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Re: Re: starship-design: Ramjet workings?

In a message dated 9/11/97 8:26:50 AM, TLG.van.der.Linden@tip.nl wrote:

>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
>>>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.

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

>>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
>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.

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.

Also the more thrust load the scop systems requires, the less advantagious a
ramscoop would be in comparison to just caring the fuel on the craft.

>>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.

>>>If the particles wouldn't fuse, they would expand again in the output
>>>(which also is a magnetic funnel, but now expanding in the direction of
>>>particles movement). The outward expansion of the particle will push
>>>the widening magnetic funnel and accelerate the ramjet forward so that its
>>>final velocity is the same as before the pellet entered it. (Pushing agains
>>>a widening magnetic funnel, is like pushing against a slanting plane: the
>>>direction of your push is partly converted into forward motion and partly
>>>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
>>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
>>the widening magnetic funnel".  
>>I think the relative velocities would give a 1 to 8 to 1 to 15 angle.
>>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.

>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.

Maybe, but that seems pretty quick manipulation of a magnetic field.  Come to
think of it, that wouldn't help.  In the worst case situation the fusion
product forward velocity would still be up to 90% of the rearward velocity
for the fuel stream.