propulsion and power satellites.

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>       To me the beauty of laser propulsion comes from its ability to use
> water rather than hydrogen as propellant.   Water is stable, dense and
> user-friendly, all the advantages that hydrogen lacks.

True statements.  However, water isn't going to give a better exhaust
velocity than SSME of 4.5 km/s, nor can it be burned with air to get
the first 25 km and 1600 m/s (2000 m/s if you launch from the
equator).  Starting that high, the aerodynamics of the larger vehicle
to hold hydrogen doesn't matter much because it's in close to a
vacuum.  LEO is 8 km/s so the needed delta V is 6 km/s.

The exhaust velocity ratio for the same temperature will be square
root of the molecular weight ratio.  For hydrogen and water that's
2/18, so the difference is a factor of three.  Hydrogen at 2700 deg K
is about 7.5 km/s, so steam at that temperature would be about 2.5
km/s.

The mass fraction for hydrogen would be 1 - e^-(6/7.5) or 55% fuel,
45% payload, structure, etc.

For water, 1 - e^-(6/2.5) or 91% water and 9% for everything else.

For a 20% structure fraction, a laser heated hydrogen vehicle gets
~25% to LEO, water heated to the same temperature gets -11% to orbit
(both cases starting at 2 km/s).

Hydrogen has been used in big rockets all the way back to the second
stage of the Saturn V.  It isn't _that_ hard to work with.  In fact,
the hydrogen load in a Skylon is a few tons less than was used in the
Saturn V second stage.

The difference is we would be flying 3 times an hour.

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I don't know how to express non-linear equations to where the masses
(or even technically astute people) are going to appreciate the
difference in performance between laser heated water and laser heated
hydrogen without looking into the "rocket science" of the situation.

This is part of a proposal to displace fossil fuels with less
expensive solar energy from space.  It's the only energy source I know
about that scales large enough and is potentially cheap enough to do
that.  The consequences of not doing something about low cost energy
could be a die back of several billion people.  It should be important
to the masses since they are the ones who are most likely to die, but
I don't have a clue as to how the concepts could be explained.

Any ideas?

Keith