A quick check on my math in this old argument...

[Nerhesi]

Striking a target at distance in space:

Speed of light c, approx 300,000,000 m/s, or 300,000 km/h. Assuming a light speed weapon and sufficiently advanced turrets, sensors and computers:

Sensor ping to identify target at a distance (light speed).
Return sensor data confirming target distance/velocity/etc (light speed).
Some-sort-of-calculation (? time)
Aiming the beam weapon (? time)
Firing the beam weapon (light speed).

At very long range (50,000 km): Each light-speed traversal is 0.167 seconds, meaning a minimum of 0.5 seconds (half a second) assuming negligible calculation/weapon aiming time (perhaps this happening in the 0.333 seconds that the sensor ping is being sent and received)
At long range (25,000 km): 0.25 seconds - obviously half the above.

Obviously the above numbers are heavily skewed (that it takes zero time to calculate and aim a weapon at a target..).

Anyways the my question is regarding the calculation of the displacement of the target in that 0.5 seconds and 0.25 seconds. Assume 6g and 10g acceleration... is it simply:

6g = up to 14.7 and 4.3625 meters displaced?
10g = up to 24.5 and 6.125 meters displaced

 

[hdan]

Yep, your math is sound. Distance = acceleration * time^2.

Though I seem to recall that Traveller re-defined "1 G" as 10 m/s^2 instead of 9.8 m/s^2.

 

[Nerhesi]

Excellent - thanks. And can I get a quick followup calculation:

A spherical small-craft lets say, 50 displacement tons, would be approximately 5.5 meters radius? This is with the assumption that One displacement ton is approximately 14 cubic meters, and using V=4/3π(r^3).

Something recently made me rethink this when you put aside inertia (and the obviously slower than light speed calculation and aiming time), to see how much of a useful displacement a small-craft could have at current mongoose traveller combat distances. Of course, I believe 2300 AD uses much longer distances resulting in greater evasion potential.

 

[tolcreator]

This idea was hashed out in glorious detail on the Traveller Mailing List, something like 15 years ago... an alternate space combat rules for TNE and T4 were developed. If I recall it made meson guns much harder to hit with, as they have to aim for a volume rather than an area.

In TNE and T4 (I think), space combat was in hexes of 30,000 km (i.e. 0.1 light seconds) with close range for high end weapons and sensors being 10 hexes (300,000 km, 1 light second), giving ships much more time to not be where the attacker thinks they will be. Turns were 30 *minutes* long, so 1G of acceleration over the 30 minutes gave you a vector change (vectors! aiiieee!) of 1 hex/turn. It reminded me a lot of Harpoon (80s hyper serious hyper accurate naval wargame, which I played once or twice I think).

My google-fu is failing me in trying to find those alternative rules... I'll post again if I find them.

 

[AdrianH]

Yep, your math is sound. Distance = acceleration * time^2.

Distance = 1/2 * acceleration * time^2

The reason being:
velocity = acceleration * time

But the object is not travelling at that velocity the whole time; it starts at 0 and accelerates to that velocity.
average velocity = 1/2 * velocity
Distance travelled = average velocity * time = 1/2 * acceleration * time^2

Thus, for 0.5s and 0.25s:
6G = up to 7.35m and 1.84m
10G = up to 12.25m and 3.06m

 

[Anonymous]

If I may, a question on a tangent to this thread:

Does anyone allow/use passive sensors to get a targeting solution?

I don't know enough to know current tech and where its heading to extrapolate for a game set in the (far) future. I am thinking there will be a time that passive sensors will be fine to target from.

Any thoughts please?

(Nerhesi, I am happy to take this to a new thread if you prefer)

 

[Matt Wilson]

I would think, as hot as ships are compared to everything around them, that passive sensors would be pretty effective.

 

[Anonymous]

Yeah Matt, that's where I'm coming from but with Traveller having stealth in space the seemingly scientific approach of hot ships in an environment that's pretty close to absolute zero doesn't amount to much. If you disregard stealth, which seems the logical thing to do then I can see a few centuries advancement in passive sensors being more than up to the job. I was wondering how others dealt with active/passive or perhaps people just hand wave it and lump the different types into one and call it good.

 

[phavoc]

From what we have learned through our own technology slog, the detection aspect of sensor technology usually emerges first, and then later an offset spoof one is invented, and the detection one has to go back to the drawing boards.

Today the argument is "there is no stealth in space!" because of heat. Since nobody has yet invented a manned ship with anti-gravity technology, fusion power and the ability to travel between the stars, I'd say we have a lot of catching up to do before we say such things. Maybe in the future a way is invented that will convert heat to energy, or we find a better way to use hydrogen fuel to store heat, or who knows what.

We've already seen in reality how technology has changed what we used to think of as immutable laws. For now I'm comfortable with giving the benefit of the doubt and mixing those things together to make for a more enjoyable game.

 

[Anonymous]

Well said

 

[Nerhesi]

I probably misled with what I meant was sensor ping. By that, I meant the simple read-out of where the target is.

Even with passive sensors - My poor choices of words were pretty much just to describe that you'd have to wait some period of time before accurately knowing where you should tell your turret to aim...

Perhaps not for an unsuspecting a target, but for that is actively evading... Maybe passive sensors would shave off a small amount of time..

 

[Anonymous]

What's of interest to me is how a ship might perform many random small manoeuvres incidental to its general direction and how very difficult that would make it to hit as distances increased. I guess this is the evade programme in effect and with sufficient distance, control and manoeuvrability there would come a point where it would be impossible to predict where the target ship would be. You have to get up close and personal to take the sensor lag out of the equation.

Question is, does the Traveller gravitic drive allow such manoeuvres?

 

[Nerhesi]

It would have to. g is g after all - whether it is over a 6 minute turn of a 60day turn, it 10 m/s^2 (in traveller anyways), and we know that inertia/g dont apply to occupants (at least for up to 6g). So many violent maneuvers are a way of life (evasion software, pilot evasion, etc).

 

[simonh]

Passive sensors will tell you direction, but they don't give you an accurate range to the target because youd don't know how long the signal has been travelling for. With active sensors you know when you sent the signal and when you received it, so can calculate range.

Note that this means you can't use the enemy's active pings to determine range either, so the advantage goes to side using active sensors.

Simon Hibbs

 

[tolcreator]

I sorta roll my eyes at games like Elite Dangerous (despite playing it obsessively) when they have big ships be slower than small ships. I smugly think, Traveller has it right, because if your ship is twice the size, you can fit engines twice the size to get the same accel.

HOWEVER!

Can those engines apply their thrust in any plane? Or only out the back? If they're reaction drives a la TNE, obviously only out the back. The Megatraveller starship operators manual mentioned how they could do 10% forward, 25% laterally, and 100% out the back.

If you need to turn the ship in order to change course, suddenly there IS an argument to be made that smaller ships are more nimble than larger ones: because a smaller ship has a much smaller moment of inertia. You can't just put x2 engines onto your x2 size ship, because the moment of inertia scales not just with mass, but with length as well.

So a small ship can easily change heading, to make those random course changes that make it harder to hit. But a big ship turns slower. Not sure how you'd represent this in game of course... just how much torque can can the gravitic engines apply to a ship?

If we take the megatraveller "25% lateral thrust" as canon, then a 4G ship can apply 1G of lateral thrust. Take say a 200 ton spherical hull... it has a radius of 8.74 meters. A 200 ton ship has a mass of roughly 2800 tons. Warning: Very very rusty (and googled) physics to follow. Please feel free to point out mistakes!

Moment of inertia of a sphere is 2mr^2/5 = (2 (2.8 * 10^6) (8.74)^2)/5 = 8.4 * 10^6 kgm^2

Torque of our "1G accel" applied at 8.74 meters:
Force of ~2.8 * 10^7 N
T = 2.4 * 10^8 Nm

We get an angular accelration of 28.6 radians/s^2. We want to flip by say, 90 degrees. That's spin up for 45 degrees, then spin down again. 45 degrees = 0.785398163 radians, so it will take us... damn. Ok lets pretend this is a linear accel of 28.6 m/s^2 and we want to go a distance of 0.785398163 meters...
s = ut + 1/2 at^2
0.785398163 = (0) + 1/2 (28.6) t^2
t = sqrt (0.054922948)
t = 0.234 seconds

So total time to flip 90 degrees is: About half a second. In a turn of 6 minutes (or 30 minutes), well, that's not a lot. I think I've just convinced myself out of my own argument: Even for a very large ship, the time taken to change course is tiny compared to the time needed on that course to clear the ships own diameter.

(ok take a very large ship: a 200,000 ton battleship, also with 4G engines).
Radius is now: 10 times larger. Mass is now: 1000 times larger.
Torque is 10,000 times larger
Moment of inertia is 100,000 times larger
Angular Accel is 10 times less.

Time taken is sqrt(1/10) more.
So a big ship like that will flip 90 degress in... 1.5 seconds.

Ok! Assuming I've not made any horrible mistakes, I've utterly dismantled my own argument. It doesn't matter really how big your ship is, as long as Gs are the same.

 

[Reynard]

We regularly assume Flash Gordon engines (except the smoke doesn't rise 'up') with fins directing the flow for direction by the looks of ship drawing in Traveller. Traveller has described what and if there's something exhausting out the back and how it moves a ship so it's the ten people giving eleven answers.

Mongoose Traveller uses reactionless drive plates which mean there is no reaction mass to throw out the back. Those ports in back are either energy sinks from the reactor or... they look cool. Drive plates push and pull on the ambient quantum wells around it but that's fairly macro. The ship would also have thruster ports all over to pitch, yaw and roll the ship for fine maneuvers.

 

[hdan]

Distance = 1/2 * acceleration * time^2

I wasn't going to respond so late in the thread, but I felt a "mea culpa" was in order, along with a "thanks AdrianH!", and "Yes, what AdrianH said!"

Sigh. I must be getting old to forget even this basic of a calculus problem.

 

[Nerhesi]

Whats interesting is that M-Drives are not reaction engines, so you must assume 6G in any direction. I only base this assumption on the fact they're not really.. throwing anything out the back. So technically, one could assume it's like some-ring around the craft providing that movement.

Of course - this isnt supported by the floorplans of any ship at all Just by the fact that M-drives dont have to "throw anything out the back".

 

[Anonymous]

Yah know, that's exactly what I've been trying to say for the last 24 hours.

Traveller meets disc shaped space ships capable of manoeuvreing in any direction almost instantaneously.

Oh my!

Roswell was true!

 

[Reynard]

There have been illustrations of the M-drives including deck plans and it's been mostly the imagination of the artist. Many illustrations show the engines aft and contacting the hull. Still, Mongoose does differentiate by having reaction (rocket) drives as an alternate. I know one edition of Traveller describes the drive plate 'pushing' the craft forward and has some force that drops off from the devise's center allowing maneuver as well as thrust.