Michael Rigby-Jones wrote:

> Only if you let go the stone, whereupon it's potential energy is
> converted into kinetic energy.  If you put the stone on a shelf it would
> stay there, and you don't have to provide power to a shelf do you? 

But you have to provide force. Movement is not only about energy, it is
primarily about (resulting) force. 

> The whole point of this drive is that as little energy as possible is
> lost from the cavity.  The energy which is lost as radiation does not
> contribute to the thrust produced. 

I have a problem with the explanation of how the thrust is generated. As
with any propulsion mechanism, this is not primarily a question of energy,
it is a question of force.

>From the article:

"Shawyer calculates the microwaves striking the end wall at the narrow end
of his cavity will transfer less momentum to the cavity than those striking
the wider end (see Diagram). The result is a net force that pushes the
cavity in one direction. And that's it, Shawyer says."

See also section 2.4 in
http://www.newscientist.com/data/images/ns/av/shawyertheory.pdf. 

He is only considering the forces generated by the reflections on the end
pieces. That's fine as long as the waveguides are cylindrical, because then
the forces generated by the reflections on the sides cancel themselves out.
But with a cone-shaped waveguide, the reflections on the side walls have a
resulting force towards the smaller end, which possibly cancels out the
difference between the forces generated by the reflections at the end
pieces. 

I haven't made the calculations to show how much force this reflection on
the cone shaped side walls generates and whether it matches the difference
in force generated at the end pieces, but neither has he in his paper.
Which seems strange. To me, it may be that there is a net difference, but
it at least would have to be considered -- and in any case it would reduce
the effect.

Gerhard

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