Re: starship-design: Pellet track

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

Timothy van der Linden wrote:
>Isaac
>You wrote to Kelly:
>>Well, then I guess my idea is more original than I thought.  So here is
>>my concept for the acceleration track:

>>1.  An relatively slow moving acceleration track is set up so that
>>    somehow a track of small fission/fusion pellets are spaced along
>>    a line.  (I imagine a string of pellet shooting fuelpacket ships,
>>    to minimize the time/distance between firing the pellets and
>>    their target pickup points.)

>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.
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.
3. The fuel requirements for a given cruise velocity go up roughly
   linearly, as opposed to exponentially for a fusion drive.  I am
   not even convinced that we will even develop sustained fusion
   reactions in the next millenia, given the technical problems
   with the high magnetic fields and pressures needed (magnetic
   fields are "leaky" and plasma just loves squeezing out and
   amplifying those "leaks").  Needless to say, I'm skeptical about
   the potential to develop fusion drives, and looking at the designs
   envisionned for normal fusion drive starships, I'm _very_ skeptical
   that we can hope to acheive any large fraction of the theoretical
   maximum Isp.

and

4. The effort involved may be spread over a long period of time.
   For the acceleration track, this might not be much of an issue,
   because all the time spent setting up the acceleration track
   is simply delaying the completion of the mission that much
   longer.  For the deceleration track, this is a critical bonus
   because you can manufacture and send fuel packet drones over
   a period of years while the starship is progressing toward
   the target system.

And no, you do not "catch" the pellets in the sense that you brake them
up to the ship's speed.  The plasma remains of the pellets blow through
the hollow central core at high speed.  It's a ramjet.

>>The idea for the deceleration track?

>>Same idea, but with the ship turned around 180 degrees.  Yes, this
>>implies having some shielding in the back of the ship, but the
>>deceleration run shouldn't last too long.

>Still you have to launch the pellets many many years in advance.

No, I assume that all resources prior to launch were devoted to
the acceleration track and the starship itself.

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.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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