Re: Quick question about a STL trip - and another...

On Aug 21, 11:28 am, Jacey Bedford <lookin...@xxxxxxxxxxxxxx> wrote:
In message
<dbfdda79-0083-4bbb-8545-51a908596...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
IsaacKuo <mech...@xxxxxxxxx> writes

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.

Probably a dumb question, but this impacts on something I'm just
starting to work on right now...

In a generation ship what kind of changes would the passengers and crew
feel/hear/sense on board ship between a) normal drive, b) deceleration
and c) we've arrived?

If we're talking about a generation ship, then it's okay to have
acceleration and deceleration times on the order of decades.
This reduces the required power level of the drive, and may
have other practical benefits. In particular, there are no issues
with slanted artificial gravity. Assuming the starship uses
spin gravity, an acceleration or deceleration of 0.1m/s produces
a small slant.

What is heard or felt depends on the type of drive. For the
relativistic kinetic impact drive I propose, there will be a low
constant hissing of the propellant nozzles puncutated by a
rapid series of impacts from the explosions. The microsails
won't be perfectly evenly spaced, so it will sound like popcorn
popping or rain falling on a tin roof. Note that most of the
plasma doesn't actually impact the outer hull of the ship;
most is deflected by the magnetic field. But I think some
amount of direct impact will occur; enough to be audible.

If there are windows to see outside, each explosion will be
visible as a brilliant flash around the impact point. The
impacts won't be perfectly lined up so it will look a bit like
crackling fireworks. Note that each flash will radiate soft
x-rays, but these x-rays are far less penetrative than natural
background radiation.

I appreciate there'd be masses of shielding between drives and the human
payload and I reckon people would treat base line background noise as
silence and only notice when it changed.

This depends on the nature of the drive in question. For
a relativistic kinetic impact rocket or an aneutronic fusion
reactor, no such shielding is required. Natural background
radiation is far more of a concern. For an antimatter drive,
highly penetrative gamma radiation may be a concern.

With the relativistic kinetic impact rocket I propose, the
drive system is more or less "around" the payload. In
principle, you could use a smaller central magloop and
space away the crew compartments on the end of tethers.
However, it would be better to simply make the magloop
that much larger. Even if you keep the mass constant,
a larger magloop is able to more effectively deflect
particles--including background cosmic radiation.

What about crew? How close would they have to get to the drives/working
parts for maintenance? (I'm talking about shipboard not external.)

Within maybe 1,500km? The crew can and should use
remotely operated robots for maintenance. You need
them anyway in order to work where there's dangerous
radiation and/or electricity and/or temperatures and/or
pressure. These robots will also probably be essential
after arrival at the destination in order to get cracking
at mining local resources.

What would be the range of a ship outward bound from Earth for eighteen
generations (a generation is flexible, say 20 - 30 years per generation
or even longer if life expectancy has increased) - anything from 350 to
up to 500 - 600 years in transit?

This depends on the cruise velocity. For a 350+ year
journey, I'd say you'd want to spend the energy to
get the thing up to a decent relativistic speed--maybe
0.75c (a relativistic gamma of ~1.5). That implies maybe
a 400+ light year range.

There's a fleet of 50+ generation ships, I guess there's been some
natural wastage - would 10% losses be unreasonable? Some in disasters,
some cannibalised to repair failing ships with their passengers and crew
redistributed, to other ships causing brief overcrowding until the
population evened itself out in a generation or two.

This sounds like the fleet is all traveling together toward
the same destination. Is that right? I would tend to use
the word "convoy" to reduce confusion.

Is there an optimum size for a generation ship (despite the fleet, if
necessary each one has to be self-sufficient)?

There will be an optimum size, but this will depend very
strongly on your assumptions. On the one hand,
economies of scale favor many small ships in a convoy.
This is especially true if you're using external power.
It may be much easier to launch many small ships
one by one than just one or two big ships. On the
other hand, some forms of drive scale up well,
favoring big ships. It may be optimal to start off as
a bunch of small ships, but to "link up" into one
really big "ship" for most of the journey.

Once a fleet is launched it has to be self-sufficient but presumably
they'll keep in contact with Earth for as long as possible. How long
will that be?

However long you want it to be. Most plausibly, they'll
keep periodic communications for the entire journey.

And after 500 years of subjective shipboard time at those speeds, how
much time will have passed on Earth?

This depends on the cruise velocity. With a velocity
of .75c, about 750 years will have passed on Earth.
The starship will be around 550 light years from
Earth, so don't forget to add that time delay to
any communications.

Sorry - a lot of vary basic questions for many of you but complete
stumpers for me and I'd really appreciate some idea, even if it's only a
broad range of either/or answers. The story is obviously NOT about the
technical aspects of the voyage, but I'd like to get the background as
close to un-ludicrous as possible.

The answers can be greatly altered if you go much
faster than .75c. However, doing so requires far
greater amounts of energy.

Isaac Kuo
.

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