Greetings fellow travellers!
This is a very long file about gravitics.
The following information is highly UN-official. I have taken
pains to try to reconcile what I know about the universe with
what I know about MegaTraveller from the rules and accessories,
but am not about to guarantee an official approval of any of it.
You can use and distribute this material freely, so long as I
get some credit for working it all out and making it available.
(and while your at it, remember to thank JamesP and Tek for the
list...)
Some of this is still pretty rough, but I think you'll find
it mostly useful.
Richard Johnson
Hillsboro Oregon
- --------------------------cut here if you like--------------------------
Gravitics in MegaTraveller
- ------by Richard Johnson
It is a misconception among several of my players that the
primary function of gravitics on space vessels is to hold
crew members to the deck. I suspect this attitude is common.
In actuality (sic) the science of gravitics has profound
implications for space travel, and the development of
gravitics has been a primary mover the direction of space
technology.
The "Starship Operator's Manual, vol 1" gives, on page 56,
under "Gravitics Technician", introduces the idea of
gravitics. It states that:
"The artificial grav plates and inertial compensators
within the ship are not critical to its performance...
More importantly, though, the maneuver drive's thruster
plates operate on a principal that takes advantage of
gravitic science..."
This article explores the theory and applications of gravitics,
and how gravitic design limitations affect the maneuvering of
space vessels. I strongly suggest you read the appropriate
sections of the operator's manual, and information about ship
design in the Referee's manual, to better understand what I'm
saying here (and to catch me where I've blown it).
- ----------------------------------
Theory of Gravitics:
Two fundamental properties of matter concern us when space
vessels maneuver. These are inertia and gravity. Inertia is
the tendency for matter to resist any changes in speed or
direction. Gravity is the tendency for all matter to attract
other matter (or anti-matter, for that matter). It is
important to understand that these are *fundamental properties*.
All that exists demonstrates these tendencies; in fact, inertia
and gravity are what we use to define matter.
Both inertia and gravity are effects that we see, in "normal
space", of matter being highly concentrated energy (1). Energy
concentrations sufficient to create solid matter are more than
enough to interact with higher-order universal dimensions (2).
These particles interact in higher dimensions in ways not
possible in "normal space". The *effects* of these
hyperspacial interactions are then seen in "normal space" as
inertia and gravity. (3)
- ------------------------------
Gravitic Devices:
There are four basic gravitic devices:
Maneuver drives
Repulsers
Grav plates, and
Inertial compensators.
These all tap into these hypserspacial interactions, and
concentrate and direct the resulting force. In a sense,
so do jump drives, but there is a critical distinction.
Jump drives use the hyperspace/space-time interactions
to loft the entire ship into a bubble in hyperspace,
while gravitic devices use the hyperspace/space-time
interactions to manipulate space-time events.
Drives and repulsers generally utilize a repulsive force,
while grav plates generally use an attractive force. In all
three devices, the "effects" being manipulated are gravitic.
Inertial compensators, on the other hand, intercept inertial
"effects" and use feedback techniques to reduce or nullify
inertia on selected bodies. Looked at this way, we can see
that inertial compensators are not *truly* gravitic devices,
but that the technology is similar enough so that we need
make no distinction, except in the scientific community.
For the remainder of this article, I use "gravitic devices"
to refer to inertial devices as well. I do not use the term
to apply to jump drives, even though they have many features
in common with gravitic devices.
- --------------------------
Basic Design of Gravitic Devices:
Gravitic devices operate much like valves in a water hose. A
small change in the characteristics of the device (valve)
produces a large change in output (flow). That is to say that
the energy you feed your drives (from the power plant) is not
converted to thrust. It is used to control and direct the
flow of "effect" from an interaction that occurs in hyperspace.
All gravitic devices have the same general design. A typical
device has a "concentrator", a "plate", and a "governor".
- ------------------------------
|Conc. || || Output |
|Plate |--------| Gov. |------| Plate |
| || | | |
- ------------|----- ------------
\ | /
\---------- /
\_____| Comp. |---/
- ----------
The concentrator plate is basically a hyperspace receive antenna.
It collects and concentrates wade-ranging "effects" into a small
region of space-time. The concentrator does not create hyper-
space interactions; that would take too much energy. It does tap
directly into activity in hyperspace and intercept the
interactions already there.
The output (or thruster) plate is basically a space-time output
antenna. It directs and releases the concentrated "effects"
gathered by the concentrator. Examine drive plates for a
classic example. (Refer to the "Starship Operator's Manual,
vol. 1, pages 2-4.) Notice how the output is strong in the
plate's basic direction, but weakens sharply in other directions.
Notice also that all plates are invariably placed so that
*nothing* gets within about a meter of them. We'll discuss
this more in a minute.
The governor is the part of the device that monitors and
controls the concentrator and the plate. It matches input,
output, and demand (from the ship's computer). Needless to
say, this is where most of the technology, and most of the
problems occur.
- ----------------------------
Limits of Gravitic Devices:
There is a serious side effect to using a gravitic device
because the device operates both in hyperspace and in
space-time. Using a gravitic device "warps" space-time
around the concentrator. This warping is very like the
warping of space-time around black holes, only the radius
of curvature of space is much shorter, usually measured
in meters rather than kilometers. This warping becomes
more intense as the acceleration produced by the device
increases. Beyond about 60 m/s/s acceleration (6 g) (4),
the device becomes inoperable because of relativistic
effects (5).
The warping of space-time caused by gravitic devices is
identical to the warping to space-time by a gravity field,
only intense and of short range. It is critical, especially
in jump-capable vessels to have space-time as smooth as
possible in and around the ship. The amount space-time is
warped represents how much is not known about the ship's
present position, or its destination. Hence -- misjumps.
If these "warps" caused by using gravitics are left
unaltered, time would flow at different rates in different
parts of the ship, sensors would not operate properly,
thrusters would fight themselves, and jumps (if possible)
would be wholly unpredictable.
The plates associated with all gravitic devices are large
(and in the case of drives, massive) mainly to deal with this
warping of space-time. The plates function as antennae that
receive hyperspacial energy and broadcast "spacial" energy.
They could be microscopic and do this.
However, the plates also "sink" or dissipate the space-time
warping associated with the use of the gravitic device. For
this task the plates need to be large, to "spread out" the
effects of the warping of space-time.
In the case of jump-capable ships, even this is not enough.
For a few microseconds before a jump, all gravitic devices on
starships are switched completely off. The transition to
jumpspace is made, and gravitics are switched back on. This
enables the computers to more properly enable the jump vector,
eases the load on the power plant during its most critical
mission, and allows the gravitic devices to switch to the
different circuits they need to operate from within jumpspace.
It is probably this short hiatus in gravity that causes the
momentary disorientation and nausea felt by even experienced
space hands at the moment of transition.
- ---------------------------------
Effects of Gravity on Maneuvering Vessels:
Gravity has little effect on most vessels unless that vessel has
lost its maneuver drives or is near a large celestial body (one
large enough to orbit - or perhaps a runaway maneuver drive?).
Thrust from maneuver drives so overwhelms most gravitational
forces in deep space so as to make them inconsequential.
Near large bodies, though, gravity causes us to radically alter
our flight paths and operational methods. The force of gravity
decreases as the distance from the attracting body increases (6).
In many instances, this force can be greater that what we can deliver
with thrusters; we must orbit the body.
This also means that for a ship in orbit, for instance, the
"bottom" of the ship (closest to the planet) and its occupants
have more gravity on them than the top of the ship and its
occupants do. This phenomenon is called microgravity, and it is
used to stabilize satellites in almost every spacefaring culture.
Satellite Top ------------Gravity - a little
||
||
Center of Mass ---| *|Gravity at orbit
||
||
Satellite Bottom ------------Gravity + a little
Ordinarily, microgravity is no problem. However, in close orbits
of very large bodies, stresses can build up in a ship's hull both
from the gradient of microgravity and from the relativistic
stresses caused by the sheer speed of the vessel.
The main problems from gravity are long-term. On-board gravitic
devices, interacting with gravitational forces, cause stress on the
ship's frame and hull. This stress is normally minor, but can be
severe close to large bodies, especially if the vessel is
maneuvering. In deep space, away from gravity wells, a ship's
gravitic devices are tuned to minimize stresses on the vessel.
- ------------------------------------
Effects of Inertia on Maneuvering Vessels:
Inertia is a much more severe problem for maneuvering. It is easy
to imagine maneuvers that could quickly make almost any sapient
creature strictly two-dimensional. Additionally, rapid changes
in speed and direction cause enormous stress on the ship's frame.
We are most familiar with inertia as a force that tends to pull us
toward the outside of turns or pushed back in our seats when we
accelerate. The magnitude of the force we feel depends only on
how fast we are changing our speed or direction.
In a space vessel these inertial forces are almost always enormous
because the ship is travelling very, very fast. For instance,
making a 5-g (2) 180-degree circular turn at just over Terran escape
velocity (a veritable snail's pace in space) creates a circular
path over 10,000 km long, and a corresponding flight time of over
13 minutes to execute the maneuver (7).
Incidently, it would also take 13 minutes to reverse course by
rotating the ship 180 degress (it's now flying backwards) and
apply a continuous 5-g thrust. No one can turn around quickly
enough to surprise and overtake his pursuers.
High agility doesn't provide this ability either. Agility is
simply a measure of a ship's ability to change *facing*, not
to change it's speed and/or direction. That is, agility is a
figure expressing a vessel's ability to pitch, roll, or yaw.
"Maneuverability" is a figure that expresses, in g-forces, the
maximum linear acceleration a space vessel is capable of.
"Normally" (who knows what those TL 21 miscreants are up to!?)
this is a figure from 1 to 6 (10 to 60 m/s/s).
This means that space battles are inherently linear. Much like
the days of sailing, with ships in a line. Only in space, guns
can be aimed (turned away from the beam of the ship) and it is
possible to sail (fly) over and under, as well as around (or into)
your opponents. (Ships in a plane?) I guess there is a good
reason to teach formation flying!
- ----------------------------
Inertial compensators:
My first thought on seeing their availability was that this was
the way out of the inertia trap just presented. "I could build
a ship with maneuver 12 and inertia dampers 6, exposing the
maneuver drives to only 6-g acceleration," I thought. It doesn't
work that way though.
The maneuver drives, and the inertial compensators work on effects
of hyperspatial interactions. Their use causes space-time to warp.
Remember that it is acceleration, not force, that determines the
extent of warping of space-time within and around the ship. Because
of this, gravitic devices (like maneuver drives) can be of any size.
They cannot however, produce more than 6-g of acceleration. Period.
Inertial compensators don't do anything for the space-time warping
caused by the maneuver drives. In fact, they make it worse. As the
maneuver drive creates more acceleration, space-time warps more, and
more strongly. Inertial compensators counter-warp space-time locally,
and in so doing exacerbate the overall effect. Your ship's computers
must be bigger and faster to handle the load, and you suck up even
more fuel.
What this means is that you cannot design a ship with a maneuver 12
and agility 12 and inertial compensators 6. It would be nice if you
could, but those big massive driveplate antennas would wipe out all
of your available volume and then some. The best you can hope for
with inertial compensation is to provide a feeling of "motionlessness"
for your crew and passengers.
- -----------------------------------
Repulsers and Grav Plates:
These are basically straightforward maneuver drives, adapted for
special uses. Grav plates primarily hold things down, and
repulsers hold things away.
Grav plates are tied in with the inertial compensator circuitry.
Grav plates need to be large in area, but light and small in
cross section. They are used throughout the ship, and produce
from -6 to +5 g (if your maneuver drive is that good - if it
isn't, buy cheaper inertia systems) to compensate for the ship's
maneuvering. Notice that the plates can produce one less g in
the "down" direction (letting inertial hold you to the deck),
but that on "outside" turns, you will sometimes experience free
fall. That's just as much force as we can produce.
(The rules say so.)
Additionally, grav plates need to produce an exceptionally
uniform field. Studies as early as TL 7 terra (really!) showed
that if your perception of "down" changes often or radically,
you *will* get at least nauseated. This is commonly known as
sea- or air-sickness. It is especially acute when the victim
lacks outside reference and/or feels confined (like in a space
ship).
Repulsers are high energy, high curvature, effect projectors.
That is, they are maneuver drives with small area, high mass
plates. This produces a "beam" of gravitational energy, almost
always away from the projector.
"Reversing" the field effect (to make a tractor beam) usually
causes more damage to the ship trying it than to the ship in
tow. In my campaign, I allow limited use of "tractors" for
in-system towing around TL 16.
- ----------------------------
Maneuver Drives
Maneuver drive plates are usually mounted across the aft end
of a starship. In this configuration, they can provide the
most force in the most desired direction.
Region of Low Force
__
| ----
Region of| | ---------
High Force| |<-plates-------- Region of *really*
| | -------- Low Force
| | ---------
| ----
- -
Region of Low Force
Since the plates also deliver force to the sides, a ship can "hover"
horizontally if the gravity is not too high. This is how air rafts
work. Space ships eventually rotate into a more conventional
attitude to leave the atmosphere of a planet. (See the "Starship
Operator's Manual, vol. 1, page 3.)
The plates of the maneuver drive must be extremely large, to
"linearize" the fields around the whole ship, and must be massive
so that a great deal of energy can be pumped through them. Look
at the information about the mass and volume of the drives (in the
Referee's manual, page 65) based on ship's mass and maneuver
number. Notice that mass and volume increase linearly with ship's
mass, and exponentially with acceleration.
Maneuver drives take the most power of any ship's system. (Well,
a jump drive takes a lot, too, but it's antenna is the entire
ship's hull.) This is because of the sheer quantity of energy
being converted.
When in use, maneuver drives leave tell-tale perturbations in
space-time. Sophisticated sensors can detect the slight
perturbations in stellar signatures that indicate the passage
of a ship, and it's course.
- -----------------------------
Summary
Gravitics in the service of humaniti. By TL 10, gravitics
are as ubiquitous as computers. Every air raft, every ATV
(or AFV), every air craft, space craft, and starship use
gravitics for propulsion. Gravitics are great for hauling
and lifting heavy loads, and for medical assistance to the
invalid. And it all came about because spewing chemicals
into space is much too expensive a way to get anywhere.
- -------------------------------------------------------
Notes:
1.This equivalence courtesy A. Einstein.
E=mC^2
says, mathematically, that mass (inertia) and
energy are *equivalent* modified by a known
constant (the speed of light, C).
2.This calculation arrived a little later than
Einstein's. Planck gave us E=hu with h being
a physical constant, and u being the frequency
of the wave being measured.
But, frequency is (velocity/wavelength), so for
electromagnetic waves, E = hC/l with l being the
wavelength and C the speed of light.
For typical masses (grams - kilograms - tons),
a typical wavelength for the energy composing
the matter is ~10^24 meters. This is roughly
as many times smaller than an atom as an atom
is smaller than us.
Energy densities this high *cannot* exist in
normal space-time. We can only speculate at
how they might actually interrelate in dimensions
we can't measure. High-energy supercolliders
are attempts to release enough energy in a
short period of time to hopefully catch some of
the affects of these interactions "in the act".
3.This is where I actually begin "honest" Mega-
Traveller speculation about what it all means.
4.A "g" is defined as the acceleration due to
gravity at mean sea level on Terra
(Solomani Rim/Sol 1827 A867A69-F). This is
roughly 9.8 m/s/s.
5.Relativistic effects include time slowing down
(literally!) in some parts of the ship, disa-
greement at a molecular level what the length
and density of ship's (or crew's) members should
be, and so forth. In general, "relativistic
effects" means we become totally uncertain about
everything; we can't "know" anything without
altering the conditions upon which we "know" it.
6.Actually, gravity decreases with the square of the
distance. Newton's formula F=G(m*M/r^2) gives the
exact description of the force of attraction between
any two bodies, m and M at a distance r from each
other.
This rapid fall off in the potential for hyperspace
interaction is serendipitous. Imagine the trouble
at Capital if the Solomani figured out they could
cause space-time warping remotely because the
interactions they tap into in hyperspace reach that
far...
7.To compute the length of the path, we first determine
the radius of the circle (r) followed by the ship,
using the general formula for centripetal acceleration
(a = v^2/r). We assign a to 50 m/s/s (about 5-g) and
v to our ship's speed (13 km/s - roughly escape velocity).
Then:
r = v^2 / a
r = 13 km/s * 13 km/s / 0.05 km/s
r = 3380 km
Next, we use the formula for the circumference of a
circle to determine the length of the path travelled.
We cut our determined value in half because we only
travel half the circle.
d = 2 * pi * r
d = 0.5 (2 * 3.14159 * 3380)
d = 10 618 km
To determine the time it takes to travel this far,
we use the formula
t = d / v
t = 10 618 km / 13 km/s
t = 817 s
If we divide by 60 to get minutes, we arrive at
13.61 minutes to execute that U-turn.
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