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Re: starship-design: Ramjet workings?

Hi Kelly,

>>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.
>Given that these drones would need to do that at up to 2.5 light months of
>distence, after floating in space for a decade, I'ld be far less confident of

I can't answer this, maybe Isaac can convince you (and me) better.

>>This [delayed fusion] indeed might be a problem, though I think that in
>>theory the particles could be decelerated to fuse while still within the
>>magnetic field.
>True, but that that involves a lot of delta-V on the ship in the wrong
>direction.  Given the deceleration of the fuel stream could need to be
>greater then the acceleration possible by fusing the fuel.  Ram scoops could
>well be incapable of boosting themselves against their fuel stream.

I think this is why Isaac almost turned purple on you ;)
If I understand corrrectly: When the particles decelerate into the magnetic
funnel, their kinetic energy will be turned into potential energy (ie. they
will be pushed into a small area). As soon as the magnetic funnel widens
again the process will be an exact reverse, the particles potential energy
will be turned back into kinetic energy again (while accelerating the scoop
in the right direction). The only losses I can imagine are those of
increased radiation during the time that the particles were close together
and thus were being hotter.

>>>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
>The maximum velocity of the fision products are limited by the physics of the
>fusion reaction.  That speed is, as I remember, about an order of magnitude
>less then the maximum relative velocity of the fuel stream to the ship.  At
>best the fusion motor could only add a trivial amount of speed to the exaust
>stream.  If the fuel stream had to be decelerated to much, the exaust speed
>could well be less then the initial fuel stream speed.

As above, the particles will regain their initial velocity because the same
process that slows them down, will also speed them up.
If somewhere in the middle you add some energy, you may speed them up to
just a bit more than their initial velocity.

>>>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.
>Not really.  The intake has to be very broughd to scoop up the dispersed
>stream.  So it needs to be wide, and presumably short to limit the power and
>structural loads.

I think I misunderstood your angle notations. You may want to explain them.

I'm not so sure if we can compress the plasma much faster than it can
expand. In the case that the plasma can expand faster than we can compress
it, the intake has to be than the maximal size of the exhaust nozzle.

However this is becoming so practical that 'I' don't have the answers,
you've to ask Isaac.