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Re: starship-design: Ramjet workings?

Kelly, you replied to me:

>>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.) 
>I don't assume it must be stoped but I assume:
>The fuel will be accelerated inward in order to get it into the ships axis
> quickly, and this will need to be a major delta-v due to the high relative
>That this delta -v will heat the fuel and put a major structural and power
>load on the fields.

Whether one can catch the fuel will of course greatly depend on the accuracy
of the pellet-track and the strength of the magnetic fields.
Isaac mentions using "fuel-drones" to enhance the accuracy, my guess is that
one indeed can use these to deliver pellets with an accuracy of meters.

>That particals don't instently fuse when they ram one another.
>That given the time delays I'ld expect in fusion, and the high relative
>speed, I'ld worry the fusion won't release usefull energy until its past the

This indeed might be a problem, though I think that in theory the particles
could be decelerated to fuse while still within the magnetic field.

>Also due to the high relative speed and the comparativly meager exaust
>velocity, I'm not clear how you can gat any thrust out of the system.

What other reasons than those you mention above would give a meager exhaust

>>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.)
>I'ld agree with this if it wasn't for the high relative speed.  Since the
>plasma can only expand outward at a certain maximum speed due to internal
>presure.  If the relative angle formed by the lateral velocity of the plasma
>relative to the 'rearward' velocity of the plasma stream, is shallower then
>the slat angle of the magnetic exaust nozzel, you won't get any "push against
>the widening magnetic funnel".  
>I think the relative velocities would give a 1 to 8 to 1 to 15 angle.  Pretty
>hard to use in a magnetiv rocket nozzel.

The intake velocity and exhaust velocity are probably rather similar, since
the fusion reaction will likely add only a little bit of velocity. So both
intake and exhaust nozzle have a similar geometry. The angle depends on how
far the magnetic field can extend. A longer exhaust nozzle means more time
for the plasma to expand and thus a smaller angle can be used.

It might even be possible to pinch the magnetic field after the pellet went
through. Then there is no other way for the plasma to go than through the
exhaust. Maybe Isaac can give you a closer explanation of the configuration
of the magnetic field.