Interstellar Travel w/o Magic 1

Let’s say, for whatever reason, someone wanted to come up with an
interstellar propulsion system that didn’t require any “magic” and
included the really tense requirement of the interstellar starship diving
through a hollowed out asteroid (or similarly visually impressive structure) on
the way out of the solar system.  Building some sort of (probably
electromagnetic) linear accelerator into the asteroid and using that to boost
the starship to relativistic speeds seems an obvious possible excuse for this
arrangement.  Unfortunately, if you want the starship to include
passengers and arrive at another star system within reasonable times on the
human time-scale this simple method doesn’t work (again, without
“magic”).  Even at the body-crushing acceleration of 10 gees, it
would still take 3.5 days of acceleration to reach even 10% of the
speed of light.  Clearly, there is no asteroid in the solar system with a
sufficient diameter for that.  Even using thousands of asteroids (perhaps
around the whole of the “asteroid belt… with a slightly non-linear
accelerator path to point the ship at the next target ) could not achieve this.

But if someone, for whatever reason, really wanted an excuse for that
“dive through the asteroid-based linear accelerator” to be a part of
the system, there might be some excuse to get there: Interstellar Ramjets.  An Interstellar Ramjet is a propulsion
concept where the spaceship collects some or all of the needed propellant from
the very interstellar medium* that it crosses between stars.  The first
version of this concept, that used a magnetic “scoop” to collect
interstellar Hydrogen to fuel a fusion ramjet engine was proposed by Robert Bussard.  As a result, the concept often referred to as
Bussard’s Ramjet. There are other variations of the concept as well, such as
the Ram
Augmented Rocket and the Laser Powered
Interstellar Ramjet**.  All of the
Interstellar Ramjet concepts fell out of favor for a while due to concern that
the intake “scoop” would generate more drag than the engine’s thrust.  Cassenti, however, has
recently shown that it is theoretically possible to make interstellar
ramjets with low enough intake losses to have positive net thrust. 

Just like a terrestrial ramjet an interstellar ramjet requires a minimum
velocity (relative to the interstellar medium) to function.  That requires
some other propulsion system to accelerate the starship to that minimum
velocity.  Perhaps the aforementioned asteroid-based linear accelerator
could be part of that system to give the starship just enough velocity before
it leaves the solar system for the ramjet (which will do most of the propulsion
work) to function properly.  The exact
minimum operating velocity that the Interstellar Ramjet would need is highly
dependent on the design of the engine, the size of the “scoop”, and the density
of the propellant in the space it is travelling through.  It would be easy, therefore, to justify
whatever duration and level of acceleration was desired for the cinematic
effect of the asteroid-based linear accelerator “dive” (though obviously if the
velocity boost is sufficiently small, then it would be hard to justify the risk
of the maneuver).  As an example,
if a spaceship that had an initial velocity of 10,000 ft/s entered a linear
accelerator with a length of 592 miles (the diameter of Ceres***) and was
accelerated through it at 10gees, it would exit with a final velocity of
46,0000 ft/s. 

Whatever structure the linear accelerator is mounted on, it would have
to either be repositioned between starship launches or else be considered “disposable”
as the equal and opposite reactions from launches pushed it out of any useful
location.  This could be done by firing
other objects (perhaps other starships?) in the opposite direction.  It might also be done via laser or microwave
beamed propulsion (or even really huge solar sails) applied over a long time to
kill the velocity imparted by the launch.  
The launching accelerator could be sufficiently massive so that it
wouldn’t pick up enough velocity to move out of range of the beamed propulsion
transmitter(s) like a rapidly departing starship would.  That would mean the beamed power transmitter
(probably located in the inner solar system) could get by with a much smaller
transmitter diameter than if it was used to accelerate the lighter-weight and
faster moving interstellar starships directly.

There are other possible dramatic and interesting methods of
accelerating an Interstellar Ramjet to its minimum velocity that could be used besides
or in concert with the aforementioned linear accelerator.  Fuel pellets could be placed (by an
earlier and slower “fuel tanker”, by beamed propulsion,
or even by the same linear accelerator the ship will use) at dramatically
appropriate intervals along the planned path of the spaceship.  The starship would fly so that the fuel
pellets would enter the engine intake scoop and detonate in the engine’s (probably
magnetic) nozzle to provide a momentary boost of acceleration.  The spacing between these fuel pellets could
either increase to maintain a constant detonation rate, or be constant (which
means that the increasing frequency of detonations would provide a sense of
increasing speed).

Another factoid on Interstellar ramjets that might interest prospective
writers and/or amateur astronomers is that our solar system sits in a region of
space known as the Local
Bubble where the density of the matter in the interstellar medium
is at least an order of magnitude less dense than the average for the rest of
our galaxy.  Since Interstellar Ramjets
are fueled by scooping up matter from the interstellar medium, it is at least ten
times harder for us to make an Interstellar Ramjet function around our local
part of the galaxy than is typical.  If
there are other interstellar spacefaring races in the galaxy, then that might
explain the Fermi Paradox that we don’t see any visiting us.  Perhaps they would avoid our region of space
the same way that sailing ships would avoid the doldrums!  On the other hand, if a race (such as our own)
within the Local Bubble did develop interstellar ramjet powered starships, they
might emerge from it to find that not only were there one or more other
interstellar races with colonies and trade routes in the denser regions of the
interstellar medium, but that their own “primitive” interstellar ramjet
starships were actually far more powerful and had far higher accelerations in
normal interstellar space than any of the races who weren’t forced by their
circumstances to squeeze every erg out of their engines.  It would be as if a nation had built EVERY vehicle as a hot-rod
dragster, and then discovers only later that everyone else built only
economical and easy-to-maintain vehicles. 
The availability of such speed, the compromises in other areas of
spaceship design, and the ability to travel in regions of space no one else
could, might create some interesting literary opportunities.

* Not to be confused with concepts that collect fuel from space itself,
such as a quantum vacuum thruster.

** Since the Laser Powered Interstellar Ramjet requires the spacecraft
to depend on a supply of laser light from some  other source, it might be a poor choice for a
story about a group of people wanting to cut ties to the existing society.

*** There is no reason, however, that you’d need to bore through a solid
spherical asteroid.  If you wanted to
make a giant facility from an asteroid it could be made “simply” by heating it
with concentrated solar light and/or beamed power until it melted, blowing it
into the shape desired with gas pressure and/or centrifugal forces like a giant
piece of blown glass sculpture, then letting it cool by radiation until solid
again.