Star Trek II
CUT TO BRIDGE, USS ENTERPRISE.
KIRK: He's followed me this far, he'll be back. But from where?
SPOCK: He's intelligent, but not experienced. His pattern indicates two dimensional thinking.
KIRK (SHOWING SLIGHT TRIUMPH): Helm, Z minus 10,000 metres; stand by photon torpedoes.
And so marks the beginning of the end for USS Reliant, and Khan Noonian Singh.
Although the distances between starships in the usual space combat are huge, measuring hundreds of thousand, if not millions of kilometres, there are occasions when the combat is at such close quarters that the third dimension will make a difference. The above is just one example, where one combatant (Khan) used WWII naval tactics, whereas the other (Kirk) took advantage of the special circumstances of space combat.
On the whole, though, in wargaming starship combat, the third dimension is, on the whole, ignored. Why?
Well, the most obvious answer is simplicity. A major problem with using the third dimension in space combat, is that, unlike aircraft combat, there is no "minimum" (i.e. the ground) or "maximum" (i.e. the aircraft's ceiling) altitude. Two starship models could be an inch apart, but due to "height" differences, separated by two or more feet. The second problem is firing-arcs. Exactly how far can a phaser bank elevate? Are disrupters fitted on swivel mounts? However, with the world of Star Trek, most of these problems can be overcome.
These rules are an addition for Star Trek - Full Thrust, although they can easily be adapted for any rules or universe in use.
There is a Level 0, that represents the standard "Galactic Plane", from which all starship navigation is taken. The other levels run from +30, to -30, giving a total of 61 Levels. Each level, for firing purposes, counts as one inch of range, and when firing, the number of levels between the two vessels are counted.
Example: USS Excelsior is at Level +8, at a distance of two inches from a Klingon Bird of Prey. The Klingon vessel is at level -2, so the overall range is 13 inches ( two inches, plus eleven inches for the 'height" difference). When calculating the height difference, Level 0 is also counted if passed through.
The firing arcs do not have to be revised. The USS Enterprise is quoted in most technical journals as having six phasers, in three banks of two. However, if you study the model, she has three banks on the underside of the saucer, and three banks atop the saucer. My own personal viewpoint is that she has six phaser "power cells", but twelve phaser "firing points" This means that only six phasers can be fired at a time, but it enables her to fight effectively in three-dimensional combat (a blatant fudge, I know, but it works for my purposes. And if anyone mentions the Enterprise-D in "Best Of Both Worlds".... ).
Okay, so on this one, there is no easy answer. The best that I can come up with is this. Any starship, regardless of velocity, may travel up or down one level, without using up any movement. Up to half of a starship's velocity may be expended on vertical movement, although this is not lost (a starship is not fighting gravity in a climb). Additionally, a starship may expend Thrust points for level changes.
Example: A starship travelling at a velocity of ten, is being pursued by three enemy vessels on the same level. In an attempt to confuse the pursuers, the Captain elects to try to alter levels. The starship has a Thrust Rating of 4, and applies all that to increase levels. She also uses it's free altitude change, and half it's movement, to gain a net increase of ten Levels. This may seem a trifle excessive at first, but, in theory, a starship could do a "course change,, to straight up, at whatever velocity the ship is travelling at currently.
Another urgent note: Any ship that travels either "above" Level +30, or below Level -30, has disengaged, and uses the regular rules for attempting to rejoin the battle.
Well, there you have it, I leave the means of marking the starship's height to you. One good way is to use "poker chips", one colour for positive, and a different colour for negative Levels. Another way is to use dice, but even scribbled notes on paper will suffice.