Re: starship-design: Pellet tracke

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

I feel like I'm talking in circles, constantly having to repeat the same
things to the same responces.  We already went over this at the start of
this pellet track discussion.

KellySt@aol.com wrote:
>In a message dated 8/19/97 11:21:57 AM, kuo@bit.csc.lsu.edu (Isaac Kuo)
>wrote:

>>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).

>Oh.  NO That wasn't clear from your descriptions.

I did describe it precisely before.  I only quote it again here
so hopefully I won't have to repeat it yet again.

>True., that overcomes my comment.  But then the fuel packets are useless.
> They have to fly as far, and about as fast as the ship.  So you might as
>well just dock them and off load the fuel, or integrate them into the ship.
> Effectivly you have an exotice, fragmented, fuel/sail configuration.

>How do you intend to slow the fuel launchers at the target star system?
> Unless they are going slowly, they couldn't launch to the ship without the
>ship haveing high speed impacts.

Oh NO!  Not this again!  This is how it all started in the first place!

1. The advantage over integrating the fuel in the ship is that you
   can spread the launch of the fuel packets over a long period of
   time.  My example before was how a .5 cruising starship to Bernard's
   Star could have the deceleration track fuel packet drones launched
   over a period of 3 years (starting after the starship's acceleration
   run--all resources prior to launch are devoted to the acceleration
   run and its track).

2. There is no intention to slow the pellet launchers at the star
   system.  There never was, there never will be.  Anything that
   could have slowed down the pellet launchers would work just as
   well for the starship.

3. The pellets fired from the pellet launchers will indeed be moving
   fast compared to the starship.

4. There aren't any high speed physical impacts.  Remember the whole
   discussion of how the magnetic fields of the magscoop conservatively
   accelerates the incoming plasma?

5. The high speed of the pellets is an inherent part of the ramjet,
   in that it provides the energy to compress the pellet's plasma
   to initiate fusion.

6. There is an advantage to the ramjet over a normal rocket in that
   its propellant requirements grow more gently than a rocket does,
   when the desired delta-v is much greater than the Isp * gee.

>>>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.

>Exactly.  Fuel impacts frount of ship.  That thrust pushes backwards on the
>ship through the magnetic fields.  Unless the later fields accelerate the
>fuel backward, you have a negative thrust.

This is exactly what the magnetic nozzle does.  But how can it do so
if it inputs no energy into the plasma, you ask?  Because the ramscoop
didn't remove energy from the plasma in the first place.  Magnetic
fields are conservative.

Like I said, you have to look at in terms of the inertial frame of the
ship.  If you don't, then it's a lot more complicated because of the
changing (moving) magnetic fields, which aren't conservative.

A bit of plasma enters the viscinity of the starship in the "backwards"
direction with a certain speed V.  That means it starts off with a
kinetic energy of 1/2 V*V*M (M is mass of the bit of plasma).

Somewhere--it doesn't really matter where--the kinetic energy is
increased to some higher value due to fusion.  This kinetic energy
will be 1/2 S*S*M, where S is a certain value greater than V.

The bit of plasma will leave the viscinity of the starship with
kinetic energy of 1/2 S*S*M, because the only forces acting on
it are from the fixed magnetic field, which is conservative.
Thus, it will leave with a speed of S.  The only question is,
in what direction?  Assuming none of the fusion products escape
out the front (because their speed overcame their incoming ramming
speed), they are moving almost straight backwards, because of the
magnetic nozzle.

So afterward, we have a bit of plasma which is moving backward
with velocity S, greater than the initial speed V.  That means
that the momentum of the bit of plasma has changed in the
backward direction.

But wait--there's conservation of momentum!  In a closed system
(the ship + the pellet), there can't be any overall change in
momentum.  We know the pellet has a change in momentum in
the backward direction.  That means _something_ has a change
in momentum in the forward direction.  The only other thing
out there is the ship.

Therefore, there is a forward change in momentum in the ship.

>>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.

>I was assuming a slow speed fuel track ahead of the ship.

So was I, and you are right in that immense accelerations are
applied to the pellet.  However, these accelerations are applied
evenly to the pellet, leaving just the "tidal force" of the magnetic
field gradient to compress/expand the pellet.  It's like how the
force of gravity acts on an object in free fall.

>The high G thrust
>on the pellats needed to accelerate slow fuel, up to relatavistic speeds ship
>speeds.  Obviousl the ship can't get boost out of fuel blasting threw the
>engines in a 1/100,000th of a secound.  You'ld be hard pressed to get a
>fusion reaction that fast.

No, because the faster the ramscoop goes, the faster it compresses
the pellets.  The implosion speed is proportional to the speed of
the incoming pellets, so the fusion cross section is proportionately
increased, so the rate of the reaction is proportionately increased.

The yeilds would be fantastic--if we could inject solid fuel pellets
at relativistic speeds on the ground into a reactor, we'd get awesome
fusion yeilds.  Unfortunately, that requires getting those solid
fuel pellets to relativistic speeds in the first place, something
which is probably impossible in practice, and it also requires an
efficient way to tap energy from the resulting reaction (otherwise,
you lose a good fraction of the energy used to accelerate the
pellet in the first place).
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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