Re: Quick question about a STL trip

On Aug 19, 1:06 pm, sigidu...@xxxxxxxxx wrote:
Assume our starship masses a cool one million tons.  The trip is 100
light years.  The ship's top acceleration is 20 cm/s^2, or 2% of a
gravity; its top speed is 0.25c.  (These numbers chosen for story
purposes, obviously.)

So it accelerates for about 12.5 years, coasts for nearly a century,
then decelerates the same.

You mean four centuries, right?

With a four century long journey, then it's plausible for Zubrin's
magsail brake to be able to provide braking with very low mass
cost.

http://www.niac.usra.edu/files/library/meetings/fellows/nov99/320Zubrin.pdf

With an advanced magsail design, it may be possible to brake
down to 1680km/s within a couple years--which is still way too
fast but an advanced high specific impulse drive could brake you
the rest of the way.

My prefered braking drive is a kinetic impact powered rocket.
The main starship is a large magloop with perhaps a dozen
payload modules strung along it like necklace beads. This
can use magsail braking to reduce its speed. Behind the
main starship is an auxiliary ship containing a payload of
zillions of tiny little robot chips, as well as a nuclear powered
laser to provide power to the chips. Before arriving at the
destination system, the auxiliary ship releases the chips,
and they fly themselves into a linear formation along the laser.

Since the main starship has used magsail braking, the
chip formation catches up. The starship maneuvers to line
itself up with the laser, so the chips pass through the
loop-shaped starship. It puffs propellant gas in front of
it. Relativistic kinetic impacts with the robot chips cause
explosions of plasma far more energetic than nuclear
reactions or even antimatter reactions. The resulting
relativistic charged particles get deflected by the starship's
magnetic field, producing backwards braking thrust.

This sort of braking drive may be very effective and
cost only a modest amount of propellant. We can
assume the following proposed drive systems are used
only for the acceleration portion (up to 25%c).

Questions:

1)  Assume a straight fusion drive. What's the mass ratio?

Well, let's assume a magical 100% efficient D-He3
reactor, which magnetically deflects the products
perfectly rearward. This means a 14.7MeV proton
and a 3.6MeV alpha particle for each reaction, or
an mass averaged exhaust velocity of 7%c.

With an exhaust velocity of 7%c, you need a mass
ratio of 35. It's perhaps exceedingly optimistic to
reach a 7%c exhaust velocity with fusion...if we
cut that back to 3%c, then the mass ratio is 4000.

2)  Okay, what about an antimatter drive?  How much better?

Perhaps very much better. It may be possible to
acheive an average exhaust velocity of 30%c
(assuming the charged pions are efficiently
deflected during their extremely short lifespans).
This would require a mass ratio of only 2.3.

Of course, antimatter is plausibly extremely
expensive and handling these large amounts
of antimatter may be problematic/impossible.

Neither fusion nor antimatter propulsion are remotely
doable with known technology, but 25%c may be
doable with particle beam propulsion or laser based
propulsion. Particle beams have the best potential
efficiency, but little is known about aiming particle
beams over long distances. Photon beams are less
efficient, but we have good ideas about how to
focus them over long distances.

For your mission profile, I'd actually suggest using
a relativistic kinetic impact rocket for the initial
acceleration as well as for braking. The main
drawback is that it may take over a year to accelerate
the tiny robot laser sails up to relativistic speeds.
This isn't a problem for the deceleration phase of
a journey, but it can be a problem for the acceleration
leg of a "short range" interstellar journey.

Isaac Kuo
.

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