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Re: starship-design: Pellet tracke

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

>>>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).

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

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.

>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
>>into a scatter shot.  Like a shotgun blast.  You couldn't 'follow' the
>>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
>>>>heating as the pellets slam into the scoop fields would cause uneven
>>>>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.

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.  Which would have to be overcome
by the thrust of the drive system.  If (as I was assuming the fuel was going
much slower than the ship).  This drag would be considerable.  So you'ld wind
up having to accelerate the fuel up to ship speed.  With the obvious
geometric explosion of fuel needed.

>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.  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.

>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.

That also implies the fuel is only within the influence of the ship foe
1/30,000th of a secound.  In that time you must compress, fuse, and tap
thrust out of the fuel stream.