Re: starship-design: Re: Decelerating a Starship

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

KellySt@aol.com wrote:
>In a message dated 8/8/97 8:41:56 AM, kuo@bit.csc.lsu.edu (Isaac Kuo) wrote:

>>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
>>2.  The starship is a ramjet design, with a powerful magnetic ramscoop
>>3.  Fuel pellets are scooped up by the magnetic ramscoop and fused.
>>4.  The fusion reaction ideally takes place beyond the main body of
>>    the ship within the magnetic rocket nozzle in the rear.  This

>This is a complicated system using technology (the magnetic scoops, mag
>thruster external to the ship, interstellar fuel targeting, etc..) we can't
>figure out how to design, or feel is unlikely to work.  So we went for the
>lighter and more compact laser fuel launcher system.

The magnetic scoop isn't particularly mysterious technology, nor is
magnetically confined pulsed fusion.

Building a magnetic scoop expansive enough to scrounge up
interstellar hydrogen is dubious, but we have plenty of experience
with manipulating high energy plasma and even higher energy particle
streams with magnetic fields.  I'd say most of the aspects of my
proposal are a lot more implemented and researched than high power
lasers.

The "interstellar fuel targetting" problem is not intrinsicly much more
difficult than the Voyager missions.  The fuel packets being launched
are drones with a pellet shooter which could account for a great
deal of error, and during their launch acceleration run, course
corrections could send them on a very precise path.  (Assuming a
100 gee launch to .33c, that still gives 100,000 seconds for course
corrections over a length of 33,000 light seconds.  If continual
course corrections can keep the drone within 1m, it will be less
than 10km off course by the time it reaches Bernard's Star.)

One of the biggest hurdle for my proposal is the development of
RPB propulsion for the fuel packets.  As should be pretty obvious,
this technology would be suitable for an unmanned flyby interstellar
mission.  I expect that long before any manned interstellar mission
could be taken seriously, unmanned flyby probes would be developed
and launched.  In the process, a lot of the technical problems with
large powerful superconducting magsails could be solved, which
would provide extra experience for the design of a magscoop.

The relatively recent public failure of the tether experiments
shows how far we have to go before making practical magsails,
but these first steps are a lot further than any solar sails
(much less laser sails).

>>As you can see, the pellets don't hit the front of the ship, and
>>the ship actually _depends_ upon them entering at a high enough
>>velocity.

>>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.

>The decel run would last for months.  <more below>

Yes, probably so.  However, during this run the ship will be
decelerating, and it has it's easiest time decelerating early
on.  The amount of rear shielding could be much less than the
forward shielding.

>>That leaves accelerating pellets by laser sail or RPB magsail.  Of
>>the two, RPB magsail is more efficient (and has the added bonus of
>>eliminating the problem of heating up the sail), but it's still
>>not very efficient (compared to an EM mass driver).

>If your going to launch the fuel with a sail, you might as well add it to the
>ship like in my Fuel/Sail system.  Other wise your launching multiple vessels
>for no advantage, and extra complexity.

The biggest advantage to launching multiple vessels is that the amount
of power needed in the launching beams is reduced.  For instance,
given the choice between 1 launch requiring a 600 TW beam or 100
launches requiring a 6 TW beam, the latter looks pretty good.

>>>However this is talking about the accel track.  In the decel track its
>>>irrelavent since you can't catch fuel runing faster then you.

>>You turn the ship around 180 degrees.  I thought that was obvious,
>>but I guess it requires pointing out.

>You expect to be ramed from behind by fuel doing from zero up to .4 c
>relative velocity?  This isn't the way 'scooping' is normally described.

Well, the track is heading towards you from "behind".  How else would
you scoop it?

> Thou it would get around the problem described.  On the other hand since the
>fuel would all hit it going .4c (or whatever the cruse speed is) absolute.
> It seems to offer no advantages.  Assuming you fired the fuel accurately
>enough to hit the ship (assuming it never had to move, and the fuel stream
>never hit anything on route), why not pick it up on route and store it
>onboard?

Because then you'd have to use it somehow with a normal rocket, and I
make the assumption that a normal fusion rocket isn't practical.  If
it were, then it would be an ideal candidate for the acceleration
run, wouldn't it?

My proposal only makes sense if no traditional rocket available has
sufficient Isp for the deceleration run.  If some traditional rocket
_does_ have sufficient Isp for the deceleration run, then it also
has sufficient Isp for the acceleration run!  (You can use en route
refueling to overcome the problem of squaring your fueled/unfueled
mass ratio.)
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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