Re: starship-design: Pellet tracke

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

KellySt@aol.com wrote:
>In a message dated 8/18/97 9:46:31 AM,
>kuo@bit.csc.lsu.edu (Isaac Kuo) wrote:
>>KellySt@aol.com wrote:

>>>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.

[...]

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

>And the structural loads.  Both are defined by the fuel flow mass and
>magnets.

Yes, that too.  However, TV tubes aren't limited by either the
strength of the opposing magnetic field nor structural loads
from deflecting the measly electron beams.

Neither criteria is appropriate for the "scaling up a TV tube"
factor.  Only the radius of curvature of the beam deflection
is a shared factor, and as I said before, this doesn't scale with
the flow mass at all.

The seriousness of the structural loads will have much to do with
how off-center the fuel pellets are.

>>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.

>Over interstellar distences?

Do I have to repeat myself?

The pellet shooters are installed on the fuel packets.

The fuel packets are accelerated to relativistic velocities
with constant course corrections during the acceleration
run so that they arrive near the target system with an error of
10km or less.

They then shoot pellets along a track to intercept the starship's
ramscoop with the pellet shooter, which has a muzzle velocity of
around 1km/s (easily enough to make up for a 10km error in the
packet's position).


These fuel packets are a big tank of fuel pellets along with a
relatively small fission power supply and a pellet shooter.
Considering the plasma dynamic accelerator acheives a muzzle velocity
of 80km/s in a few centimeters, a 1km/s muzzle velocity pellet
shooter shouldn't have to be very big.

>Anyway a multiple projectile stream wouldn't follow a line.  It would diverge
>into a scatter shot.  Like a shotgun blast.  You couldn't 'follow' the stream
>to keep it centered.  If you could, you wouldn't need a scoop structure.

Each fuel packet is responsible for laying down quite a lenth of
track.  Assuming a length of 100km and a muzzle velocity of 1km/s,
that means firing pellets with at least 50sec between firing and
interception (probably much more, depending on how long it takes
for the pellet shooter to fire the entire load of fuel pellets).

It would be naive to think that at even 100km a pellet gun could
hit a bullseye.

>>>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.

>No the energy would be released in the scop system ahead of the ship.

This is just fine.  The energy is still in increased kinetic energy
of the particles, which are still funnelled along the magnetic lines
of the ramscoop.  It will still be turned into extra forward thrust
when the magnetic nozzle directs those products rearward.

You have to look at this in terms of the inertial frame of the
starship, because that is where the magnetic field of the ramjet
are conservative.

Anyway, I must repeat that I really doubt the flash heating alone
could initiate fusion.  As I said before, there is no compression.
It's not like the front part of the pellet suddenly hits a brick
wall of magnetic field while the rear part of the pellet slams
into it--only the differential between the strength of the magnetic
field encountered by the front part and the rear part is significant,
and this is minimized by having a small pellet.

If you look at inertial confinement concepts, you see that the
primary purpose of the lasers impacting the fuel pellet is to
_compress_ the pellet, at which point it heats up due to the
compression.  Even in H-bombs, the way to acheive high yields is
by using the fission bomb to implode the fusion warhead (which
heats it up to fusion levels).

>>_Stable_ magnetic confinement beyond what we can already acheive is
>>unnecessary for pulsed fusion designs, of which this ramjet is one.

>Given the power levels we'ld need for these ships you'ld need a prety
>continuous flow of a lot of fuel mass.  I doubt you can assume the magnetic
>fields would have time to stabalize out.

It depends on what you mean by "continuous".  For purposes of being
a pulsed fusion system where each pulse has no significant effect
on the next, let's say only one pellet is processed through the
ramscoop at a time.

Let's say each pellet is 1g, the ramscoop is 1km long, and the
current relative speed to the pellets is .1 c.  That still implies
a mass rate of 300kg/s.  Assuming the added velocity is merely
100km/s (equivalent to 10,000 secs Isp), the thrust provided is
30,000,000 newtons.  If the starship is 10,000,000kg, then
this thrust provides .3 gees.

In practice, 10 pellets at a time would still be separated by
100m each, effectively separating their reactions from each
other while evenning out the loads on the starship.  That would
provide 3 gees acceleration.  Also, pellet separation will be
larger at velocities greater than .1 c.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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