Re: Re: starship-design: Pellet tracke

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

KellySt@aol.com wrote:
>In a message dated 8/22/97 1:51:40 PM, you wrote:

>>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.

>I think the problem I'm having is a lot of your pellate track idea is
>self-contradictory, and by the time you mention some parts, I've forgotten
>others from previous posts.

I haven't said anything self-contradictory.  You just infer nonsensical
things.

>>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.

>Neat trick for an automated probe.  The way you phrased this is sounds like
>you stop in the star system.  I'm assuming you actually mean fly through the
>star system?  

Here's an example.  Where does my wording imply anything about
stopping? If I say I fire a bunch of machine gun bullets at a Teddy
Bear 500m away with an error of 50cm, does that imply in any way
that I mean the bullets to stop when they arrive?

Sure, I _might_ mean that the bullets are supposed to decelerate
themselves near the target Teddy Bear, but since I don't explicitely
mention this difficult to accomplish task, why assume?

I find this particular point annoying, because right from my very
first e-mail, I explicitely described the concept of the
deceleration track moving at relativistic speeds catching up with
the starship, and from you very first responce you mentionned
the problem of these things slamming into the ship, destroying it.

>>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).

>Not really.  If the fuel launchers are to be able to fire the fuel into the
>following starship, they can't be to far ahead of it, or going at a radically
>different speed.

Oh NO!  Not this again!  Get this notion straight--the fuel launchers
and fuel pellets are going to have very high relative velocity with
the starship (once it makes headway in the deceleration run--just like
the acceleration run, the pellets at the start aren't moving with much
relative speed).

>They can't just drop the fuel in their wake, even assuming
>the following ship could follow in that exact track.  The fuel would drift
>away over a period of time from interstellar effects, or be blasted out of
>the way from the fuel tankers drive beam or nav thrusters.  (you did mention
>the tankers are in powered, course correcting, flight.)

Get this straight.  The fuel packet drones have their course
corrections during their acceleration run, and this is sufficient
to acheive an accuracy of 10km or better (I analyzed this before
assuming the target system of Bernard's Star about 6 LY away).  I
did not explicitly explain that the packet drones don't make
course corrections during the cruising run, because:

1. It wasn't necessary.  Course corrections during the acceleration
   run (accomplished by using the accelerating RPB and varying the
   magnetic field of the magsail) were enough to acheive the desired
   accuracy of 10km.  (The pellets which are shot have to be able to
   hit an area the size of the starship's ramscoop at 10km.)

2. I would have had to explain about the extra overhead involved
   in built in thrusters not being worth it.

3. I would have had to explain about the fact that although the
   pellet shooter itself could be used as a rocket thruster, it
   has a pathetically low Isp (1km muzzle velocity is merely 100sec
   Isp).

The fuel pellets aren't fired until a short time before reaching
the deceleration track's position.  Why?  To make those
interstellar effects irrelevant.

>>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.

>Then the pellats will have to be fired at the rear of the ship.  For
>collection in rear scoops.  An done befor the ship is passed by the tankers.

Oh NO!  Not this again!  How many times do I have to explicitly
repeat that the ship turns around 180 degrees for the deceleration
run?

How many times do I have to explain how the deceleration run is,
as far as the starship is concerned, almost identical to the
acceleration run?  How many times do I have to repeat that the
starship turns around 180 degrees for the deceleration run?

How many times do I have to repeat that the starship turns around
180 degrees for the deceleration run?

How many times do I have to repeat that the fuel pellets travel
at a high relative speed to the starship (once it has made significant
progress in the acceleration or deceleration run)?

How many times do I have to repeat that the acceleration run and
deceleration run are essentially identical as far as the starship
is concerned?  In both cases, the pellet track starts off at a
relatively slow speed w.r.t. the ship, and ends up at around .5c
w.r.t. the ship.

How many times do I have to repeat that the starship turns around
180 degrees for the deceleration run?

How many times do I have to repeat that the deceleration track is
travelling at relativistic speeds w.r.t the target star system?

>>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 scoop 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.

>Your mising the fact that this doesn't happen at once.  Collecting the
>fuel as you run over it subjects the scoop system to a serious reward
>thrust as he scoop tries to diver the fuel into the engine.  This load
>must be supported by the scoop fields.

I'm aware of this.  Which is why I explicitly explained that the
magnetic nozzle is what provides the forward thrust.  In order
to calculate which effect is greater, you need only consider
what effect they have on the momentum of the pellet--the ship
will necessarily experience an opposite momentum change.

>If the fuel is successfully funneled into the engine.

We understand how to manipulate plasma with magnetic fields
well enough to do this.

>The engine can then try to fues it before it blast out the
>rear of the ship.  After it fuses.

You can't have it both ways.  First you _insist_ that it will
fuse before even entering the scoop.  Now you question whether
it even has time to fuse.

Anyway, I did already explain how the fusion cross section
increases linearly with the rate of compression, and how
the rate of compression in this design is proportional to
the speed of the incoming pellets.  The time allowed to
fuse with sufficient yield is, of course, also proportional
to the speed of the incoming pellets.  If this thing works
at any speed, it will work at high speed.

>Also since the closing velocity is up to .5 C, and the scoop fields arn't
>thousands of kilometers deep, it is going to be like hitting a magnetic wall.

Make up your mind.  Will the pellet fuse in time or will it not?
If so, then remember that magnetic fields apply conservative forces,
and this extra energy will _still_ produce thrust even though the
fusing takes place ahead of the ship.

>>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).

>It takes power to compress those pellets that fast in the magnetic scoop.

Yes, but the power is entirely from the speed of the incoming
pellets.  The magnetic scoop, being superconducting, does not require
energy to maintain its magnetic field.  The pellets, OTOH, see a
different thing.  They see a changing magnetic field (because to
them, the magnetic field is moving at high speed toward them).
As they pass through the "bottleneck", it looks to them like the
magnetic field lines got compressed.

> Also fusion reactions don't happen instently.  Even after the particals are
>fused, even if every partical in the fuel stream is shoved together
>instently, it takes a finite time for the particals to fuse, shuffel neclear
>particals, and then throw the resulting fussion particals outward.  By then
>these particals could be well past the ship.

The speed of fusion is dependant upon the fusion cross section.  The
higher the cross section, the faster the fusing.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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