I know this is the sort of question that comes up every 6 months to a
year in RASF but here we go anyway. (Mainly because I'm interested in
the discussion that it will likely provoke.)
I have been working off and on for several years on a setting/world
for some stories I want to write. With a minimum of handwaving I want
to use a setting where the solar system is well populated in the inner
and outer system. This in itself will take close to a hundred years or
more probably (to generate the kind of density I'm looking for), but I
don't want to use any technology that we do not have a workable/
plausible theory for right now. So no artificial gravity (except that
which comes from spinning your habitat) and no technology that
"changes the world as we know it." I'm willing to do a little
handwaving to achieve this, but I'd like to keep it to a minimum. If
at all possible I want to use technology that has at least been used
experimentally, but I recognize that without a lot of handwaving 100
years from now we will have far better technology. However, I still
want to limit it to things that we are almost positive will work and
have a reasonable plan of action for achieving.
The assumption for the setting is that it takes weeks to a month or
two to travel in and around the inner system and as much as 6 months
to a year to travel in and around the outer system. (6 month round
trip to say Saturn, but a year round trip to points further. Though
the availability of refueling at the destination would reduce this
significantly.) The propulsion system that I am looking at is
magnetoplasma (VASIMR for example) drives powered by pebble bed gas
cooled fission reactors. There are some assumptions about space combat
and available technology that I make (and are detailed below) and I
would like to stick to those assumptions. I'm more than happy to
entertain suggestions for changes or additions to those assumptions,
but my main interest is what sort of propulsion would a military craft
use if it wanted something better than a magnetoplasma drive? One of
the assumptions is that fusion is only recently been made reliable
enough to use for power generation, it is large, expensive, massive,
fragile, and twitchy which means that it is prone to shutting down
unexpectedly and occasionally tends to melt itself spectacularly. The
prospect that fusion could be made into something that would compete
with fission for a spacecraft power plant is still many decades away.
(Since fusion is still considered experimental for power generation,
such installations are not emplaced on planetary surfaces, and instead
placed in high orbit.) Also stealth is by far more im****tant than
minor improvements in acceleration, so exploding nuclear devices
behind your craft for thrust would be a bad idea. Finally, the
propulsion needs to be able to send the craft to the outer system and
back in a reasonable timeframe without refueling. Keeping the fuel to
payload ratio down as much as possible is also ideal, since the larger
the craft is, the more vulnerable it will be to enemy fire.
I assume that nuclear thermal rockets (and/or chemical rockets) would
be used for maneuvering during combat because the need for large
thrust to change the craft's delta-v over a short timeframe would
trump fuel efficiency (ISP) for that purpose. However, those types of
rockets would not be used for main propulsion due to lack of specific
impulse. (Ion drives having ISP ranging up to 6,000 for
magnetoplasmadynamic thrusters using hydrogen and variable specific
impulse magnetoplasma rockets having a variable ISP ranging from
roughly 3,000 to 30,000. While the best ISP for chemical rockets I
have seen is roughly 550 for a toxic tripropellant type (binary types
rate about 450). Nuclear thermal rockets (fission powered) have a 800
to 900 ISP range for solid cores (up to 1,000 for a complex and
advanced type). Liquid and (especially) gas core nuclear thermal
designs seem to have significant materials science issues to overcome,
but could generate 1,300 to 5,000 ISP. The dangers associated with
running a reactor as hot as a gas core type causes me to consider that
it would probably be unsuitable for a manned military vessel however.)
So without fusion or nuclear pulse propulsion what would be the next
step in interplanetary propulsion beyond magnetoplasma drives? Also,
does anyone know the detectability of the plasma ejected from a
magnetoplasma drive?
*******************************
Ground Rule Assumptions
Stealth: The easiest way to detect another craft in space is via
Infrared (IR) detection. Simply put, the power requirements needed to
run a magnetoplasma drive and power weapon systems is immense. This
(using current and foreseeable technology ) means that there will be a
lot of waste heat to get rid of. This in turn will mean that without
means to ameliorate this any spacecraft will ****ne like a beacon in
the IR band. The setting assumes that this is resolved by insulating
the hull of the ****p with advanced materials that reduce the IR
detectability of the craft to a relatively short range from most
aspects. (MS Word insists that detectability is not a word, but I'm
going to use it anyway.) This leaves a large amount of waste heat that
needs to be released somehow. War****ps use a modified photonic rocket
technology (using blackbody radiation to provide thrust) to get rid of
waste heat from the rear aspect of the craft, consequently providing
some small additional thrust. However, some back-of-a-bar-napkin
calculations I did some time ago (and can't at the moment find) based
on the melting point of tungsten indicated that war****ps would
probably also need additional radiator area. The assumption is that
specially designed radiators that limit the viewable angle of the
emissions to between 1 and 10 degrees (the closer to 1 the better) are
used on the rear aspect as well as the top and bottom aspects of the
craft. Since most of the im****tant real estate in the system is on the
ecliptic this is a relatively safe option. (Consequently this will
limit the "any direction is up" mentality and align up and down as
right angles from the ecliptic.) Of course sending sensor satellites
into orbits that take them high and low over and under the ecliptic
would be a normal precaution to detect such craft, but they would also
be the target of close scrutiny of the opposing side to track their
positions. This would result in war****p captains needing to turn off
top or bottom radiators if they would swing into the view of such a
satellite, or alternately dumping the heat into internal reservoirs of
water or hydrogen fuel in order to hide from said sensors for a short
period of time. The rear aspect is difficult or impossible to disguise
however due to the active drive system, but since in general your aft
end is going to be pointed at where you came from and not where you
are going (before turnover assuming you intend to do so) this would
seem to be a manageable threat. You'd have to be wary of "neutral"
****pping and spies though. This leads to a submarine type paradigm
rather than a battle****p paradigm for space combat. Use stealth to get
into position, the other side only has a general idea of your
location, but once battle is joined you become detectable. In the case
of war****ps once the power plant is ramped up to provide energy for
directed energy weapons, additional waste heat would need to be rid
of. This additional heat (according the aforementioned back-of-a-bar-
napkin calculations) would possibly exceed the surface area of the top
and bottom of the craft depending on what assumptions you made about
power generation so military craft also have one or more large sail-
like radiators that can be used during combat and angled to radiate
away from hostiles but would be cold when not in combat. The intense
IR emissions from the top and bottom of the craft (in addition to
likely lighter armor (not that craft carry much armor in any case) on
the top and bottom aspects) mean that most missiles try to go high or
low (or to the rear) and get top down or bottom up looks at the target
for acquisition and homing.
Weapons Systems: Tooth-and-nail range directed energy weapons (DEWs)
have the power to destroy incoming missiles. (DEWs include only lasers
and masers since neutral particle beams are difficult to focus and
charged particle beams are twisted aside by EM radiation ****elding of
the attacking craft itself let alone a defender.) Knife-fighting
range DEWs can blind the seeking systems of incoming missiles and self-
guided projectiles, as well as provide hard targeting returns for
sensors. (Conventional chemical massdrivers (gatling guns, like the
Phalanx system in use with the US Navy) bridge these two ranges for
anti-missile protection.) Short range includes anti-missiles that seek
out and engage incoming missiles. Short range would be mutual suicide
for war****ps to engage each other. At short range electromagnetic
massdrivers have a high probability to score direct hits. Medium range
is the territory of the railgun. Using disposable and replaceable
rails to fire (limited) self-guiding projectiles at high speed a
single hit from one of these weapons would cripple or destroy any
war****p in space. For higher hit probability fragmenting projectiles
are also used, but a successful hit by one fragment would cause
relatively limited damage (though still quite serious). (Due to the
assumptions associated with fragmenting projectiles, multiple
fragments from the same weapon would not strike a war****p because of
the dispersal pattern.) Long range is the domain of the missile. With
ranges of up to 10 light seconds, missiles are large, expensive, and
easy to shoot down. But they are the only game in town since a
projectile from a rail gun would be very unlikely to score a hit at
this range.
Engagement Range: From 1/2 light second to 10 light seconds.
Weapon Destructiveness: Nuclear tipped missiles would range in the
kilotons to megatons giving them some small standoff range. The energy
liberated by a direct hit by a railgun projectile is on the order of
400 megajoules for a light railgun, up to 1,200 megajoules for a heavy
railgun. (Velocity 20km/s. Delivered mass is 2 to 6 kilograms.)
(equivelent to roughly 100 to 300 kilograms of TNT.) (Note these
figures are for craft at rest relative to one another. Converging
vectors will impart additional energy.) Fragmenting projectiles,
obviously deliver less energy. (Pro****tional to the fraction of the
mass represented by the fragment.) These weapons will generally
penetrate the entire length or breadth of the war****p they hit. The
chemical point defense guns use cartridges modeled on light gas guns
to produce hyper velocity projectiles. While the velocities are not
equivalent to railgun projectiles they are significantly higher than
normal chemical fired projectiles, (energy is 1.6 megajoules and
velocity 8km/s with 50 gram projectiles. Roughly 350 grams of TNT
energy) making them more accurate over the range they need to operate.
Lasers and Masers generally have delivered energy of 1 to 16
megajoules.
Nanotechnology: Nanotechnology is available and can do wondrous
things, but it is expensive, not self-replicating, not autonomous, and
strictly limited in scope. (i.e. It must be used in special chambers
with large sup****t apparatus.) This is due in part to natural
limitations of the technology and level of advancement, and in part
due to universal government regulations. It can however produce some
expensive but incredibly intricate parts and small devices. (Computer
processors, other small electronic parts, etc...)
Computing: Quantum computing is still just a theory. Computers are not
different in kind than those we have today, just more efficient,
smaller, and use less energy. Copper based circuitry is common, but
silicon based circuitry is still common too. The computer technology
curve (Moore's Law) began to level off rapidly during the 2030s and
has been improving only incrementally as transistors reach near-
molecular scales. Massive parallel processing has taken up some of the
slack however.
Artificial Intelligence: AI has advanced greatly, but is still limited
by processing power and programming technique. AI is used to automate
many functions on war****ps as well as target discrimination and
limited tactical evaluation for munitions, but is no substitute for
the human mind. Individually the pieces for autonomous androids are
available: Robotic bodies nominally human in appearance are relatively
inexpensive and are in use for things that can be done with pre-
programmed responses. Power supplies with sufficient energy density
for autonomous operation. Programming capable of simulating human
emotion and interaction (in a relatively narrow context). Programming
capable of limited problem solving. Technology suites and software
able to evaluate and interact with a real world environment. Computing
power that could conceivably put all that into a human sized chassis.
There are rumors that it is being done, but not at a price tag that
could replace humans. (or again, match the flexibility of the human
mind.)
Unmanned Craft: Unmanned munitions and scout vehicles are common, but
after several horrific incidents involving compromised AI weapon
systems in the previous war international treaty places strict
limitations on them.
Size Matters: Since weapons are far more destructive than armor is
protective (even including reactive armor) there are practical
limitations of war****p size. The lower end is bounded by the minimum
mass and volume of certain systems (such as drives and power plants)
and other such concerns. The upper end is bounded by the practical
issue of weapon destructiveness. A larger craft is in general not as
fleet of foot and also a larger target. Larger size also does not
allow war****ps to carry enough armor to be notably more survivable
against projectile weapons. In the other direction economy of scale
does factor in, and active defenses such as anti-missiles, DEWs, and
chemical propelled projectile weapons do markedly increase ****p
survivability against missiles. War****ps able to cruse independently
in general range at the low end from around 10 men to the upper end of
around 100 men on what might be considered "capital ****ps." In mass
this is roughly equivalent to a WWII attack submarine on the low end
to perhaps twice the mass of a modern ballistic missile submarine on
the high end. Differences in crew size between the comparison craft
are attributed to extensive automation.
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