[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Re: starship-design: Pellet tracke

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.

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

>>>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).

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.  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.)

>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.

>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?

Which was wrong, see below.

>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.

If the tankers are launched after the main ship.  They will have to be going
faster than the ship to get to the starsystem before the ship gets there.
 But the ship can't scoop up fuel thats going faster then it.  It might be
able to catch the fuel fired at it from behind.  But since the impact of the
fuel on its catcher feilds would shove it forward.  This would accelerate the
ship, increasing fuel consuption.  (Sorry, regardless of your fram of
reference the fuel hitting from behind at high speed would exert a forward
thrust on anything that interacts with it physically or magnetically.  Can't
be helped.)  Also since the fuel lanchers are racing past it.  They would
have to launch the fuel up to months before it reaches the ship.  I.E. from
light months distence.  So hiting the ship would be dificult.

>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
the scoop tries to diver the fuel into the engine.  This load must be
supported by the scoop fields.  If the fuel is successfully funneled into the
engine.  The engine can then try to fues it before it blast out the rear of
the ship.  After it fuses.  The drive system has to tap out enough forward
momentum to shove the ship against the scoop with enough force to accelerate
against the drag.  This may not be possible.

If the fuel fuses.  The plasma products are probably blasting outward at
random directions at about 20,000,000 meters per secound (rough number off
top of sleepy head).  Note that at some points the fuel stream and ship will
have 150,000,000 mps relative velocities.  So assuming the fuel isn't changed
to ship speeds before fusion.  The fuel will still be blasting backwards
relative to the ship.  Their will be a big delta v change in the fuel.  But
unless you have some tricky interaction of fusion plasma and ship I don't
know about.  Thats not going to effect the ship much, except for the heavy
drag from the forward scoop.

>>>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.

But if the fuel tankers and ship are moving at such high relative velocities,
you can't launch the fuel to a speed near to the ships speed.  I.E. no slow
fuel track.  No slow acceleration on the fuel by the scoop track.

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.

>>The high G thrust
>>on the pellats needed to accelerate slow fuel, up to relatavistic speeds
>>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).

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