starship-design: Ramjet workings?

[TLG.van.der.Linden@tip.nl (Timothy van der Linden)]

Kelly,

In a letter to Isaac you wrote:

>>Also since the ship would be decelerating in the deceleration track.  At some
>>point it will be going at minimal speed, geting impacted at fuel still going
>>at high relatavistic speed.

The fact that your fuel is moving fast relative to your engine isn't a big
problem, as long as you add momentum to the pellet-stream that goes through
the ramjet.

I think one of the problems in you letters with Isaac is that you assume
that the pellet has to be stopped first before it can be fused. And that
thus most kinetic energy of the incoming particles is lost. (If this is not
the problem, then likely you can neglect the rest of my explanation.) 

The particles will enter the mouth of the ramjet, which is a giant magnetic
funnel that gets smaller the further it goes towards the heart of the
ramjet. This funnel pushes the particles towards the central axis of the
ramjet. Since they don't like to be in a more convined space, they'll to try
to slow down (kinetic energy is transferred to "heat" energy).  While
slowing down they'll also accelerate the ramjet backwards. (This is the
wrong direction for a ramjet!)
However they have too much kinetic energy to stop and turn around, so they
will fly through the smallest part of the magnetic funnel of the ramjet.

If the particles wouldn't fuse, they would expand again in the output nozzle
(which also is a magnetic funnel, but now expanding in the direction of the
particles movement). The outward expansion of the particle will push against
the widening magnetic funnel and accelerate the ramjet forward so that its
final velocity is the same as before the pellet entered it. (Pushing against
a widening magnetic funnel, is like pushing against a slanting plane: the
direction of your push is partly converted into forward motion and partly to
sideward motion.)

If the particles would fuse they'd become hotter. That means that they do
like it even less to be so close together (mass is transferred to "heat"
energy).
Now that they want to expand even more, they'll push harder against the
magnetic funnel of the output nozzle and thus the ramjet will gain more
speed than it lost while the pellet entered it.
(Also the exhaust velocity will be bigger than you'd expect from a fusion
reaction where the particles had no preferred movement just before they fused.)

So in the end, even though the particles came in with relativistic
velocities, they will come out with an even higher velocity.

(Note: several concepts have been simplified a bit, but I believe the
general idea is correct.)

Timothy

P.S. Isaac, if you feel I made a large error, don't hesitate to tell me.