Tactical Movement in Space Combat

[Ezra]

The best I can come up with is that a ship with 1G sAccel moves 1 hex. Every time I work on my space campaign, I hit a snag here. None of the books explicitly state how many hexes the ship can move per turn, but I feel the "Slow Maneuvering" section at the top of p. 222 in Traveller: Interstellar Wars implies such.

I don't know why the writers were so cryptic about movement (in both Traveller: Interstellar Wars and GURPS Spaceships 3). In Interstellar Wars, it says that 1 hex a round is 33,000 mpg, and the very next sentence it says "acceleration is measured in Gs" (p. 219). Nowhere does it say how many mph a G is though. If you look it up and do the math it doesn't add up.

In GURPS: Spaceships 3 the rules assume you're using delta-v, but most space settings have reactionless thrusters, and Traveller definitely does.

I can't help but feel this is broken. I wish they had just said, a ship can move x, y, or z hexes with b movement in space combat.

[Anthony]

Originally Posted by Ezra

Nowhere does it say how many mph a G is though.

That's because it's a unit of acceleration, not a unit of speed. One G for one second is 22 mph, one G for one hour is 79,000 mph.

[Anaraxes]

There's a fairly simple way to track movement with a couple of counters, seen long ago in GDW's Triplanetary and later in other games like Mayday.

Each ship has two counters, the ship's current location, and its current vector on the map, which is to say its future location if it just keeps moving -- some distance and direction from the ship. Acceleration moves the vector counter, not the ship, some number of hexes. When it comes time to relocate the ship, the ship moves to the spot the vector counter is on, while the vector counter moves the same distance and direction the ship does. (You might find it easier to use a temporary counter to mark the location of the ship; move the ship to the vector; then move the vector marker so it reflects the temporary counter on the other side of the ship.)

A ship's G rating can then be used to count hexes the vector counter moves on the map.

There was also a somewhat less elegant system used in the SSI game Battlefleet: Mars, which played on a square grid, and in which each ship had two counters underneath showing the X and Y components of its movement. Acceleration could be used to add or subtract to those numbers. Movement just consisted of relocating the ship (and its movement counters) however many squares in the X and Y directions the counters said. Doesn't handle thrusting in arbitrary directions as well, but the map isn't cluttered with all the future position vector counters.

[Ezra]

Originally Posted by Anaraxes

A ship's G rating can then be used to count hexes the vector counter moves on the map.

So, if I'm reading this correctly, a ship with 1G sAccel could move 1 hex/turn and a ship with 2G could move 2 hexes. A ship with less than 1G doesn't move every turn (per "Slow Maneuvering" on p. 222 of Traveller: Interstellar Wars or "Slow Acceleration" on p. 166 of GURPS Traveller 3rd ed.) That makes sense to me.

[Fred Brackin]

[QUOTE=Ezra;2316388]So, if I'm reading this correctly, a ship with 1G sAccel could move 1 hex/turn

No. A ship with a velocity of 1 hex per Turn (whatever your scale is) can move 1 hex per turn unless they change that velocity by accelerating or decelerating.

Acceleration is the ability to change velocity. It's is not a velocity itself.

[Ezra]

[QUOTE=Fred Brackin;2316390]

Originally Posted by Ezra

So, if I'm reading this correctly, a ship with 1G sAccel could move 1 hex/turn

No. A ship with a velocity of 1 hex per Turn (whatever your scale is) can move 1 hex per turn unless they change that velocity by accelerating or decelerating.

Acceleration is the ability to change velocity. It's is not a velocity itself.

There's no cap on how many hexes they could theoretically move then? Only on the rate at which they can move more or fewer? The GM decides arbitrarily how many hexes per turn they were moving? How do I tell the players how many hexes they can move?

[RyanW]

Originally Posted by Ezra

There's no cap on how many hexes they could theoretically move then?

Only how long they can accelerate - usually based on fuel.

One thing to keep in mind is that in space, there isn't really a stationary. You can declare any inertial frame of reference (i.e. anything that is moving at a steady speed) to put your hex grid on, and only worry about motion relative to that. If everything is in a similar orbit at five miles a second, just call that "stationary."

[Anders]

[QUOTE=Ezra;2316392]

Originally Posted by Fred Brackin

There's no cap on how many hexes they could theoretically move then? Only on the rate at which they can move more or fewer? The GM decides arbitrarily how many hexes per turn they were moving? How do I tell the players how many hexes they can move?

The speed of light is the traditional cap.

[Balor Patch]

[QUOTE=Ezra;2316392]

Originally Posted by Fred Brackin

How do I tell the players how many hexes they can move?

They can move the same number (and direction!) of hexes they moved the previous turn, plus or minus their acceleration.

Expect combat to involve fast passes rather than dogfights.

[Anaraxes]

Originally Posted by Ezra

The GM decides arbitrarily how many hexes per turn they were moving?

On the very first turn? Yes. Your battle scenario has to have some sort of starting conditions, and if you don't know what those are, you're going to have to make them up. Ships will start with some velocity (speed with a direction), even if that velocity is zero.

- If ships are in orbit, they're moving at orbital velocity. That depends on the mass of the planet and the height of the orbit.
- If the ships are between planets, and "stopped", they're still in orbit around the central star, moving at a significant pace. But you probably ignore this value for the sake of combat, since all the "stopped" ships would be moving at that same speed, and it's really the relative motion that counts -- until there's something you can run into.
- If they're travelling between planets, they're likely to be moving pretty fast relative to "stopped". This will depend a lot on the technology. Ships capable of continuous thrust that's not a tiny fraction of a G can build up quite a high velocity (speed).
- If the ships just "jumped in" to a system, they might be at a dead stop relative to the system, or an orbital velocity appropriate to their destination world, or retaining the relative velocity of their departure system (probably a bad idea :)). Those details depend on the pseudo-science tech the setting uses.
- If the ships "warp in" with some sort of FTL drive, the setting again tells you what that looks like from a normal-space point of view. Perhaps those warp bubbles do come blasting in, but lurkers can't tell because they're, well, FTL -- unless you throw in some FTL sensors.

i don't know whether you're looking for a "realistic" answer, nor what sort of "reality" your game is set in. So it's hard to give an answer like "all ships begin combat moving at 5 hexes per turn directly toward the opponents' side of the map".