I considered vector movement for the TL9 ships of Orbital, but started going crazy trying to tie in the rules in the core book. Instead I compromised and based the combat rules on the Core book, with one or two significant amendments to give the sense and the feel of slow, rocket-based space combat.
We talk about 'burn allocations' rather than thrust ratings, ranges are reduced and there is a section on trajectory, which I will paste here:
Traveller players may be used to encountering other ships in space, hailing them, and then rendezvousing to dock, ever in deep space. In Orbital, the trajectory is king. What the pilot has decided to do with the ship dominates its entire voyage and makes deviations difficult, if not impossible. Imagine a DSV as a bullet, fired from a rifle that slows itself down to come to rest as it reaches the target. If it spies an enemy bullet shooting across its path, it can do little about it! It has used a third of its fuel in the acceleration, and saves another third for the deceleration. Space combat out in the vacuum between planets is virtually impossible. Space combat occurs around, or near, worlds. Spacecraft do not move wherever they please, as if on a hex map. They are all in orbit around something, even if it is the Sun. All vehicles are moving in circles or ellipses. Few are pointing toward their destination, they are arcing their way there in long orbital curves around the Sun or a planet. Instead of thinking about starship combat as a hex map, it is more helpful to think of spacecraft travelling along a racecourse around the Sun, the Earth, the Moon, Jupiter, Titan, etc. Each world has its own racecourse of tracks, each track a higher and higher orbit. Flying from Earth to Mars, for example, could be seen as leaving one racetrack, to temporarily follow the Sun’s racetrack, finally arriving at Mars and joining that planet’s racetrack. Everything is circles and ellipses.
We assume that the attacker and the target are in approximately the same orbit, going the same direction. Of course vehicles can change their orbit to intercept if desired, but this takes time ... and fuel. All space combat is interception, an aggressor is closing on a stationary target (perhaps a OPM in orbit), or on a fleeing target. Sometimes the aggressor may be stationary and not manoeuvring, waiting for a target vehicle to approach. When two craft are moving towards one another, in the Orbital setting, this is achieved by the aggressor accelerating toward the target and the target carrying out braking manoeuvres to slow itself. None of this alters the rules given on TMB p. 147, but should always be borne in mind.
