Re: starship-design: Pellet track

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

KellySt@aol.com wrote:
>In a message dated 8/8/97 9:49:36 PM, kuo@bit.csc.lsu.edu (Isaac Kuo) wrote:
>>Timothy van der Linden wrote:

>>>If the pellets are slow moving, then what is the use of pellets?
>>>Catching pellets like this, will not give a significant energy advantage
>>>over taking the pellets with you (=attached to the starship) from the
>start.

>>The advantages are:

>>1. You only have to accelerate the unfueled ship.

>Well not really.  The ship slams into the unaccelerated fuel and has to
>accelerate it up to most of the ships velicity in order to scoop it into the
>engines.  Otherwise it would blast through the scop mechanism and past the
>ship before the scoop could shift it inward.

First off, I'm going to view this in terms of the inertial frame of
the ship.  Thus, the ship has no kinetic energy but the incoming
pellets have a lot of kinetic energy.

When the fuel first hits the magnetic field, induced electric fields
would heat it up and turn it into plasma.  For the most part, this
energy is lost forever (because the effective expansion ratio of
the magnetic rocket nozzle won't cool the exhaust down to such a low
tempurature).

However, once broken down into charged particles, the plasma can
be directed with the magnetic fields in an energy conservative
way.  The electrons in a TV travel at around .75 c, and yet
TV tubes don't have to be many times longer than they are wide.
You could do the same thing with protons with magnetic fields
only a 1000 times stronger than a TV CRT's.

So the individual particles will continue to travel at the same
speed that they entered the magnetic fields at (despite having
their path radically bent).  The challenge is to direct all these
particles rearward with the magnetic rocket nozzle.  Here's
really where the losses can really creep in if the nozzle isn't
designed just right.  All the kinetic energy of each particle
is still there (magnetic fields are conservative), but the nozzle
has to direct the particles in the rearward direction.

>>2. You don't have to bend over backwards trying to ignite fusion (since
>>   you're using the pellet's kinetic energy).  It's actually _easier_
>>   to implement a fusion ramjet than it is to implement a onboard
>>   fusion drive.

>Ah, but that would ignite the fuel during the point of Max acceleration in
>the system.  Which would be when it hits the scoop, not when its in the
>engine?  The fuel would produce nothing but drag.

No it wouldn't.  Acceleration isn't the same thing as compression.
A magnetic field affects charged particles by making them tend to
follow the field lines (actually, they make a helical path around
a field line).  Thus, compression is caused where field lines
get closer together.  A ramscoop design uses several coils to
generate a magnetic field with field lines converging toward
the center.  This has the inevitable effect of compressing whatever
it scoops up in the center.

>I also doubt magnetic confinement fusion will ever be practical.  Certainly
>the systems the U.S. gov. research group are coming up with suggest that.
> Thats why I'm using the voltage compression system described by Bussard (and
>decribed in the web site).  You however ARE assuming we can megnetically
>control ultra hot plasmas.  Thats how your scoop/drive system works.

I assume we can magnetically control ultra hot plasmas because we _can_
and _do_ control ultra hot plasmas.  Demonstrating fusion in a magnetic
confinement research reactor like Tokomak is routine.  Demonstrating
_sustained_ fusion isn't (obviously).  That's why I'm so much more
optomistic about pulsed fusion.

Anyway, magnetic confinement is a very mature technology, thanks to
millions upon millions of dollars worth of research over the last
several decades.  At least we're already familiar with it and its
technical pitfalls.

>>The deceleration track, which takes longer to set up and requires
>>much more power, is launched _after_ the starship, at velocities
>>somewhat higher than the starship's cruise speed to catch up with
>>it.

>>Assuming a mission to Bernard's Star at .5c, these deceleration
>>track packets drones could be launched over a period of 3 years,
>>at a maximum velocity of .67c (for the ones launched at the end),
>>and all arrive at around the same time.  Before arriving at the
>>intercept points, each drone would fire its load of fuel pellets
>>to arrive in a steady line to meet up with the starship.

>>Now, this still requires a pretty heavy set up of RPB emitters
>>to accelerate the packet drones up to relativistic speeds,
>>but presumably these same RPB emitters could have been the ones
>>used to accelerate the starship up to the minimum speed needed
>>to operate the ramscoop (for the acceleration track).

>>The difficulty of launching the packets gets progressively more
>>difficult as time goes on, because they have to get faster and
>>faster to catch up with the starship in time.  However, this
>>is perfectly in line with the idea of manufacturing more and
>>more (and hopefully better) RPB emitters as the mission continues.

>What is a RPB emitter?  All I can think of is relatavistic partical beam
>emitters, but that doesn't seem to make sence in this context.

That's what I'm talking about.  The individual packet drones are
launched to relativistic speeds using RPB propelled magsails.
The two big advantages a magsail has over a laser sail are:

1. Significantly better potential energy efficiency.
2. No waste heat rejection problem (a superconducting magsail
   doesn't absorb a significant amount of energy from the
   particle stream, while a laser sail will absorb a significant
   percentage of energy from the laser).

The big disadvantage of a magsail is that it's a lot harder to
focus a particle beam over a long distance than it is to focus
a laser.

I get around this problem by assuming there is a string of RPB
emitters along the acceleration run, each one only responsible
for a relatively short length of the acceleration run.  This
is obviously very wasteful for a single launch, but makes sense
if you have to launch a whole bunch of vessels one after another.

The deceleration track will require a lot of mass in fuel packet
drones--probably a _lot_ more total mass than the starship itself.
However, they are launched over an extended period of time with
that string of RPB emitters.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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