Real Thrust

The Physics Model Movement System for Full Thrust

Copyright 1995 by Mark Langsdorf

Purpose

The Real Thrust rules are meant to replace the standard Full Thrust movement rules with rules that more accurately model the motion of powered vehicles in a vacuum. These rules are not playtested nor or they guaranteed to be balanced, but they are physi cally more correct.

Assumptions

Real Thrust assumes all vessels have powerful drives that require minimal fuel as a function of ship mass- thus, ships' acceleration will not change during the course of the game, nor will ships run out of fuel.

Definitions

A ship's COURSE is the current direction of its movement.

A ship's FACING is direction that the ship's bow is pointing (the other side of the ship from its engines). A ship's FACING and COURSE do not have to be the same.

Example: A ship can have COURSE 8 and FACING 2, meaning that its nose is pointed in the opposite direction from its movement.

A ship's VELOCITY is the speed at which a ship's is moving along its COURSE, the same as in Full Thrust.

Basic Movement

Ships move in the direction of their current COURSE 1" per 1" of VELOCITY. No ship may have a negative velocity. A ship's FACING is irrelevant to its straight-line movement.

Example: A ship with COURSE 9, FACING 6, and VELOCITY 5 moves 5" straight to the left. Its weapons in the Fore-arc can cover ships at the bottom of the map.

Changing Course, Facing, and Velocity

A ship can expend 1 point of Thrust to change its FACING by up to 4 points. This change can be made at either the start or the end of the move.

If a ship's FACING and COURSE are the same, a ship can increase its VELOCITY by 1" per point of Thrust it expends. If a ship's FACING and COURSE differ by six (the ship is facing 180 degrees away from its COURSE) then the ship can decrease its VELOCITY by 1" per point of Thrust.

When a ship's FACING and COURSE are not the same and do not differ by 180 degrees, things get complicated. Attempts to accelerate will change the COURSE, and maybe the VELOCITY, of the ship. The amount COURSE and VELOCITY change depends on the VELOCITY of the ship, the amount of Thrust used, and the difference between its FACING and COURSE.

The following chart shows how much Thrust a ship needs to apply to change its' COURSE by one point. The ship's COURSE will always try to align itself with the FACING by using the shortest path. The ship's VELOCITY will usually change as it turns.

Timetable

VELOCITY 1 2 3 4 5
1 3+3V 1+1V 1 0* 0*
2 6+5V 2+1V 1 1 1-1V
3 9+8V 3+2V 2 2 2-1V
4 12+11V 4+3V 2 2 2-1V
5 - 5+3V 3+1V 2 3-2V
6 - 6+4V 3+1V 3 3-2V
7 - 7+5V 4+1V 3-1V 4-3V
8 - 8+6V 4+1V 4-1V 4-3V
9 - 9+6V 5+1V 4-1V 5-4V
10 - 10+7V 6+1V 4-1V 5-4V
11-15 - 12+9V 7+2V 6-2V 7-5V
16-20 - 10+7V 9+2V 9-2V 10-7V
21-25 - - 11+2V 11-3V -
26+ - - - - -
In order to read the chart, first subtract the smaller of FACING and COURSE from the larger. Cross-reference the number you get with the VELOCITY of the ship. The first number is the amount of Thrust necessary to change the ship's COURSE by one. The number after the +/- is the amount the ship's VELOCITY changes by. If a ship needs to change its COURSE by more than one point, just multiply the cost of changing one point by the the number of points changed and add one to determine the number of Thrust points necessary (the extra point represents changing the ship's FACING as it moves to keep the same difference of angle.).

Ship's with a velocity of 1 and a COURSE-FACING difference of 4 or 5 may change their COURSE by one point each turn at no cost. Changing it by two or three points uses one Thrust point, four or five by two Thrust points, and so on.

Moving a Turning or Accelerating Ship

When moving a ship whose VELOCITY changes during the turn, the total length of its movement is its VELOCITY at the start of the turn plus one-half its change in VELOCITY.

Ships that are changing COURSE make one-half the COURSE change at the start of their move (round down) and the other half of the CHANGE mid-way through their move.

Example A: A scoutship is moving at VELOCITY 5, COURSE and FACING 3, when it spots an opposing battlegroup. It wisely decides to disengage and starts to turn. It spends a Thrust point to change its FACING to 6, and then spends 6 Thrust points to change its COURSE to 5 (each turn increasing its VELOCITY by 1), and its last Thrust point to increase its VELOCITY by 1. From its original position, it turns to COURSE 4 and moves 3.25". It then moves to COURSE 5 and moves the remainding 3.25". At the end of the game turn, it has VELOCITY 8, FACING 6, and COURSE 5.

Movement Orders

The format for movement orders needs to be changed slightly. Instead of just P/Sx to indicate turns, use FP/Sx to indicate FACING changes, and CP/Sx to indicate COURSE changes. Acceleration is still marked with a + or -. Note that ships can only accelerate when they have the same FACING and COURSE, and they can only decelerate when their FACING and COURSE differ by 6.

Example A: The movement orders for the scoutship mentioned above would be FS3 CS2 +1.

Implications of Real Thrust

Space combat just got very complicated.

Ships have to travel slowly in order to be able to change course, and ships that are moving quickly will make very wide turns that will take several game turns to complete. Ships with high thrust ratings have an advantage- they can easily slow down in one game turn, make a tight course change, and accelerate again. A scoutship moving at Velocity 12 can decelerate down to 5, making a 180 degree turn, and accelerate back up to 12 in the course of four game turns. Comparatively, a dreadnought with 2 Thrust points and Velocity 12 will need about four turns just to slow down to Velocity 5, and then another six turns to change its Course, and then another four turns to speed back up. For most ships, it will often be more efficient to come to a dead stop, change Facing, and then start up again, especially for 4-6 point turns.

On the other hand, it is very hard to get "behind" someone, and not necessarily very useful. All ships can change their Facing by 180 degrees every turn, so even if you were in the Dreadnought's blind spot last turn, it will have turned to face you this turn. Maybe if you accelerated past it and turned around...

I'm not sure if these rules increase the tactical complexity of Real Thrust enough to justify the increase in the rules complexity. See for yourself.

About the Author

Mark Langsdorf is a junior Engineering and Computer Science major at Trinity University in San Antonio, Texas. He can be reached for questions, comments, criticisms, or flames via email at mlangsdo@io.com or mlangsdo@engr.trinity.edu. His qualifications for writing Real Thrust are twofold: an understanding of vector-based, mechanical physics and the willingness to write Real Thrust.

Real Thrust was posted to rec.games.miniatures.misc on 9/16/95. It is available via the World Wide Web at http://www.engr.trinity.edu/~mlangsdo/RPGs/FTRealThurst.html.