My motivations for developing the Graphical Inertial Movement System (GIMS) are threefold:
I have attempted to keep the GIMS rules simple and have based everything on standard B5W stats wherever possible to maintain play balance. Of course any alteration in rules will result in some imbalances, but I think that they are not severe for this supplement. While the physics is not simulated exactly in GIMS, the rules come very close with minimal complexity.
This document is freely distributed as an optional supplement for use with the Babylon 5 Wars gaming system and is not intended to be a competitive product or to infringe on the copyrights or liscenses of Agents of Gaming who publish it.
The underlying concepts behind the Graphical Inertial Movement System (GIMS) are Newton's laws of motion, namely that an object in motion will keep moving in the same direction at constant velocity unless acted on by a force, i.e. thrust. Likewise an object that is rotating will continue rotating until a counter rotational thrust is applied.
Inertia is simulated in GIMS by tracking a velocity vector for each ship, indicating the current speed and direction of motion of the ship. This velocity vector is independant of the current facing of the ship. The vector is recorded directly on the hexmap by using an additional counter for each ship. This counter represents the projected location and heading of the ship at the end of the movement phase. If the ship makes no alterations to its velocity then it continues along this vector turn after turn. Example: The Vorchan cruiser is facing in the direction of the small arrow, while it is moving at a speed of 3 hexes/turn in the direction of the long arrow.
During each ship's movement phase, changes to the ship's velocity are plotted on the hexmap directly by rotating and moving the vector counter, not the ship counter. Once the changes have been plotted, then the resulting vector (drawn from the ship counter to the vector counter) is the new velocity vector for the ship. The ship counter is moved to the position and heading of the vector counter, and the vector counter is moved an equivalent distance in the same direction, setting up the initial conditions for the next turn's movement.
By placing vector counters directly on the hexmap it is possible for players to immediately assess the current movement status of all ships in play. This greatly simplifies strategy since players will not need to count out hexes to estimate where other ships in motion are likely to end up.
GIMS statistics are as closely based on the standard B5W statistics as possible. The Accel/Decel cost operates as in the standard rules. All ship rotations are based on the Pivot Cost and Roll Cost; the Turn Cost and Turn Delay are ignored. The Pivot Cost for ships with a rating of "n/a" is assigned to be (Accel/Decel Cost) (Turn Cost), which for most ships is equal to their given Pivot Cost. Ships that are not normally allowed to roll in the standard rules may not roll in GIMS either.
Velocity vectors for ships are plotted directly on the hexmap by using an additional set of counters. These vector counters should have an arrow (to indicate facing); sample counters can be found on this site in a PDF file. For large scenarios with many ships, it is useful to use numbered counters to match ship numbers. Vector counters may also be placed on top of duplicate ship counters to help identify which counter goes with which ship.
The only way to turn the ship is to execute a pivot, and the laws of inertia apply to pivots as well. Once a ship is rotating, it will continue rotating until thrusters are fired to halt the rotation. Unchanged from the standard B5W rules are the thruster combinations for pivoting (port/fore or starboard/aft to go clockwise and starboard/fore or port/aft to go counterclockwise) and the need to distribute thrust points evenly between the two thrusters.
It is possible for a ship to rotate more than one facing per turn. For each pivot thrust made during the same or subsequent turns, the rate of pivot increases by one facing per turn. The pivot is halted by applying an equal number of pivot thrusts in the opposing sense. Pivots are executed at the beginning of each movement phase and set the rate of rotation for that phase. Each player should keep track of the rotation rate of each ship on its control sheet.
Pivot costs are usually the same for every firing; if the ship's pivot rating is "3+3" then every pivot will take 3 thrust points. However, if the ship's pivot cost is listed as two different numbers then the cost alternates between the two values on each firing, independant of whether the rotation is being increased or decreased with thrust. For example, a ship rated at 3+2 would burn 3, then 2 thrust points to increase its rotation to 2 hex sides per turn, or it could burn 3 points to start rotating at 1 hex side per turn, then 2 points to stop the rotation (effectively this means the pivot cost is 2.5, alternately rounded up and down).
The maximum pivot rate of a ship is limited by its size; if a large vessel spins too quickly it will suffer structural damage due to the centrifugal stresses. Commercial/Civilian & Capital Combat Vessels are allowed one facing/turn while Heavy Combat Vessels and Medium Warships may rotate two facings/turn. Fighters and shuttles have no restrictions on rotation. A ship may exceed this maximum rotation limit but it will suffer structural damage from the strain. For every excess point of rotation roll 1d10 of damage and apply it randomly using this chart:
1: Fore Structure 2: Aft Structure 3-4: Port Structure 5-6: Starboard Structure 7-10: Primary StructureFighters and shuttles use a slightly different rule for pivoting. Because of their maneuverability, their pivots snap the ships around rapidly and do not result in any continuing rotation. They simply pay 1 thrust point per change in facing. Fighters and shuttles have no limits on the number of pivots they can make in a single movement phase.
As in the standard rules, the Accel/Decel Cost indicates how many thrust points are required to produce a change in velocity of 1 hex/turn. However, there are no facing restrictions on when a ship can accelerate! It can change its velocity regardless of how its facing compares with the direction of motion. Accelerations are generally allowed before each rotation of a ship's facing (agile ships may also accelerate after the final rotation in a movement phase).
Normally the main rear-facing thrusters are used for accelerations. When a point of acceleration is applied, the vector counter should be moved one hex in the direction it is currently facing. If that facing happens to be along the direction of motion then the ship will be speeding up (the vector counter will move further away from the ship counter), and if it is pointing opposite the direction of motion then the ship will be slowing down (the vector counter will move towards the ship counter). If the ship's facing is not along the direction of motion then the ship will be changing some combination of its speed and direction of motion (students of physics might notice this is vector addition).
The lateral (side) or retro (forward) thrusters are intended primarily for pivots and rolls, though they can sometimes be used for accelerations. Since they are generally not designed to handle the thrusts required to move large ships they must burn twice the normal number of thrust points when making accelerations. The retro thrusters will push the vector counter opposite the ship's facing, while the lateral thrusters can push the vector counter into either of the opposing side hexes (player's choice). Such accelerations are very costly and for many ships would require overthrusting for even one point of acceleration.
Fighters and shuttles follow the same basic acceleration rules. They also require twice the thrust to accelerate using lateral or retro thrusters. One exception applies to Starfuries; these craft have powerful retro thrusters and may freely accelerate either forwards or backwards. Fighters and shuttles may accelerate before or after any pivot.
Rolls are executed almost the same as described in the B5W rules. The main difference is that they are always initiated or terminated at the beginning of the Vector Plotting Segment. Once initiated, the ship is considered to reach its new orientation at the end of the Movement Phase. Until the ship terminates its roll it will keep swapping its port/starboard orientation at the end of every Movement phase. A rolling ship suffers a -3 penalty for weapons fire.
A rolling ship may not use its lateral thrusters while rolling (except to stop the roll). It therefore cannot make a pivot burn, though it does maintain its current rate of rotation from previous pivots and may use the fore and aft thrusters for acceleration.
Movement is executed for all ships within a ship class, in order of increasing initiative, as described in the standard rules. Each ship's movement can be broken down into several basic steps. The vector counter should begin the sequency pointing along the ship's current facing
Fighter/shuttle movement is simpler than ship movement since there is no carryover rotation from pivots. Thrust points may be expended to achieve any desired sequence of pivots and accelerations, plotting the effect on the vector counter. Once finished, counter movement is executed exactly as for ships.
In this example, two ships are moving. The first, an Omega Class destroyer, has a velocity of one hex/turn and is facing its direction of motion. The second, a Vorchan Cruiser, is facing the top of the map while its velocity is 4 hexes/turn mostly to the left; it is assumed to begin the turn with a pivot rate of 1 facing/turn in the clockwise direction. Note that the "X" in the figures is included as a stationary reference hex.
The Omega destroyer moves first. It chooses to merely accelerate along its current direction of motion. It starts with no pivot, so it applies its maximum thrust of two. Its vector counter is displaced two hexes forward, resulting in a new speed of 3 hexes/turn. At the end of the movement, the ship counter is moved to the final position of the vector counter, and the vector counter is moved the same distance ahead.
Note that if the Omega destroyer takes no further action in subsequent
turns it will now continue advancing 3 hexes/turn. To come to
a complete stop the ship would have to rotate a total of 3 hex
facings (180 degrees) and accelerate a total of three hexes per
turn. This would move the vector counter on top of the ship counter;
this indicates a stationary object.
The Vorchan executes a more complex move. First, it increases its pivot rate to 2 facings/turn by making an initial pivot burn. Now, before its first rotation it accelerates a single hex/turn at its current facing (1st arrow). Its first pivot is recorded by rotating the vector counter one facing clockwise. Now it applies a second acceleration of one hex/turn at this new facing, moving the vector counter one hex ahead. It now makes its final clockwise rotation (it may not accelerate after its final rotation). The ship counter is moved to the same hex and facing as the vector counter, and the vector counter is moved the same distance ahead.
Note that the Vorchan has not actually changed its speed in this maneuver. It is still moving 4 hexes/turn. The direction of motion has changed by about 30 degrees, however. While it has successfully closed on the Omega with forward weapons bearing, the new positions of the vector counters show that the distance between the ships will increase dramatically in the next turn unless the ships change their velocities substantially.
There are a few special case scenarios and optional rules that are included in the GIMS.
A number of turn rules and special-case turn modes are no longer relevant in GIMS and may be ignored. These include: Turn Cost, Flat Turn Cost, Turn Delay, Extended Turns, Snap Turns, and Slides. Thrusters (except for the limits on fore and lateral accelerations described above), Engines, and Overthrusting operate exactly as described in the B5W rulebook.
Initiative may be determined and played as described in the standard B5W rules. However, speed modifiers may be neglected since, by the laws of relativity, a ship that appears moving to one observer may be stationary to another.
Agile ships have two basic advantages in GIMS. The first, as described above, is the ability to make pivots and/or accelerations at the end of the Vector Plotting Segment as well as at the beginning. This makes it possible to better manage thrust usage. The second advantage is that their maximum rate of rotation is one larger than other ships of their class. Thus an agile Capital Combat Vessel could pivot 2 hex faces per turn instead of the typical 1 hex face per turn.
Many ships will waste one or two points of thrust every turn because their acceleration and pivot costs do not factor neatly into the available thrust points. To partly compensate for this, a ship may carry over a limited number of thrust points at the end of one movement phase to the beginning of the next. The limitations of this are twofold: (1) it can only be applied to the first acceleration burn that precedes the first facing change and (2) the number of carryover points can be no more than one less than the number of thrust points required for one point of acceleration from the main thrusters. The purpose of this rule is to simulate the effect of the thrust applied right at the end of one movement phase as it carries over to the next phase.
Should all the ships begin moving in the same direction at large velocities, it is possible to renormalize the velocities to keep them from drifting off the map. To do this simply shift all vector counters on the hexmap by exactly the same amount. This is not really changing any ship velocities; it is more like moving the hexmap "camera" to keep up with the action. This should only be done if there are no stationary objects like planets or asteroids in the scenario. Of course one can also recenter the action on the hexmap by moving all counters by exactly the same amount.
In a real combat situation using inertial movement, it is not reasonable to allow ships to disengage by simply moving 25 hexes or so away from the nearest ship. Generally speaking, a ship can only escape its pursuers if it can muster a larger acceleration every turn; if not, even if it adds significant distance between it and its pursuers, the faster accelerating ship will always be able to catch up. Rules for disengagement in a scenario should be modified to reflect this.
It is important for players to get the "feel" of an inertia-based movement system, perhaps by playing through a quick movement-only scenario before a full engagement. Often players are tempted to make too much course correction within a single turn. Rembember, one turn is only 10 seconds; capitol ships do not maneuver very much in such a short period of time!
Ships are most effective when they use intertia to do the work for them. This requires a little patience. Starting a slow pivot and waiting until the ship comes about to the desired heading is often much more useful than attempting more rapid pivots (some of which can cause structural damage). Likewise while it may be tempting to dump everything into acceleration to quickly engage an opponent, it is important to remember that it takes just as many turns to decelerate as it does to accelerate. It is very common to find ships careening out of control past the battle, wasting valuable turns trying to decelrate and return to the engagement.
Fighters can be quite powerful as they have enough thrust and maneuverability to keep up with almost any ship, usually staying within any desired firing arc. As such they exel at beating down the systems and defences on specific sides of a capitol ship
The current version of these rules should always be available
at my personal web site:
http://members.aol.com/roberthurt/b5wars.html
There you can also find a few other game aids as well as some B5W links (including the Agents of Gaming home page).
I'm happy to hear comments on GIMS and hope to keep the system updated as new B5W rules become available. Please let me know if you have tried the system and if you find it enhances the game. You can send e-mail to me at roberthurt@aol.com, but make sure the subject line doesn't look like junk e-mail or it will be deleted unread!
1/4/98 - Limited acceleration burns after final rotation to agile ships only.
2/4/98 - Streamlined rules and eliminated Inertial Vector Charts in favor of vector counters displayed directly on the hexmap.
This This material is ©1998 by Robert L. Hurt, though this document and associated graphics may be copied for personal, non-commercial use and freely distributed so long as they remain unmodified from their original form.