Re: starship-design: Pellet tracke

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

KellySt@aol.com wrote:
>In a message dated 8/14/97 2:19:57 AM, kuo@bit.csc.lsu.edu (Isaac Kuo) wrote:

>>First off, I'm going to view this in terms of the inertial frame of
>>the ship.  Thus, the ship has no kinetic energy but the incoming
>>pellets have a lot of kinetic energy.

>>When the fuel first hits the magnetic field, induced electric fields
>>would heat it up and turn it into plasma.  For the most part, this
>>energy is lost forever (because the effective expansion ratio of
>>the magnetic rocket nozzle won't cool the exhaust down to such a low
>>tempurature).

>>However, once broken down into charged particles, the plasma can
>>be directed with the magnetic fields in an energy conservative
>>way.  The electrons in a TV travel at around .75 c, and yet
>>TV tubes don't have to be many times longer than they are wide.
>>You could do the same thing with protons with magnetic fields
>>only a 1000 times stronger than a TV CRT's.

>Note the mass of the TV tubes deflector magnets in comparison to the mass of
>the beam they are deflecting.  Now scale such a system up to handel tons of
>fuel over tens if not hundreds of miles.

It doesn't scale linearly with the mass of the beam, and the size
doesn't scale up at all.

For a given magnetic field, the charged particles are deflected into
helical motion with a radius determined by their charge/mass ratio.
This does not go up at all with mass of the beam.

What does go up is the strength of the opposing magnetic field.

>Oh, you forgot the lateral thrust needed to bend the fuel stream, has to be
>countered by a similar force on the ship.  Assuming a balenced and centered
>partical streeam (unlikely) that would translate into structural loads on the
>ship and scoop mechanisms.

The pellet stream will be balanced, because the ship will maneuver as
best it can to keep it centered along the incoming pellets (it can
do so by manipulating the magnetic fields of the nozzle).

The lateral loads on the scoop is a problem, which is why the mass of
each pellet must be minimized, and the precision of the pellet
shooters maximized.

>>>Ah, but that would ignite the fuel during the point of Max acceleration in
>>>the system.  Which would be when it hits the scoop, not when its in the
>>>engine?  The fuel would produce nothing but drag.

>>No it wouldn't.  Acceleration isn't the same thing as compression.
>>A magnetic field affects charged particles by making them tend to
>>follow the field lines (actually, they make a helical path around
>>a field line).  Thus, compression is caused where field lines
>>get closer together.  A ramscoop design uses several coils to
>>generate a magnetic field with field lines converging toward
>>the center.  This has the inevitable effect of compressing whatever
>>it scoops up in the center.

>Since the plasma starts out as frozen particals (high density).  The flash
>heating as the pellets slam into the scoop fields would cause uneven inertial
>confinement fusion in the particals as they hit the collector fields.

I'm pessimistic about how much heating could acheive just from the
flash heating alone.  If it's enough to initiate fusion, then so much
the better--it saves the trouble of doing so magnetically.  The
kinetic energy from the fusion is still in the products and will
still be there when the pass through the magnetic nozzle.

This assumes the products don't fly forward fast enough to overcome
the initial backwards velocity they had.  In that case, a pellet
can be merely plasmatized with an initial, low strength magnetic
field far forward of the main ramscoop.  Gas pressure and the
expanding magnetic field lines from behind this field will expand
the plasma puff down to a level which can be scooped up without
initiating fusion.

>>I assume we can magnetically control ultra hot plasmas because we _can_
>>and _do_ control ultra hot plasmas.  Demonstrating fusion in a magnetic
>>confinement research reactor like Tokomak is routine.  Demonstrating
>>_sustained_ fusion isn't (obviously).  That's why I'm so much more
>>optomistic about pulsed fusion.

>>Anyway, magnetic confinement is a very mature technology, thanks to
>>millions upon millions of dollars worth of research over the last
>>several decades.  At least we're already familiar with it and its
>>technical pitfalls.

>Actually stable magnetic confinement of dense high energy plasma streams is
>the critical failure in our magnetic fusion research.  Since its the brick
>wall we can't get past now, why are you so confident that its the easiest
>tech for a future rocket system?

_Stable_ magnetic confinement beyond what we can already acheive is
unnecessary for pulsed fusion designs, of which this ramjet is one.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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